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You have to be able to do it with the tools at hand. My rules for identifying tray malfunctions are: You have to do it in one day. But most towers do not have such luxuries. Distillation Troubleshooting. How does this set of symptoms relate to other experiences I've had with similar equipment?
Is this a problem with heat transfer. They simply do not have the ability to bring the vapor and liquid into intimate contact. I've not found it—and I've tried often. They are widely used in slurry oil pumparound i. As my coworker. What questions should I ask the plant operators? Maybe it's not a process equipment malfunction at. My intent is not to write a reference book. What you want is a basic understanding of process and chemical engineering concepts. Author's Observations: There are better books available to guide one in making engineering calculations.
TYPES OF MAINTENANCE
For further information about this site. Distillation Tray Malfunctions. Privacy Notice and copyright information. Pretty far down on my list of concerns is how to quantify the malfunction. Like any puzzle. Chapter McGraw-Hill Professional. Understanding the nature of the question is the real challenge in correcting process equipment malfunctions.
The one exception is closure of the tray deck manway. When a tower is 1 meter or less in ID. Packing has a lower delta P than trays and hence may be favored for vacuum tower service.
After the trays are installed. I can crawl through every tray and inspect each component for proper installation and cleanliness..
I imagine that packed towers have a potential for greater capacity than trayed towers. Once the packing is installed in a fractionator. But the advantage is small. I prefer to use structured-type packing. I'm quite sure this statement may be refuted by vendor-published. NBA basketball coach I never really liked packed towers.
Even if there is only a single vessel manway. For wash oil service i. Packed Tower Problems The price we pay for success is the willingness to risk failure.
More likely. I was assigned to consult on a research project at the University of Chicago. The percent of absorbed radiation is measured on the opposite side of the tower. This measurement is done continuously up the length of the tower.
A radiation scan Gamma scan or TruTec scan are common trade names was performed. The scrubber was a inch-ID packed tower. This is a type of packing that is downloadd in blocks. But these correlations fail to take into account real-world installation.
The fractionation zone consisted of 12 layers of structured-type packing. An obstruction had been left between the layers of packing.
I studied the tower's operation and design for several weeks and issued the following brilliant report: A rather similar but more complex problem occurred last year in Lithuania.
Areas of low absorption correspond to the vapor phase. They were developing a coal-to-gas process. Syn Gas Scrubber Flooding In In a packed bed. But when the scrubber was opened later. The packing consists of thin sheets of perforated. I was working for American Oil in Chicago.
Each block is about 10 inches high. A source of radiation is placed on one side of a tower. Areas of high absorption correspond to a dense liquid phase. This incident bothers me to this day. I had to make a decision. As layer number seven was removed to expose the top of layer eight. It forms part of my bias against the use of packing in fractionation service. During my breaks. This is true for structured packing as well as more conventional dumped or random-type packing.
Reduction in Percent Open Area Packed beds must be supported. I personally supervised the removal of each layer of structured packing.
I had another. So the new packing. I mean: A typical random-type packing is 1-inch pall rings. All work had stopped. I found … nothing! It was getting dark and cold. We'll install all new layers of structured packing. But what is wrong with this structured packing? Packed beds must be supported on a grid support. By the percent open area. A distillation tray. You need to acidify the absorber.
I have found that these deposits accumulate at the interface between the packing itself and the grid support. I think your theory. How do I know this? Toward the bottom of the packed bed. They have to be supported by a packing grid support. The openings in the grid support have to be less than 1 inch to prevent the rings from slipping through the grid. In my long and unpleasant experience with packed towers. The packing was clean. It wasn't until the last few feet of packing was removed in the plastic buckets that the packing was found to be mixed with large amounts of black.
Circulate from the bottom up. The rather complex. After this malfunction was corrected and the packing was again reloaded. When I inspected the tower. Circulate from the top down? The acid will bypass the most fouled portion of the packing. I saw that the corrugated grid support see Figure had been misassembled after it had been cleaned and replaced.
When the cleaned packing and grid support were replaced. Cathy's beautiful. Cathy had all the packing removed in plastic buckets a second time. There must be an alternative. It was you who ignored my advice to use a trayed tower and not packing when you designed this absorber four years ago. And there is an alternative to acid washing the packing.
