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When not to use oil rings

09.01.2012  |  Bloch, H. P.,  Hydrocarbon Processing Staff, 

Keywords:

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
  right side.1,2


Manufacturer’s limits

These various parameters are probably within limits on the pump manufacturer’s 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.

Final word

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 ring’s original width
  as-installed and also its
  after-removal width. 


LITERATURE CITED

1 Bloch, H. P. and A. Budris, Pump User’s Handbook—Life 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 User’s 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 Chemical’s 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.  

 



Have your say
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L N Prasad
11.20.2012

Dear Sir
The article and piece of information published is outstanding and it will boost the knowledge in the field of rotary equipments, especially in pumps. I have been associated with maintenance of rotating equipments since last twenty years and i used to replace the metallic oil ring with non-metallic such as teflon/PEEK in order to establish reduced wearing pattern on shaft as well as oil ring .
Regards.

Kiran Rajgor
10.03.2012

Tom,
Good suggestion.
However we can print this article in PDF format and save for our referance / database.

Best Regards,
Kiran Rajgor
Rotating Equipment Engineer
Ranhill WorleyParsons| Kuala Lumpur

Sumeet Beniwal
09.20.2012

I agree with Hamid's comment on measurement of metal particles in lube oil. We do the lube oil analyses regularly (monthly and bi-weekly) and it keeps the maintenance Pro-active rather than reactive.

Tom Davidson
09.20.2012

I've been a subscriber to this wonderful magazine for over 25 years. It would be a great service to us readers if you could make these and other articles available for download (PDF) for us to store and reference at will. You give us the option to Print but not Download. I like many other readers would much prefer an e-copy than a paper copy.

Why is this not an option?

Best Regards,

Tom Davidson
Chief Mechanical Engineer
Linde, LLC

hamid jahanian
09.17.2012

it seems the measuring of metal particle in oil can be helpful

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