This Month’s Puzzler
The pipeline company I work for replaced some centrifugal pumps (CPs) that performed well for over 20 years with progressive cavity (PC) pumps at an old oilfield gathering station. (See online figure.) After two months’ operation, the PC pumps suddenly failed. Because the station is down, the company decided to do some much needed maintenance. While sniffing for leaks before construction could begin, we discovered several, in particular around the new pumps. In addition, it looks like regulators will ding us for over-using our flare and for complaints about hydrogen sulfide emissions because the flare failed to ignite properly.
My first problem is finding out why the PC pumps failed and what to do about the flare. We had relied on a world-renowned engineering company for the previous work; it had highly recommended a particular make of PC pumps. However, my company opted for a different pump vendor because maintenance insisted on the brand for “consistency” in the equipment inventory. The engineering company also installed a new flare ignition system about the time of the pump replacement.
Why do you think the CPs ran so well for so long and the PC pumps failed? Is the flare problem somehow connected with the pump failure? What other thoughts do you have?
Inspect The Pump
Consider the following:
1. If elastomers are used in the stator/rotor, they would fail due to high temperature or dry run, among other reasons. Ensure you have continuous flow. You may monitor temperature (suction/ discharge) or flow rate to the PC pumps.
2. Inspecting the internals of your failed pumps could point to some possible causes of the premature failures: abrasive wear may indicate rubbing of metallic parts, which in turn could have been caused by high temperatures or vibration; chemical attack may indicate presence of corrosive chemicals.
3. Is your lube compatible? CP lube may not be compatible with PC requirements.
4. Cavitation could have damaged the pump. Because (I assume) you didn’t have cavitation with the CPs, the feed to the pumps might have become lighter, thus increasing the chance of cavitation. One early indication is erratic flow and discharge pressure. Check to make sure you have sufficient net positive suction head (NPSH). For your well-head application, this is not easy to do. You may look at the speed of the PC pumps and CP pumps. If the PC pump speed is excessive, it could contribute to cavitation problems.
For extreme variations in temperatures, some vendors recommend metal rotor/stator combinations.
GC Shah, senior HSE consultant
Wood Group, Houston
Why The Change?
“If it’s not broke, then don’t fix it” still rings true today. The CPs “performed well for over 20 years” and operations and maintenance had accepted them. This is a good run for centrifugal pumps in sour, dirty service. I generally am in favor of trying new products, but how much more effort and money are you willing to spend on the PC pumps instead of replacing them with the tried and proven CPs?
The vapors contain 2% (20,000 ppm) H2S (which is lethal) and indicates that the oil and water phases are also sour. Add the CO2 and water vapor, and the mixtures are very corrosive. Corroding through 3-in. standard pipe (0.218-in. wall thickness) in 20 years gives a corrosion rate of 0.011 in. per year, which is acceptable for sour service. In this service, liquid velocities should be kept below 5 ft/sec. Cut open some of the pipe and inspect for a solid layer of FeS that helps reduce/prevent further corrosion/erosion. Is the problem corrosion, erosion or sulfide stress cracking? Although changing pipe material may give you a longer life, it probably is not worth the added cost.
Figure 1. One reader recommends a variety of significant changes, as shown in red.
The PC pumps failed after only two months. Did the alternative pump vendor meet all of the engineering specs? Were the manufacturer’s installation, operations and maintenance recommendations available and followed? Is there adequate NPSH for all operating conditions? Send the pump(s), with history and operating conditions, to the manufacturer for inspection. That vendor can tell you a lot. In many cases, it will bear or share the repair cost under warranty. Consider sending the second pump to an independent pump repair shop.
The PC pump will move 48 gpm but you only need 25 gpm. Consider the changes in red in Figure 1. Because the PC pump essentially is a positive displacement pump, either speed control or recycle is required for a steady 25 gpm. Alternatively, the pump can be an on/off operation, but initially pumping only water may flush the lubricant on the seals and lead to early seal failure.
Based on the 15-hp motor, the pump discharge is about 180–250 psid. Unless the oil storage is a long way off, you only need about 10–30 psig pump discharge to flow to an atmospheric storage tank. A small centrifugal pump, operating at 1,200–1,800 RPM to minimize NPSH-required, should work well. The CP should be able to handle solids up to about ¼–½ in. so use a suction strainer without the screen to minimize suction pressure drop.
The block labeled “VRU” looks more like a separator and the box next to “Carseal Open” would be the VRU. The separator should operate at lowest possible pressure so as to minimize vapors at the oil separation/storage area. A VRU package normally tries to maintain a suction pressure between 1–8 oz. (2–14 in. H2O). The piping and components between the separator and the VRU will cause pressure drop and surges, especially if the piping has pockets. If the VRU is not very close to the separator, then locate the suction pressure transmitter on the separator. Take precautions that the VRU does not create a vacuum and pull oxygen into the system.
Larry Tarkington, consultant
San Antonio, Texas
Give More Details
The text said: “After two months’ operation, the PC pumps suddenly failed.” Is it possible to know why the pump failed?
Ernesto Calderon, consultant
Techint Engineering and Constructors, Quito, Ecuador
Why Was The Switch Made?
PC pumps excel where different liquids can be expected and where the viscosity and density change. They can easily handle slurries that would destroy a CP. In such cases, PCs are the pump of choice. However, keep in mind that changing the pump type may not be the most effective solution if a problem exists and, indeed, may create other problems.
Don’t blindly choose CPs above a viscosity of 100 cP or where the density changes significantly. In fact, carefully review the use CPs when the viscosity is much above 50 cP. Operating a CP pump with a medium viscosity fluid increases the horsepower draw and decreases the throughput.
A CP performed well for 20 years. So, the first question to ask is why was the low-cost CP replaced by an expensive, apparently unreliable PC pump? Is someone at corporate expecting a denser, falling API crude oil? There must be more to it than a maintenance engineer’s wish to cut down on inventory.
Another concern is seals. PC pumps aren’t designed to prevent leaks like CPs. Small wonder the leak detection pegged the meter. Another concern may be that the PC pump bearings aren’t protected against corrosion like those on the CPs. Without data on pump performance before it failed it’s difficult to find the failure root cause — that’s one problem with remote locations.
Dirk Willard, consultant
My company is trying to decide whether to sell or expand a facility it purchased. Unfortunately, we inherited a mess from the previous owners. There are no equipment files, and many equipment nameplates are missing or are only partially legible. Some pumps and blowers were made by vendors that have been taken over by other manufacturers; they can’t find the files for such legacy products. Much of the equipment was purchased used. Some equipment was imported from Korea, Japan and Germany. No drawings exist at all. We brought in an engineering firm for the first two months but it over-ran its budget; we fired the firm for lack of progress.
What can we do? How can we determine equipment capacities and create data sheets for equipment like pumps, blowers and instruments? I don’t even have an equipment list.
Send us your comments, suggestions or solutions for this question by November 10, 2017. We’ll include as many of them as possible in the December 2017 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.
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