To reduce this limitation. So all's well that ends well. Between the grid support and the regular packing. Rings crush rather easily if handled roughly. A corrugated grid support increases the open area at the bottom of a packed bed. That's also the reason I avoid aluminum rings. For example. There is a reasonable. download these larger rings with the maximum thickness available. Corrugated Grid Support Packed towers are limited not by the open area of the packing.
But it's just at this point that fouling deposits tend to accumulate. If properly designed and installed. The rings had drained through this relatively small opening. It had been ground up into tiny metal fragments.
The broken bits of rings spun round and round through the reboiler and through the bottom of the regenerator. Process Design for Reliable Operations. Most of these fragments had plugged the shell side of the circulating thermosyphon reboiler see Chapter 9.
The maintenance manager developed an excellent method to rigidly secure these beds using layers of sturdy grids laid cross-wise and vertical half-inch steel rods. The circulating catacarb potassium carbonate solution had carried the rings into the reboiler. Bed Hold-Downs Structured packing or grids are often used in the wash oil or de-entrainment sections of vacuum and crude distillation towers.
A similar failure in a trayed tower would have had relatively small consequences and could not have led to a loss in thermosyphon circulation in the regenerator reboiler. What force had ground up these metal rings into such tiny fragments?
My inspections indicated a small portion of the packing support grid had come loose. Not a dozen intact rings could be found. The remainder of the broken and ground-up rings were lying in the bottom of the regenerator.
This is an excellent application for such packing. There must have been a channel somewhere in the reboiler bundle large enough for the rings to pass through. The packing in the Catacarb Regenerator Tower.
Sometimes the packing vendors will claim that the weight of the packing will. Always insist that the upward force that the packing hold-down structure must resist must be equal to at least the weight of the grid itself.
The liquid distributor above the grid wash oil section may then be damaged by impact with the grid. I have summarized in Process Design for Reliable Operations one practical mechanical design to handle this critical problem.
This is a pipe grid. Attached to the center pipe and arms are perhaps 15 to 20 spray nozzles. This is simply not true.
Liquid Distribution to Packed Beds Packing is employed in towers in three distinct services: Pumparound heat removal see Chapter 6 Wash oil de-entrainment Fractionation distillation In wash oil and pumparound services. To prevent this sort of upset. Full cone.
These are like shower heads. The standard spray nozzle used in the industry has the following characteristics: Model number corresponds to the maximum free passage of the nozzle. I say this not by calculation. The grids. But because of this inherent distribution problem. In all cases. Liquid is redistributed through progressively smaller and more. One day. The term spray angle is just the angle at which the spray leaves the nozzle.
The term. More on this critical subject later. The term maximum free passage means the maximum-size particle that can pass through the nozzle without plugging the nozzle. Tests have shown that packed beds do not redistribute liquid. The lie is that the liquid is equally dispersed in the area encompassed by the spray cone. A wider spray angle increases the wetted perimeter on the packing. I removed every drinking glass from the kitchen.
This is not just my opinion. For fractionation service.
This is bad. Nozzle plugging is by far the major malfunction encountered with packed beds in pumparound and wash oil service. I'll discuss this in detail later. I set up a solid array of glasses in my driveway. The least guilty nozzle in this liquid maldistribution problem was the Bete nozzle. A properly designed distributor can cost more and take longer to install than the packed bed itself.
Or when they are damaged due to pressure surges. The most common malfunctions with gravity distributors occur when their various components are not installed level. I applied the design water pressure of 15 psig to each nozzle and measured: A three-stage gravity distributor used in fractionation service. Or when they are removed for cleaning and are not reinstalled properly. Gravity distributors for larger-diameter towers are very complex and very costly.
But maybe you have read enough. This is an environment suitable only for rugged.
Or when they plug due to fouling deposits. I had each nozzle unscrewed for testing using the equipment shown in Figure The point is. The water spray angle. I mean zero holes! To prevent spray nozzles from plugging.
Larger openings will result in the nozzle plugging. The one-third value is derived from experience. This happens most often. The nozzle internals were missing.
Most of the remaining nozzles were partly or totally plugged with green glass from a broken beer bottle. Two of the nozzles failed to develop any spray at all.
But in theory. Plastic Packing. And you all will understand how I've become so smart on this particular subject. Nozzle plugging will surely follow. Another problem with a diminished spray angle occurs when using subcooled liquid.
