10.2.1 Preinstallation Dos and Don’ts for Tray Columns

Inadequate preparation of trays prior to installation may prolong the turnaround, adversely affect column performance, and even lead to safety issues. The guidelines below (43, 192, 307, 319) can help minimize these problems:

1. Installation of trays while the tower lies horizontally is generally not a good practice. Saving by shop-installation is often offset, or surmounted, by reinstallation and repair costs. One can get away with it in small (<10 ft ID), short towers. In towers with larger diameters, it is difficult to reach the top of the trays to inspect them. Taller towers (>60–80 ft) often acquire a “banana” shape while horizontal, which throws off dimensions, requiring reinstallation after the tower is raised. Finally, internals can get damaged during transportation and uplifting.
   
2. Adequately detailed installation drawings need to be available prior to assembly.
   
3. The tray supplier should be required to clearly identify all parts and pack them separately for shipment.
   
4. Tray parts should not be removed from the crates prior to installation. Early removal can lead to rusting, dusting, or loss of tray components. The crates should be stored and kept in a dry, covered area.
   

   Carbon steel trays and internals need protection from a damp atmosphere. Inside storage is recommended even when these internals have surface protection. Stainless steel does not require such protection, so the conventional wooden crate with tarred paper or plastic lining is often considered adequate (25).

   
   
5. Valve tray panels should never be shipped or placed “cap to cap” or “legs to legs” in order to prevent interlocking of valve floats. Panels with interlocking valve floats are extremely difficult to separate without damaging the valves (307).
   
6. Masking tape must not be used as flange covers. In one case (222), an erratic reboiler action resulted from a piece of masking tape left in a reboiler flange. Plastic flange covers are better because they need to be removed before bolting.
   
7. It is a good practice to order about 10% spares on bolts, nuts, and clamps in case some become lost or damaged. A higher percentage of spares is often advocated for some fouling or corrosive services (192). In such services, spare trays are sometimes justified in order to minimize downtime (319).
   
8. Construction supervisors should be familiarized with the column internals, their functions, and any unique requirements of the service. For instance, the construction crew should be made well aware of any collectors or seal areas from which leakage must be minimized. They should also be alerted to the common installation traps that deserve specific attention, as discussed later in this chapter.
   
9. A mock-up tray installation outside the tower is a valuable tool for familiarizing the installation crew with tray parts and training them in the installation procedure (319).
   

   This is imperative with specialty and proprietary trays. Supplier drawings often omit key details, in some cases due to confidentiality concerns, leaving installers with no idea how certain parts fit in. The mock assembly unveils such details in time to contact the suppliers and obtain their guidance. In one case (43), such a mock installation identified a fabrication error that would have prevented the high-capacity trays from attaining their high capacity. Early detection enabled a remake within the time constraint. The author had similar experiences.

   
   

   A mock-up installation on the ground is also useful in other complex tray designs. In one case (43), a mock-up installation identified an issue with a modification of a complex chimney tray that decanted water. Had this modification been installed as shown on the drawings, the tray would have been dysfunctional. In another case, a mock-up installation of specialty trays at the vendor shop allowed a thorough inspection and was invaluable in reducing the tight installation time in a revamp.

   
   
10. Prior to any work inside the column, it is essential to implement measures to prevent small parts such as bolts, nuts, and clamps from finding their way into downstream equipment, such as heat exchangers, pumps, and control valves. In one case (192), a piece of rope ladder reached the bottom pump inlet and lodged there, causing the pump to frequently lose suction. Temporary plugs in the tower base and other drawoffs are effective in preventing such incidents. It is imperative to positively ensure that these plugs are removed prior to startup.
    

    In addition, temporary strainers can be installed in outlet lines, especially those feeding pumps. Plugs are more effective than strainers alone, because some debris can get through strainers or damage strainer elements and then pass through them. In one case (434), blockage by debris broke strainers, and pieces of strainer casings damaged the pump. Strainer casings should be mechanically strong enough to withstand pump suction when fully blocked.

    
    
11. Keep track of all replacement parts’ orders, especially small parts. Keep a record of who ordered them and when, who received them and when, and where they went after receipt. Disappearance of items after receipt is not uncommon, as described by two case studies (42).
    
12. In one case (360), where many specialty trays were to be installed in a very short time interval, a training column at the vendor shop was used to train the crew.

10.2.2 Preinstallation Dos and Don’ts for Packed Towers

Inadequate preparation of packing prior to assembly may prolong the turnaround, adversely affect column performance, even lead to safety issues. The guidelines below can help minimize these problems. Steps 2, and 4–7 primarily apply to random packings; steps 1, 3, and 8–10 apply to both random and structured packings.

1. Installation of packings while the tower lies horizontally is bad practice. Distributors cannot be properly leveled, and the packings can be easily compressed and damaged in transportation and when the tower is lifted. Wall wipers are one of the weakest links and get damaged most easily. Random packings cannot be randomly installed.
   
2. New packings often have an oil film coating. The oil film may be lubricants used in the packing press or used to inhibit packing corrosion during shipping or storage. For sea transportation of carbon steel packings, an oil coating is often necessary (e.g., 143). This oil film may inhibit wetting of the packing surface, especially in aqueous systems. Certain lubricants may induce foaming in high-pH aqueous systems. The oil may also be a fire hazard during hot work or hot commissioning/startup operations.
   

   It is necessary to be familiar with the nature of the oil and, if needed, to adequately remove it. It is best to seek the supplier’s advice on the preferred removal procedure. Alternatively, the supplier can be requested to use a water-soluble lubricant in the press, which can be washed during commissioning, or to degrease the packing with solvent after pressing. Early removal of the oil may cause corrosion and should be avoided.

   
   
3. Packings should be stored and kept in a dry, covered area well protected from sunlight. Packings left standing in the rain may corrode or oxidize rapidly. Oil-coated packings may collect dust. In the sun, metal packings heat up and the oil coating becomes fluid and flows down until trapped, forming hot oil pools, which are a fire hazard. Plastic packings may be attacked by ultraviolet rays from the sun.
   
4. Drums used for packing storage should be clean and free of chemicals that may attack the packing or of materials that stick to the packing surface and later inhibit liquid film formation or cause undesirable effects (e.g., foaming). Oversized containers should be avoided as they are hazardous to workers lifting them.
   
5. New or reused ceramic packing should be screened to remove broken pieces. In some cases (267), up to 40% of the ceramic packings were damaged during transportation. Experiences of damage to ceramic packings during service are abundant (31, 68, 192, 222, 421).

   

   
   

   Figure 10.1 shows a few samples of ceramic saddles as received from shipment. The breakage (Figure 10.1a) is mainly of chipping at the corners (267). The large pieces can still be used, but the chips must be screened out as they are likely to lower the column capacity and increase its pressure drop. Screening must be carefully performed and closely watched; otherwise, it may cause more particles to break than it removes. In some cases (e.g., 267), it is necessary to pick chips out by hand. Figure 10.1b shows the size nonuniformity of saddles of a single nominal size as received from one shipment.

   
   
6. Plastic packings should be checked for brittleness. Plastic can become brittle in some environments and temperatures. Grab a few pieces and squeeze them to see whether they shatter. In case of problems, the supplier should be contacted.
   
7. About 10% of the spare packing volume (in case of ceramic packings, about 20%) should be ordered. The packing volume supplied is usually based on the supply containers’ volume. Typical packings supplies are in 1- or 2-ft³ boxes or 25-ft³ shipping containers. When emptied into the column, the total packed height may be less than the specified height (267). The difference may be due to unfilled space near the walls of the box, underfilling of boxes, interlocking of packing particles, compression of packing when the column is filled, and using dry packing techniques. The author is familiar with cases where installed packed beds were shorter than intended.
   

   Instead of ordering spare packings, the supplier can be requested to provide enough packings to fill the bed to the required height. The supplier will then allow for the spare volume. With ceramic packings, additional allowance should be made for any breakage occurring past the point where the supplier’s responsibility ends.

   
   
8. In reactive chemicals services, the resistance of the packing materials to a chemical attack should be audited by lab testing under simulated process conditions. The ability of a ceramic or plastic packing to weather a chemical attack depends on its texture and composition, and this varies with the manufacturing process. Porous surfaces may be more prone to attack than smooth surfaces. In one case (421), samples of ceramic packings from different suppliers were lab-tested over several days under simulated hot potassium carbonate (hot pot) absorber–regenerator conditions, revealing wide variations in the rate of loss of silica from the packing. The silica loss from one sample was so rapid that the packing was branded unsuitable for the service. Packing deterioration in service can induce fouling, plugging, and poor performance.
   

   Before selecting a supplier, manufacturers should be requested to submit samples for testing, and any unsatisfactory test results should be discussed. A desirable (or acceptable) performance specification should then be developed and included in the purchase order. Samples of the final shipment should be retested.

   
   
9. When replacing trays by packings in a tower section, it is ideal to remove all internal support rings and downcomer bolting bars to approximately ⅜″ to ½″ from the shell (68, 174, 307, 340, 462). Complete removal (i.e., grinding flush) is expensive and time-consuming, can damage the tower wall, and is rarely justified.
   

   Horizontal support rings left in the tower interfere with liquid distribution and reduce the available open area, with possible adverse effects on packing efficiency and capacity. Straight downcomer supports or other vertical bars are generally far less detrimental to distribution and open area, but make the installation of structured packings difficult. These are usually removed only if they are expected to adversely interfere with the new internals, their installation, or liquid flow through them (461). Some designers (340) prefer to always remove them. Inward projections of manholes can also interfere with the installation of structured packings, in which case they need to be removed (84).

   
   

   Strigle (461) states that with random packings, tray support rings need only be removed when they occupy more than 10% of the column cross-sectional area. Strigle’s recommendation for the maximum acceptable support ring width depends on column diameter, as shown in Table 10.1.

   
   

   In one 13.5-ft-diameter column (413), random packings performed well even though the tray support rings were not removed. In another case (432), the replacement of poorly performing trays by random packings in an amine absorber was only partially successful in improving performance, presumably because the support rings and downcomer bolting bars were not removed.

   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   

   Column diameter (ft)	Acceptable ring width (in.)	Excessive ring width (in.)
   4	1¼″	1⅝″
   6	1⅞″	2⅜″
   8	2½″	3¼″
   10	3⅛″	4″
   12	3¾″	4¾″

   
   

   With structured packings, it has been recommended to always remove the support rings prior to packing installation (174). Tests in a 3-ft-diameter column with a 14-ft-tall bed of structured packings showed that leaving in the 2″ tray support rings resulted in a 10% efficiency loss and a 40% rise in pressure drop. In larger columns, the support rings usually occupy a smaller fraction of the column area, and the pressure drop rise is likely to be smaller. The efficiency loss may escalate in taller beds due to distorted distribution profiles and in larger-diameter towers due to the higher ratios of tower diameter to packing diameter, which diminish the effect of lateral mixing (Section 4.3).

   
   

   In order to minimize irregularities and dead liquid pockets at the tower wall, it has been recommended to internally blind unused nozzles (340) in the packed zone.

   
   
10. Closely review possible interference of manholes with distributors, redistributors, and parting boxes. The location of manholes may require distributors or parting boxes to be offset from the feeding nozzle, leading to challenges and errors during installation (84).
    
11. Steps 2 to 4, 6 to 8, 10, and 11 in Section 10.2.1 also apply to packed columns and their internals.

10.2.3 Removal of Existing Trays and Packings

Inadequate removal of existing trays or packings can endanger workers, damage equipment, and prolong the shutdown. The following guidelines can help minimize the above problems:

1. The likely conditions of the internals, such as degree of fouling, corrosion, toxicity, and potential trouble spots, should be evaluated. This information can be obtained from past inspection reports, by questioning operating personnel, from similar columns, from pressure drop measurements, and from gamma scans. Based on this evaluation, the proper tools, procedures, work and personnel schedules, and safety equipment can be planned.
   
2. Before removing trays or packings, obtain as much information as safe practices would allow about the condition of the trays, packings, distributors, nature of deposits, corrosion, and damage.
   
3. The ability to remove tray panels and other internals through the manhole should be carefully reviewed. If not possible, the old trays may need to be burned out. In one case experienced by the author, this burning out placed a relatively small tower on the turnaround critical path. In another case (398), the time required for tray removal in two large towers increased from five to nine days due to the need to oxy-cut tray parts.
   
4. When the old trays are to be burned out, additional special safety measures and equipment (e.g., fire extinguishers inside the column, explosivity monitoring, good forced-air circulation for removing fumes, water flooding, tarps) are likely to be required. Steps also need to be taken to prevent insulation damage due to overheating. If the tower wall is fitted with wall cladding for corrosion prevention, flammable material may be trapped behind the cladding, and burning trays out is best avoided.
   
5. When pyrophoric deposits are expected inside the column, all trays should be flooded with water before dismantling. The water level should be progressively lowered to just below the tray that is being cleaned and dismantled. Any freshly exposed tray or wall section should be immediately cleaned.
   

   The air above the water level should be continuously monitored for explosivity and toxicity. When a hydrocarbon or water-insoluble organic is present in a small concentration in the water, the high repulsion of the water phase imparts a very high activity coefficient to it. Its volatility becomes several thousand times that of the pure component, which sends it into the air, where it can exceed the lower explosive or the allowable exposure limit.

   
   
6. When removing damaged or bumped trays, special safety precautions are required. Often, pieces are left hanging, and even those in place would not support a worker (307).
   
7. In corrosive services, workers should bring various sizes of sockets (307) as several bolt heads will be corroded.
   
8. Some experiences have been reported (307) on blowing highly plugged or coked trays out by explosives. This technique is infrequently used and not always successful (307). There have been cases where the explosions damaged support beams and tower walls.
   
9. Hand-cleaning and high-pressure water jetting are the common methods of cleaning trays. Water jetting is reported to work well for sieve and valve trays and for cleaning the shell and support rings after the trays are removed (307), but not for bubble-cap trays (307).
   
