Pump manufacturers have, for years, made strides to improve their products. However, the primary emphasis has often been on efficient hydraulics, low initial pump cost, and standardization of buy-out products such as bearings and seals. When that happens, a pump’s mechanical end tends to be neglected, as demonstrated in a recent case history.
FACING DOWN PUMP FAILURE
When failure happens, pump users—the manufacturers’ customers—are under pressure to get the equipment running again. While there may be many competing priorities, users should be encouraged to take good notes and to collect as much data as possible. Only then will vendors’ representatives and competent consultants be most effective.
In this case study, commendation was due when an oil refinery had collected much data and followed the right troubleshooting sequence. The events involved two new process pumps over a period of only three days in early 2015. The bearing housings of the new pumps are similar to the “generic” image shown in figure 1.
WEAK DESIGNS CAUSE TROUBLE
These identical process pumps—we will call them P-45C and P-45D—were ready for startup at a U.S. oil refinery in April 2015. The first of the two, P-45D, was started up without bearing housing cooling water, and the outboard bearing failed in 10-15 minutes. Removing the bearing housing revealed a broken oil ring such that there was a loss of lubrication. Both inboard and outboard bearings were replaced and the pump was run for four to five days. But serious oil discoloration was quickly noticed and oil from both bearing housings had to be replaced twice per twelve-hour shift.
Because the housing temperatures at the P-45D outboard bearing location were observed at approximately 190 degrees Fahrenheit (88 degrees Celsius), cooling water was connected. The outboard housing temperature then dropped to around 174 degrees Fahrenheit (79 degrees Celsius). The inboard bearing housing was in the stable temperature range of 130-140 degrees Fahrenheit (54-60 degrees Celsius) with cooling water, but also without cooling water. It should be noted that this again corroborates industry findings that cooling water is not needed on process pumps with rolling element bearings (see reference 1).
When P-45C was started up, it ran only a short time before its inboard oil ring also broke. The pump was immediately shut down before severe damage could be caused.
As part of the resulting repair, the bearings were replaced and new smaller diameter and thicker cross-section oil rings were installed on both bearings.
The two pumps had been manufactured with roller bearings at the inboard housing location. Upon dismantling, the inboard roller bearing outer race was found to be running off-center relative to its roller elements; there were no housing locating shoulders to position the outer race. Pressed for time, the refinery fabricated a new bolt-on bearing housing end plate which incorporated a shoulder to set the race inboard position and a sleeve to locate the outboard race location. This modification was quickly incorporated into the P-45C, but not the P-45D pump.
INVOLVING SERVICE ORGANIZATIONS
Without much delay, field representatives from the pump manufacturer and from an independent contract service organization arrived on site for a field assessment. Regrettably, neither the refinery nor the two service representatives could persuade the manufacturer’s engineering department to give a timely response.
By now, four days into the problem, the refinery had some basic engineering questions relating to design and factory fabrication issues. In next month’s conclusion, we will take a closer look at the on-site testing as well as illustrate the dangers of treating failure symptoms instead of the root causes of unsatisfactory bearing and lubrication performance.
REFERENCES
Bloch, Heinz P.; “Pump Wisdom—Problem Solving for Operators and Specialists” (2011), John Wiley & Sons, Hoboken, New Jersey
Bloch, Heinz P. and Allan R. Budris, “Pump User’s Handbook: Life Extension,” 4th Ed., (2013), Fairmont Publishing Company, Lilburn, Georgia.
ABOUT THE AUTHOR
Heinz P. Bloch, P.E., is one of the world’s most recognized experts in machine reliability and has served as a founding member of the board of the Texas A&M University’s International Pump Users’ Symposium. He is a Life Fellow of the ASME, in addition to having maintained his registration as a Professional Engineer in both New Jersey and Texas for several straight decades. As a consultant, Mr. Bloch is world-renowned and value-adding. He can be contacted at heinzpbloch@gmail.com.
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MODERN PUMPING TODAY, August 2015
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