4.2 Low pressure system configuration - part 4
On all 500 MW units and the earlier 660 MW units with DC heaters, cooling of the generator by con-densate was incorporated in the feed train immediately after the extraction pump, as illustrated in Fig 3.41.
The reason for placing it in the feed system was to reclaim the generator losses as heat input to the feed. In practice, it was found that for large modern power stations the following difficulties were experienced:
- On extraction pump trip, the residual heat in the generator could boil the condensate in the system and cause thermally-induced hydraulic shock .
- To provide an acceptably modest condensate pressure within the coolers on the generator, two pressure stages are needed. Two pumps are used and the condensate is discharged from the first pump through the generator coolers and then into the turbine moisture extraction condenser (TMEC) with consequent complicated pipework arrangement. The TMEC extraction pump then discharges to the lowest pressure DC heater.
- To prevent limitations on unit output during the summer when condensate temperatures may be too high to cool the generator adequately, coolers to reduce the condensate temperature before it passes through the generator may be required in the feed system. The introduction of raw water coolers in the condensate system carries the risk of leakage of the cooling water into the system.
Whilst these problems can be overcome, the resultant plant is expensive and needs special care to operate. Consequently an auxiliary CW system using indirect coolers has been developed by the main manufacturers and is fully described in Chapter 6.
On all modern feed systems, protection against the use of contaminated feedwater is provided. On some units, this takes the form of visual alarms to alert the operator of high conductivity levels in the feed leaving the extraction pumps. In this case, the operator has to use his judgement as to the subsequent actions, for example, to close sectionalising valves and unload the unit.
For AGR feed systems, as illustrated in Fig 3.5, a full-duty polishing plant is provided to ensure high quality water for boiler feed. As their boilers need special protection against chloride ingress, valves are provided upstream of the polishing plant but downstream of the outsurge connection.
In the event of a condenser tube leak, the conductivity sensors close the protection valves and trip the extraction pumps; should there be an accidental release of chemicals from the polishing plant, the second line conductivity sensors also close the protection valves. The valves used for this duty are butterfly valves of the spring closing/air opening type similar to those used for bled-steam isolating valves.