10. Removal of random packings from the column is usually performed by opening a manhole or handhole located right above the packing supports and letting the packing pieces roll into a collection drum. Special care is required with ceramic or carbon packings to avoid breakage.
    
11. Suction equipment is often connected to the manhole (or handhole) via a wide flexible tube to speed up the removal of random packings. This is especially important in large towers. If the packings have sharp edges, the tube should be fabricated from metal, not plastic, and care should be taken not to damage the tube. This operation can damage packing particles, especially with thin-gauge packings, but usually, the damage is slight (407) when done properly. The author had one experience with a Pall-ring type packing where the damage was quite significant, with many pieces caught in each other and unrolled; maybe the operation was not done properly. Care should be taken to minimize packing damage, but having replacement packing pieces on standby is worth considering. This operation is practiced with plastic and metal packings and should be avoided with ceramic or carbon packings where it is likely to cause breakage.
    
12. With columns constructed in flanged sections, removal of random packings is usually done by dismantling the top distributor (or redistributor) and retaining device, then lifting the packed section (including the support plate), and tipping it over. Extreme caution is required with ceramic or carbon packings as packing pieces may break if dropped from heights exceeding about 2 ft.
    
13. Care is required with hand-cleaning random packings outside the tower. Thin-walled packing can easily be damaged or deformed by water jets as reported in one case study (290). Thicker-wall packings can also be damaged or deformed, especially when hit with high-pressure water jets, as the author experienced. Placing the packings in baskets before hydroblasting them appears to help at least to some extent. What worked for the author in one case is loading baskets of packings on a flat-top truck and taking them to the car wash (no detergent, takes a few passes through the wash with each load, and makes a special attraction to spectators) or to wash them in a cement mixer.

10.2.4 Tray Installation

The objective of correct tray installation is to follow safe practices while minimizing installation errors and installation time. Practices prior to installation that help achieve the above installation objectives are presented in Section 10.2.1. This section discusses practices that help achieve the above objectives during the installation period:

1. Close contact with the work crew is essential. Lack of communication may result in a bad solution to an installation problem that later adversely affects column performance.
   
2. Any work that can be performed on the ground is easier and safer than inside the column. It is therefore preferred to preassemble each piece on the ground as it comes out of the crate. Clamps, bolts, nuts, washers, and seal plates should therefore be preassembled to each part (307), with care taken to ensure correct preassembly.
   
3. To minimize the time spent by the installation crew inside the column and to speed up installation, the preassembly line should be kept ahead of the installation (307). As soon as one tray arrives at the manhole or enters the column, the next tray should be hoisted to the platform, and another tray preassembled on the ground, ready to hoist. Parts of only one tray should be passed on at a time, so that no pieces remain when the tray is installed.
   
4. Major support beams should be installed first, with the top of the beam flush with the tray support rings. Shims are often added to minimize out-of-levelness.
   
5. Tray installation usually begins at the bottom and proceeds upward. Tray pieces should be passed to the work area in the order they go in. The preferred order (307) is downcomers first, under-downcomers next, then side-tray floors, center-tray floors, and finally manways.
   
6. To minimize the impact of support ring out-of-levelness on tray levelness, bolts and clamps that fix tray panels to support beams should be fastened before those that fix the panels to the support ring. To ensure the manway fits in, it should be temporarily set in place before tray panels are fastened.
   
7. Downcomer panels should be installed on the downcomer side of the downcomer support bars (e.g., the wall side of single-pass trays). With this technique, the weight of the liquid tightens the joints, and the bowing out of the downcomer panels is minimized.
   
8. To set the required clearance under the downcomer, the length of downcomer panels should be carefully adjusted. Wooden blocks cut to the required dimensions to act as spacers are useful for achieving this (307).
   
9. The process or operations engineer should periodically spot-check that trays and components are correctly installed and immediately inform the construction supervisors of any features that can adversely affect column performance.
   
10. Rough handling of valve tray sections can damage valve legs and must be avoided. It can result in valve floats sticking shut, sticking open, or falling out when the column is placed in service.
    
11. When shutdown time is critical in large-diameter and tall columns, and if manhole location permits, two or even three tray installation crews have been used simultaneously inside the column. This practice suffers from the potential hazards of falling objects and/or poor ventilation and should be avoided whenever possible. If it needs to be used, precautions must be taken to mitigate the risks. Bulkheads are often used in the tower, and these need to be inserted in a way that will permit good ventilation throughout.
    
12. Ideally, any seal welding of a tray or part of a tray should be performed after completing tray installation (307). This, however, requires large enough tray spacing (at least 24 in) and good ventilation to disperse fumes.

10.2.5 Dry versus Wet Random Packing Installation

Random packings can be either wet-packed (Figure 10.2a) or dry-packed (Figure 10.2b,c). With wet packing, the column is filled with water following the installation of the bottom support plate. Packing pieces are gently poured from a short distance above the water level and are “floated” to the bottom. The water cushions the fall and promotes random settling. With dry packing, the packing pieces are dumped from a certain height above the top layer of packing.

Wet packing minimizes breakage, compression, and mechanical damage to packings and supports. It also maximizes random settling and reduces packing density. This reduced packing density provides a small capacity gain and pressure drop decrease compared with dry packing and minimizes the required number of packing pieces per unit volume. Billet’s experiments (34) with 1½” Pall rings in a 20-in-diameter column showed that changing the packing technique from dry to wet increased column capacity by about 5%, lowered column pressure drop by up to 10%, reduced the number of packing particles by about 5%, and had a negligible effect on efficiency. The author also observed a 5% capacity improvement after switching the packing technique of a 3-ft column from dry to wet. Ludwig (305) reported cases in which the pressure drop was lowered by 50 to 60% and even more by switching the packing technique from dry to wet. The author believes that Ludwig’s cases are for ceramic packings in which breakage played an important role in increasing the pressure drop of the dry-packed beds.

Dry packing avoids the introduction of water into a dry process, high hydrostatic heads, minimizes rusting of metal packings, and is quicker and less expensive. It is generally preferred in most modern metal and plastic packing applications, especially in the following applications:

1. Plastic packings, as plastic typically floats on water.
   
2. Large-diameter (>10-ft) columns, as the cost and speed advantage of dry packing are the controlling considerations.
   
3. Where downtime is critical, as dry packing is faster and also eliminates the need for leak testing prior to packing installation.
   
4. When the random packings are only in the upper sections of a column. Filling the entire column with water in order to wet-pack the upper sections is rarely justified.
   
5. When either water presence or some corrosion to the packing cannot be tolerated.

Listed below are applications where wet packing is advantageous. Wet packing is strongly preferred for breakable packings (step 1). For the other steps in the list below (2–4), wet packing offers a smaller advantage:

1. Packings constructed of ceramic, carbon, or other breakage-prone materials.
   
2. Small-diameter columns (<2 to 3 ft), where lowering boxes of packing into the column is difficult while dry dumping from high elevations may damage packings and supports.
   
3. Where column capacity is near a limit.
   
4. Where dry packing offers little advantage. Wet-packed beds perform a little better.

10.2.6 Dos and Don’ts for Random Packing Installation

Poor packing installation may induce maldistribution, flooding, loss of efficiency or capacity or both, instability, and excessive pressure drop (31, 305, 339, 518). Bad packing installation practices may also damage packings and supports.

When correctly done, either wet or dry packing can provide good performance. Both techniques have traps. Failure to avoid these pitfalls, rather than a nonoptimal choice between wet packing and dry packing, is usually responsible for column malfunctions. Several guidelines for avoiding these pitfalls are listed below.

1. The supplier’s advice on packing procedure should be sought, reviewed, and in the absence of major issues, followed. Any deviations from the supplier’s recommended procedure should be discussed with the supplier.
   
2. Always remove all the previous packings before adding new ones. Failure to do this can lead to density gradients in the bed as observed in one case (2).
   
3. Packings should be spread as evenly and as randomly as possible, and formation of “hills” of packing (i.e., buildup of layers sloping away from the dumping region) strictly avoided. Biales (31) reports an experience where forming a “hill” (Figure 10.2d) caused severe maldistribution and lowered efficiency and capacity. Pilot-scale experiments by Billet (34) verify that hill formation during packing installation leads to a lower packing efficiency than when the packing is uniformly spread.
   
4. To reduce the liquid tendency to accumulate at the wall, it was recommended (305) to lay each horizontal layer of packing starting from the edges and working toward the center.
   
5. Packing pieces should not be pressed into place. Pressing them may increase bed density, compress them, and cause maldistribution and high pressure drop.
   
6. In dry-packed towers of larger diameter (>4-ft), it is usually necessary for workers to stand on top of the bed to spread out and level the packing pieces. Standing directly on the packing should be avoided as it may compress and crush the packing. Sheets of plywood, or other rigid material, with an area of 4 ft² or higher, should be used to stand on (68). With plastic packing, it is best to avoid standing on the packing altogether. Care should be taken not to introduce dirt into the tower during installation.
   
7. It is important to positively ensure that plywood and other foreign objects are removed and not buried in the bed. These have caused malfunctions in the past (68).
   
8. Figure 10.2b shows a frequently recommended technique of dumping the packings from buckets lowered into the column. The buckets should be emptied at several locations, starting from the edges. A worker is often lowered into the column with the bucket and pours the packings out at a height of 6 to 10 ft above the top of the bed. An alternative to this technique (337, 478) is to construct a simple frame that will hold a shipping carton of the packing, with a trip cord attached to the frame. The carton is lowered into the column, and the cord is tripped to dump the packings onto the bed.
   
9. Figure 10.2c shows the “chute and sock” method (68, 305), which is probably the most popular technique practiced. From an inclined hopper (the “chute”), packing pieces roll into a sheet metal cylinder (the “sock”) and from there are dumped at several locations, starting at the edges. A flexible large plastic hose has more movement flexibility and is usually used as the sock instead of the cylinder. This method breaks the packing fall height and evenly spreads the packing. A worker usually stands on top of the bed per step 6, moves the plastic hose to direct the packing pieces to different locations, and uses a device similar to a garden rake to spread the packing. Packings should be raked at frequent heights and definitely no more than every 10 ft (140). This technique is fast and is often preferred in large columns (68). When the tower diameter exceeds 10–15 ft, a number of workers can spread the packings inside the tower to speed up installation.
   
10. In dry packing, pieces of packing should not be allowed to fall a large distance onto the bed. The fall height for ceramic or carbon packings should not exceed 2 ft (68, 305, 337, 339, 478). Higher fall heights can be tolerated with metal and plastic packing, but should not exceed 10 ft (192, 339, 478). One designer (68), however, believes that with metal packings, a fall height of up to 20 ft may be permissible.
    

    Caution is required when dumping the first 2 to 3 ft of packings, as excessive fall height can damage the support plate or crush the packings. The support plate is usually of the gas injection multibeam type (140, 192, 407), clamped to a support ring. The levelness of the ring is not critical, and normal construction tolerances are usually acceptable (407).

    
    

    In near-freezing conditions, the fall height of plastic packings should not exceed 4 ft. The impact resistance of plastic often diminishes at low temperatures, and it is prone to breakage. In one case (192), polypropylene rings were dropped from a height of 23 ft onto a steel support at about 15°F, and 20% of them were damaged.

    
    
11. Dropping packings directly from a column manhole promotes excessive fall heights and hill formation and should be avoided. Instead, the techniques in step 8 or 9 should be used. One designer (68), however, believes that in small-diameter (< 6-ft) columns, dropping packings from the manhole may be permissible as long as excessive packing fall heights are avoided and the packing spread is visually inspected to avoid unevenness.
    
12. In wet packing, the column shell and supports must be designed to withstand the full hydrostatic head.
    
13. In wet packing, at least 4 ft of water should be kept above the surface of the packing at all times (337, 339). Ideally, the water level should be kept up to the loading manhole. This should be combined with the bucket dump method mentioned in step 8.
    
14. When wet-packing reused packings, consider the relevance of step 5 in Section 10.2.3.
    
15. The distance between the top of the packing and the distributor should be as specified in the drawings. Packing suppliers usually supply 5–10% or more spares in case the bed is too short. The author has seen poor packing efficiency resulting from a foot overlong bed which grew all the way to the distributor as the installation crew kept pouring in the spare packings. The top of the packing should be one ring size below the bed limiter (140).
    
16. Precautions are required to avoid interchanging when using different packing sizes or types in different beds or column sections.
    
17. During the loading of metal or ceramic packings into a column, the ground area surrounding the tower should be out of bounds for personnel and should be wired off. Sharp edges of the metal packing particles falling at high speed, or the heavy ceramic packing particles, can cause serious injuries to workers underneath. The author is familiar with one accident where a serious injury to a passer-by was caused by a falling piece of sharp-edged metal packing.
    
18. Bed limiters should be installed so that they do not interfere with liquid distribution.
    
19. Thermowells are best inserted before dumping the packings.
    
20. Debris often remain lodged inside the packings or distributors and can later bottleneck the tower. Throughout the construction, it is important that the debris are removed. In particular, any wooden sheets or planks used must be all accounted for and removed.

10.2.7 Dos and Don’ts for Structured Packing Installation

Incorrect installation has caused numerous failures in columns containing structured packings. Inclined layering, incorrect layer orientation, heavy-footed stepping on structured packings, failure to achieve a snug fit of packings to the tower wall, and excessive compression of packings in an attempt to achieve a snug fit are a few of the many causes of poor performance. Some of the lessons learned are described below:

1. Welding of internals such as packing supports, redistributors, and any other hot work inside the tower needs to be completed before packing is installed (174). Hot work inside the tower while the packings are in place is a serious fire hazard. For both safety and liability concerns, a number of companies and contractors are now requiring the removal of structured packings before any hot work is started (108). Others (97, 108) strongly recommend taking the packings out before hot work. If hot work above the packings is absolutely necessary, extensive safety measures such as water flooding need to be implemented to protect the packings (97, 108).
   
2. The edges of structured packings are razor sharp, so any handling of these packings needs to be done with the appropriate protective clothing and cut-resistant protective gloves.
   
3. Distributors and redistributors used in packed towers can be complex. Installing complex distributors and redistributors on the ground ahead of the turnaround is worth considering. Any issues can then be brought to the supplier’s attention.
   
