To ensure optimum reliability of the trip system, use multiple solenoid control oil dump valves in parallel and in series, with on-line solenoid test facilities for each valve.
One solenoid valve installed in the control oil system exposes the turbine to a spurious trip if the solenoid should open; and to catastrophic damage if the valve fails to open on command.
Installing two solenoid valves in parallel with another valve in series in each loop (total four solenoid valves) will result in the highest trip system reliability, since this will provide optimum assurance that one valve will open on command in addition to preventing an incidental solenoid valve opening.
In addition, having an online solenoid test for each valve every three months will provide optimum assurance that the solenoid trip system will function as necessary.
The function of the steam turbine protection system is often confused with the control system, but the two systems are entirely separate. The protection system only operates when any of the control system set point parameters are exceeded and the steam turbine will be damaged if it continues to operate.
The protection system monitors steam turbine total train parameters and ensures safety and reliability by the following action:
- Start-up (optional) provides a safe, reliable fully automatic start-up and will shut down the turbine on any abnormality
- Manual shutdown
- Trip valve exerciser allows trip valve stem movement to be confirmed during operation without shutdown
- Rotor over speed monitors turbine rotor speed and will shut down turbine when maximum allowable speed (trip speed) is attained
- Excessive process variable signal monitors all train process variables and will shut down turbine when maximum value is exceeded
A multi-valve, multi-stage turbine protection system incorporates a mechanical over speed device (trip pin) to shut down the turbine on over speed (10 percent above maximum continuous speed). Centrifugal force resulting from high shaft speed will force the trip lever which will allow the spring loaded handle to move inward. When this occurs, the port in the handle stem will allow the control oil pressure to drain and drop to zero.
The high energy spring in the trip and throttle valve, normally opposed by the control oil pressure will close suddenly (less than 1 second). In this system there are two other means of tripping the turbine (reducing control oil pressure to 0 psi):
- Manually pushing spring loaded handle
- Solenoid valve opening
The solenoid valve will open on command when any trip parameter set point is exceeded. Solenoid valves are designed to be normally energized to close.
In recent years, the industry has required a parallel and series arrangements of solenoid valves to ensure increased steam turbine train reliability.
Most speed trip systems now incorporate magnetic speed input signals and two-out-of-three voting for increased reliability. The devices that trip the turbine internally are:
- Loss of control oil pressure
Spring force automatically overcomes oil force holding valve open (approximate set point 50-65 percent of normal control oil pressure)
- Manual trip (panic button)
Manually dumps control oil on command
Turbine excessive axial movement
That is, they directly reduce the control oil pressure causing a trip valve closure without the need of a solenoid valve (external trip method).
Two popular types of steam turbine shutoff valves use a high spring force, opposed by control oil pressure during normal operation, to close the valve rapidly on loss of control oil pressure.
It is very important to note that the trip valve will only close if the spring has sufficient force to overcome valve stem function. Steam system solid build-up, which increases with system pressure (when steam systems are not properly maintained), can prevent the trip valve from closing.
To ensure the trip valve stem is free to move, all trip valves should be manually exercised online. The recommended frequency is once per month.
All turbine trip valves should be provided with manual exercisers to allow this feature. Facts concerning manually exercising a turbine while online:
- Trip valve is only as reliable as valve to move
- Should periodically (minimum one per month) exercise valve to ensure movement
- Exercisers will not trip turbine
- If valve does not move, must be remedied immediately
Protection system philosophies have tended to vary geographically with steam turbine vendors:
- Most domestic vendors rely only on trip valve to shut off steam supply. (Throttle valves remain open.)
- European vendors close both trip and automatic throttle valve on trip signal.
Malfunctioning solenoid valve trip control systems have been responsible for catastrophic turbine failures and spurious trips on low control oil pressure due to solenoid valve failures.
The majority of older control oil trip systems used neither parallel trip valves nor online test facilities.
This best practice has been used since 1997, when research into the causes of turbine trips showed that the highest cause of turbine trips were improper functioning of the solenoid trip valves in single and parallel trip systems.