3.6 Protection valves
The bled-steam non-return valve is power-assisted. Figure 3.32 shows a typical flap type non-return valve with a spring-closing/air-opening actuator. There is lost motion between the actuator and the valve flap which allows the valve to function as a free-acting non-return valve.
The actuator is there primarily to give the valve a positive closing force to overcome any stiffness in the bearings, etc., when the valve is commanded to shut. The valve is shut by dumping the air from the cylinder which allows the spring to close the valve.
If the air supply is lost, the valve will fail to the closed position, although it will not shut until the forward steam flow falls to a low value. This is because the steam flow on the underside of the disc creates an opening force which is greater than the spring-closing force. The bled-steam isolating valve is a butterfly type valve of the spring-closing/air-opening type. Figure 3.33 shows a typical butterfly isolating valve with a spring-closing/air-opening actuator. This form of actuator ensures the bled-steam isolating valve fails-shut on loss of air pressure. On a signal to trip, the air is dumped from the cylinder and the multiple springs exert sufficient force to close the valve against the full differential between heater pressure and turbine condenser pressure to keep it shut. This is needed as the IP/LP turbine pressure falls to condenser back pressure in approximately one second when the interceptor valves close due to a unit trip. The heater pressure remains constant for seconds or longer according to circumstances, and then reduces slowly, hence the isolating valve must close against the full differential pressure.