Page 471 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Be st Practice 7 .33 Lube, Seal and Control Oil System Best Practices

Best Practice 7.33

Exercise all critical service trip valves (un-spared turbines) this un-spared turbine to be immediately shut down using the inlet
to ensure movement a minimum of once a month assuming steam isolation valve with exposure to the corresponding revenue
that the steam system is properly maintained and treated. losses until the valve problem is corrected. It is better to err on the
All critical service steam turbine trip valves are closed by spring conservative side if there is any question about the condition of the
force when the opposing control oil force is suddenly reduced allowing steam system.
Finally, confirm with the vendor during the design phase that it is
the spring force to close the valve in one second or less.
impossible to cause a trip during the trip valve exercise. Then dem-
However, the effective spring force to close the trip valve is
onstrate to each operating shift that the exercise of the trip valve during
a function of the closing spring force minus any trip valve packing
commissioning and start-up with the vendor representative present
friction force.
that a trip and/or upset of the steam turbine is not possible during a trip
The trip valve packing is effectively a filter with a differential pres-
valve exercise.
sure equal to the inlet steam pressure minus atmospheric pressure.
The higher the steam pressure, the greater the driving force to lodge
any steam contaminants into the packing and to increase the packing Lessons Learned
friction force. Also, the higher the steam pressure the more difficult It seems that the majority of operating plants do not ex-
boiler treatment becomes and the greater the possibility of steam ercise the trip valves periodically, causing unscheduled
contaminants (silica, calcium, etc.). shutdowns (when an exercise was performed without
Especially during the start-up of a new plant with very high pressure valve stem movement) or catastrophic damage to the
steam (over 100 barg), consider the plant experience and boiler turbine and/or driven equipment.
treatment equipment in operation to determine if a monthly trip valve
exercise is appropriate. FAI has, in some cases, recommended daily Benchmarks
exercise in new plants with a gradual weekly incremental increase to
one month valve exercises. This best practice of including more frequent exercise intervals for new
Please remember that we are concerned with a trip valve exercise, plants, has been followed since the mid-1980s to achieve maximum
not a test! If the trip valve fails to move, global safety practices require critical steam turbine driven train reliability (greater than 99.7%).

B.P. 7.33. Supporting Material

See B.P. 7.32 for supporting material.

Compressor Liquid Seal Best
Practices
potential weak points that can significantly reduce the re-
Liquid seal systems have generally not been used since the liability of the critical compressor trains that they service.
acceptance of dry gas seal systems by all end users in the late This section therefore presents the best practices used by
1980s. However, most compressor trains designed before the author to optimize the safety and reliability of these
1985 still use liquid seal systems. These systems have many systems.

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