This is not a problem I have observed personally. Most likely you will wind up with a slug of water when the steam valve is tripped open. If the delta P is high perhaps above 50 psi. It's only for a few shifts. This may cause the spray cone to collapse. Spray Nozzle Pressure Drop Spray nozzles do not have a very large operating range. If the delta P is less than 5 to 8 psi. The plastic rings were dumped onto a big pile in the equipment storage area.
The pyrophoric iron will autoignite when dry. Steam out the packing under pressure. Allow air to enter a packed bed contaminated with Fe HS 2. The lesson is. If you wish to duplicate our achievement. Exposed to sunlight.
I have designed such packed towers using standard correlations. I did not design the distributor myself.
CCP SOLAR THERMAL POWER PLANTS
I had downloadd a large quantity of plastic packing. While the theoretical advantages for capacity. Packed Tower Problems. They were fractionating between benzene and toluene. The liquid on each tray is at its boiling or bubble point. Texas City. Distillation Tower Pressure and Composition Control 3. Relatively small pressure increases would cause a surge of liquid to drain down through the packing and contaminate the toluene bottom's product with benzene.
An erratic tower pressure creates several problems in meeting a column's fractionation objectives. If the liquid on each tray foamsup. If the tower pressure suddenly drops to 90 psig. Relatively small decreases in the tower pressure would cause a surge of heavier toluene vapors to pass up through the packing and contaminate the benzene overhead product with toluene.
I recall a packed tower that I was working on for a Texaco Chemicals plant. An analogy is rapidly opening a warm bottle of beer. The smaller the residence time on the trays or in the packing. Towers running with materials that tend to foam i.
Steam Trap Stability Malfunctions For a steam trap to function. I can draw several conclusions about the relative importance of pressure control in various distillation columns: The tighter the specs in both the distillate and bottoms. If it is not. The chimney trays increased the inertia of the system. For an exchanger with steam on the shell side. For an exchanger with steam on the tube side. From these observations. We installed chimney trays between the packed beds.
Steam was blowing out of the drain. This reduces the exchanger surface area exposed to condensing steam. Believing that the trap was malfunctioning. The general lesson of the preceding example is that the use of a steam trap on a reboiler is almost certainly going to cause distillation tower pressure instability malfunctions.
I was working last week with a young engineer who had this problem on a liquid O2 vaporizer. My client should have used warm water from a cooling application. As the condensate was being drained to an open sewer. Note that the pressure downstream of the steam trap in this application was atmospheric pressure. I could see that he had made the problem worse. This will blow the condensate out through the reboiler's steam trap. To correct the problem with the existing facility. He had observed that the steam trap was not constantly draining.
The rate of heat transfer will drop. By "substantially above. On occasion. I've dealt with this problem in greater detail in Chapter This covers some tubes in the shell. The level in the condenser shell is not known. I call this the Heat Makes Pressure theory of control. I reviewed the good. Troubleshooting Process Plant Control. I'll only detail malfunctions with the good methods. Hot vapor bypass.
You can consult my Process Control book if you wish to read about the bad and ugly methods. The surface area exposed to the condensing vapor is reduced. Heat Makes Pressure In my book.
Both good methods work on the principle that adding heat increases pressure and subtracting heat reduces pressure. There are no level taps on the shell. For this common application. By the heat input. The rate of condensation falls. When liquid ethane. But at Texas City. Thus the proposed need for hot vapor bypass pressure control. The overhead product from this tower was typically only butane.
A leaking vent valve is a serious malfunction for "Flooded Condenser Pressure Control. Regardless of the sort of pressure control selected. Install a leakproof vent valve that really seals tightly when shut. Gas blanketing: It's just that. Excess Condensing Capacity For any pressure control scheme to work. Hot vapor bypass: This is not a malfunction of the pressure control instrumentation. Backpressure control on vapor line: At Texas City.
Throttling on cooling water: Convert to one of the Bad or Ugly sorts of pressure control that I have listed. A little bit of liquid hydrocarbon leakage will cause a large accumulation of ice downstream on the vent valve as shown in Figure Flooded condenser control: But I didn't have the time or patience to explain this concept to the plant people in Montreal.
The correct way to resolve this malfunction was to change the control scheme. Oversized Control Valves Placing a backpressure control valve in the overhead vapor line of a distillation tower is a poor method of pressure control. What I found. I recall working as a subcontractor for Glitsch. Very small movements in the valve position resulted in relatively large changes in the upstream tower's pressure. The lab data supported their complaint.
