Page 66 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

Installation, Maintenance, and Repair of Horizontal Pumps 51

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0 200 400 600
CAPACITY IGPMI

Figure 1-18. Lack of hydraAic balance may overload thrust bearings.

Design-related Problems

Although statistically less prevalent, design problems do occur and may
have to be addressed. Lack of hydraulic balance may overload thrust bear-
ings (refer to Figure 1-18)? Here, the pump manufacturer had calculated a
load of 1,000 lbs. at zero-flow conditions. Frequent bearing failures were
explained when field tests showed actual loads in the vicinity of 2,600 lbs.
Because ball bearing life changes as the third power of the load ratio. this
2.6-fold load increase would reduce the probable bearing life to 1/17 of the
pump manufacturer's anticipated bearing life. The problem was solved by
redesigning the pump impeller.
Older pumps that were origindly designed for packing as a means to con-
tain the pumpage may be prone to frequent mechanical seal failure. The root
cause of the problem may well be related to shaft slenderness; that is, the old
braided packing acted as a stabilizing bushing. Insertion of a suitably dimen-
sioned graphite bushing often proved to be beneficial.
There may also be pumps with less than optimum internal gap or clear-
ance values..". Incorrect internal dimensions may cause pump component
breakage, involving impellers, fasteners: bearings, or mechanical seals. Cor-
rect gap values are given in Table 1-6.
Installation Problems

A surprisingly large number of installation-related deficiencies continue to
plague literally thousands of centrifugal pump users worldwide.