Page 596 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Be st Practice 1 1.4 Preventive and Predictive Maintenance Best Practices

Best
Best
Best Practice 11.4Practice 11.4Practice 11.4
PM critical (un-spared) machinery train oil system and dry system control valve, valve replacement during plant
gas seal system valves and sensing line hardware at every operation exposes the plant to an unscheduled shutdown
turnaround to achieve optimum oil system and train and corresponding large revenue losses.
reliability (>99.7%). During site auxiliary system audits I have observed isolated control
The operation of control valves, during steady state and transient valves and the associated manual bypass valve in operation. Problems
conditions, in critical (un-spared) machinery train oil and dry gas seal had existed with the valve and the plant was waiting until the next
systems directly affects the train reliability. shutdown for replacement. Needless to say, any transient condition
For this reason, we recommend that every control valve in the oil, (main pump trip, two pumps operating, etc.) would expose the plant to
dry gas seal and associated buffer gas systems be overhauled (di- a shutdown.
aphragm, packing, controller [if applicable]) and then checked for
proper response as well as position indication. Benchmarks
If associated pulsation suppression devices (ball check/needle This best practice has been recommended since the late 1980s during
valve, needle valve and/or orifice) are installed, these devices must be turnaround planning meetings for a large petrochemical complex.
inspected, cleaned and replaced if necessary. Note: Pulsation sup- Since that time, this best practice recommendation, where followed,
pression devices must be installed in the proper orientation. Always has resulted in optimum compressor train reliabilities (> 99.7%).
match mark these devices to ensure they are re-installed in the proper
direction.
Lessons Learned
Even though industry specifications require isolation
valves and a manual bypass around each oil and gas

B.P. 11.4. Supporting Material Excessive valve stem friction
Control valves should be stroked as frequently as possible to
In order to prevent the potential reliability issues noted below, ensure minimum valve stem friction. Excessive valve stem
we recommend that this best practice be executed during friction can cause control valve instabilities or unit trips.
every turnaround for critical equipment oil and dry gas seal
systems.
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
Concerning auxiliary system control and instrumentation, be signaling excessive flow to the unit. Remember the concept
a number of reliability considerations are worthy of mention. of control valves being crude flow meters. Observation of valve
travel periodically during operation of the unit will indicate any
Control valve instability significant flow changes.
Control valve instability can be the result of many factors, such
as improper valve sizing, improper valve actuators, air in hy- Control valve sensing lines
draulic lines or water in pneumatic lines. Control valve sensing Frequently, plugged or closed control valve sensing lines can be
lines should always be supplied with bleeders to ensure that a root cause of auxiliary system problems. If a sensing line that is
liquid in pneumatic lines or air in hydraulic lines is not present. dead-ended is plugged or closed at its source, a bypass valve will
Thepresenceofthese fluids will usually cause instability in the not respond to system flow changes and could cause a unit shut-
system. Control valve hunting is usually a result of improper down. Conversely, if a valve sensing line has a bleed orifice back to
controller setting on systems with pneumatic actuators. You the reservoir (to ensure proper oil viscosity in low temperature
are advised to consult instruction books to ensure that proper regions), plugging or closing the supply line will cause a bypass
settings are maintained. Direct acting control valves frequently valve to fully close, rendering it inoperable, and may force open
exhibit instabilities (hunting on transient system changes). If the relief valve in a positive displacement pump system.
checks for air prove inconclusive, it is recommended that
a snubber device (mentioned previously) be incorporated in
the system to prevent instabilities. Some manufacturers install Valve actuator failure modes
orifices which sufficiently dampen the system. If systems Auxiliary system control valve failure modes should be designed
suddenly act up where problems previously did not exist, any to prevent critical equipment shutdown in case of actuator
snubberdeviceororifice installed in the sensor line should be failure. Operators should observe valve stem travel and pressure
checked immediately for plugging. gauges to confirm valve actuator condition. In the event of

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