Page 64 - Machinery Component Maintenance

Maintenance Organization and Control for Multi-Plant Corporations 49

Figure 2-12. Bar charts are part of the job plan for major machinery turnaround.

In the case of steam turbines, the overspeed trip bolt and the steam trip
and throttle valve have proven to be the least reliable-and yet most im-
portant-safety devices in the train. A check of these two components is
mandatory during major shutdowns, and checks should be made at every
other opportunity. We believe these are the most important checks per-
formed during a shutdown. In addition, one should "exercise" the trip
and throttle valve weekly by moving the stem in and out manually several
turns on the hand wheel to preclude the buildup of deposits that would
prevent the machine from tripping during a shutdown condition.
Consider an example of the penalty associated with the failure of a trip
circuit at one plant. A 10,OOO horsepower, steam turbine-driven com-
pressor train failed to trip during a condition which had caused the com-
pressor to fill with liquid while at full speed. The resulting loads led to
catastrophic failure of both the compressor and coupling and allowed the
steam turbine to overspeed to destruction. The repair bill for parts and