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Alignment choices have consequences

02.01.2011  |  Bloch, H. P.,  Hydrocarbon Processing Staff, 

Keywords: [pumps] [bearings] [vibration failure] [reliability] [pump housing]

Optimists tell us about steady industry trends toward reliability-imparting procedures and work processes, while realists continually make us aware of pressures to reduce expenditures. As outside observers, we affirm that striving to reduce monetary outlay is quite commendable, but only as long as these aims don’t run counter to the professed longer-term reliability improvement objectives. Conflicting issues are often alluded to in queries that we receive from readers. For instance, we were asked if we knew of literature that quantifies the merits of precision alignment for pumps, and if it’s really appropriate to shun old-style methods.

Modern vs. old school.

With modern alignment methods taking no more time than tinkering with old-style methods, common sense should point toward using precise methods. Inadequate alignment still causes major calamities (Fig. 1); whereas, the results of sound alignment approaches typically show up as improved mean time between repairs (MTBR) and a reduction in maintenance outlay. The November 2006 HPIn Reliability column shed additional light on this topic.

 
  Fig. 1. Process pump failure that started with
  misalignment, high vibration and bearing
  distress. Source: Murray and Garig Tool
  Works, Baytown, Texas. 


A likely consensus among reasonable people holds that precision alignment typically lowers vibration to half of the value of “conventional” alignment. Fig. 2 represents an estimate of bearing operating life extension due to reduced vibration velocity for typical process pumps. Fig. 3 gives an indication of how a major bearing manufacturer rates the effects of misaligned bearings. Most rolling element bearings fit somewhere between the two curve boundaries and at tangents below 0.001; bearing life is thought to exceed a relative rating of 1.2


 
  Fig. 2. Bearing housing vibration velocity vs.
  bearing life for process pumps.1 

 
  Fig. 3. How tangent of misalignment angle
  affects bearing life.  



On average, there is then reason to believe that precision alignment alone would result in a pump MTBR multiplier of somewhere between 1.4 and 1.7. The problem is that best-of-class performers inevitably implement additional upgrades and they will seldom confine their work to just better alignment. That is why Ref. 1 puts the pump MTBR of a very marginal performer at 1.6 years, while best performers often get 9 years or more between pump failures. HP

LITERATURE CITED

1 Bloch, H. P. and A. Budris, Pump User’s Handbook: Life Extension, Third Edition, 2010, Fairmont Press, Lilburn, GA 30047; (ISBN 0-88173-627-9).
2 Leibensperger, R. L.; “Look beyond catalog ratings,” Machine Design, April 3, 1975.

The author 

Heinz P. Bloch is Hydrocarbon Processing’s Reliability/Equipment Editor. A practicing consulting engineer with almost 50 years of applicable experience, he advises process plants worldwide on failure analysis, reliability improvement and maintenance. He has authored or co-authored 18 textbooks and close to 500 papers or articles dealing with related subjects.  




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03.03.2011

Proper run-in of the pump unit (shaft/bearing/motor) after alignment is essential to a longer operational service life of the pump system, run-in speed is advised to be on a gradually increasing load basis with time so as to allow the alignment to adapt within tolerable limit to conditions of vibration & heat development


02.17.2011

Antifriction bearings- fatigue life is determined by load and speed, for most applications speed tends to be constant as per motor driven pump. Misaligment adds to the loads hence reducing overall life.
Of note though, the late Charlie Jackson wrote an article where he conducted an experiment on apump with increasing misaligment vs vibration - the vibes increased then flattened out, so the bottom line- vibes is an indirect means of determining load but does not tell the full story. Until we start integrating load sensors on bearings we will have to depend on vibes, heat, ulrasonics etc.


02.17.2011

Article is excellent and the two charts in Figure 2 and 3 are very useful in practice

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