4. Redistributors that look similar may have subtle differences between them, like different hole sizes. In these situations, special care is required to prevent interchanging. The author has seen poor efficiency caused by interchanges.
   
5. The support grid should be leveled to the supplier’s specifications. If it is not level, the out-of-levelness carries all the way to the top of the bed (407). It was recommended (174) to fasten the sections of the packing support grid only loosely until all parts have been assembled. This allows adjusting the overlap of the rim of the support grid and the support ring before final clamping and tightening (174).
   
6. Labels, plastic sheets, and plastic bags should be removed before loading the packings into the tower. Care should be taken not to introduce dirt into the tower during installation.
   
7. The packing is usually supplied in elements, or “bricks,” typically 8 to 12 in tall. In smaller-diameter columns, an element can be as wide as the wall-to-wall distance, but to minimize the possibility of damage during handling, it is better to make up this distance in the central portion of the column using a number of elements (Figure 10.3). Ropes should be used to lower the elements into the column (Figure 10.4a) as metal hawsers and chains can damage the packings (174).
   
8. The workers should not stand directly on the structured packing as this may compress and damage the packings. Sheets of plywood, or other rigid material, with an area of 4 ft² or higher, should be used to stand on (68). Any wooden planks or boards used should not be prone to splinter. The installation crew should be instructed not to drop or drag heavy objects or elements across the packings. The author is familiar with cases where such mishandling caused indentations, depressions, and other damage to the packing layers.

   

   

   
   
9. Usually, each layer of packing requires a wall wiper. Some wall wipers are incorporated into the packing elements, whereas others are supplied loose. Any wall wipers should be bent outward prior to element placement (174); see Figure 10.4b. These wall wipers catch liquid flowing down the wall and return it into the packing. It is important to check that the packing reaches the tower walls; in one case (450), gaps as large as ½” were found in an inspection. The author is familiar with other cases of very poor efficiencies in wire-mesh packings due to ½” gaps between the packings and the wall.
   
10. Figure 10.3 shows a typical procedure for the installation of the packing elements. The elements are fitted in beginning from the sides. A slide plate (or “shoe horn,” Figure 10.4c) is used to fit centerpieces. In Figure 10.3a, element 1 on the left is first inserted. Next, elements 2 and 3 on the left are inserted and pressed against element 1. Element 1 on the right is then inserted, and finally, elements 2 and 3 on the right are fitted in.
    
11. It is important to avoid vertical misalignment of the installed elements. The author is familiar with cases of misalignment defects, as much as 4 in. minimum to maximum in a single layer, as well as wavy surfaces of the packing layers. Where these were not caught at the inspections, tower performance was poor.
    
12. The elements of the next layer of packings are rotated 90° so they have flow channels running at 90° to those in the layer beneath (Figure 10.3b). Failure to rotate the layers may result in severe channeling and extremely poor efficiency, as was experienced in one case that the author is familiar with.
    
13. The orientation of the bottom layer is critical (43, 140, 407) and is set to orient the distributor at a desired angle (usually 90 degrees) to the top packing layer. Further, the bottom packing layer usually needs to be perpendicular to the support bars to prevent sagging into the spaces between the bars. It is therefore mandatory to ensure that the bottom layer is oriented per the supplier’s drawings and procedures. With grid packings, each element is usually rotated 45° (instead of 90°) to the one below. In one case (409), grid reinstallation was needed because this was not followed.
    
14. Column out-of-roundness or other irregularities may make it necessary to reduce or increase the width of some center elements. It may also form gaps between elements (Figure 10.4d). Reducing the width of elements can be accomplished by removing a few strips of the packing material. Increasing the width, or filling gaps, is accomplished by inserting filler sheets of packings. The filler sheets can be inserted perpendicular to the packing layers (Figure 10.4e) or parallel to them. They can also be inserted between the packings and the wall. It is best to seek and follow the supplier’s recommendation as incorrect installation may later show up as maldistribution and loss of packing efficiency. In one case (450), gaps as wide as ½” were found in the middle of the bed upon turnaround inspection.
    
15. The supplier supervisor, or trained process/operation personnel, should be required to inspect each layer of the structured packings after it is installed. Good-quality 360° photographs of each layer of the packings should be taken and clearly labeled to indicate its exact location. This information will be invaluable for a troubleshooting investigation should the packing perform poorly. The inspector should also monitor the packing height as the layers are installed; it is better to identify deviations sooner rather than later.
    
16. Tight fit of structured packings may result in a “growth” of the overall packed height; in one case (405), a 3-in. growth occurred. The additional height is often sawed off. The supplier’s advice should always be sought as sawing may damage the packing and cause maldistribution and/or premature flooding.
    
17. It is important to keep the exact count of the number of packing layers going into each bed. The author is familiar with cases in which post-startup gamma scans showed layers intended for one bed installed in another: in one bed leaving a large gap under the distributor, in the other bed leaving no disengagement space under the distributor.
    
18. Inserting thermocouples into the bed requires special caution. The author is familiar with a case in which packing elements were simply piled up above a thermocouple, forming inclined layers in the bed. Fortunately, this was caught upon inspection, which saved the tower from inferior performance, but required removing the packings and reinstallation. Drilling or bayonetting the packing by sharp-pointed rods to insert the thermocouple is often needed. To minimize packing damage, two or three rods of progressively increasing diameter, the last being slightly larger than the thermowell, are sometimes used. If drilled, there is a question of where the drilling bits will end up. Again, it is best to seek and follow the supplier’s recommendation as incorrect installation may later show up as packing damage, maldistribution, and loss of packing efficiency.
    
19. Steps 16–18 and 20 for the installation of random packings (Section 10.2.6) apply also for the installation of structured packings.

Because of the multitude of installation traps that can impair the performance of structured packings, it is best to have the supplier provide an installation team. If impractical or too costly, the supplier should be requested to provide detailed installation instructions, which must be strictly adhered to, as well as a supervisor (at cost) to supervise the installation. The supervisor’s scope of work should include thorough hands-on training for the installation crews and watching that the packings are correctly installed.

10.2.8 Some Considerations for Towers Out of Service for a Time

At times, a tower is to remain idle until there is economics to activate it again. Below are some considerations:

1. Ensure all health, safety and environmental regulations are followed.
   
2. Keep manholes closed to protect the tower from water and animal entry. In one case (284), the carcass of a dead rat lodged in the kettle reboiler inlet nozzle backed up liquid into the tower upon restart, initiating premature flood.
   
3. Keeping the manholes closed is especially important in packed columns, where moisture, rain, or dust can lead to rust and particles that plug distributor holes. In one case (264), moisture entering through open manholes of a large tower during an extended outage caused significant rusting on the tower inner walls. Upon startup, the descending liquid washed away the rust particles, which plugged the distributors.
   
4. Before boxing the tower in, make sure it is dry. If the column was water-washed or chemically washed, make sure it has been adequately dried immediately after the conclusion of the wash. It is amazing how fast carbon steel, and even stainless steel, rust in the presence of oxygen and water.

10.3.1 Strategy

In general, any departures from the design and fabrication drawings are a potential source of trouble. Added to these are any details that are not clearly marked on the tower drawings. Finally, anything that does not make sense should be questioned. “How is this supposed to work?” is an excellent key question.

This section emphasizes some common traps that deserve particular attention, either because of their high frequency of occurrence in practice or because of the severe consequences of overlooking them.

The discussions in this section center on inspections carried out by process and operations personnel. The discussions exclude column inspection for corrosion, fatigue, damage, and equipment mechanical integrity. These inspections should be carried out separately by inspectors specifically trained in these disciplines. However, the operations or process inspectors should be on the look for such issues and call on the right inspection personnel to evaluate them in detail.

The earlier a fault is discovered, the less costly and time-consuming it is to correct, and the less likely that time pressure will prevent the correction altogether. For new installations, it was recommended that the inspection be carried out simultaneously with the assembly of new internals, as demonstrated by field experiences (329). Existing towers should be inspected as early as practical after an entry permit is issued. When safe and feasible, an inspection before cleaning and disassembly (322) can provide valuable data on fouling, corrosion, and internal damage. A second, briefer inspection may be needed after cleaning and repairs.

It was suggested (322, 409) that trained tech service personnel are generally most suitable for performing inspections. The author endorses this recommendation. This helps balance the turnaround workload (operations personnel are usually the busiest during turnarounds). Tech service engineers generally have close familiarity with equipment fluid flow and mass transfer and a good understanding of the consequences of deviations from design dimensions. It has been stated (293), “The most important job of the process engineer working in a refinery or chemical plant is to inspect tower internals during a turnaround.”

The inspections are best performed by new graduates and less experienced engineers (or operators). The experts and experienced engineers can follow later with checks of their own. Column inspection is an invaluable training exercise and provides the inexperienced with insights essential for improving their design and operation skills. It has been stated (293) that “Crawling through the trays is the only real way to understand how they work and what malfunctions can be expected.” The author concurs.

Prior to the inspection, it is critical to provide the inexperienced inspectors with adequate training in “what to look for.” Usually, this is a classroom training 4–8 hours long by an experienced engineer knowledgeable of the towers to be inspected. The author is familiar with cases in which this step was skipped, leaving the inexperienced engineers to “swim or sink,” and when they missed some details, they were yelled at. This scenario is unfair, demotivating, and counterproductive and must positively be prevented.

To maximize effectiveness, and to get the most out of the inspection as a training exercise, especially for the inexperienced, it is important to do homework prior to the inspection. What has been the past experience in the tower (plugging, corrosion, damage, where)? What is being changed (trays, distributors, feed pipes)? Why?

Follow-up on the recommended action items is critical. Incorrect implementation is a common occurrence, and under time pressure, the tower is bolted up shortly after. A poorly implemented cure can be more lethal than the original illness and worsen tower performance. Do not let this happen. Keep close to the action, and always reinspect the implementation as soon as it is done. This is also recommended by troubleshooting maestro Lieberman (288).

10.3.2 Should the Tower be Entered at the Turnaround?

The answer to this question should be primarily based on the potential hazards of tower decontamination and, secondly, on history, monitoring key variables, and looking for problems well before the turnaround. Health, safety and environmental considerations prevail. Process-wise, if the tower is in clean service, with no history of, no evidence of, and little potential for corrosion, fouling, or damage; no signs of performance deterioration during service; and no statutory, safety, or environmental requirement to enter, then there is a good case for not entering the tower.

In many cases where the tower ran well, the tower is opened with or without access to the spaces at the manholes. It is important to make the most of the opportunity and look or enter through the open manholes to the extent permitted by safe practices. Attention should be focused on packings, packing distributors and their piping, feed piping, cleanliness, corrosion, and damage. In one case (43), a small void at the top of a structured packed bed gave reason to remove additional layers of packings, which showed a large, corroded void further down in the bed. The entire bed needed replacing with corrosion-resistant packing.

Monitoring the key variables also provides a guide to focus on problematic sections of the tower. For instance, if flooding was experienced near the top of the tower toward the end of the run, the inspection should closely look at fouling or damage near the top of the tower. Differential pressure monitoring (Section 2.8) and gamma scans (Chapter 5) can help identify the sections to be singled out for special attention.

There are three common shortcuts that the author strongly recommends closely reviewing and in many cases avoiding altogether:

1. “Not going into the column at the turnaround.” Shutdown planners often argue that the need to remove the chemicals, gas-free the tower, clean it for personnel entry, and later recommission it for service is costly and will prolong the turnaround. This argument is often valid in clean, noncorrosive, trouble-free services as described above. In many services, however, this is false economics. The author has seen far too many cases where upon startup a column experienced poor operation or performance due to plugging, corrosion, damage, and other operating problems that either existed before the turnaround or crept in while shutting down, remained undetected as the tower was not opened, and intensified or just persisted upon restart. An additional outage became necessary weeks after the turnaround. The cost of lost production was orders of magnitude larger than that of entering the tower. In other cases, columns suffered substandard performance for years due to unrectified faults.
   
2. Partial cleaning of trays or other tower internals. To minimize turnaround work, shutdown planners often limit tower inspection and cleaning work. This may be OK in clean, noncorrosive services with low potential for damage and with no history of operation problems. This can also be justified if the potential trouble areas are inspected, while sections with trouble-free history are skipped. But like in step 1, no-inspection can breed poor performance. In Figure 10.5 (280), for 20 years, the panels of one of the two passes in the tower were inspected and cleaned, but on the other pass, the manways were not removed and the panels were not cleaned. Following one restart, the tower performance was so poor that the unit needed to be shut down again to clean the panels that were not inspected.
   
3. Turnaround wash without inspection may be counterproductive and needs to be carefully evaluated. Many good experiences with on-line or shutdown washes without tower entry (e.g., Cases 12.3 and 12.4 in Ref. 201, many others abstracted in Section 12.7 in Ref. 201, as well as the cases described in Refs. 353, 384) have been reported, especially where the nature of the fouling was well understood and there was a clear path for removing the foulant-rich spent wash solution.

   

   
   

   In contrast, the author is familiar with turnaround chemical washes with no entry resulting in premature flood upon restart. Deposits moved by the washes blocked a couple of downcomers in one case and sumps in another. In the experience cited in step 2 (280), water washing is believed to have moved deposits to the unaccessed tray panels. Lieberman (290) reports water washing a 25-ft-tall bed of 3-in. Pall rings in an 8’-6” scrubber to wash off salts. Some of the salts settled lower in the bed, causing channeling and poor separation upon restart. Sloley (443) reports a case in which replacing a coked vacuum tower wash bed grid took “too long to wait,” so instead it was cleaned in place from the top with a high-pressure water lance. Upon restart, the grid pressure drop doubled; the water pushed down the solids, increasing the blockage of the area near the bottom of the bed. In all these cases, the plant needed to be shut down a short time after restart in order to clean the tower.

10.3.3 Inspector’s Checklist

Table 10.2 lists the common assembly mishaps reported in the literature as surveyed by our malfunction survey (198). Sections 10.3.4 through 10.3.11 will elaborate on these. Any inspector needs to focus on the items on this list.

