Page 153 - Improving Machinery Reliability

Machinery Reliability Audits and Reviews 125

Table 3-9
Primary Failure Causes, Centrifugal Pumps
(Four Overseas Plants)

Repair Frequency (Percent)

Part 1976 1977
Mechanical Seals
Antifriction Bearings
Impellers
Shafts
Casings and Diaphragms
Sleeve Bearings
Piping
Conventional Packing
Lip-type Seals 2
Couplings 2
Suction Strainers 127 ( 7)
Others 569 (34)
1,736 (100) 1,769 (100)

Auditing and Reviewing Centrifugal Compressors
There is little difference in how an experienced engineer approaches audit and
review tasks for centrifugal compressors as opposed to those for turbines, gears, and
other machinery. In each case he must obtain drawings and other technical data from
equipment vendors. He will then review all pertinent documentation for consistency,
safety, compliance with specifications, etc., and document all areas requiring follow-
up. Of course, he will also initiate and review the rotordynamic design audits
described earlier in this chapter.
Centrifugal compressor documentation requirements probably exceed those of
most other machinery with the possible exception of large mechanical drive steam
turbines. A listing of relevant documentation is contained in API Standard 617.
Using the API tabulation facilitates outlining the items recommended for review:

1. Certified dimensional outline drawing, including:
a. Journal-bearing clearances
b. Rotor float
c. Labyrinth, packing, and seal clearances
d. Axial position of impellers relative to guide vanes
e. List of connections

Journal-bearing clearances may be required for rotor sensitivity studies. Bearing
dimensions allow rapid calculation of bearing loading and serve to screen for poten-
tial oil whirl.
Labyrinth, packing, and seal clearances may be too tight for normal process opera-
tion. The vendor may attempt to show good efficiency (low recirculation) during
shop performance tests.