Page 442 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

420 Major Process Equipment Maintenance and Repair

5. Slowly turn the inlet block valve to full open to feed gas to the ex-
pander. This should cause it to rotate at reduced speed.*
6. Use the variable inlet nozzle control to increase the speed of the
expander in 10 percent speed increments, at intervals of 20 to 30
minutes.
7. As the expander flow is increased, the flow passing through the
Joule-Thomson bypass valve should be correspondingly decreased
and, finally, flow control switched to automatic nozzle control.

A minimum flow of approximately 65 percent of design must be main-
tained through the compressor to avoid surge.
Check the expander and compressor inlet screens during startup, and
repeatedly during the first few hours of operation, for blockage by mate-
rial from the plant lines.
Adjust the seal gas flow to the minimum found to be required to pre-
vent perceptible cooling of the oil drain line when the unit is operating on
cold gas.

Normal Operation and Periodic Maintenance

Normal operation requires only routine monitoring of system controls
and gauges.
The following should be checked during each shift during initial opera-
tions, and daily later on, and the readings logged:

1. Reservoir lube oil level
2. Seal gas flow
3. Lube oil pressures
4. Lube oil temperatures
5. Thrust meter readings

The lube oil should be changed every three to six months, or whenever
there are indications of oil contamination, as evidenced by reduction of
the oil viscosity. Frequent oil checks are necessary after startup to estab-
lish the best timing and frequency for oil inspections.
Filters do not require frequent cartridge change, except to remove con-
taminated oil.
Annual inspection of the expander-compressor is not essential, unless
damage is suspected.

* Some system attain reduced speed and pressurization prior to opening the trip valve by
slowly opening a bypass valve around the trip valve. If such a bypass valve is used,
great care must be exercised to be sure the bypass valve is closed and kept closed while
the trip valve is open. Otherwise, the trip valve loses effectiveness.