Page 371 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

350 Major Process Equipment Maintenance and Repair

on the nameplate of the unit itself. A direct-reading, tachometer is pre-
ferred for reading the speed. A digital readout instrument is also satisfac-
tory, provided the operator is familiar with its use and can read the speed
correctly to a very close value.
During these tests, the speed should be increased slowly and the
tachometer watched very carefully. An operator should stand by, ready to
trip the mechanism by hand instantly if it does not trip automatically at
about 5 percent above the specified overspeed. If the mechanism does not
trip at the proper speed, it should be inspected and adjusted. The over-
speed test should be made periodically, throughout the life of the ma-
chine, to ensure that this important safety device is kept in good working
condition. Do not put the unit into service, or keep it in service, if it is
known that the overspeed trip is not functioning properly.

Shutting Down

1. Decrease the load to about 20 percent of full load; except in an
emergency shutdown, load should be removed gradually.
2. Then remove all load and shut down the turbine by tripping the
overspeed trip mechanism manually.
3. Be sure that the oil supply is maintained until the machine becomes
relatively cool. If this is not done, the heat conducted along the
shaft from inside the turbine may injure the bearings.
4. When the turbine comes to rest, close the exhaust valve and open
all drains between the throttle valve and the exhaust valve.
5. Shut off the air ejectors. Open the vacuum breaker if one is pro-
vided.
6. Shut off the water to gland condenser and steam to the ejector.
7. Shut down the condensate pump.
8. Open all blow-down drains.

How to Avoid Steam mrbine Distress

All steam turbines, like other turbomachinery, are prone to suffer from
certain maintenance-causing problems. They are those triggered by:

Oil contamination
Foundation difficulties
Alignment problems
Piping loads
Bearing difficulties
Unbalance conditions
Operating errors