Combustion Gas Turbines - Turbine Overspeed Trip Mechanisms PDF Print E-mail
Written by Norrie   
Wednesday, 13 January 2010 10:12
Article Index
Combustion Gas Turbines
Principles and Operation
The Air Compressor
The Combustion Chamber
The Compressor (HP) Turbine
The Variable-Angle Nozzles Load Turbine
Turbine/Compressor Lube Control Oil System
Turbine Hydraulic Oil Trip System
Turbine Overspeed Trip Mechanisms
Turbine System Details
All Pages

TURBINE OVERSPEED TRIP MECHANISMS

A turbine may exceed the safe speed for a number of reasons. One could be the failure of the speed control systems. Mainly however, overspeed is caused by a sudden drop in or loss of the load on the turbine. This, in the case of a gas compressor driven by the turbine, will occur if the gas supply to the machine is suddenly decreased or fails. The resultant compressor surging due to the back and forth gas flow through the compressor does not allow the governor to control the correct speed quickly enough. The speed increases rapidly and the overspeed trip mechanism is activated to shut down the machine by dumping the hydraulic oil and thereby closing the fuel gas stop valve. The operation of these mechanisms is graphically explained in the following Figures: 24 & 25.

OVERSPEED TRIP IN 'NORMAL' CONDITION

Figure: 24

In the above diagram, the turbine is at its operating speed and overspeed bolt is in the normal position inside its cavity in the shaft. The shuttle valve is held to the left against spring 'A' by the trip latch. The right end piston of the valve is closing off the oil dump line, so holding the oil pressure between the two valve pistons. Hydraulic oil pressure is also passing to the Fuel Gas Stop Valve actuator, holding the valve open against spring 'B', allowing fuel to pass to the turbine. (A VERY SMALL leakage of oil is passing between the pistons and the cylinder walls of all trip systems to allow lubrication).
OVERSPEED TRIP IN 'TRIPPED' CONDITION

Figure: 25

In this diagram, the turbine has exceeded the maximum speed and, due to the centrifugal force of rotation, the overspeed bolt has moved outwards from the shaft cavity. The outward movement causes the bolt to strike the trip lever and lift the trip latch, releasing the shuttle valve piston. Spring 'A' pushes the shuttle to the right which opens the oil dump line. The hydraulic oil pressure immediately drops to zero, taking the pressure off the gas valve actuator diaphragm. Spring 'B' pushes the diaphragm down closing the gas stop valve which instantly shuts down the turbine. (The oil pressure is also taken off all other trip systems being supplied). The sudden reduction in oil pressure to zero is due to the oil flow to dump being much higher than the supply oil flow through the restriction orifice in the hydraulic oil feed line; – look back at Figure: 23.

TURBINE COOLING SYSTEM
(Figure. 26)
In desert locations, cooling water is used as the heat absorbing medium for the lube oil cooler, the turbine shell, second-stage nozzle stems and the turbine support legs. The cooling water system is a 'Closed Loop' type in which the water itself is cooled by an 'Air-fin' cooler and re-circulated around the system. The water temperature is maintained at 100 to 110 °F. (This system is just like the cooling system in a car engine, on a very much larger scale). Two, A/C motor driven, centrifugal pumps are the prime-movers for the cooling water - one operating & one standby. A make-up head tank is incorporated into the system to maintain the necessary water volume and the suction head to the pump to prevent cavitation. The discharge from the pumps at about 90 Psi is piped through check valves to feed the turbine sections as listed above. The water passes through the lube oil cooler tubes and returns to the air-fin cooler to begin the cycle again. On the outlet of the cooler, a thermostatic valve is installed which is controlled by the temperature of the lube oil header feeding the turbine bearings. A D/C driven, emergency pump is also installed in the turbine to supply cooling to the turbine parts in the event of main pump failure, power failure or on low water pressure in the turbine shell cooling system. The emergency pump will run until the main pump is returned to operation or the shell cooling is restored. (Total main pump failure will trip the turbine). A test valve is installed for checking the operation of the emergency pump. When the test valve is opened, pressure is dumped into the return line simulating a low water pressure and a low pressure switch brings in the emergency pump. When the test valve is closed, pressure returns to normal and the D/C pump will shut down again. The air-fin water cooler has two motor-driven fans. One is operating during normal weather conditions, the other will start on demand from the temperature control when water temperature tends to rise above its set point. Fan High vibration and water low pressure alarms and shut-downs are also incorporated.
TURBINE COOLING SYSTEM

Figure. 26



Last Updated on Wednesday, 24 February 2010 20:06