3.5 Protection systems
If the de-aerator pressure falls rapidly, as would happen on a unit trip, steam will be evolved from the large mass of stored water within the tank.
To prevent the reflux of the steam to the turbine, a nonreturn valve of the power-assisted closing type is provided in the bled-steam line between the de-aerator and the bleed point on the turbine. As the amount of steam evolved would overspeed the turbine, the nonreturn valve is of a high integrity type specially developed by the manufacturer for this duty. The valve is air-opened, spring-assisted closed and is commanded to shut by the action of a switch on the turbine governor gear when the unit trips. As a back-up, the bled-steam isolating valve is tripped so that it also closes. In the event of an HWL in the tank, the bled-steam and power-assisted non-return valves are shut. In addition, a high integrity valve in the condensate line to the de-aerator is also shut. The route to the turbine is therefore closed and the source of flooding water is isolated.
Inflows to the de-aerator tank for which automatic isolation on a high water level is not needed are the HP heater drains and the feed pump leak-offs. This is because the feed pump leak-offs are purely circulatory flows between the tank and the boiler feed pump, and the incoming HP heater drains are always less than the outgoing feed flow from the tank needed to generate the drains.