An automatic temperature control system is fitted to keep the LP exhaust steam temperature within acceptable operational limits during no-load or light load operation. A temperature detector, incorporating thermocouples, is situated in one exhaust flow of each LP cylinder and this measures the exhaust temperature. In operation, a signal is passed to the temperature controller, via an amplitude selector (this ensures that the LP exhaust with the highest temperature is selected), which operates the spraywater control valve.
When any individual sensing element detects a preset low exhaust steam temperature, the controller will open the spray control valve to pass a minimum quantity of water. Further increases in temperature will allow proportionally-greater quantities of water, rising to a maximum water flow at a preset high exhaust steam temperature.
The temperature detectors can have an inherent time lag and, under certain circumstances, an overriding control is necessary to produce a two-minute burst of spraywater at maximum flow.
In the event of the condensate extraction pumps failing, the emergency spraywater pump is brought into operation automatically, provided that the turbine gland steam isolating valve is open. This condition is detected by a limit switch situated on the isolating valve. Loss of supply from the extraction pumps is indicated by a pressure switch located on the supply line from the pumps, which is able to detect low water pressure. The emergency pump continues to run until stopped by the operator.
Should the automatic system fail, a manually-operated by-pass is provided round the spray control valve to enable the operator to maintain the spraywater supply to LP exhausts. This valve is normally closed.
The pressure reducing valve in the line from the condensate pumps is set during initial commissioning of the system to give the required pressure at the spray valve inlet (while it is open). Once set, it should not require any further adjustment.
The emergency pump discharge isolating valve is activated by the upstream pressure and is arranged so that it remains closed against the static head of the feedwater tank, but opens when subjected to the emergency pump discharge head. This feature automatically prevents the condensate storage tank draining via the sprays during shutdown.
If the steam temperature in any of the exhausts reaches a predetermined level, an alarm is given. If the exhaust temperature exceeds this level or the load falls below a certain value, a turbine trip will be initiated. These levels are determined by the turbine manufacturers.