Page 443 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Be st Practice 7 .22 Lube, Seal and Control Oil System Best Practices

Best
Best Practice 7.22Practice 7.22Practice 7.22
Best
PM control valves and pulsation suppression devices at Preventive maintenance (PM) of the above items during a turn-
every turnaround by changing diaphragms, checking valve around is good insurance for a trouble-free run of the highest reliability.
stem freedom of movement and cleaning pulsation sup- Note that most plants today (2010) are targeting six year uninterrupted
pression devices to ensure optimum control valve runs of critical equipment.
reliability.
A friction bound control valve (tight and/or oil sludge in packing) can Lessons Learned
cause a unit trip during a main oil pump trip if sufficient system oil Plants frequently experience unscheduled unit trips
(because the valve does not immediately close) is not available during caused by control valve issues. It is good practice to PM
the transient event to prevent a low oil pressure unit trip. the control valve systems during a turnaround.
A failed control valve diaphragm, which can be replaced on-line,
exposes the plant to shutdown, since improper adjustment of the Benchmarks
manual bypass valve around the control valve and/or taking out or This best practice has been used since the mid-1980s, when I served
putting the affected control valve back into service can cause a low oil as an interim reliability manager at a large petrochemical plant. It has
pressure trip. been recommended to all our clients since that time, and has resulted
A plugged control valve sensing line pulsation suppression device in trouble free control valve operation during steady state and transient
can cause an oil actuated control valve to go unstable (hunting), which conditions.
can also cause a unit low pressure shutdown.

B.P. 7.22. Supporting Material Control valve excessive noise or unit trips
Squealing noises suddenly produced from control valves may
System reliability considerations indicate valve operation at low travel (C v ) conditions. Valves
installed in bypass functions that exhibit this characteristic may
be signaling excessive flow to the unit. Remember the concept
A number of reliability considerations are worthy of mention
concerning auxiliary system control and instrumentation. of control valves being crude flow meters. Periodic observation
of valve travel during operation of the unit will indicate any
significant flow changes.
Control valve instability
Control valve instability can be the result of many factors, such Control valve sensing lines
as improper valve sizing, improper valve actuators, air in hy- Frequently, plugged or closed control valve sensing lines can be
draulic lines or water in pneumatic lines. Control valve sensing a root cause of auxiliary system problems. If a sensing line that is
lines should always be supplied with bleeders to ensure that no dead ended is plugged or closed at its source, a bypass valve will
liquid is present in pneumatic lines or air in hydraulic lines. The not respond to system flow changes and could cause a unit
presence of these fluids will usually cause instability in the shutdown. Conversely, if a valve sensing line has a bleed orifice
system. Control valve hunting is usually a result of improper
back to the reservoir (to ensure proper oil viscosity in low tem-
controller setting on systems with pneumatic actuators. Please perature regions), plugging or closing the supply line will cause
consult instruction books to ensure that proper settings are a bypass valve to fully close rendering it inoperable and may force
maintained. Direct-acting control valves frequently exhibit in- open the relief valve in a positive displacement pump system.
stabilities (hunting on transient system changes). If checks for
air prove inconclusive, it is recommended that a snubber device Valve actuator failure modes
(mentioned previously) be incorporated in the system to pre-
vent instabilities. Some manufacturers install orifices which Auxiliary system control valve failure modes should be designed
sufficiently dampen the system. If systems suddenly act up to prevent critical equipment shutdown in case of actuator
where problems previously did not exist, any snubber device or failure. Operators should observe valve stem travel and pressure
orifice installed in the sensor line should be checked immedi- gauges to confirm valve actuator condition. In the event of ac-
ately for plugging. tuator failure, the control valve should be designed for isolation
and bypass while on line.
This design will permit valve or actuator change-out without
Excessive valve stem friction
shutting down the critical equipment. During control valve on-
Control valves should be stroked as frequently as possible, to line maintenance, an operator should be constantly present to
ensure minimum valve stem friction. Excessive valve stem monitor and modulate the control valve manual bypass as
friction can cause control valve instabilities or unit trips. required.

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