Page 152 - Improving Machinery Reliability

124 Improving Machinery Reliability

Table 3-7
Primary Causes of Gas Turbine Failures in the USA

Primary Failure Cause, Faulty Design Operating Bad
Gas Turbines Driving: Maintenance Problems Problems Repairs Other
Generators 42% 19% 14% 4% 21%
(utility units)
Compressors, pumps, 60% 14% 18% 6% 2%
etc. (mechanical-
drive units)

relevant specifications. For instance, the typical yearly repair summary given in
Table 3-8 would prove informative to administrative personnel, but would not yield
much material of technical relevance.
Once the failure causes are broken down into such elements as O-ring failures,
bearing overload failures, incorrect metallurgy, etc., it may become possible to trans-
late failure analysis results into better specifications. For instance, swelling of Viton
O-rings in services containing certain chemicals may lead to the specification of
Buna or PTFE elastomers; thorough analysis of bearing overload failures could point
the way toward requiring pump vendors to provide bearing load values at off-design
conditions; and frequent erosion damage in a given pumping service could lead to
specifying hard-metal overlays.
Nevertheless, Table 3-8 does represent fairly accurately the pump failure picture
of large U.S. refineries. As can be seen, mechanical-seal failures seem to predomi-
nate. Similarly, the tabulation summary given in Table 3-9 points to mechanical-seal
distress as an area of concern at four overseas plants, also.

Table 3-8
Typical Yearly Repair Summary, Centrifugal Pumps
~ ~
Centrifugal pumps installed: 2560
Centrifugal pumps operating at any given time: 1252 (average)
Total pumps repaired in 1979: 768
Pumps repaired at site location: 382
Pumps repaired at own shops: 267
Pumps repaired at outside shops: 119
Failure Causes Repaired on Site Repaired in Shop
Mechanical seals 112 153
Bearing distress 72 83
Vibration events 8 13
Packing leakage 103 22
Shaft problemskouplings 21 60
Case failure/auxiliary lines 35 2
Stuck 20 13
Bad performance 4 18
17
22
Other causes - -
Total 382 386