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STEAM CONTROL TO A TURBINE
In order to control a turbine speed, a method of controlling the steam supply is needed. To do this, the turbine steam inlet first enters a 'STEAM CHEST'. (Figure. 8) The steam chest contains a series of steam valves which can be opened gradually as required. As each valve opens the flow of steam to the nozzle(s) is increased thus increasing the turbine speed.
CONTROL SYSTEM DESCRIPTION
(See Figure 9).
The hydraulic/control oil enters the unit and is piped to the following parts of the system. (The oil pressure depends upon the maker's specifications).
- Power Piston
- Trip and Throttle Valve
- Overspeed Trip Mechanisms & Slide Valves 'A' and 'B'
- Trip Solenoid Valve
1. The Power Piston - The oil passes to the power piston via Port 'Y' of Slide Valve 'A'. The Governor, on the signals from the control system will, through a Servo-Mechanism', adjust the hydraulic oil to the power piston. This in turn, controls the steam flow via the steam chest valves to the nozzles, thus controlling the turbine speed.
2. The Trip and Throttle Valve - This valve passes the H.P. steam to the steam chest. On startup of the turbine, the steam chest valves are fully open. The T/T valve is opened slowly by a hand-wheel until the machine comes to Minimum Governor Control setting - - the power piston falls and the steam supply to the nozzles comes under Governor control. When the turbine is under the control of the governor, the T/T valve hand-wheel is swung to the fully open position. (This will not increase steam flow to the turbine due to the governor control of the steam chest valves). The high pressure hydraulic oil passes to the T/T valve cylinder via a restriction orifice and the ' Y ' Port of Slide Valve ' B '. This oil pressure acting on the piston keeps the main steam valve fully open during normal operation. In order to periodically check the operation of the T/T valve, a 25% stroke solenoid operated valve is fitted. On operation of the stroke check button, the valve is energised and bleeds off an amount of oil from the T/T valve cylinder. The T/T valve closes down by 25% without affecting the steam flow to the turbine. When the check button is released, the T/T valve goes to the fully open position again.
3. The Overspeed Trip Mechanism & Slide Valves ' A ' and ' B '
The O/S trip, as its name implies, is a Mechanical shutdown device in the event of turbine excessive speed. (Overspeed trips are discussed later). The slide valves are kept in the ' RUN ' position by applying oil pressure to the valve piston against a return spring. The oil feed to these mechanisms also passes through a restriction orifice. From this feed line, oil is also piped to the 'Trip Solenoid valve’.
4. The Trip Solenoid Valve - This is an Electrical shut-down device which receives a signal from the electrical trip circuit which includes - High vibration, Low lube oil pressure, High bearing temperature, Low hydraulic oil pressure ... etc. The electrical signal energises the solenoid which opens the valve and dumps the hydraulic oil back to the reservoir. The oil pressure is dumped to zero Psi due to the oil flow rate through the restriction orifices being LESS than the flow of oil returning to the reservoir. When the oil dumps, Slide valves ' A ' & ' B ' are pushed across by their springs. This CLOSES the oil supply to the T/T valve and to the Power piston via the ' Y ' Ports and OPENS the ' X ' Ports to dump the oil. The T/T valve closes and the steam flow is stopped. At the same time the Power piston rises to fully open the steam chest valves. (No steam can flow as the T/T valve has closed). Before resetting the trip condition - electrical or mechanical, the T/T valve hand-wheel must be spun to the closed position and made ready for start-up and the governor control system set to minimum governor. When the trip system is re-set, the hydraulic oil pressure is restored and the two slide valves move across to the ' GO ' position again. The machine can now be re-started.
TURBINE CONTROL SYSTEM