During a revamp, it is essential to critically examine the modification areas and to identify the liquid and gas passageways. Several cases have been reported (8, 43, 44) where following a modification, the passage of liquid to reach the section below was blocked.

The prime column inspection tool is a checklist. Table 10.3 provides a generic master checklist. For each tower, a similar list should be prepared, including the relevant items, deleting the irrelevant items, and adding items not included on this checklist. For each satisfactory item, the inspector enters a tick, and for each unsatisfactory item, a number, with a comment of the same number spelled out on a separate sheet. Inapplicable items are left blank. Prior to inspection, design values and relevant tolerances are entered in the right-hand column of the table.

Any alternative checklist is satisfactory as long as it is sufficiently detailed and the user is comfortable with it.

Miller (322) and recently Bouck (42) proposed “inspector survival kits.” The author has supplemented this list. Additional items, dictated by safety requirements, need to be on the list, but as these vary with the specific process, they were excluded from the list below. It is mandatory to verify with the safety department whether there are any specific requirements for taking any of the items from the list below into the tower.

The updated list includes:

• Equipment drawings and simplified sketches. The drawings should be used for preparing the sketches. They are too cumbersome to take into the tower. The simplified sketches show what the internals should look like and what the inspector is looking at.
  
• A checklist.
  
• A high-intensity flashlight and spare, or at least spare batteries. Losing the source of light while in the tower is a common issue. A well-fastened miner’s lamp should be considered (safety permitting) and can help free the inspector’s hands. A high-intensity LED light has been advocated by some to give high intensity and long life.
  
• A color marker pen that writes on metal. Yellow shows up best. The marks should be waterproof. In stainless steel columns, the ink needs to be low in chlorides. Low-chloride pens may require special ordering ahead of the turnaround. Check that the pen does not leak; the paint may be difficult to wash off the skin.
  
• A bound pocket notebook and two pencils and ballpoint pens. The pens should be waterproof.
  
• A short (6-in.) steel ruler. The ruler can also be used as a deposit scraper. Bring Ziplock bags to collect the deposits.
  
• A 6- to 10-ft tape measure.
  
• An autofocus pocket camera. Photographing damaged, corroded, and plugged regions, as well as flaws and critical equipment items, provides management, designers, suppliers, and experts with factual information essential for immediate decision-making. It also supplies an invaluable record for future troubleshooting. It is important to mark each photograph with the location, tray number, and the direction (e.g., east–west) at which it was shot. The author has seen many photographs becoming useless as nobody knew what part of the tray or tower they came from. It is worth considering a camera with a video-taking capability that the inspector can directly talk to. Keep a spare camera handy in your office or locker in case the camera gets damaged.

  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  

  Column	Inspector										Date
  Tray no.	Top								Bottom		Design
  	1	2	3	4	5	6	7	8	9	10	Value ± Tolerance
  Points of transition
  Orientation of internals
  Passage obstructions
  Possible impingement
  Watermarks
  Instrument location
  Chimney trays
  Riser-hat clearance
  Riser diameter
  Riser number
  Hats correctly mounted
  Flow obstruction to outlet(s)
  Unintended liquid descent
  Hats strong, supported?
  Vapor rise obstruction
  Vapor impingement
  Buckling, riser bow
  Instrument taps
  Trays
  Downcomer clearance
  Weir height
  Downcomer width
  Downcomer brackets/bow
  Hole diameter
  No. of holes/valves
  Valve direction
  Light/heavy valve arrangement
  Panel alignment/overlap
  Valves secure?
  Cracks and crevices
  Holes/valves under downcomers
  Tray levelness
  Tray spacing
  Tray/downcomer materials
  Trays/downcomers secure
  General
  Deposits found? Sampled?
  Corrosion found?
  Bolting firm?
  Double nutting?
  Fastening materials
  Gaskets
  Absence of debris
  Damage: bow, warp, cracked
  Unexpected features
  Sumps
  Inlet weir/seal pan width
  Inlet weir/seal pan depth
  Weep hole area
  Weep hole diameter
  Gaskets: tightness
  Others

  
  

  The camera needs to be approved by the safety department.

  
  
• A small backpack or waist pack to carry tools.
  
• Templates for measuring repetitive measurements.
  
• A 9/16″ wrench (will fit most tray hardware) and pliers to check tightness.
  
• Knee pads and/or shin guards.
  
• A magnetic rod (a good one). Magnets have come off the rods in some cheap ones.
  
• An unbreakable pocket mirror with a stick to attach to for looking around corners.
  
• A self-leveling laser light that levels itself with a small internal pendulum (288, 293). This device is small and can be purchased for about US $150 from hardware stores or on the web. Suitable where levelness is not critical. Where levelness is critical, see Section 10.3.4.

A few additional hints from an expert who inspected many a column (288):

• Always verify that all items on your punch list have been corrected to your satisfaction. Review and address any deviations.
  
• Do not wear loose-fitting coveralls or anything that is likely to catch on tray parts.
  
• Use the toilet before entering the tower.

10.3.4 Packing Distributor Checks

As shown in Table 10.1, mishaps in packing liquid distributors are foremost among the assembly mishaps. Below are some guidelines for inspection:

Distributor Levelness Distributor levelness is central to good liquid distribution in packed towers (Section 4.8). Levelness checks are a critical part of distributor and redistributor inspection. The out-of-levelness tolerance used should be that specified by the supplier, with a typical value for high-efficiency separations 1/8″ high to low (43, 140). Lower out-of-levelness values may be difficult to achieve in the upper sections of tall (> about 100 ft) towers due to tower sway. In as much as practicable, distributor leveling, especially in such sections, should be performed under calm conditions.

An adjusting leveling mechanism is built into the troughs and parting boxes for easy leveling. Following the checks and adjustments, it is important to check that all the leveling screws are tight and double-nutted (140).

Three different techniques have been used for checking levelness:

ZIPLEVEL^® (Figure 10.6) This technique rose from a complete unknown in the field of packing distributor leveling to perhaps the most popular. Unlike laser methods (below), it does not require line-of-sight, does not amplify error with distance, and does not require calibration. It is simple, accurate, and of low cost, all of which contribute to its success.

ZIPLEVEL® (519) measures the weight of a proprietary liquid sealed within its cord relative to a reference cell in the hub of its reel. It is a high-precision pressurized hydrostatic altimeter that reads elevations by measuring the pressure developed by gravity acting on the net height of liquid between the measurement module and the base unit. This makes it immune to both barometric pressure and altitude changes. Its proprietary liquid is sealed within, does not move in its cord, and is pressurized with a special gas to prevent bubble formation. It is stated not to be damaged by stepping on or kinking the cord though readings will be affected briefly.

The procedures for using ZIPLEVEL in towers are simple and similar to that of a water level or other instruments. Its standard precision of 0.050″ (519) is usually sufficient for distributor levelness measurements, especially considering the tower sway mentioned above. ZIPLEVEL is reported to operate from −22°F to 158°F for up to a year of daily use on a single 9-V battery. It is reported to be set up in seconds and offers true one-person operation with no line-of-sight, no error with distance, no factory calibration, and no math.

If the interior of the tower is more than a few degrees different than the outside, ZIPLEVEL requires acclimation to avoid error. The length of the cord to be used needs to be unpacked inside the tower, so the unit acclimates to the tower temperature for at least 10–15 minutes before use and up to 20–30 minutes with subfreezing ambient temperatures. Double-check your starting benchmark zero for repeatability at the end of the measurements.

The popular ZIPLEVEL PRO-2000 consists of a base unit that stores its handheld measurement module, 100 ft of interconnecting cord, and accessories. Accessories include a rubber protective boot for the measurement module, a unipod for measuring without bending, and a pair of stakes to secure the base unit on a hillside. The fully extended unipod doubles as a 4-ft vertical calibration standard for the system. Other models and accessories are also available, allowing sharing and plotting profiles, 3D maps, and data on photos.

Laser techniques create a flat, level reference plane. The distributor level is measured with respect to this plane. A rotating laser is mounted to a tower attachment that projects a 360° light circle on the column wall. Readings are taken in reference to the light circle. Laser techniques can measure 1/16″ out-of-levelness in a large (<30 ft) distributor. Calibration is needed. Accuracy can be checked by marking the walls of a conference room and then rotating the laser and monitoring the reproducibility. Although most experiences with the laser techniques have been good, the author is aware of some that were less satisfactory.

The water manometer technique is the time-honored way of checking distributor levelness. It uses a flexible plastic tube with two vertical steel rulers – one stationary, held by one person, and the other, held by the other person, is moved around with the tube. The tube is filled with water, and the hydrostatic level establishes a level reference plane. To get a good reading, the water should be colored (typically with food coloring). Some of the water often spills while moving around, so it is important to use rubber stoppers and bring some additional water to make up for losses. One needs to watch out for bubbles in the manometer water and eliminate them. Oil and dirt may introduce errors. Accuracy and reproducibility checks are mandatory, including a side-by-side check, as well as reversing the ends of the tubes along the full length. Photographing the readings can overcome meniscus reading problems.

Orientation of Distributor and Collector Pipes Misorientation of distributor and collector pipes is a common flaw. In Figure 10.7a, an absorber did not absorb because its distributor and its parting box were rotated 90° from its intended orientation, so the feed pipe sparger became oriented 90° to the parting box. In Figure 10.7b, the feed pipe was too long, leaving a small clearance from the tray floor.

Sections 8.18–8.23 describe additional cases of flawed orientation of distributor and collector pipes. The cases described there were due to design oversights, but a good inspection would have identified the flaws and possibly led to modifications to circumvent poor performance. Only one of all the cited cases (Figure 10.7b) was picked up in inspection before startup; the others were all picked up in inspections after the tower performed poorly.

For effective inspection, it is important to address not only the question “Does the piping orientation match the drawings?” but also the question “Would it work?” Section 10.3.8 addresses this in detail and also presents many other watchouts for points of transition that apply to packing distributors.

Special Checks for Spray Distributors Special considerations that apply to spray distributors are listed below. Some of these are described in detail in Section 4.15:

1. The checks for spray distributors in existing columns should begin before the turnaround. The first check is by installing a calibrated pressure gauge upstream of the spray header and downstream of any control valve, filter, or other obstruction that gives a high pressure drop. A detailed description of this check is in Section 4.15.
   
2. Consideration should be given to water-testing spray distributors as described in Section 4.15. Figure 4.27 shows a water test, and Figure 4.29 shows how water testing quickly identifies a plugged nozzle. Figures 4.29 and 10.8a show plugged nozzles. Many cases of plugged nozzles impairing tower performance have been reported (e.g., 89, 209, 211, 409, 505).
   
3. Plugged nozzles indicate that upstream piping needs cleaning.
   
4. Spray distributors tend to vibrate, which can loosen the flange and support bolting (43). Bolting should be closely inspected. Double nutting is recommended (43).
   
5. It was recommended (43) that spray nozzles are removed and inspected (43) because plugging on the upstream side cannot be identified from below. Caution is required during the removal, as pockets of hydrocarbons, H₂S, or other toxic materials trapped behind plugged nozzles may be released when the nozzles are loosened (287). Where liquid distribution is not critical (e.g., pumparound sections in refinery fractionators) and the plugging potential is not high, removing the nozzles and inspecting them is sufficient and there is no need for a water test.

   

   
   
6. Check that the nozzles spray the bed and not pipes or supports (see Figure 10.8b).
   
7. Check that the flanges in the spray header and laterals contain the correct gaskets. One case was reported (443) in which left-out gaskets caused poor performance.
   
8. Because spray nozzles need to be special-ordered, it is a good practice to have spare nozzles and, in the case of a fouling application, a complete new set, on hand (43).

Other Distributor Checks During inspection, it is also important to check the following:

1. Look for watermarks. Watermarks in one notched trough distributor (Figure 4.26) showed the liquid exiting with a horizontal momentum from right to left. This was neither intended nor expected and created liquid maldistribution.
   
2. Look for plugging, corrosion, damage, sag, or sloping in the distributor and parting box. When suspected and it is difficult to see due to congestion of internals, consider dismantling parts to properly inspect and clean. Look for signs of bowing in long troughs or chimneys. They may need straightening and bracing to prevent recurrence.
   
3. Look for any interferences of supports, internal pipes, or other internals. Case 2 in Section 8.12 and another case reported in Ref. 450 are examples of interference of supports. Refs. 257, 261 describe a case where 4″ × 8″ distributor supports blocked 8% of the packing cross-section area.
   
4. Look for broken or bent drip tubes (e.g., Figures 4.30d and 10.8c) or other channels guiding liquid to the packings or distributor troughs.
   
5. Check that parting boxes properly discharge into the distributor troughs. The openings in the parting boxes should align with those in the distributor. Sometimes the parting box liquid is directed to the troughs by shields and baffles. Check that these are all there and none is damaged or broken.
   
6. Check that holes in the distributor and parting box are of the correct sizes and punched in the same direction. The side through which the punch entered should feel smoother, and both sides should be free of burrs. Small hole diameters can be checked with taper gauges. These checks are best done at the supplier shop during the water tests, but an additional check during inspection will not hurt. Hole size checks of distributors that have been in service are important in corrosive or scaling services, where the holes may expand or scale during operation.
   
7. Look for features that obstruct liquid flow or cause excessive hydraulic gradients that lead to maldistribution or overflows. Case 1 in Section 8.18 and Figure 8.21 describe closely spaced large chimneys restricting liquid movement between them, causing maldistribution and poor separation in the bed below.
   
8. Deck distributors that are supported and sealed to the tray support ring can leak at the joints. It is essential that the deck distributor panels are sealed to the support ring, as well as to one another, with gasketing of the correct materials.
   
9. Ensure that all gaskets are in place and in good condition. Disintegrated gaskets not only generate leaks that promote maldistribution, but also plug distributor holes. In one case (264), disintegrated gasket pieces falling off from downpipes (Figure 10.12b) were found in six distributors of a large fractionator, where they plugged orifice holes in the main channels and led to poor performance of the entire tower.
   
