Page 325 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Be st Practice 5 .12 Steam Turbine Best Practices
- Manually pushing spring loaded handle
The protection system monitors steam turbine total train parameters - Solenoid valve opening
and ensures safety and reliability by the following action:
The solenoid valve will open on command when any trip
Start-up (optional) provides a safe, reliable fully automatic start-
up and will shut down the turbine on any abnormality parameter set point is exceeded. Solenoid valves are designed to
Manual shutdown be normally energized to close.
Trip valve exerciser allows trip valve stem movement to be In recent years the industry has required parallel and series
con firmed during operation without shutdown arrangements of solenoid valves to ensure increased steam tur-
Rotor overspeed monitors turbine rotor speed and will shut bine train reliability. Figure 5.12.3 shows two popular methods
down turbine when maximum allowable speed (trip speed) is
attained of overspeed protection used in the past.
Excessive process variable signal monitors all train process Today, most speed trip systems incorporate magnetic speed
variables and will shut down turbine when maximum value is input signals and two-out-of-three voting for increased re-
exceeded liability. Figure 5.12.4 presents the devices that trip the turbine
internally. That is, they directly reduce the control oil pressure
causing a trip valve closure without the need of a solenoid valve
Fig 5.12.1 Protection
(external trip method).
Two popular types of steam turbine shutoff valves are shown
Centrifugal force resulting from high shaft speed will force the in Figure 5.12.5. Both types use a high spring force, opposed by
trip lever, which will allow the spring loaded handle to move control oil pressure during normal operation, to close the valve
inward. When this occurs, the port in the handle stem will allow rapidly on loss of control oil pressure.
the control oil pressure to drain and drop to zero. The high It is very important to note that the trip valve will only close if
energy spring in the trip and throttle valve, normally opposed by the spring has sufficient force to overcome valve stem friction.
the control oil pressure will close suddenly (less than Steam system solid build up, which increases with system
one second). In this system there are two other means of trip- pressure (when steam systems are not properly maintained) can
ping the turbine (reducing control oil pressure to zero): prevent the trip valve from closing.

Fig 5.12.2 Typical steam turbine protection (Courtesy of Elliott Co.)

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