Sealing shut-off valves in nuclear power plants

Issue and challenge:

The turbine shafts of power station turbines can weigh several hundred tonnes. In combination with the comparably high rotation speeds, enormous forces are generated. In unfavourable cases, a malfunction can cause the turbine to burst; flying broken fragments could, for instance, damage a nuclear reactor. To prevent this possibility, double-fast-closing shut-off valves regulate the steam-controlled rotation speed of the turbine. In an emergency they must be able to close within 0.3 seconds. In the past, the seal between the flap and the steam pipeline was implemented with packing glands. Their disadvantage is the high breakaway torque, which is compensated for by very large hydraulic drives with high inertia. A new sealing system will allow these drives to be made significantly smaller. Regulating the speed of rotation by means of steam valves has also been found to be too sluggish. The fittings manufacturer was therefore looking for an appropriate sealing system.

Solution from EagleBurgmann Espey:

By modifying the double-fast-closing shut-off valve and integrating the Espey WKA600S chamber seal with leakage disposal for a shaft diameter of 240 mm (9.45 “) in order to seal the flaps, the breakaway torque could be reduced so far that the flaps could be shut within 0.3 seconds at the same time as designing the drive smaller. The seal has 7 seal rings in 3 ring groups. This, together with their detailed implementation, allows the pressure of the steam pipeline – 11.6 bar (168.24 PSI) at an operating pressure of 311 °C (591.8 °F) – to be dropped with extremely low leakage.