It was barely open. As I explained in my book. I checked the backpressure control valve on the overhead vapor line. This was a consequence of the designer oversizing the control valve. The trick is to close the vent when the liquid level reaches a maximum elevation in the drum. So I slowly closed the isolation gate valve upstream. To restore the condenser capacity. With the control valve now operating around the half-open position. I stopped. Hydrogen 0. A control valve does not produce a linear response throughout its range of operation.
TYPES OF MAINTENANCE
C2 components. The higher molecular weight components help absorb the hydrogen. Raising the reboiler duty raises the tower top temperature. Don't you always teach that heat makes pressure? Increasing the reboiler duty will lower the tower pressure.
But what will happen to the tower pressure? I recall my friend Steve. Steve's error was obtaining the feed sample for his design of this debutanizer in an open glass bottle. I noticed an interesting trend.
But why? The vapor leaving the top of a tower is at its dew-point temperature and pressure. As the ethane content of the depropanizer feed became erratic.
When the ethane content of the propane product was high. He should have used a sample bomb. The ethane makes the propane more volatile. As I explained in Troubleshooting Process Operations. The higher reboiler duty would have increased the condenser duty. What I eventually did on my depropanizer was to integrate the online gas chromatograph GC to reset the tower-top temperature set point for ethane and butane composition. The use of a condensate collection drum requires the use of a balance line between the steam-side channel head and the condensate drum to achieve tower stability.
When using lower-pressure steam. Use of Balance Lines When using high-pressure steam to a reboiler. Placing the balance line on the vent on top of the channel head connection where it too often is connected will promote tower pressure instability due to the resulting variable heat input. Neither balance line. I have never understood their intended purpose. And in my mind. I have blocked these lines in. When possible. Another common use of a balance line is in the distillation tower overhead system.
After all. By "lower-pressure. I use the concept of tower pressure and heat input control as if they were the same parameter. Ambient Conditions and Air-Cooled Condenser. But I felt none. The louvers have to be automated. Sticking Control Valve On some towers.
So I approved their recommendations. Most of the plants I have operated had water-cooled overhead condensers. Louvers placed on top of the air-cooled bundles. One tower had a long history of instability. My understanding of the problem with air coolers is that tower pressure instability during a sudden rain can be best avoided by the use of: Variable-speed fans.
As the vent valve was an air to open valve i. It's much easier to feel a very small control valve movement than to see it. When I read their report. My experience with air-cooled exchangers during sudden rainstorms is quite limited. The drum pressure slowly rose above its set point.
At my recommendation. And when their recommendations were all implemented. When they close. I sprayed it with the lubricant. The better or more experienced console operators will begin making these same corrective moves as soon as the warm-up vapors are partly diverted into the cold and empty coke drum. I observe two classes of reactions.
So I went back to my truck and got a can of WD But I'd like to propose a more productive method. The coke drums work in pairs. As the panel operator stroked the valve i. The drum pressure then rapidly dropped below its set point.
I felt the valve stem jump up a quarter of an inch.. With a sudden partial loss in hot vapor feed. The objective of the process control engineer is to automate the response of the experienced operators. Experienced plant operators learn to cope with this problem by not taking samples during coke drum warm-up. This diverts part of hot vapors away from the fractionator. The proper way for the control engineer to proceed is to observe. Should the bottoms liquid level rise above the reboiler return nozzle.
Reducing the reboiler duty i. If the reboiler is served by a forced circulation pump. Instability Due to Flooding Often. It is probably best to size this line a little too small rather than a lot too big.
As the heat balance on a distillation tower ultimately sets the operating pressure. I call this line the "emulsion outlet.
This is a somewhat more complex application of standard feed-forward process control design. This is an indication of quite a serious process malfunction. Feed Instability. If the return line velocities are much lower. Thermosyphon Circulation Instability Most of our towers are served by thermosyphon or natural circulation reboilers. Thus the observed pressure instability.
A second source of thermosyphon reboiler instability is a consequence of oversizing the reboiler return line. How can the novice engineer grasp the underlying principles if he or she is only performing the clerical function of computer-aided calculations? The larger issue is that instructors concentrate on the theoretical and mathematical aspects of the process phenomenon without explaining the practical application or the conceptual basis of the principle involved.
