9 LP exhaust spray cooling system
9.1 Function and system layout
During the low-load running of a turbine-generator unit, the volume of steam flowing through the last row of the LP turbine blades is reduced considerably and the smooth aerodynamic flow of steam through these blades is lost. The LP exhaust steam is recirculated and, as a result, energy is transferred from the blades to the steam, resulting in a significant temperature rise of the circulating steam.
In addition, if the condenser back pressure is high, a smaller volume of steam will flow through the system, causing a significant worsening of the situation.
The excessive temperature rise caused by these effects can result in an undesirable restriction on the length of time that the turbine can run with no load, and possible uneven heating of the exhaust casings leading to serious distortion and adverse effects on turbine alignment.
The LP spray cooling system is provided to ensure that exhaust temperatures under these conditions are maintained within defined limits, thereby minimising the possibility of excessive casing distortion. This is done by spraying cooling water into the exhaust space, whenever necessary, downstream of the exhaust guide vanes. The exhaust steam is cooled and then recirculated to cool the casing. A typical LP exhaust cooling system is shown in Fig 2.78.
During normal running of the turbine, the conden-sate extraction pump supplies the spraywater to the system. The condensate supply, before reaching the spray nozzles in each end of the LP turbine cylinder exhaust, normally passes through a manual isolating valve, a pressure reducing valve, a strainer to remove dirt particles, a flow indicating switch (measuring water flow to sprays) and a pneumatic flow control valve, with manual inlet and outlet isolating valves. A pressure switch is usually fitted across the strainer to measure any difference in pressure, thus indicating whether blocking has occurred. A by-pass, with a manual isolating valve, is usually provided round the spray control valve and its associated isolating valves.
An emergency spraywater pump driven from a DC protected power supply will cut in automatically in the event of a failure of the normal spraywater supply. The emergency pump draws its water from an auxiliary water tank and discharges into the spray-water line upstream of the strainer. The emergency supply line normally contains a manual isolating valve, an emergency spraywater pump and a self-activated isolating valve.
Each LP turbine exhaust flow incorporates a split ring of spray nozzles, the ring being located outboard of the exhaust guide vanes. Initially, a number of these nozzles may be blanked off, to be used as additional spray nozzles if extra spraywater is found to be necessary during the subsequent running of the turbine. The arrangement of the spraywater nozzles is shown in Fig 2.79.
Each nozzle is designed to provide a spray of atomised particles which will evaporate quickly, thus promoting rapid cooling of the exhaust steam.
Suitable safety measures are built into the system to correctly maintain spraywater supplies so that an effective atomised spray is available when required. This is normally achieved by installing an automatic valve in the supply system to ensure that the spraywater flow is above a set minimum level. This valve is arranged to open or close rapidly near this minimum set point. The minimum flow requirement may result in a slow cycle of valve opening and closing during low load, but this is acceptable.
A temperature detector is located in the path of the exhaust steam downstream of the spray nozzles to provide the signal for the automatic start and control of the spraywater quantity.