7.2.1 Temperature control
The temperature of the steam supplied to the glands is controlled by two desuperheaters. A typical desuperheater is shown in Fig 2.73.
The two types of desuperheater are basically similar and each consists of a cylindrical steel body, provided with a steam outlet branch, which has a cover incor¬porating a steam inlet branch with a nozzle bolted to it.
In operation, the steam enters the desuperheater through the nozzle plate to form a high velocity jet. The water being injected through the nozzle is entrained by this steam jet, atomised and evaporated, thus desuperheating the steam before it is discharged via the outlet branch.
An electronic or pneumatic controller is provided for each of the HP/IP and LP sealing lines which, via a temperature transmitter, senses the line temperature and transmits electric signals to the controller mounted near to the desuperheater nozzle.
The desuperheaters require cooling water which is supplied from the reserve feedwater tank through a motorised isolating valve. The water supply is then controlled by regulating valves. Each valve is adjusted by a controller on receipt of a signal from a sensor in the appropriate desuperheater outlet. Any increase or decrease in temperature at the desuperheater is ompensated by the automatic opening or closing of the valve to admit the amount of water necessary to maintain the correct outlet steam temperature.
The steam temperatures are indicated in the control room and locally; high and low temperature alarms are included to indicate a fault condition.
There are drain connections on the desuperheaters which allow drainage to pass to the clean drains tank via an isolating valve, a strainer and a drain trap.