10. Check that redistributor hats are correctly installed and are not upside down. Upside-down hats are a common error that can bottleneck the tower, similar to Case 11.12 in Ref. 201. For hats equipped with weirs, the liquid collected between the weirs should pour out over a closed portion of the chimney (as in Figures 10.34 and 4.22) and not descend into the vapor space between the chimney and the hat. Long hats need to be adequately supported; if not, additional support brackets may need to be added. In one case (496), a loose distributor hat fell off blocking a circulation loop, forcing a shutdown.
    
11. Check that redistributor hats are not oversized. Oversized hats may restrict the vapor ascent area, leading to flood or vapor maldistribution to the bed above. In Case 3 in Section 8.11 (Figure 8.12b), oversized hats led to premature flood. The author has seen similar cases. In case studies 2–4 in Section 8.12 (Figure 8.13b,c), oversized hats and their interaction with support beams led to vapor maldistribution.
    
12. Critically review drain and weep holes, checking that they are of the correct sizes and located at the correct locations. Seal off any unneeded drain holes. In spray distributors, high-velocity jets issuing from drain holes can damage the packing.
    
13. Check that all deflector baffles are installed, are firm, and would not bow.
    
14. Check that all the nuts and bolts are tight and that any gaskets are properly installed and are of the correct sizes and materials. In one case, improperly installed hold-down clamps and missing bolts led to a draw pan being dislodged (259).
    
15. Check that all the appropriate packing hold-downs are there and do not interfere with liquid distribution.
    
16. Do not rule out the “impossible.” A case was reported in which a distributor was switched from feed to reflux due to a drawing error and another in which a distributor was improperly sized due to a design error (112). Everything looked good except for the separation.

10.3.5 Packing Assembly Checks – Existing Columns

Assembly of packings was discussed at length in Sections 10.2.5 through 10.2.7.

When inspecting a bed of existing packings, the following should also be checked:

1. Where fouling, solids, corrosion, or damage may be suspected, monitor bed condition well ahead of the turnaround. This can be done by pressure drop monitoring (Section 2.8), gamma scans (Chapter 5), or other techniques. Based on this, order any needed replacement. Last-minute ordering of replacement packings may be too costly or even impractical, as proved in some cases (e.g., 443).
   
2. Are the packings there at the expected height? Cases of disappearance of packings due to corrosion, or shrinking due to compression, are common. Sections 5.5.12 to 5.5.14 describe such cases.
   
3. Are the packings clean? Corroded? Damaged? Compressed? Sagged (structured beds)? Figure 10.9a, b shows some examples.
   
4. If damaged, how? Pushed up? Pushed down? Corroded? Eroded? Compressed?
   
5. Look for flattened packing channels on the top structured packing layer of each bed. This is especially important in services where the distributors above are regularly dismantled for cleaning. In one case (264), maldistribution due to such flattened layers reduced bed efficiency by 25–30%. Similarly, check for crushed or deformed random packings at the top of each bed.
   
6. For ceramic and carbon, are the packings broken or chipped?
   
7. For plastic packings, are the packing brittle? Squeeze a few pieces and check whether they shatter.
   
8. Are the supports plugged or damaged? Figure 10.9c, d show examples.
   
9. Is the gap between the distributor and the top of the bed per supplier specifications?
   
10. Are there any watermarks on top of the packings, suggesting dry areas or preferential flow patterns?
    
11. Is there any mixing of random packings of different sizes?
    
12. Are there any random packing particles migrated via the bottom support or the top bed limiter?
    
13. Are there gaps through which random packing particles can escape (e.g., gaps between the support ring and the packing support, bed limiter not a tight fit)?
    
14. Does the open area of the supports or bed limiters appear restrictive? In metal and plastics, the open area is usually 80–100%+ of the tower cross-section area.
    
15. For manholes inside the beds, are manhole inserts in place?
    
16. Are there large gaps between layers of structured packings or between the packings and the wall?

10.3.6 Untightened Nuts, Bolts, Clamps, and Downcomer Panel Assembly

Trays and downcomers should be firmly bolted to their supports. Loose bolting may result in excessive deflection, leakage, deficient mechanical integrity, and flow restriction.

Downcomer Panels, Panels Under Downcomers, and Their Fastening Tray active panels containing valves or holes must not be installed under the downcomers from the tray above. This includes false downcomers installed at feeds or refluxes. If present, they are likely to cause premature flooding or excessive inlet weeping, or both, as experienced by the author and others (112). In some cases, column out-of-roundness moves holes or valves from the inlet active panels to the area under the downcomers, and these should be blanked. For each downcomer, carefully look down from above, making sure you cannot see holes or valves.

Loose or improper bolting of the panels beneath downcomers (e.g., Figure 10.10) or holes due to corrosion in those areas are likely to cause excessive leakage, which must positively be avoided. Liquid leaking from this area is likely to completely bypass two trays, making such leaks highly detrimental to efficiency. This area also has the highest leak potential because of the high liquid head. An inlet weir or a recessed seal pan (if present) further raises this leakage potential, and all its bolting should be firm and carefully checked. The above could not be emphasized more for leak-tight services.

Downcomer panels must be firmly fastened to avoid leakage. They should be installed on the wall side of the bolting bars and not on the tray side to prevent leakage when the downcomer plate is pushed by the hydrostatic head of the liquid. Bolting should be audited for tight fastening. Z-bars, commonly used in revamps that modify downcomers, should fit tightly against the new downcomer panels and the existing weld-ins. Downcomer plates should be checked for firm support, so they do not bow under the downcomer liquid hydrostatic pressure. They should firmly fit into their bottom support brackets. Center downcomers should have spacers to maintain the downcomer width. Figure 10.11a shows a downcomer that bulged under the liquid hydrostatic head. Bulging is especially a problem when the bulge nears the front row of holes or valves, creating a path for vapor to enter the downcomer. Figure 10.11b shows a downcomer that came off its bottom support bracket. A downcomer bracket should be installed about every 3–4 ft (140, 288, 293). When an inlet weir or recessed seal pan is present, an improperly tightened downcomer may bow toward the weir, restricting flow at the downcomer outlet. The downcomer panels should be inspected for cracking (Figure 10.11c), and any cracks repaired.

Tray Panels Fastening Tray panels and all internal parts should be adequately secured and tightened. Otherwise, they may lift during even minor pressure surges or loosen during operation. All clamps should fully grip the tray support ring or the bolting bar. In one case (42), a piece of an outlet weir was missing probably due to poor tightening of the bolting hardware or loosening of the bolting hardware by vibrations. Clamps are normally spaced about 6–7 in. around the trays’ periphery, 5 in. when close to the downcomer, and downcomer clamps are spaced about 4 in. (25).

Double nutting should be used where bolting has shaken itself loose during the previous operation or where there is a potential for vibrations. It is also a good practice to double-nut tower attachments for major support beams and internal pipes. Some experts (140) require double nutting on all bolts to beams and support rings. In one case (254), an entire grid bed fell to the bottom of the column due to poor tightening; in another (254), poor tightening led to a loud banging noise from an operating column. Proper overlaps should be used when fastening tray panels. Washers should be of the correct size in friction fit assemblies; using the wrong size washers is a common error (140).

When workers rush to open the manways at the turnaround, they often hammer the manway clips to the open position, which may bend the clips (42; Figure 10.13f). Clips with a permanent bend need to be replaced. There are special considerations for Nutter-style internal sliding manway clips, and these are discussed elsewhere (42). When dismantling manways, it is a good practice to keep the bottom manway bolted. Workers have almost fallen through an open bottom manway not realizing there were no more trays below.

Special care should be taken to ensure proper tightening of the nuts and bolts at the drawoff trays (particularly for total drawoffs). At least one case (79) was reported where poor column performance was due to failure to tighten the nuts and bolts on a drawoff box. In another case (256), improperly installed hold-down clamps and missing bolts resulted in a draw pan being upset during startup.

In leak-tight services, it is important to ensure that washers and gaskets are installed as specified. In one revamp (192), leaving out gaskets on a total drawoff chimney tray rendered the column inoperable. Gaskets must be carefully checked to ensure they are suitable for the service, are properly cut, and are securely held by the joints. In existing towers, prior to inspection check whether the gaskets are fabricated from asbestos, and if so, seek guidance from HSE for inspecting damaged gaskets. Tray gaskets need to be closely inspected to ensure they are in good condition. Torn or damaged gaskets (e.g., Figure 10.12) should be replaced, and the pieces removed from the column.

When shear clips are used, they should not be welded to the tray support ring (288).

Figure 10.13 shows a sample of miscellaneous issues with nuts, bolts, and clamps that were observed during inspections and that inspectors should be on the look for. The diagram is self-explanatory, and there are more examples in Ref. 42.

10.3.7 Tray Assembly

Tray Panels Leakage from the tray active area is less critical than leakage from panels under the downcomers, but still should be minimized, especially where high efficiency at turndown is important. Bolles (in Ref. 192) recommended that cracks and crevices should not take up more than 2% of the tray area. If they do, excessive weeping and/or channeling may result. Figure 10.14 shows a sample of cracks in the tray floor observed during inspections that the inspector should be on the look for.

Time pressure during turnarounds often breeds the question of whether to fix the cracks or not. As a general rule, if it is easy to seal the cracks, then it is best to seal them. If sealing the crack is costly and/or time-consuming, the magnitude of the crack, the cost and schedule impact of sealing it, and the consequences of not fixing it should be evaluated. As stated in Section 10.3.6, cracks in the panels under the downcomers (e.g., Figure 10.10), as well as cracks at the tray inlet, from which the weep will bypass two trays, need to be sealed. Large cracks (e.g., Figure 10.14a,d) also need to be sealed. In contrast, one can get away with leaving smaller cracks (e.g., Figure 10.14b,c) unsealed, assuming their area is small compared with the tray hole area and the trays operate near maximum rates most of the time.

With new trays, and also in existing trays where this has not been done before, the hole area or number of valves should be checked, together with the diameters of the sieve holes or the dimensions of the valves (particularly the valve slot height or open float lift above the deck). This open area should be within 3–5% of the design (192). Care should also be taken to ensure that the holes or valves are of consistent dimensions and that any blanking strips are correctly positioned. This is especially important when the hole area or dimensions change from section to section, as panels are often interchanged. Premature flood due to panel interchange is common (e.g., 112). The author is familiar with one case of a tower performing below the design expectation, but the panel interchange was so extensive that the plant accepted the substandard performance rather than attempting to correct the problem in the short turnaround. The author is also familiar with major capacity losses incurred by grossly diminished hole areas, in one case on only one tray and in others in a section of the tower. Similar issues were reported by others (41). Finally, the author is familiar with many cases in which the sieve tray hole area increased due to corrosion and fewer cases in which it decreased due to scaling.

Counting holes or valves is best performed outside of the tower. In new installations or revamps, it is best to install one or more trays on the ground, which makes valve or hole counting and checking easy. If practical, this should be done before the turnaround so that any corrective action can be taken in time.

Most fixed valves, as well as some moving valves, are directional (e.g., Figure 10.15a,b). Suppliers’ drawings show the required direction, and the direction is often also etched on the valve cap. The inspectors should ensure that the panel installation adheres to these requirements. As a general rule, unless the drawings show otherwise, the wide legs of the fixed valves should face the liquid flow (as in Figure 10.15b) to minimize weep. Nonetheless, misdirection of panels has been common, especially on manways. Misdirecting may lead to an increased weeping and a slight loss in capacity and efficiency. In most cases, misdirecting only leads to a relatively small loss of performance. Should redirecting a large number of panels threaten to prolong the turnaround, or be costly, redirection may not be justified.

The opposite applies to trays installed upside down (e.g., Figure 10.15c). Such installation leads to a major loss of performance and must be corrected. In one case (254), bubble caps were installed under the tray panels; this column flooded at 30–40% of the design.

Another situation where misdirection cannot be tolerated is when it affects the functionality of devices like weirs or downcomers. A case history by Golden cited in Ref. 192 is of tray panels in a low-liquid-rate amine contactor that were rotated 180° to the desired orientation. The inlet weirs were therefore installed near the tray outlet and were ineffective for sealing the downcomers. Vapor entered the downcomers and interfered with liquid descent, which caused excessive liquid carryover from the tower. To minimize the carryover, the liquid rate was lowered to 20% of the design rate, which provided poor tray washing. The tray valves plugged, and frequent cleaning became necessary. After the fault was corrected, normal liquid rates were reinstated and the plugging problem disappeared.

It is important to closely audit any plugging, corrosion, or damage (Figure 10.16). Deposit samples are invaluable in shedding light on the foulants, even if these are known. There have been many instances in which deposits of a new foulant looked the same as those of a past foulant, but were entirely different and required a different mitigation strategy. Explore for bent, warped, or bowing tray parts. Good photographs should be taken of corroded regions, damaged regions, warping, and cracks. These should be forwarded to personnel who have expertise in the relevant discipline to see whether any action is needed to prevent recurrence. For instance, a crack shown in Figure 10.16d indicates possible fatigue or vibrations that need to be further investigated.

Tray levelness is normally not critical in conventional one- and two-pass trays (192, 303). Since tray-by-tray levelness checks are tedious, it is often satisfactory only to spot-check their levelness. Checking tray levelness becomes important in multipass trays, where it may affect the liquid distribution to the passes, in dual-flow trays where vapor can preferentially channel through shallower parts, and in specialty trays.

With moving valve trays, inspectors should also look for popped-out valve floats (Figure 10.17a). This is very common. The popped-out floats should be properly reinstalled (if the floats are found) or replaced. If only a few floats are randomly missing (<3%) and timely replacement is impractical, then “do nothing” is generally acceptable. Some experts accept even a higher percentage of missing floats, up to 5–10% (293). While replacing valve tray floats, attention should be paid to replace by the same type whenever possible. Many valve trays contain alternate rows of light-gauge and heavy-gauge floats. The light floats should be replaced by light floats and the heavy with heavy floats. Failing to do so (very common) may detract from the optimum performance, but seldom causes a major operating problem. What is critical is that the installers correctly replace the floats. There have been incidents where the new floats were a loose fit and all popped out again in the next run (42); the author had similar experiences. In another case, the installers bent back the legs of the floats so the floats could not rise above the tray floor, causing premature flooding and forcing an unplanned outage to fix the problem. The symptoms were like severe plugging of the bottom trays.

