Page 324 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Steam Turbine Best Practices Best Practice 5 .12
The solenoid valve will open on command when any trip All turbine trip valves should be provided with manual ex-
parameter set point is exceeded. Solenoid valves are designed to ercisers to allow this feature. Figure 5.11.21 presents facts
be normally energized to close. concerning manually exercising a turbine while on-line.
In recent years the industry has required parallel and series
arrangements of solenoid valves to ensure increased steam tur-
bine train reliability. Figure 5.11.18 shows two popular methods Trip valve is only as reliable as valve to move
of overspeed protection used in the past. Should periodically (minimum one per month) exercise valve to
Today, most speed trip systems incorporate magnetic speed ensure movement
input signals and two-out-of-three voting for increased re- Exercises will not trip turbine
liability. Figure 5.11.19 presents the devices that trip the tur- If valve does not move, must be remedied immediately
bine internally. That is, they directly reduce the control oil
pressure causing a trip valve closure without the need of a sole- Fig 5.11.21 On-line manual exercise of trip valve
noid valve (external trip method).
Two popular types of steam turbine shutoff valves are shown
Protection system philosophies have tended to vary geo-
in Figure 5.11.20. Both types use a high spring force, opposed by graphically with steam turbine vendors. Figure 5.11.22 presents
control oil pressure during normal operation, to close the valve
rapidly on loss of control oil pressure. these facts.
It is very important to note that the trip valve will only
close if the spring has sufficient force to overcome valve stem
friction. Steam system solid build-up, which increases with Most domestic vendors rely only on trip valve to shut off steam
system pressure (when steam systems are not properly supply (throttle valves remain open)
maintained) can prevent the trip valve from closing. European vendors close both trip and automatic throttle valve
To ensure the trip valve stem is free to move, all trip valves on trip signal
should be manually exercised on-line. The recommended
frequency is once per month.
Fig 5.11.22 Protection system philosophies

Best Practice 5.12Practice 5.12
Best
Exercise very high pressure turbine trip valves (inlet steam Daily exercise of VHP trip valves will ensure freedom of movement
pressures exceeding 100 barg) daily to positively prevent of the trip valve and positively prevent unnecessary unit shutdowns.
their failure to close upon command.
It can be hard to maintain VHP (very high pressure) steam systems, Lessons Learned
and to prevent contaminants (calcium, silica, etc.) from forming inside Failure to exercise VHP trip valves daily or HP trip valves
the turbine. monthly has resulted in catastrophic machinery failure,
Trip valve packing is essentially a filter that will trap any contami- personnel lost time and loss of life.
nants between the trip valve and the packing which can prevent the trip
valve form closing. Benchmarks
Failure of the trip valve to close on command can cause cata-
This best practice has been recommended since the 1990s. When
strophic machine failure and expose personnel to safety issues.
followed, it has resulted in zero lost time accidents and failure-to-trip
Periodic or infrequent exercise of trip valves can result in failure of
incidents. When not followed, more than one catastrophic machine
the valve to move which, considering plant safety requirements, will
outage in critical (un-spared) machinery has occurred, that has
necessitate immediate turbine shutdown.
exceeded three months in repair time.

B.P. 5.12. Supporting Material when any of the control system set point parameters are
exceeded, and the steam turbine will be damaged if it con-
tinues to operate. Figure 5.12.1 defines the typical protection
Protection methods.
A schematic of a multi-valve, multi-stage turbine protection
The function of the steam turbine protection system is often system is shown in Figure 5.12.2. This system incorporates
confused with the control system, but in fact the two systems a mechanical overspeed device (trip pin) to shut down the
are entirely separate. The protection system operates only turbine on overspeed (10% above maximum continuous speed).

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