The pressure of the feed slowly rose by 3 or 4 psi. The cycle continued hour after hour. To correct this malfunction. At some point the liquid would blow clear and the cycle would be repeated. I had the operators increase the duty of the feed preheat exchanger. I interpreted this as liquid slowly accumulating in the feed riser line. At the resulting higher feed vaporization rate and line velocity. Engineering calculations should never be done with computer programs.
This is an example of phase separation in the rise line. The way chemical engineering is currently taught in universities is a waste of time and money. After a few minutes it would rapidly drop. Too often. I've had over Vapor—liquid equilibrium is needed to distill brandy from wine. Textbooks should be written by professors who have actually worked with the technology that they are describing.
Distillation Tower Pressure and Composition Control. We do not use higher math in industry. I've worked with 5. Instructors themselves should have practical process plant experience. My experience is that chemical engineering is taught incorrectly in universities.
Too many university texts are simply reworks of older university texts. The second law of thermodynamics is used to explain how gas compressors work.
In 46 years. Only after the concepts and applications are explained should calculation procedures be presented. Salt and Rust Formation on Tray Decks 4. Flooding in the sense that increasing the tower's feed rate increased the water content of the overhead methanol. The methanol overhead product spec was not less than It's no one's fault. The NutraSweet product. By-products from the NutraSweet production reaction were water and acetic acid. The water was removed in a trayed dehydration tower.
Aspartame or NutraSweet is now mostly marketed as Equal in little blue packages. There's no way to test the theory. Nothing can be done to correct the problem. Salt and Rust Formation on Tray Decks A politically acceptable theory presented to management as the cause of a process malfunction must have these characteristics: Thinking that the trays were salted out with NaOH.
Step 2—Raise the reboiler duty. The purpose of steps 2 and 3 was to increase the water content in the normally dry. Step 5—Continue until the tower delta P lined out.
After 75 minutes. NutraSweet production was then restored to 7 tons a day. So when I checked the water bottom's product.
Step 4—Monitor the falling tower delta P. This incident. The J. If caustic is injected into the feed on a continuous basis. The operators were using 10 times the amount of NaOH in the feed than they should have. I have presented this story in greater detail in my book. Searle Corporation. The objective of the caustic injection was to neutralize the acetic acid in the tower's feed. My clients. I proceeded as follows: Step 1—Measure the tower delta P. When the tower was commissioned in the previous year.
I observed. This method takes a while. For the naphtha reformer stabilizer. The overhead product will be quite heavy during this period. The catalyst employed is impregnated with chlorine. The tower bottoms temperature must be reduced so that a free water phase can leave the bottom of the column.
One method. The tower bottoms temperature has to be less than the boiling point of water at the tower's operating pressure. I estimated that the operators' confusion over the pH paper cost the J. Many malfunctions sound that way. There are two methods used to remove sublimed salts from the trays. The second. For a psig. This salt will sublime out vapor changing to a solid on the upper tray decks. This will only move the NH4Cl deposits from the upper trays down to the middle trays.
Naphtha Reformer Stabilizer Aromatics xylenes are used to make polystyrene. Searle Co. Washing these trays just transferred the NH4Cl salts to the downstream debutanizer and plugged the trays in that column.
I realize this story makes it seem that the malfunction. The circulation pumps had a tendency to cavitate when free water entered their suction during onstream water washing. This prevented tray salting with NH4Cl. I convinced them to stop the potentially dangerous water washing. It's a serious problem when running high-nitrogen FCU feed. The trays above the wash water inlet are protected from salt deposits because the salt has been removed in the lower trays.
This cavitation blew out the pump's mechanical seal. The culprit is ammonia salts. Continuous Water Wash On some columns. So I suggested that they operate their upstream feed hydrotreater at a high enough hydrogen partial pressure to remove nitrogen from the naphtha reformer feed.
But then they complained about the pentanes and the NH4Cl contamination of their overhead stabilizer product. Some hydrocarbon entrainment in the water is unavoidable. Continuous water wash circulation to remove salts. Croix U. Amoco downloadd a used propylene—propane splitter from Union Carbide in Whiting. Ten percent is a typical target. I've designed but never operated the system shown in Figure Indiana see Figure Virgin Islands. The tower was equipped with Linde high-capacity trays.
