Centrifugal pumps in U.S. oil refineries and petrochemical plants typically reach mean-times-between failures (MTBFs) ranging from barely three years to as much as ten years. This leads to the conclusion that the MTBF of these simple machines is largely influenced by issues or parameters that “someone” either misunderstood or chose to disregard. Therefore, many pump failure incidents could be avoided by acting on the information contained in this checklist.
PUMP FAILURES: THE AVERAGE AND THE IDEAL
It has been estimated that in the majority of pump failures investigated since 1962, the average pump reliability improvement implementation cost about 20 percent of the pump’s original cost. Another estimate held pumps responsible for one fire per one thousand pump repair events. With the average pump fire causing about two million dollars’ worth of damage, achieving fire risk reduction by component upgrading could be worth considerable money. Smart users are, therefore, factoring the imputed value of fire avoidance into their cost justifications.
In the late 1990’s and early 2000’s, the average repair of a refinery pump cost approximately $11,000—a figure that included burden and overhead. (Ref.1). By any measure, implementation of the average improvement item costs less than $2000 and is worth every penny of it. Our checklist of implementation items lists “things to consider” when pursuing reliability improvement. All are certainly known to best practices performers and further descriptions can be found in the literature.
A CHECKLIST OF OPTIONS AND CONSIDERATIONS
Part 1 of this 3-part comprehensive checklist starts with cooling issues, items 1 through 6, followed by bearing topics (7 through 17). Notice that many of these issues overlap; i.e. a cooling topic overlaps certain lubrication topics or issues. These, in turn, overlap with bearing issues, etc.
Therefore, being familiar with all checklist items should be important to the reliability professional. Equally important should be the realization that a checklist is never intended to take the place of the rigorous explanations that can be found in books and articles. These details are certainly available in both of the books listed in our references.
COOLING PUMP BEARING HOUSINGS
1. Do not allow pedestal cooling of centrifugal pumps, regardless of process fluid or pumping temperature. Calculate and accommodate thermal growth by appropriate cold alignment offset. Consider using hot alignment verification measurements (see references 1 and 2).
2. Do not allow jacketed cooling water application on bearing housings equipped with rolling element bearings. Note that water surrounding only the bearing outer ring will often cause bearing-internal clearances to vanish, leading to excessive temperatures, lube distress, and premature bearing failure.
3. Understand and accept the well-documented fact that on pump bearing housings equipped with rolling element bearings, it is possible to delete cooling of any kind. Simply change over to the correct type (synthesized hydrocarbon lube preferred) and generally somewhat more viscous, grade of lubricating oil.
4. Recognize that cooling coils immersed in the oil sump may cool not only the oil, but also the air floating above the oil level. The resulting moisture condensation can cause serious oil degradation. This is one of several reasons to avoid cooling of pump bearing housings, if at all possible.
5. Recognize that stuffing box cooling is generally ineffective. If the seal must be cooled, consider introducing a flush stream that has been cooled in a pump-external cooler before entering into the seal cavity. In the 1960’s it was proven that with stuffing box cooling the face temperature of mechanical seals is decreased by only about 1.8 degrees Fahrenheit (1 degree Celsius).
6. In water-cooled sleeve bearing applications where cooling water may still be needed, excessively cold cooling water will often cause moisture condensation. Even trace amounts of water may greatly lower the capacity of lubricants to adequately protect the bearing. On sleeve bearings, close temperature control is far more important than on rolling element bearings.
That, then, leads into a number of points on
BEARING-RELATED ISSUES
7. Do not use filling-notch bearings in centrifugal pumps. Inevitable axial loading will cause rapid bearing degradation. Replace filling-notch bearings with dimensionally identical Conrad-type bearings.
8. All bearings will deform under load. On thrust bearings that allow load action in both directions, deformation of the loaded side could result in excessive looseness—hence, the skidding—of the unloaded side. This skidding may result in serious heat generation and thinning-out of the oil film. Metal-to-metal contact will now destroy the bearing. Always select bearing configurations that limit or preclude skidding. The better bearing manufacturers have application engineering departments that can advise suitable replacement bearing upgrades. Work with these manufacturers and be prepared to pay a little more for their bearings. It’ll be well worth it in the long run!
9. The API-610 recommended combination of two back-to-back mounted 40-degree angular contact bearings is not always the best choice for a particular pump application. Matched sets of 40-degree and 15-degree angular contact bearings (SKF’s “PumPac” brand) are designed to avoid, or greatly reduce skidding. While not a cure-all, they may thus be more appropriate for a given service application.
10. Keep in mind the possibility of using a 9000-series thrust bearing together with a 7200 or 7300-series angular contact bearing in the thrust location of certain pumps. Seek factory advice and understand best-of-class lubricant application method before proceeding.
11. Thrust bearing axial float, i.e. the total amount of movement possible between thrust bearing outer ring and bearing housing end cap, should not exceed 0.002 inch (0.05 millimeter). Some hand fitting or shimming may be necessary to thus limit the potentially high bearing-internal acceleration forces.
12. Consider replacing old-style double-row angular contact bearings (bearings with one inner and one outer ring) with newer, Series 5300UPG, double-row angular contact bearings. These bearings have two brass cages, one outer ring and two inner rings per bearing. Note that axial clamping of the two inner rings will be necessary, but that these newer double-row bearings resist skidding.
13. Observe allowable assembly tolerances for rolling element bearings in pumps:
- Conrad, single angular contact and double-row bearings: bore- to-shaft: 0.0002 to 0.0007 inches (0.005 to 0.018 millimeters) interference fit.
- Bearing outside diameter-to-housing fit: 0.0007 to 0.0015 inches (0.018 to 0.037 millimeters) loose fit.
14. Be careful when allowing bearings with plastic cages in process pumps intended to operate dependably for years:
- Plastic cages tend to be damaged unless highly controlled mounting temperatures are maintained
- Plastic cage degradation will not show up in conventional vibration data acquisition and analysis.
15. Be certain to use only precision-ground, matched sets of thrust bearings in either back-to-back thrust or tandem thrust applications. Matched bearings must be furnished by the same bearing manufacturer. Verify precision grinding by observing that appropriate alphanumerics have been etched into the wide shoulder of the outer ring.
16. Use radial bearings with C3 clearances in electric motors so as to accommodate thermal growth of the hotter-running bearing inner ring. Note that modern polyurea greases (“EM” greases) are much preferred for electric motors.
17. Investigate column bearing materials upgrade options on vertical deep-well pumps and compare available high-performance (HP) polymers. Understand all physical properties and the intended service before picking the right HP polymer for the job.
But bearings require lubrication and bearing issues must often be dealt with in conjunction with lubricant performance parameters that even include lube application. We will consider them in part 2 of this series.
A LOOK AHEAD
Part 2 (next issue) will continue by addressing lubrication and mechanical seal issues. The third and final entry will delve into hydraulic and installation issues. There, too, we will grapple with misunderstandings and oversights.
REFERENCES
1. Bloch, Heinz P., and Allan Budris. Pump User’s Handbook: Life Extension, Third Edition (2010), Fairmont Publishing Company: Lilburn, GA (ISBN 0-88173-627-9).
2. Bloch, Heinz P. Machinery Reliability Improvement, Third Edition (1998), Gulf Publishing Company: Houston, TX (ISBN 0-88415-661-3).
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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 2013
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