Avoiding Destruction of Steam Turbines
PROPER CONTROLS AND PROTECTION TECHNOLOGY SUCH AS TRIP-AND-THROTTLE VALVES CAN SHUT DOWN EQUIPMENT AND AVOID OVERSPEED EVENTS
Important to the performance of any steam turbine is the ability to start and stop, especially under emergency operating conditions. Shutting off the steam is normally accomplished by properly maintained trip or trip-and-throttle valves that are typically located at the turbine inlet or inlet piping.
The purpose of the trip-and-throttle valve is to protect the turbine by immediately closing off (or tripping) steam supply when an abnormal operating condition occurs. Such conditions include overspeed, low lube oil pressure, rotor overspeed, high rotor vibrations and high bearing temperature.
Perhaps the most destructive incident a steam turbine can experience is an overspeed condition resulting from residual energy trapped in the trip or trip-and-throttle valve, steam chest and turbine casing after a unit trip. This may cause the turbine rotor to operate at speeds exceeding its design limit.
An overspeed event can cause a catastrophic failure, and perhaps pose a severe threat to personnel. Given these consequences, it is incumbent upon operators to confirm that an appropriate trip or trip-and-throttle valve is installed on the unit, and operated and maintained properly.
Trip-and-throttle failure
There are various reasons why a trip-and-throttle valve could fail including poor maintenance, improper testing and substandard replacement parts. Other factors include poor steam quality and contaminated oil.
One U.S.-based operator’s trip-and-throttle valve failed to close on one of its steam turbines, resulting in catastrophic damage to the turbine. Following the incident, the first order of business was to get the turbine rebuilt and back online, and the plant running as quickly as possible. The scope of work included engineering, turbine case, diaphragms, valves, bearings, turning gear, and rotor rebuild, as well as foundation repairs.
It was recommended that the plant install a fail-safe control system featuring triple redundancy and containing operating logic to help secure the safety functions, and improve the availability and reliability of the turbine. Proper controls and protection technology can shut down equipment and avoid an overspeed event.
Figure 1: Gimpel valve[/caption]
In addition, a Gimpel oil operated positional trip valve (OOPTV) (Figure 1) and associated instrumentation were added to the overspeed protection arrangement. The OOPTV has an emergency closure rate of no more than 0.3 seconds and stops steam flow at the turbine inlet. The Gimpel valve trips due to the loss of oil supply pressure sensed by a solenoid valve located in the unit oil system and reopens after oil pressure is re-established.
The OOPTV was equipped with a linear variable differential transmitter to provide valve position feedback to the control system. It remotely monitors valve positioning during start-up, without operator assistance.
This is accomplished using hydraulic oil pressure supplied by the turbine control oil system to compress a fully enclosed, large diameter trip spring. The valve is equipped with a pull-to-close operation and features a back-seated stem eliminating steam leakage when the valve is operating in its normal, fully open position.
The valve operating system uses an electro- hydraulic or electro-mechanical actuator for the Gimpel valve’s operation and a spring load of up to 10,000 lbs. for closing. It is available in ANSI nominal pipe sizes 4-inch to 24-inch (102 mm to 610 mm) and pressure classes 150 psig to 2,500 psig. The OOPTV can be custom engineered to accommodate a range of temperatures.