Popped-out valve caps are also experienced in large-opening fixed valves where the valve caps are fastened to the tray floor by tabs (rather than an integral part of the tray floor). This problem is far less severe and less frequent than in moving valves and is experienced when the tabs are not properly bent (a manufacturing issue) or corrode.

The inspection should determine what caused the floats to pop. Are the legs worn or corroded? Are the holes expanded due to wear or corrosion (if so, adding light-gauge retainer rings is needed before replacing the floats)? Were the floats blown out by a process upset? By impingement by high-velocity flashing feeds (in this case, the feed distributor should be modified, like in the case study in Refs. 60, 61)?

Valve floats may stick to deposits on the tray floor, and their opening may be restricted. Check that the floats can move freely. Float movement may also be restricted by deposits at the valve legs; check for these too. Floats in moving valves also stick open (Figure 10.17b) or spin (watermarks in Figure 10.17c). Sticking open reduces the turndown. Spinning may damage the tray floor and is usually not a big issue, but may justify using valves with stops (a small tongue projecting into the hole to stop the spin) during the next turnaround. It is important also to ensure that moving valves do not easily come out of their orifices. If they do during inspection, they are likely to do the same in service.

With bubble caps, at least a sample of the caps should be removed, and the clearance between the riser and cap measured and inspected for cleanliness. Dirt accumulating in this area can cause premature flooding (293 describes one case). When replacing the caps, check that the nuts that secure the caps in place on the threaded nut protruding from the bubble cap are not overtightened (288). One case was reported (144) in which overtightening these nuts reduced the clearance between the caps and the risers, inducing premature flood. In another case, not knowing the clearance between the caps and the risers, which was not on the drawings and not in decades of inspection reports, led to an expensive replacement of welded-in bubble-cap trays that was completely unnecessary. Also check that the outlet weirs are shorter than the risers but are taller than the bottom edge of the caps.

Downcomer Clearances and Weir Heights The only tray internal issue that specifically made it to the top 10 assembly mishaps (Table 10.2) is the downcomer clearance. This is both because it is common to find incorrectly installed clearances and because incorrectly set clearances generally have a greater adverse impact on tower performance than most other tray internals installation errors. Clearances that are too small can cause premature downcomer backup flooding or can easily plug and cause the same. Clearances that are too large can allow vapor entry into the downcomer with possible downcomer unsealing flood (Section 2.11) or just a loss of tray efficiency. Clearances that vary along the downcomer length maldistribute liquid to the tray. With multipass and specialty trays, incorrectly set clearances can lead to liquid and vapor maldistribution to the passes with efficiency and capacity penalties.

In one case (161), a column flooded prematurely after replacing valve trays by sieve trays. The cause was scale left on the support rings raised many panels beneath the downcomers, reducing the clearances from 1″ to 5/8″–3/4″. In another case (292), reducing clearances from 2″ to 1″ in a stripping section caused premature flooding. In another case (286), a stripping tray was installed with zero clearance, flooding the entire stripping section and propagating to the rectifying section above, causing off-spec product. In another case (93), premature flooding started three trays from the bottom due to vapor entering the downcomer. The latter two cases teach that it is sufficient to have one incorrectly set clearance to bottleneck an entire tower. In one more case (254), the downcomer clearance was about 7 to 8 in. at the feed tray due to miscommunication, and premature flooding (due to lack of downcomer seal) resulted. In another case (9), excessive downcomer clearances caused downcomer seal loss accompanied by the loss of tower capacity and product purity.

Figure 10.18a shows an invaluable tool for inspecting downcomer clearances. This template should be inserted under the downcomer clearance in a number of locations along the downcomer length. The template is marked at quarter-inch intervals, which permits clearance measurement within ±1/8″, which is almost always satisfactory. Most major tray suppliers are delighted to provide templates like this (upon request) as they want their trays to work. Of course, short rulers or tape measures (Figures 10.18b,c) can also be used to improve accuracy, but are much slower. Some inspectors use wooden blocks (of different sizes) that are inserted under downcomers. They work, but the blocks often slip out of the inspector’s pocket, leaving the inspector without a tool until the slipped block is found or replaced.

In large-diameter towers (>10 ft), it was recommended (140) to take at least three measurement points along the clearance and allow variations of no more than 1/8″ along the length.

Special caution is required when the downcomer clearances change from one tower section to another. A common error is to interchange tray parts, and this may cause incorrect clearances to be set. One incorrectly set clearance is sufficient to bottleneck an entire tower.

If the column was previously in service and no changes were made, clearance measurements need only be spot-checked to ensure the absence of corrosion, scaling, or fouling effects.

The guidelines above extend to outlet weir heights. Fortunately, in conventional one- or two-pass trays, incorrectly set outlet weir heights are less likely to cause a spectacular tower failure like those described above. However, outlet weir heights are frequently incorrectly set, which may reduce tray efficiency and should be avoided. Correct setting of outlet weir heights is critical for multipass and specialty trays, as variations in weir heights will maldistribute the liquid to the passes, accompanied by efficiency and capacity penalties.

Other issues to look for include excessive gaps near the walls, weir pieces displaced and pushed away from their retaining bolting (Figure 10.19a), or missing weir pieces (Figure 10.19b). As above, this is critical for multipass and specialty trays.

Keep an eye open for trusses and draw sumps mounted over outlet weirs that the restrict downcomer entrance area, especially in foaming or high-pressure systems. These have led to premature flooding (Section 8.5).

Picket-fence weirs (192, 245) contain pickets or weir blocks to reduce the effective weir length. In the spray regime, this design is used to keep the liquid on the trays and, in multipass trays, to balance the liquid flow between passes. The dimensions of the pickets should be per the drawings, but reasonable deviations can usually be tolerated when approved by the supplier. The pickets should be held together by stiffening brackets and not “flap in the wind.”

Incorrect installation of inlet weirs can be a lot more troublesome. In services where they are installed on every tray, usually low-liquid load services, their function is to give adequate seal to the downcomer and good liquid distribution to the tray. Incorrect setting may allow vapor into the downcomer, causing a premature downcomer unsealing flood, and/or maldistribute liquid to the tray, which at low liquid loads may lead to drying of tray sections, liquid entrainment, and poor efficiency. If installed too close to the downcomers or if the downcomer plate bows onto them, they may cause restriction and premature flood. In one case (486), bowing of several downcomers over the inlet weirs led to a premature flood and capacity restriction.

In most services, inlet weirs are only installed on selected trays, typically the top tray, to distribute liquid to the tray. A common error is to have the inlet weir installed in some other location in the tower. In one case (254), an inlet weir intended for the top tray was located five trays below, effectively closing off the downcomer. In one more case (112), an inlet weir on a tray blocked the flow from the tray above, restricting capacity. In another case (61), during the replacement of a corroded feed tray by an in-kind, an unintended inlet weir was installed on the tray, resulting from the supplier’s misinterpretation of the tray drawing that called for an inlet weir on the top tray only. The narrow <0.5″ gap between the downcomer and inlet weir (Figure 10.20a) precluded the installation of a downcomer brace, leaving the downcomer dangling against the weir. Fortunately, due to the low liquid load and the extended tray spacing at the feed, no operational issue resulted.

With circular moving valve trays, a short inlet weir (about ½″–¾″), referred to as “interrupter bar” or “breaker bar,” is often installed to prevent liquid from opening the front row of valves and weeping through them. In one case (409, Figure 10.19b), a panel was rotated, rendering the interrupter bar ineffective.

The inspection tools for weir heights are the same as those described above for downcomer clearances, except that the template in Figure 10.18a is turned upside down to fit over the weirs and marked accordingly.

Seal Pans A seal pan is provided below the bottom tray, often also at the tray just above the feed, to stop vapor ascending the downcomer. In some chemical towers, especially with smaller tray spacings (<20″), a seal pan is provided at the bottom of each downcomer to permit a larger clearance to be used without excessive downcomer backup.

Seal pans are often awkward to access and, as a shortcut, left uninspected or only partially inspected. The author learned in the school of hard knocks that this can be fatal to the tower performance (216). Incorrect seal pan installation is one of the most common installation errors in tray towers and often leads to very poor performance. Troubleshooting maestro Lieberman (288) concurs, stating “If you have concluded that I’ve had lots of bad experiences with flooding due to seal pan malfunctions, you have drawn the correct conclusion.” The author recommends expanding every possible effort to ensure proper inspection of the seal pans, both during initial installation and again in every turnaround.

It is amazing how often seal pans are not installed at all or installed at an incorrect location. The diagram on the left in Figure 10.21 shows a seal pan that needs to be installed above the feed tray; the photo on the right shows what the photographer caught following the installation. The seal pan simply was not there. In another case (69), the bottom seal pan was inadvertently blocked off during a revamp that replaced a reboiler by steam injection. In one more case (489), an unsealed overflow pipe from a chimney tray prevented liquid descent, causing liquid buildup above the chimneys and entrainment (Section 8.6 and Figure 8.6b).

Figure 10.22 shows the clearances that need to be checked (a, b, and c). Clearance b should be less than c and less than the overflow weir height. It is also essential to check that the stiffener lip at the bottom of the downcomer plate immersed in the seal pan liquid does not significantly reduce dimension c to interfere with the flow (140) and that the downcomer plate is solidly supported. If this plate bends under liquid hydrostatic force, liquid downflow will be impeded and flooding of the bottom tray may initiate. Lieberman et al. (144, 292, 288) describe experiences with restricted “c” clearance. In one case (288), a discrepancy between the tower and tray fabricators caused dimension c to be 1″ when it should have been 3″, leading to restriction and premature flood.

Figure 10.23 shows a sloped downcomer from the bottom tray to the seal pan installed back to front (191, 216). The incorrect arrangement (Figure 10.23b) restricted the liquid flow at the bottom seal pan, producing cyclic flooding in service. This tower was the first that the author took from design to startup and turned success into failure until the flaw was diagnosed and repaired. This hard-earned lesson from the school of hard knocks taught the author, and hopefully the industry, the importance of avoiding shortcuts in seal pan inspections.

Corrosion, water trapping, and collection of debris or deposits are common issues affecting both new and operational units (Figure 10.24). The photographs are self-explanatory. Ensure that the weep hole(s) work. Trapped water from commissioning can lead to operating problems. In one case (342), plugged bottom seal pan weep holes trapped water, which later caused a pressure surge and tray damage. Good practices for weep hole sizes are in Ref. 192.

When the tower base contains a preferential baffle dividing it into a reboiler draw and a bottoms product draw compartments, it is imperative that all the seal pan liquid overflows into the reboiler draw compartment. Failure is likely to lead to starving the reboiler of liquid and to lights in the tower bottoms.

Odd Features It is important to look for and question any odd features observed during inspections. Some odd features can bottleneck or adversely affect performance; others are quite harmless. In all cases, they should be noted and evaluated. Figure 10.25a shows an obstruction at the downcomer exit. No one knew the reason, and the tower operation was fine. Figure 10.25b shows truss lugs. These are used for tray support in large towers but stick out above the tray floor, impeding liquid flow and in fouling services potentially collecting solids. Changing the truss lugs can be costly and, in the absence of operating issues, also unnecessary.

Specialty Trays These contain intricate features, many of which are omitted from the tray drawings, often due to confidentiality concerns by the supplier. When shown, they are often in insufficient detail. It is best to install a couple of trays from each section on the ground well before the turnaround, ideally by the crew that will be installing them in the tower. Often, tray parts can be installed in different ways, with the drawings providing insufficient guidance to the correct way. The supplier should then be consulted and provide guidance. Photographs and videos, emphasizing the issues in question, should be taken for future reference.

Many specialty trays contain “push valves.” These are fixed or moving valves with a long front leg and a short back leg (or closed at the back), inducing horizontal vapor flow. These push valves should be installed per the supplier drawings. If the drawings are not clear, the supplier should be questioned. As a general rule, the openings should direct the vapor issuing from them either in the direction of flow or toward the stagnant regions on the sides of the main flow path (or both).

Figure 10.26 shows incorrectly installed froth initiators (or bubbling promoters) on specialty trays. Froth initiators inject some of the rising vapor into the liquid issuing from the downcomers in order to aerate the liquid upon tray entry and also to impart this liquid a push in the direction of flow. Three of the froth initiators in Figure 10.26a, and all the froth initiators in Figure 10.26b, were installed backward, sending this vapor into the downcomer. The author has seen cases in which vapor entering downcomers by a similar mechanism choked the downcomers and incurred a tower capacity limitation. It is also important to check that froth initiators are firmly fixed to the tray and do not come loose.

Some specialty trays feature a multitude of downcomers and passes. Good liquid split to each of these passes is essential for achieving the design tray efficiency. It is therefore critical to positively ensure that tray levelness, weir heights, and downcomer clearances are within the (usually very tight) tolerances set by the proprietor. Tower sway may render these inspections difficult on windy days. The author is familiar with cases of poor separation in tall towers containing specialty trays with multiple downcomers resulting from a relatively small degree of out-of-levelness (which significantly exceeded the manufacturer’s specifications).

Materials of Construction It is essential to ensure that all tray parts, including clamps, bolts, nuts, and washers, are installed according to the materials of construction specifications. If lower-grade materials are arbitrarily substituted (even in only one tray or one section of the column), severe corrosion may occur. It is not uncommon to find that all tray parts, except for a very few, are fabricated from the correct materials, but these few are usually sufficient to cause problems.

The materials of construction are often marked on hardware and tray parts. A magnetic rod can be used to distinguish austenitic stainless steel (300 series) from carbon and martensitic stainless steel (400 series).

