Page 439 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

Steam Turbines and Turboexpanders 417

showing desired vs. actual values of operating parameters. Settings are
made at the factory but may drift during shipment and must be checked
and reset as necessary. Today’s control/annunciation systems are largely
trouble-free, using easily replaceable solid-state printed circuit cards
without the complicated wiring connections used in earlier systems. This
facilitates maintenance and checking of the electrical system.
All alarms and shutdowns should be checked under simulated condi-
tions, and all components tested through the plant annunciator control
system. Verify that the inlet trip closes within the specified time (nor-
mally one-halP second) to provide maximum expander protection. Acti-
vate all shut-down controls, both for the system and the plant, to test the
shutdown valve. Experience has shown that precise settings and chcckout
of the instrumentation at the site are key factors in smooth startup and
operation of all systems.

Pre-Startup
The first component of the auxiliary system to be started is the seal
gas. It is also the last to be shut off to prevent contamination of the pro-
cess zone by the lubricating oil. An optional interlock feature is often
used to prevent startup of the lube oil system until an adequate flow of
seal gas is established. The seal gas is filtered and regulated to a given
supply pressure. A graduated needle valve or other flow control** regu-
lates the flow and pressure of the seal gas to the labyrinth seal, providing
positive containment of the process gas and preventing its loss or contam-
ination by the lube oil and possible dilution of the lube oil by the process
gas.
To start the lubrication system, open the pump and filter bypass valves,
close the accumulator inlet valve, and then turn on the pump(s). The fil-
ter selector switch should be so positioned as to allow the purging of
trapped gas or air in both filters. If two pumps are provided, both should
be started.
After several minutes, when the system is thoroughly purged, close the
pump and filter bypass valves. Once the system is brought up to pres-
sure, the second pump should be stopped and placed in the auxiliary
”standby” mode. The oil system is normally operated with one pump and
one filter.
Failure to purge the system of gas at low pressure in this manner cre-
ates the risk of collapsing the filter cartridges when the pumps are turned
on.

* Some generator systems require more rapid response; and for some compressor-
loaded systems a slower response is specified-of the order of several seconds.
** In some systems a differential pressure control across the seal is used.