The tale was told in the snow. The KOH dryers were large. Upstream of the tray splitter. Carryover of KOH brine salts up lower trays in splitter.
Union Carbide had operated the tower for 30 years with good fractionation. The purpose of the KOH was to extract all traces of moisture from the splitter's feed. The splitter bottoms product was LPG propane. The splitter overhead product was polymer-grade propylene that was shipped to the Amoco chemicals plant in Houston. The wet. We knew the cause of the problem immediately.
Periodically this brine phase is supposed to be manually drained to the sewer by an operator. Go ahead. This was caustic—KOH brine. An aqueous KOH brine is formed. We're on the road. I wouldn't be out here freezing. We don't need a permit. See if propane or brine drains out. We drain down the brine regular. If this doesn't happen. I don't need any permit to be on this unit. If you people spent more time working and less time complaining.
Comprehensive documentation produced by proficient technical writers is very helpful, especially if it provides a theory of operation for the subject device or system. A common cause of problems is bad design , for example bad human factors design, where a device could be inserted backward or upside down due to the lack of an appropriate forcing function behavior-shaping constraint , or a lack of error-tolerant design.
This is especially bad if accompanied by habituation , where the user just doesn't notice the incorrect usage, for instance if two parts have different functions but share a common case so that it is not apparent on a casual inspection which part is being used. Troubleshooting can also take the form of a systematic checklist , troubleshooting procedure , flowchart or table that is made before a problem occurs. Developing troubleshooting procedures in advance allows sufficient thought about the steps to take in troubleshooting and organizing the troubleshooting into the most efficient troubleshooting process.
Troubleshooting tables can be computerized to make them more efficient for users. Some computerized troubleshooting services such as Primefax, later renamed MaxServ , immediately show the top 10 solutions with the highest probability of fixing the underlying problem.
The technician can either answer additional questions to advance through the troubleshooting procedure, each step narrowing the list of solutions, or immediately implement the solution he feels will fix the problem.
These services give a rebate if the technician takes an additional step after the problem is solved: report back the solution that actually fixed the problem. The computer uses these reports to update its estimates of which solutions have the highest probability of fixing that particular set of symptoms.
From there the troubleshooter forms hypotheses on potential causes, and devises or perhaps references a standardized checklist of tests to eliminate these prospective causes. This approach is often called " divide and conquer ". Two common strategies used by troubleshooters are to check for frequently encountered or easily tested conditions first for example, checking to ensure that a printer's light is on and that its cable is firmly seated at both ends.
This is often referred to as "milking the front panel. This latter technique can be particularly efficient in systems with long chains of serialized dependencies or interactions among its components. It is simply the application of a binary search across the range of dependencies and is often referred to as "half-splitting".
Often considerable effort and emphasis in troubleshooting is placed on reproducibility Intermittent symptoms[ edit ] Some of the most difficult troubleshooting issues relate to symptoms which occur intermittently. In electronics this often is the result of components that are thermally sensitive since resistance of a circuit varies with the temperature of the conductors in it.
Compressed air can be used to cool specific spots on a circuit board and a heat gun can be used to raise the temperatures; thus troubleshooting of electronics systems frequently entails applying these tools in order to reproduce a problem. In computer programming race conditions often lead to intermittent symptoms which are extremely difficult to reproduce; various techniques can be used to force the particular function or module to be called more rapidly than it would be in normal operation analogous to "heating up" a component in a hardware circuit while other techniques can be used to introduce greater delays in, or force synchronization among, other modules or interacting processes.
For example, knowing that an intermittent problem occurs "within" an hour of a particular stimulus or event Nevertheless, sometimes troubleshooters must resort to statistical methods In such cases, even when the symptom seems to disappear for significantly longer periods, there is a low confidence that the root cause has been found and that the problem is truly solved. Also, tests may be run to stress certain components to determine if those components have failed.
However, many problems only occur as a result of multiple failures or errors.A Maintenance Model is a mixture of the previous types of maintenance in certain proportions, and it responds appropriately to the needs of a particular equipment. I had to make a decision. This review will replace, in general, all parts subject to wear or failure probability over the years parts with a lifetime less than two years.
After the trays are installed. And if this observation does not coincide with an increase in the tower delta P, then the problem is ooding starting at an upper tray deck.