This Month's Puzzler
Our pumps use API Plan 53 for sealing. We’re having problems with our seal pots: high consumption of very expensive oil, and contamination of the oil in the pot (see Figure 1).
Our young inspector brought this to my attention. He wants to bring in the pump salesperson but I doubt that person has the technical expertise we need. I can’t get any help at all from the company that sold us the seal pot or the seals; the salesman familiar with our product suggested a steam seal but that’s not our corporate policy. We have trouble maintaining pressure at our air compressors, especially with the use of pneumatic pumps all over the plant. I have seen the psi at the air tank drop to the low 70s; the tank is located 500-ft away from the seal pot. I want to send the sample to a laboratory for analysis because I think it’s the seal.
The inspector wants to test the cooling water coil. He thinks it might be leaking into the seal pot. The water is from a cooling tower.
What is the cause of the high consumption of oil? Is there any solution besides continuing to go through $10/gal oil? Are there any tests we should run?
Evaluate Three Options
This is a common problem: equipment needs expensive lubrication but consumes a lot of lubricant that contaminates product and causes spills that pose risks of fire or slipping injuries. In addition, failure of lubrication may damage the equipment or even lead to a fire, as in the case of compressors where loss of lubrication is a common cause of fire.
You can approach this problem in three ways: 1) live with it; 2) manage it; or 3) get rid of it.
Continuing to live with a situation involving an expensive ingredient (lubricant is an ingredient whether or not you think it is) adds to the work load of your staff. In engineering, we are taught to put problems behind us so we can face others. I’ve worked at places that never addressed some problems and, as a result, staff became overwhelmed. Nevertheless, you should weigh this option. Sometimes, the hysteria of a problem diminishes after a time. Indeed, there’s a productivity approach called the Napoleon technique that involves deliberately not responding for a while in the expectation that someone will come up with a solution in the meantime (see https://bit.ly/3PJrQNZ).
However, you never should delay addressing certain issues — product contamination and fire risk come to mind. Quantities matter. Spending a few hundred dollars extra on lubricant per year might make sense compared to investing tens of thousands of dollars to reduce this cost slightly.
Now, on to managing. This involves automatic measurement of the oil supply (level) with some assurance that it’s getting where it is needed (temperature measurement). If product contamination is an issue, you regularly must sample the product and, if the oil and product interact, identify not only the quantity of oil in the product but any material that evolves by interaction. This analysis could be pretty intense; minor constituents in the lubricant could cause a ripple effect in your customers’ products. Customer service will want to you to identify this problem as soon as possible.
So, let’s consider what could cause the oil leak: 1) corrosion in the seal pot or pump; 2) poor operation of the pump leading to deadheading or dry-running; 3) faulty selection of the seal pot; 4) low pressure in the backing pressure — with dips in the air pressure eventually damaging the seal separating the oil and process fluid; and 5) an oil incompatible with the seal and, perhaps, the product. The corrosion problem should be the item that scares you: it won’t matter what oil you use — this one isn’t going away!
Note that I alluded to ensuring that oil is getting where it’s needed. Temperature is a clumsy way of doing this because the damage already could be done before overheating occurs. Old fashioned sampling probably is the best way; look for bits of rubber and metal that tell you what’s happening with the seals and equipment.
As always, relying on instrumentation doesn’t do away with the need for humans. Monitoring the oil requires a disciplined lubrication but also staff to regularly inspect the instruments.
Of course, before you choose this path, weigh its ongoing costs.
If, instead, you decide to try to eliminate the problem, remember that maintaining seals is expensive. So, consider magnetic-drive and canned pumps as well as seal-less equipment whenever possible. If protected against deadheading (from a closed discharge valve or blockage in the line) or dry pumping (i.e., no liquid in the pump bowl), these types of pumps can run reliably for decades. However, compared to sealed pumps, they are hypersensitive to these events; providing redundancy is crucial.
If you don’t want to bring in a consultant, coming up with a way to eliminate the problem altogether may take months. You might succeed with smaller equipment but dealing with centrifuges, compressors, grinders, etc., requires specialist expertise — which is why consultants with such knowledge are invaluable.
Also, don’t forget about the possibility of substitution. You might find an oil that won’t contaminate your product, poses less of a fire hazard, is more compatible with seals, not as corrosive, lower cost, etc.
Dirk Willard, consultant,
Originally, our column was installed with a large feed preheater. Severe corrosion in that preheater and an increase in throughput required a larger reboiler. Our internal reboiler was replaced by an external vertical thermosiphon using an available bottom nozzle. Now, 30% of the tubes in the preheater are plugged. The trouble is that the nozzle is too small for the thermosiphon.
Our control engineer at the time — he has since retired — set the tower level control as the bottom valve. This worked well enough. A new production supervisor didn’t like that arrangement because it was a challenge starting up the plant. He had a contractor create a cascade control scheme where the level controlled the steam flow to the reboiler while the column bottom temperature served as a subordinate controller for the bottom valve.
With the changes, the reboiler stopped working completely. We can hear steam getting to the reboiler but the thermosiphon is working only erratically if at all. Our plant outage has been extended while we unravel this mystery.
A committee of engineers has come up with a range of possible causes: 1) the preheater isn’t providing sufficient heat because additional tubes were plugged during the outage; 2) the level reading is faulty because there wasn’t time to clean the tank bottoms and fouling has occurred in the bottom pressure leg; and 3) the steam traps aren’t doing their job, causing surges of condensate causing control problems.
The corporate engineer suggested contacting the old engineer for some advice. What do you think caused the problem? What is the best way to address it?
Send us your comments, suggestions or solutions for this question by October 7, 2022. We’ll include as many of them as possible in the November 2022 issue and all on ChemicalProcessing.com. Send visuals — a sketch is fine. E-mail us at [email protected] or mail to Process Puzzler, Chemical Processing, 1501 E. Woodfield Rd., Suite 400N, Schaumburg, IL 60173. Fax: (630) 467-1120. Please include your name, title, location and company affiliation in the response.
And, of course, if you have a process problem you’d like to pose to our readers, send it along and we’ll be pleased to consider it for publication.