Page 464 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Best Practice 7 .30
Lube, Seal and Control Oil System Best Practices Best Practice 7 .31

Best Practice 7.30

Adjust critical equipment shaft vibration alarm settings to Lessons Learned
activate as low as possible (D50% of initial reading) to Failure to detect early rotor condition change has resulted
detect rotor condition change before component damage in damage to compressor and turbine internals which has
occurs. caused long shutdown periods (weeks or months) to repair
Present industry practice (2010) is to high speed balance rotors stationary internals which are usually un-spared.
which results in low initial field shaft vibration readings (typically less Recent experience (2010) with a generator exciter failure showed
than 15 microns peak to peak). that shaft vibration increased from 10 microns to 30 microns, but was
Normal shaft vibration alarm settings are 50e60 microns which can undetected since the alarm was set at 60 microns. Shortly thereafter
amount to a 400% increase in shaft vibration before an alarm is activated. the vibration exceeded 100 microns e at which point the exciter
Setting the shaft vibration alarm at 50% of the initial field value will disintegrated.
allow early detection of rotor condition change, and initiate in-
vestigation and an action plan for corrective action before a rotor or Benchmarks
component failure occurs. This best practice has been recommended and used on new equip-
Once the cause of change is known and accepted, the alarm setting ment since the 1990s, to closely monitor shaft vibration condition
can be increased to an increment of the new reading (suggested in- changes and to prevent rotor and/or component failure.
crement of 50% of new reading).

B.P. 7.30. Supporting Material
In a recent case (2010), threshold shaft vibration and phase head and stress, in order to warn of initiated cracks and to allow
angle alarms (þ50% vibration and þ/ 10% phase angle) were an orderly shutdown before internal compressor damage could
set on a compressor that contained prototype impellers of high occur.

Control Oil System Best Practices

Best
Best Practice 7.31Practice 7.31
Check the control oil accumulator every three months to FAI accumulator best practice is to install a bypass valve, with an
ensure desired transient response, in order to prevent orifice around the isolation valve to prevent rapid oil pressure drop
a steam turbine trip on low control oil pressure. when the accumulator is introduced back to the system.
In order to ensure proper transient response, an accumulator must
maintain the proper nitrogen pre-charge pressure and have the bladder Lessons Learned
intact (rupture free). Failure to periodically check the accumulator for pre-
Accumulators cannot be checked for pre-charge and bladder charge and bladder condition has resulted in many steam
condition on-line, since the nitrogen pre-charge pressure will be in turbine unit trips during a transient condition (rapid load
equilibrium with the accumulator oil pressure. change: the surge valve opens rapidly, process control
In order to check the pre-charge and bladder condition, the accu- system rapidly changes turbine speed).
mulator must be isolated and drained. Nitrogen is then introduced at There has been a reluctance to check accumulators in some plants,
the correct pre-charge pressure to confirm fittings are leak free and the because the check has caused a low control oil pressure trip on oc-
bladder is in good condition. casions when the accumulator isolation valve was opened too quickly.
After the accumulator check and with the accumulator pre-charge
as specified, the isolation valve must be slowly opened to ensure that Benchmarks
the control oil pressure does not fall below the low control oil pressure
setting that will trip the turbine. This best practice has been recommended since 1990, resulting in
critical equipment train maximum reliability above 99.7%.

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