Page 441 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

Steam Turbines and Turboexpanders 419

The accumulator system requires careful attention, since it maintains
oil pressure to the bearings during the first critical seconds of coast-down
in case of power failure (both pumps stopped). Ambient temperatures be-
low 50°F require heat tracing and insulation of the accumulator.
The accumulator should be flushed and checked periodically to ensure
that the oil in the holding tank is clean and uncontaminated. First, place
the seal gas and lube oil systems in operation with the accumulator inlet
valve open so that the accumulator will be filled. Then close the accumu-
lator inlet valve, and open the drain valve. Connect a pressure gauge to
the top of the accumulator and adjust the pressure, as necessary. If the oil
is contaminated, drain and refill as often as necessary to obtain a clean
flow of oil.

Pre-Start Mode

Place the system in the prestart mode by bringing the plant to startup
pressure. This is accomplished by starting the seal gas system and the
lube oil system (to protect the bearings in case pressurization rotates the
expander). Finally, pressurize the unit by opening first the expander dis-
charge block valve, the compressor inlet block valve, and the compressor
discharge block valve. krify that all atmospheric vents are closed.
Maintain the prestart mode long enough for the oil pressure to cease fluc-
tuating, and the temperature to stabilize at prescribed levels.

Startup

Note I: Prior to startup, verify that the range of the nozzle actuator rod
responds proportionally to the specified air pressure signal.
Note 2: Purge compressor (and if applicable, expander) casing for possi-
ble accumulated liquid.
Remember that turboexpanders come in different sizes and that expander,
compressor and auxiliaries may differ from model to model. Our proce-
dures are typical, but may have to be modified or adapted to a specific
situation. Always consult with the manufacturer if discrepancies need to
be resolved. On typical machines, with the seal gas flow established and
oil temperature and pressure stabilized, proceed as follows:

1. Clear all alarms and verify that the “expander ready” light is il-
luminated.
2. Verify that the expander inlet block valve is closed.
3. Set the actuator inlet vane control to a low power position.
4. Open the inlet trip valve by pressing the start button on the panel (if
provided).