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 dont 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 its really appropriate to shun
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.
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
2. Bearing housing vibration velocity
bearing life for process
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.
1 Bloch, H. P. and A. Budris, Pump
Users 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.
Heinz P. Bloch is Hydrocarbon
Processings 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