For process pumps in particular size and duty categories, oil rings are rarely, if ever, the most reliable means for lubricant application. Oil rings tend to skip and even abrade, as shown in Fig. 1, unless:
- The shaft system is truly horizontal.1 Horizontality is very rarely obtained when shims are used to manage shaft centerline alignment.
- Ring immersion in the lubricant is perfect. Consistent depth of immersion can be difficult to maintain over time.
- Ring eccentricity is closely controlled. This is rarely possible unless stress-relief annealing is included as part of the manufacturing steps
- Ring bore RMS finish and oil viscosity are maintained within close limits. Viscosities can change depending on oil temperature and contamination-related oil quality.
- Shaft-surface velocities are within an acceptable range.
| Fig. 1. Oil rings in as-new |
(wide and chamfered) condition
on the left, and abraded
(worn narrow and without
chamfer) condition on the
These various parameters are probably within limits on the pump manufacturers test stand, and the manufacturer feels exonerated. However, when considered collectively, these parameters are rarely within the suitably close limits needed in actual operating plants.1 Other vulnerabilities exist and can cause repeat pump failures.2
To avoid premature equipment failure or, alternatively, the need for frequent precautionary replacement of rings and lube oil changes, serious reliability-focused owner-purchasers often specify and select pumps with flinger discs. Reliability-focused buyers often follow the advice of prominent pump manufacturers whose 1970s-era brochures asked buyers to opt for a superior anti-friction oil thrower (a disc) to ensure positive lubrication and thus eliminate the problems associated with oil rings.1
Often, and to accommodate the correct flinger-disc diameter, the pump manufacturer will have to mount the thrust bearing set in a cartridge.
Keen on better pumps
The term better pumps describes fluid movers that are designed beyond just soundly engineered hydraulic efficiency and modern metallurgy. Better pumps are units that avoid risk-inducing components or geometries in the mechanical portion commonly called the drive end.
An over-emphasis on (initial) cost-cutting by some pump manufacturers and many pump purchasers has negatively affected the drive ends of thousands of process pumps. Flawed drive end components contribute to elusive repeat failures that often plague these simple machines. Pump drive end failures represent an issue that has not been addressed with the urgency it deserves. Remember: Repeat failures can only happen if the true root cause of failure remains hidden, or if the true root cause is known, but no corrective action is taken. Either of these two possibilities will defeat asset preservation and operational excellence goals.
Reliability specialists should actively track metal wear by oil analysis or by observing the shaft (Fig. 2), or via micrometer measurement of the oil ring width. Active monitoring can move a plant into the best-of-class category.2, 3 HP
| Fig. 2. Wear tracking on both |
equipment shaft and oil ring is of
high interest. Note: This oil ring
makes contact with housing-
internal surfaces. Best practices
companies measure and record
the rings original width
as-installed and also its
1 Bloch, H. P. and A. Budris, Pump Users HandbookLife Extension, 3rd ed., Fairmont Press, Inc., Lilburn, Georgia, 2010.
2 Bloch, H. P., Pump Wisdom: Problem Solving for Operators and Specialists, John Wiley & Sons, New York, New York, 2011.
3 Bradshaw, S., Investigations into the contamination of lubricating oils in rolling element pump bearing assemblies, 17th International Pump Users Symposium, Houston, 2000.
|The author |
Heinz P. Bloch resides in Westminster, Colorado. His professional career began in 1962 and included long-term assignments as Exxon Chemicals Regional machinery specialist for the US. He has authored over 520 publications, among them 18 comprehensive books on practical machinery management, failure analysis, failure avoidance, compressors, steam turbines, pumps, oil mist lubrication and practical lubrication for industry. Mr. Bloch holds BS and MS degrees in mechanical engineering. He is an ASME Life Fellow and maintains registration as a Professional Engineer in New Jersey and Texas.