Particular attention must be given to nuts, washers, and bolts. Often, those used are of an inferior grade. This could be caused by inadvertent mixing of nuts and bolts or installers running out of an item and using an inferior substitute they find in their toolbox. The substitute corrodes in service (e.g., Figure 10.27), causing a variety of problems such as mechanical failure, leakage, rusting, and inability to undo when required. One inspection (329) identified four installed trays as well as many nuts and bolts fabricated from 304 SS where 316 SS was specified; these would have failed in service. Another inspection (291) revealed stripping trays and bolts in a refinery vacuum tower fabricated from carbon steel when 410 SS was specified.

If different materials are used in different parts of a tower, care must be taken to ensure that no interchanging of materials occurs between sections. Internals of existing towers should be inspected for corrosion and operation damage. The materials of any changed parts should also be checked.

Damage If damage occurred, the inspection should explore and identify the mode. By “mode” we mean how it happened, not why. The reason is sometimes well known and at other times obscure. Refs. 192, 201 discuss many possible mechanisms of damage, which can be identified or at least narrowed in on once the mode of damage is determined.

Upward push on the trays is consistent with a pressure surge below the trays, high liquid level at the tower base, fast depressuring from the tower top or from a location above the damage, or a sudden step-up in condensation. Downward push is consistent with a vapor gap problem, subcooled liquid entering into the bottom sump, liquid slugging from above, or rapid depressuring from the tower bottom. Trays above the feed bent up while those below the feed bent down may suggest rapid flashing at the feed. Trays above the feed bent down while those below the feed bent up may suggest rapid quenching at the feed due to cold liquid entry into a hot column. Bending or cracking can suggest heavy stepping, overtorquing, or vibration. Loose hardware and cracking can also indicate vibrations.

Of course, there can be many other explanations for various modes of tray damage. Identifying the mode of damage can guide the user to the most likely mechanism.

Tolerances Figure 10.28 depicts typical tolerances recommended in many literature sources surveyed in Distillation Operation (192). These are shown only as a general guide. When company or supplier specifications differ, they should prevail over those shown in Figure 10.28. The inspector should be aware of the possible adverse effects of departures from the specified or recommended values, as described in sections discussing the relevant parts.

10.3.8 Feeds/Draws Obstruction, Misorientation, and Poor Assembly

What to Look for Incorrect orientation of internal pipes, baffles, and other removables is one of the most common installation errors. These are often installed in situ by people who have little understanding of column operation.

Figure 10.29 is an excellent illustration. It shows correct and incorrect ways to assemble the inlet bend of a liquid drawoff line. The correct arrangement (Figure 10.29a) was intended to prevent drawing solids into the liquid outlet line. The incorrect arrangement installed (Figure 10.29b) possibly made more sense to the installer, thinking that the pipe was to overflow liquid or to catch the falling liquid. A good, but uneducated logic. This arrangement caused 160 psig vapor rather than liquid to escape through the outlet pipe (191) into an atmospheric storage tank, lifting its roof; fortunately, no one was hurt. Hence, it is essential to ensure that such pipes have not been installed upside down or sideways in the column.

In another excellent illustration (164), a revamp added product draw sumps beneath the downcomers a few trays below the top of a tower. The product was drawn from the sumps, with tower reflux descending onto the tray below via the clearances between the downcomers and the sumps. The installers presumably did not understand the need for the clearances and welded them shut. The reflux was unable to descend, causing flooding above.

For liquid feed, critically review the possibility of vapor presence or vapor flash upon entry, especially with packed towers. Sections 4.9 and 4.13 provide detailed discussion with case studies.

When checking the location and orientation of internals during inspection, note should be taken of the possible interference of internals with instruments or other parts. It is common to have different internals shown on different drawings, and interferences between these remain unappreciated until the parts are assembled.

Explore the possibility of plugging of any internal piping and vapor channels, especially those that are difficult to check. In one case (281), plugging of internal pipes bottlenecked a tower for 12 years and evaded several inspections.

Incorrect location or orientation of instruments is a common flaw. Thermocouples may be installed in a stagnant region or in a region where the feed or weep from above hits the thermocouple, therefore giving erroneous readings. Pressure taps are sometimes installed in turbulent regions, or at the wrong nozzles, and consequently give incorrect indications.

Strategy A very useful technique, recommended by the author and others (409), for critically inspecting any point of transition (feed, draw, chimney tray, change in diameter or number of passes), is to imagine yourself as a pocket of liquid or vapor. This pocket enters at a given velocity (which you should calculate) and travels in an initial direction dictated by the entrance geometry. Once inside the tower, the pocket will seek the easiest path, be deflected by walls or baffles in its path, and change direction as it hits them. Baffles and walls also break and/or redirect its momentum. Your challenge: “Would it work? Would you (the pocket of liquid or vapor) get to your intended destination without obstruction and without upsetting or adversely interfering with the tray action?”

Closely look for watermarks. They are invaluable in identifying mysteries, often unexpected ones. The watermarks in Figure 4.26 identify a horizontal direction of liquid coming out of distributor openings. The watermarks in Figure 10.17c show that the valve floats have been spinning. The watermarks in Figures 10.30a and b show a 7-ft vortex in a tower base and leaks from a reboiler draw pan. None of these issues was expected and could have remained undetected were it not for the watermarks.

Items that Frequently Go Wrong Below is a list of items that commonly go wrong and need to be closely watched by a process/operation inspector:

• Misoriented feed pipes (e.g., Figure 10.31a) and misoriented pipe distributor holes. Also check that the number and size of the holes conform to the drawings. In one case (41, 43), a feed pipe was welded in place upside down, which could have been circumvented by a timely inspection. In another case, the feed pipe holes pointed at 45° toward the outlet weir when they should have pointed at 45° toward the inlet downcomer. Luckily, this did not cause a bottleneck.
  
• Plugged holes or slots (Figure 10.31b), corroded holes or slots, unintended holes (Figure 10.31c,d), loose nuts and bolts, and damage. Corrosion tends to target welds and areas near welds (43); inspect the bottom corners and all welded seams.
  
• Flashing feeds finding a path into the downcomers. The flash vapor is likely to choke the downcomer. Section 8.2 and Figure 8.1b show a feed pipe bringing a flashing feed into a downcomer, leading to a capacity bottleneck. Section 5.5.4 and Figure 5.15b show a high-velocity flashing feed directed at a tray floor, causing a tower capacity bottleneck.
  
• Flashing feeds causing damage upon entry. Baffles, false downcomers, and other internals in contact with a flashing feed may be subjected to severe hydraulic pounding and vibrations. They need to be secured to their supports with brackets, bracing, through bolts, and double nuts. The supports should allow pipe movement due to thermal expansion. Check for deflection, wear, broken brackets, or other damage. Audit the possibility of velocity components impinging on the tray above or below, the tower wall, and the seal pan above. These should generally be avoided.
  
• Feeds significantly hotter than the tray liquid getting into the downcomers and causing vaporization there. In one revamp (167), a feed 80°F hotter than the tray liquid was introduced a short distance upstream of a downcomer, causing erratic operation and poor separation in a column.

  

  
  
• Feed pipes obstructing liquid entry into the feed tray downcomer. Section 8.4 and Figures 8.3 and 8.4 are illustrations.
  
• Center downcomers, supports, or baffles obstructing the movement of vapor or flashing feed or reboiler return and channeling the vapor into one section of the tray. Some case studies are in Section 8.12 and Figure 8.13.
  
• Plugged, blocked, or missing drain or vent holes on internal pipes. Inability to drain or vent internal pipes can trap toxic or pyrophoric materials and be hazardous to personnel entering the tower in the next turnaround.
  
• Draw boxes or draw sumps choking downcomer entrance, as illustrated in Section 8.5 and Figure 8.5 as well as in Case 9.9 in Distillation Troubleshooting (201).
  
• Substandard flanges/gaskets in internal pipes, missing gaskets, incorrect gasket sizes, and materials. Using incorrect gaskets in internal flanges has been a common problem. The temperature rating of the gaskets should conform to both normal and abnormal operation (e.g., steamout). In many cases, incorrect gasket materials were attacked by the chemicals in the tower, leading to severe leakage. In one case (61), metallic spacers were found in lieu of gaskets in the flanges of the reflux distributor, causing a large leak, liquid maldistribution to the three-pass trays, and poor tray efficiency. This was missed in past inspections. In another case (60), many shop-fabricated plate flanges were found leaking and needed replacing (Figure 10.31e). Other cases of leakage from such flanges were described (198, 285).
  
• Impingement of high-velocity feeds (especially those containing vapor) on the tray liquid, tray floor, distributor liquid, chimney tray liquid (Section 8.13, Figure 8.14), downcomer entry, tower walls (causing corrosion or erosion), or instrument taps in the feed region. In one case (60), high discharge velocity of flashing feed from downward-pointing pipe distributor perforations damaged and destroyed valve floats located right beneath the distributor near its closed end, with weeping and penetration through the holes with the missing floats. In another case (144), a high-velocity hot flashing feed to a refinery atmospheric crude tower was preferentially deflected by a center downcomer into one quadrant of the tray above where it caused severe fouling.
  
• Impingement of high-velocity reboiler return feeds on the base liquid level, seal pan overflow, tower walls (causing corrosion or erosion), or instrument taps at the tower base region. Figure 10.32 shows reboiler return entries that impinged on the sump liquid level and caused severe bottlenecks in towers. Sections 8.14–8.16, and Figures 8.15–8.17 have additional good examples.
  
• Maldistribution of feed liquid or vapor in multipass trays, favoring some passes to others. Section 8.3 and Figure 8.2 are good examples.
  
• Poor mixing between reflux from the tray above and the feed. This poor mixing can lead to poor separation in the section below and is fatal for extractive distillation. Sections 8.18 and 8.19 and Figures 8.24 through 8.27 have discussion and illustrations.
  
• Misoriented or mislocated draws, seal pans, and inlet weirs (at feed or reflux entry).
  
• Leaking draws and chimney trays. These are discussed in this Section under Water Testing.
  
• On chimney trays and draws, features that obstruct liquid flow or cause excessive hydraulic gradients that lead to overflows. Figure 10.33a,b shows supports installed during a revamp inside the downcomer from which a pumparound was drawn. The supports impeded liquid flow toward the draw nozzle, causing cavitation of the draw pump. The problem was solved by cutting holes in the supports to allow the liquid to drain (in this case, this could be done without compromising the mechanical strength). In Section 8.10 Case 1 and Figure 8.11a (236), rectangular chimneys perpendicular to the liquid flow restricted the liquid flow area toward the outlet nozzle, causing a steep hydraulic gradient, an overflow, and excessive lights in the bottom product.

  

  

  
  
• Chimney trays and draws are often intended to be total draws or to permit only liquid overflow to descend. During inspection, attempt to visualize the flow patterns on the tray, as well as the vapor–liquid interaction, and look for hidden paths for the liquid to bypass the tray. Section 8.8 and Figure 8.8b describe vapor issuing from chimneys, horizontally deflected by chimney hats, blowing some of the liquid descending from the seal pans above into overflow downcomers that were meant to have no flow. Case 1 and Figure 8.9b in Section 8.9 describe a situation experienced by the author several times. In all these, only liquid overflowing into the downcomer was meant to descend. However, while the chimneys had hats, the overflow downcomer did not, so liquid from the packed bed (or tray weep) rained into it. Case 2 in Section 8.9 describes two other mechanisms for liquid bypassing a chimney tray and raining into the risers: by pouring into an open chimney and by wrapping itself around hats that had no weirs. In another case (56), a support for the seal pan of the tray above did not permit installing one of the chimney hats; seal pan liquid poured into the chimney. Finally, a case was described (144) in which adding manways to a chimney tray led to leakage as the manways could not be seal-welded.
  
• Oversized hats may restrict the vapor ascent area, leading to flood, or cause vapor maldistribution to the bed or trays above. During inspection, visualize the vapor rise path and evaluate whether there is an oversized hat issue. Review Case 3 in Section 8.11 and Figure 8.12b where oversized hats led to premature flood. The author has seen other similar cases. Review the case studies in Section 8.12 and Figure 8.13, where oversized hats and their interaction with support beams led to vapor maldistribution. In another case (140), a very small vapor space between chimney hats was identified by inspection and corrected prior to startup.
  
• Errors in the dimensions on the chimney tray. The chimney area and the area between the chimneys and the hats should be within 3–5% of the design. Look for any signs of bowing in long chimneys. They may need straightening and bracing to prevent recurrence. Look for signs of buckling at the tray floor, especially in large-diameter hot services. Buckling is a sign of expansion (usually thermal) between rigid supports and is a common cause of leakage.
  
• No path for liquid to go from the chimney tray to the tower section below. This path can be external to the tower or internal via a properly designed overflow pipe. In one case (56), the intended overflow was not installed; in another (41, 43), redesign of a draw tray in a revamp overlooked replacing the original downcomer to the tray below, causing flooding above the chimney tray. In one more case (41, 42) a block valve added in the draw to isolate a side reboiler for maintenance was unusable because the chimney tray had no overflow. The extra pressure drop of the block valves caused excessive backup in the tower. In another case (292), the chimney tray feeding the tower reboiler had an overflow, but the top of the overflow was 18 in. above the bottom seal pan, causing tower flooding when the reboiler fouled up.
  
• Chimney hats incorrectly installed. Upside-down installation of chimney hats is a common error that can bottleneck the tower, similar to Case 11.12 in Distillation Troubleshooting (201). The author has had a similar experience with a chimney tray. For hats equipped with weirs, the liquid collected between the weirs should pour out over a closed portion of the chimney and not descend over an open chimney window. Figure 10.34 shows a good arrangement.
  
• Chimney hats, especially long hats, need to be adequately supported; if not, additional support brackets may need to be added. The author is familiar with many incidents in which a hat breaking loose blocked or partially blocked the liquid outlet. In one case (256), a broken hat suddenly restricted the circulation of olefin plant quench water, forcing a shutdown.
  
• With very tall chimneys (>36″ height), holes found at the base of the chimneys indicate chimney sway as described in one case study (56). Stabilizing struts to connect the risers together and prevent the sway solved the problem.
  
• Vapor from chimneys impinging on the chimney tray liquid level or instrument taps.
  
