This Month’s Puzzler:
An 18-head gear-driven piston pump with a worm gear is the heart of our soap-making process. A few years ago, it became noisy, so operations installed a soundproof barrier with a door to block the deafening thump-thump of the pump. For a plant expansion, we now want to increase the capacity of the pump by a modest 20%, which is well within its range. However, the new attention to the pump has prompted a question as to why it’s noisy. The maintenance department notes: 1) a mechanic changes the oil weekly because it’s clogged with oils and caustic; 2) stripping away insulation has exposed corroded copper steam tracing that leaks; 3) there are no seal pots to handle lubricating fluids during startup and shutdown; 4) some of the type-304 stainless steel pipe has been welded repeatedly to repair leaks; and 5) the concrete is corroded below the pump base. What is causing these problems? Can we repair the pump quickly, ideally in only three days? How should we expand the capacity?
The Mounting Is Shot
I ran into an overly noisy gear pump years ago while visiting a plant for an interview for some project work. I didn’t mention it during the interview, though. (A recruiter once asked me why I didn’t give free technical advice during an interview; I explained to him I get paid for my insight.)
A vibration check of your pump likely will lead to the discovery of broken mounting bolts for connecting the gear reducer to the pump. When maintenance people at the plant I visited (and at which I did get the project job) looked at the pump, only one of four bolts remained — it was twisting into a pretzel. The motion of the shaft damages the pump head seals, causing them to leak. (Being new to the company, I enlisted the pump vendor’s engineers to warn the plant staff about the seriousness of the situation — the pump would fail within a few weeks and, thus, required immediate attention.)
Let’s dig into your problem for the root cause.
First, type-304 stainless steel is a poor choice for caustic service. I would go with a type-316L because it can operate up to 212°F with 50% NaOH. Type-304 stainless in our plant was manufactured in Italy and, for some reason, was susceptible to spider-like cracking of the parent metal when exposed to caustic.
Second, copper tubing is a poor choice for heat tracing in this service. Steam overheats the oils and caustic. Overheated organics tend to burn and discolor. So, you should replace the steam tracing with electric tracing. If that’s not feasible, consider using type-316 stainless tubing. You will need four times the number of wraps of stainless steel compared to copper because copper is four times more conductive than stainless.
Planning the repairs will pose a challenge. You should replace all the piping around the pump heads and instruments. The complication of eighteen heads oriented in their own unique ways will be tough to estimate and bid. When I did the job, I took a very specific set of photographs, not just random shots. I laid out a grid with angles on the floor. I took a long shot from each 90° angle and had a mechanic hold a level yardstick to allow measurements. I filled a binder with about 300 pictures. Apparently, this was enough to do the job. The welding was bid out as time and material. The real test came in repairing the concrete in time.
As far as expanding the pump capacity, the simplest method is to increase the stroke speed. However, simplest is not necessarily the best. Faster speed raises the risk of erosion. A better option is to lengthen the stroke. The most expensive option is to increase the piston size, i.e., the area. This is expensive not only due to equipment cost but also because these pumps are like trees with all sorts of branches to be moved.
Dirk Willard, consultant
At our refinery, the mechanical seals on the pump in the heavy gas oil (HGO) pump-around on the pipe still (Figure 1) failed about once every two years. The pump manufacturer told us the pump, which delivers 339 gpm, wasn’t operating within its most efficient range and would experience fewer problems if we moved the operating point closer to the best operating point. So, a year ago, we trimmed the pump’s impeller to 11.75 in. from 12.75 in. We also increased the minimum-flow restrictive orifice (RO) diameter to maintain 194-gpm flow. Before trimming, the 4-in. control valve ran at 50%; initially after trimming, it ran at 67% — but this has been creeping up ever since and now is at 73%. A month back, we started to notice poor distribution in the return line to the crude distillation unit (CDU). The pressure drop across the spray nozzles should be about 6 psi but now reaches 9 psi. Is the trimming causing our problems? Should we go back to the old impeller? Can we increase the RO diameter to reduce wear on the seals even more? What other symptoms should we look for? What’s our path forward?
Send us your comments, suggestions or solutions for this question by February 12, 2016. We’ll include as many of them as possible in the March 2016 issue and all on ChemicalProcessing.com. Send visuals — a sketch is fine. E-mail us at ProcessPuzzler@putman.net 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.