Page 467 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Be st Practice 7 .32 Lube, Seal and Control Oil System Best Practices

Best Practice 7.32Practice 7.32Practice 7.32
Best
Best
Use multiple solenoid control oil dump valves in parallel Lessons Learned
and in series, with on-line solenoid test facilities for each Malfunctioning solenoid valve trip control systems have
valve, to ensure optimum reliability of the trip system. been responsible for catastrophic turbine failures and
One solenoid valve installed in the control oil system exposes the spurious trips on low control oil pressure due to solenoid
turbine to a spurious trip if the solenoid should open, and to cata- valve failures.
strophic damage if the valve fails to open on command.
The majority of older control oil trip systems used neither parallel
Installing two solenoid valves in parallel significantly increases the
trip valves nor on-line test facilities.
reliability of the system since only one valve has to open to trip the
turbine on command. However if one valve opens when not required, Benchmarks
a trip can occur.
This best practice has been used since 1997, when research into the
Installing two solenoid valves in parallel with another valve in series
causes of turbine trips showed that the highest cause of turbine trips
in each loop (total four (4) solenoid valves) will result in the highest trip were improper functioning of the solenoid trip valves in single and
system reliability, since this will provide optimum assurance that one parallel trip systems.
valve will open on command in addition to preventing an incidental
solenoid valve opening.
In addition, having an on-line solenoid test for each valve every
three months will provide optimum assurance that the solenoid trip
system will function as necessary.

B.P. 7.32. Supporting Material

This system incorporates a mechanical overspeed device
Protection (trip pin) to shut down the turbine on overspeed (10% above
maximum continuous speed). Centrifugal force resulting from
The function of the steam turbine protection system is often high shaft speed will force the trip lever which will allow the
confused with the control system, but the two systems are spring loaded handle to move inward. When this occurs, the
entirely separate. The protection system only operates
port in the handle stem will allow the control oil pressure to
when any of the control system set point parameters drain and drop to zero. The high energy spring in the trip and
are exceeded and the steam turbine will be damaged if it throttle valve, normally opposed by the control oil pressure will
continues to operate. Figure 7.32.1 defines the typical pro- close suddenly (less than 1 second). In this system there are
tection methods. two other means of tripping the turbine (reducing control oil
pressure to 0 PSI):
- Manually pushing spring loaded handle
The protection system monitors steam turbine total train - Solenoid valve opening
parameters 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- parameter set point is exceeded. Solenoid valves are designed to
up and will shut down the turbine on any abnormality be normally energized to close.
Manual shutdown In recent years the industry has required a parallel and series
Trip valve exerciser allows trip valve stem movement to be arrangements of solenoid valves to ensure increased steam tur-
confirmed during operation without shutdown bine train reliability. Figure 7.32.3 shows two popular methods
Rotor overspeed monitors turbine rotor speed and will shut
down turbine when maximum allowable speed (trip speed) is of overspeed protection used in the past.
attained Most speed trip systems now incorporate magnetic speed
Excessive process variable signal monitors all train process input signals and two-out-of-three voting for increased re-
variables and will shut down turbine when maximum value is liability. Figure 7.32.4 presents the devices that trip the turbine
exceeded internally. That is, they directly reduce the control oil pressure
causing a trip valve closure without the need of a solenoid valve
(external trip method).
Fig 7.32.1 Protection
Two popular types of steam turbine shutoff valves are shown
in Figure 7.32.5. Both types use a high spring force, opposed by
A schematic of a multi-valve, multi-stage turbine protection control oil pressure during normal operation, to close the valve
system is shown in Figure 7.32.2. rapidly on loss of control oil pressure.

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