• Unsealed chimney tray overflow pipes or downcomers (e.g., Section 8.6, Case 2).
  
• When the chimney tray includes water decanting and removal features, check that there is residence time for water–organic separation, that the water path to the sump is unobstructed, and that the sump and water collection regions do not leak. One case of an obstructed path to the water draws was reported (41).
  
• Tower bottom sumps are often split into a reboiler draw compartment and a bottoms product draw compartment as described in Section 8.17. The intention is to divert the cold liquid from the tray above to the reboiler. The hot liquid also enters the reboiler draw compartment and overflows into the product draw compartment. Critically look at the arrangement in the tower and convince yourself that it will work as intended. The case studies in Section 8.17 show several arrangements that worked poorly. Lieberman and Lieberman (292) list errors in baffle arrangement among the most common refinery distillation design errors. A similar check is needed on a chimney tray that has hot and cold compartments (Section 7.3.4).

  

  

  
  
• Mislocated level taps on draw trays and tower base. Figure 10.35 shows a level tap on a chimney tray installed in a dry spot that does not see the liquid level. This is a common error and, if not picked by inspection, will leave the chimney tray with no level measurement and be very embarrassing, as it was one time to the author. A similar incident was reported by Sloley (440). Verify that each tower tap is connected to the correct instrument.
  
• Other instrument taps in regions where modifications took place. In one case (440) where single-pass trays were replaced by two-pass trays, a pressure tap that used to be in the vapor space found itself in the middle of a downcomer. In Case 25.3 in Ref. 201, pressure transmitters were installed below their taps during a revamp, giving false high readings due to condensate buildup above the taps.
  
• The upper level tap of the tower level transmitter should be below the bottom of the reboiler return nozzle.
  
• Vortex breakers are installed, firm, and clear of trash and plugging materials. When gratings are used, each grating layer is rotated 90°. The gratings support should allow free flow to the liquid outlet.

Water Testing Visual inspection for leakage can be misleading. The author has seen many situations where a pan “did not look like it would leak,” but water testing revealed a significant leak. For chimney trays, total drawoff pans, liquid outlet pans, bubble-cap trays, and seal areas behind inlet weirs, it is imperative to perform a leakage test. A leakage test is conducted by plugging weep holes and then water-filling the chimney tray or draw or seal area up to near the top of the weir, marking this level, and then monitoring how fast the water leaks down. A leakage rate of 1″ per 20 minutes for normal services (192), or as little as 1″ per hour (even less) for services where leakage is to be positively avoided, are common criteria used. Any leaking joints need to be tightened or repaired, and the test then repeated. Watch the repair work. There have been cases where the leaking joints were repaired by silica cement instead of seal welding. At the end of the test, all temporary plugs must be removed.

At the conclusion of the water test, once the temporary plugs are removed, all the water should drain out. Remaining puddles indicate low points trapping pockets of water, which may cause operating problems and, in hot towers, also pressure surges.

It has been argued that a tray that does not leak under test conditions may leak under operating conditions, and vice versa, but experience has taught that the water test generally provides a reliable indication of leakage under operating conditions.

Water testing should be conducted with solid-free, noncorrosive water and, in the case of stainless steel, also water low in chlorides. The quantity of water needed for the test should be estimated ahead of time, and additional pumps ordered as needed. Work underneath the tested pan should be stopped during the tests.

Figure 10.36 shows a massive leak from a once-through thermosiphon reboiler draw pan (60, 61). The leak induced lights into the bottoms product. Initial visual inspection did not reveal any integrity problems, but identified the watermarks in Figure 10.30b. Upon water introduction using hoses, the leak was so massive that it was impossible to build a liquid level in the draw pan. Gasketing gave some improvement, but the pan still failed the leakage test. Seal-welding the entire draw pan eliminated the leakage and the lights losses in the bottoms and reduced steam consumption, saving the refinery $1.5 million per year. In another case (168), “no amount of care in installation of the gaskets (of a chimney tray collector) prevented distortion of the gaskets in service, and liquid bypassing.”

References (60, 61) describe a second case, where the draw pans had big gaps and no gaskets. A water test proved that the draw pans were incapable of holding liquid level. Seal welding eliminated the leakage and improved product recovery and purity.

Case study 10.4 in Ref. 201 describes another case in which leakage from a diesel draw pan in an atmospheric crude tower caused a large loss of diesel product to the much lower value residue. That leakage lasted for at least 11 years, with the hidden flaw becoming the norm. When a leakage test was conducted, the water could not even fill the draw pan. Seal welding eliminated most of the leakage, giving a major improvement in diesel yield. An atmospheric crude tower in another refinery experienced a similar loss in diesel yield due to incorrect seal pan installation, which would have been detected by a water test. To improve the diesel yield, high-capacity trays were installed. During their installation, the seal pan problem was detected and corrected, which by itself would have improved the yield.

Draw sumps should drain into a drawoff nozzle flush with the sump floor, usually centrally located in the sump. With this arrangement, weep holes are not needed and should be sealed off if found.

When a preferential baffle divides the tower bottom base into a reboiler draw and a bottoms product draw compartments, there is usually no need to water test. An exception is when the bottoms product stream is small compared with the reboiler draw stream. In this case, leakage across the baffle can starve the reboiler of liquid, leading to lights in the tower bottoms. In this case, water testing of the reboiler draw compartment is a good idea.

In situ water testing of packings distributors is invaluable, which is further discussed in Sections 4.15 for spray distributors and in Sections 4.16.2 and 9.2.8 for gravity distributors.

10.3.9 Cleanliness of Internals

The column should be inspected for proper cleanliness and absence of debris. While cleaning, steps 1, 4, 5, 9, 13 in Section 10.2.3 and step 10 in Section 10.2.1 are important. Chemical cleaning is sometimes practiced. Washing and chemical cleaning are discussed elsewhere (192).

The inspection should pay special attention to the cleanliness of distributors, downcomers, seal pans, and nozzles. Debris left in the column is a very common experience, as listed in Table 10.2. Debris commonly found in columns can consist of working tools, clamps, nuts, bolts, gaskets, shipping covers, gloves, rags, ear plugs, paper, boards, masking tape, cups, and beverage containers. Each can cause bottlenecks or severe operating problems if not removed. Some troublesome experiences have been reported (88, 222, 288, 319).

In packed towers, dirt and sand may be lodged in narrow distributor or parting box troughs. These can often be sucked out by handheld vacuum cleaners. Plugged small holes can be reamed out with thin rods or metal wire.

Temporary plugs are commonly used during construction and cleaning at tower outlets to prevent small parts such as clamps, nuts, and bolts from entering outlet lines and in weep holes (for leak testing). These plugs must be removed prior to startup. All weep holes must be cleared of possible plugging; in one case (342), weep hole plugging resulted in tray damage.

Trash often travels past the bottom nozzle and finds its way to the bottom line. This line needs to be adequately flushed or blown during commissioning. Also, closely inspect draw nozzles and vortex breakers for trapped gloves and rags (288).

When the tower was previously in service, inspection by backlighting may reveal internal blockages in pipes, tubes, mist eliminators, and other internals, even when the device appears visually clean (392).

Lieberman and Lieberman (293) describe experiences where poor column performance resulted from a variety of debris left inside, including a rag caught in a vortex breaker in a jet fuel draw causing the shutdown of an entire refinery, and a case in which a complete scaffold including boards and piles was left inside a tower.

10.3.10 Final Inspection

Safety considerations permitting, immediately upon completion of column inspection, manhole doors should be shut and from then on should only be kept open while work (e.g., tray reassembly) is being performed inside. This would keep rain, sand, dust, and animals from entering the column. There have been cases (e.g., 284) where animal carcasses lodged in tower internals and caused premature flooding.

It is essential to maintain a continuous close watch of activities around the column during the period preceding the final bolting up of the column. Common errors during this period are (i) manways left loosely placed and unbolted or loosely bolted to the trays, (ii) debris reintroduced, (iii) pipe scaffolds left in the tower, and (iv) bottom baffle hatchways put into place but left unbolted.

Leaving manways unbolted or loosely bolted is a very common experience. Lieberman and Lieberman (293) state that the problem is not just common, it is universal. The author has experienced numerous cases of missing or unbolted manways, and many more are described in the literature. In the author’s experience, missing manways deserves a much higher spot in Table 10.2 than the ninth spot that it received. A few of the many cases are described below.

One 10-ft ID chemical tower (226) was returned to service after a routine turnaround in which no modifications to the trays were performed. The tray manways were dismantled for inspection but were not reinstalled following the inspection. The tower flooded at 70% of the rated jet flood, which did not happen previously.

In the most common cases (123, as well as some that the author is familiar with), poor separation and excessive entrainment were experienced due to missing or unbolted manways. In one case, separation was poor, but everything looked good on the gamma scans; in fact, the trays with properly installed manways were thought to be entraining (112).

One classic case (287, 294) showed how leaving manways uninstalled on only four trays in a 30-tray atmospheric crude tower was enough to bottleneck production and lead to building a new vacuum feed heater (with adverse carbon footprints) that would have been unnecessary had the manways been installed. The four uninstalled manways were in the section separating diesel from residue. Their absence led to more diesel in the residue and reduced bottoms temperature, both of which increased the heat duty on the downstream vacuum tower feed heater, which in turn led to building a new vacuum heater to overcome the bottleneck, which could have been circumvented by bolting the four manways.

Sometimes good luck is on your side; one of the author’s experiences had been finding a few manways left sitting on nearby tray decks from the last turnaround, leaving a large gap in the tray floor; surprisingly, the column still functioned.

There are different supervision procedures to circumvent such incidents. The level of effort expended varies with the procedure. Procedure selection should be based on the safety, environmental, and economic consequences of having to shut down the unit because of the unbolted manways, the criticality of the service, and the degree of confidence in circumventing such malfunction.

One low-effort component that is invaluable in minimizing, even avoiding, such malfunctions is to clearly explain to the supervisors and work crews the importance of properly bolting up the manways and the consequences of unbolted manways. In the author’s experience, as well as a few other experts he talked to, workers want to do a good job and will pay much closer attention when they understand why they are doing it and what is at stake. At the same time, inform the work crews that plant personnel will perform spot checks.

It is important to alert the work crews to potential miscommunication upon shift changes. In one case, the leaving shift bolted the top manway, leaving several unbolted manways below in an endeavor to prevent unauthorized entry. The next shift thought the work was complete and wanted to leave, but fortunately were stopped by the process inspector. One lesson is that it is important to make sure that all manways are accounted for. There have been other cases (293) where only the manways near the tower external manholes were bolted.

Some of the common techniques are listed below:

• As each manway is being installed, having trained responsible persons directly reporting to plant (or project) management inspect it and sign off on it. This is also recommended by Lieberman (288). Very often, process engineers are utilized for such tasks. This is probably the most reliable method, but also the most laborious. It is justified when the stakes are high. For instance, in one giant tower with 4-pass trays (242), there were more than 1000 manways, and even a few unbolted ones could have led to maldistribution and costly operating problems. The installation was very closely inspected, and the next time when the tower was entered, not one manway was found unbolted.
  

  One benefit of this procedure is affording one final check of fastening all the nuts and bolts, being able to fix any issues on the spot, and positively preventing the reintroduction of debris.

  
  
• Request the construction supervisor to number all the manways and take a photo as well as sign off as each manway is bolted in. This is reliable most of the time, but there is always a possibility that some manways inadvertently remain unphotographed and not signed off, and by the time it is realized, it may be too late to check.
  
• Rely on the work crew, with spot checks by plant personnel. This can be satisfactory when contracting a well-trained work crew (e.g., from a tray or packing supplier). With this technique, it is imperative to make sure that the supervisors as well as the work crew fully understand the importance of properly bolting up the manways and the consequences of unbolted manways – even when the crews are from a tray or packing supplier, as some of their workers may not be well trained.
  
• “Do nothing.” This is the one technique that the author does not recommend, as it has been the root cause of many malfunctions.

While the main issue is manways not being installed or adequately bolted, there are smaller issues that also need to be watched. When the tray contains directional valves, manways should be installed at the desired direction. Manway locking clamps need to be oriented in the correct direction (25). Many older installations of Nutter trays have unique manway hardware that requires correct orientation and installation, as discussed in detail elsewhere (42). When tray design varies from section to section, interchanging needs to be avoided. A good practice is to check this ahead of time and to mark the manways and the desired direction before their installation commences.

In towers that have a preferential baffle that divides the tower base into separate reboiler draw and bottoms product draw compartments, a check should be made that the baffle hatchway has been bolted. In one case (304), poor column performance resulted from a failure to detect (during inspection) that a bottoms hatchway had not been bolted.

10.3.11 Externals Inspection

The previous sections emphasized tower internals inspection, but the inspectors must not lose sight of some external units that need to be inspected while the tower is down for maintenance. These include:

• Heat exchanges: reboilers, condensers, and preheaters. These should be inspected for fouling and corrosion and leak tested.
  
• Filters, strainers, and coalescers. It is not uncommon to find filter baskets damaged or missing altogether. Experiences with missing and deformed meshes (Figure 10.37) were reported (409). Also look for watermarks. Poor mounting of filter baskets can give rise to liquid bypassing the baskets and deposits at the bottom of the filter casing (outside the baskets). The baskets should be installed securely and inspected by mechanical personnel to prevent bypassing. Filter elements are only as good as the sealing surface between the elements and the vessel (273, 274), so very close seals should be ensured. A check should also be made that all components of the filter are fabricated from materials compatible with the service.

  

  
  
• Solids in the lines leading to the tower, especially those leading to packing distributors, are likely to lead to plugging. They should be adequately flushed, blown, or otherwise cleaned before startup.
  
• Any maintenance items in lines connecting to the tower. In one case (216), a new undersized replacement orifice plate installed in the tower reflux line caused an off-spec product. In another case (Section 5.5.2) another improperly-installed new orifice plate led to premature flooding. In one more case, adding block valves in a side reboiler circuit to permit isolation introduced excessive pressure drop and premature flooding (42).