Page 148 - Improving Machinery Reliability

120 Improving Machinery Reliability

Table 3-6
Typical Distribution of Unscheduled Downtime Events for
Major Turbocompressors in Process Plants
Approximate number of shutdowns per train per year: 2
Estimated Estimated Average Downtime
Cause of Problem Frequency HrdEvent EventsNr HrsNr
Rotorkhaft 22% I22 .44 54
Instrumentation 21% 4 .42 2
Radial bearings 13% 28 .26 7
Bladeslimpellers 8% 110 .I6 18
Thrust bearings 6% 22 .12 3
Compressor seals 6% 48 .I2 6
Motor windings 3% 200 .06 12
Diaphragms 1% 350 .02 7
28
Miscellaneous causes 20% I0 .40 -
All causes 100% 2.00 I37 hours

problem-cause distribution and duration are shown in Table 3-6. The table also
shows where detailed design reviews might prove most profitable.
Experienced process plants using a conscientious program of mechanical and
instrument-condition surveillance could expect to achieve compressor train availabil-
ities exceeding 98.5%. Unscheduled downtime events in state-of-the-art facilities
occur fewer than once per year per train.
Table 3-6 shows that rotor and shaft distress rank highest in downtime hours per
year per train. Blade or impeller problems rank next, followed by motor failures.
Obviously, centrifugal compressor reliability audits and follow-up reviews should
concentrate on these areas first.

Failure Statistics for Steam Turbines

Failure statistics for special-purpose steam turbines are often separated into those
for impulse turbines and those for reaction turbines. One such important statistic is
represented in Figure 3-32. Of practical interest are primarily those failures which
have a reasonable probability of occurring within 80,000 operating hours. The
author’s experience would indicate that each turbine type is acceptable as long as
rigorous selection criteria are applied. First and foremost of these would be an inves-
tigation of vendor experience, blade stresses, and blade vibration behavior.
Operating data accrued after 1970 support the belief that rotor blades furnished
with impulse turbines rank about even with reaction-turbine blading. A prerequisite
to operation would be that they were subjected to user design audits and pre-com-
missioning reliability reviews.
Interesting statistics have also been quoted by a large North American industrial
equipment insurance company. Over a number of years, this company has insured an
average of 6,353 steam turbines. During this time period, one out of every 186 steam