Spherical valves are installed as special shut-off valves for high pressures at the end of the penstock in front of the turbine. Their main purpose is to shut-off the penstock during normal shutdown or in the case of turbine trips.
The valve bodies are either cast or steel-welded. The rotor is normally forged. The stems are either forged on or bolted on and are running in self-lubricating bearing bushings.
The metal sealing rings are normally situated on the downstream side of the body and are actuated either by water pressure or by means of biodegradable hydraulic oil, pressing them against the rotor after closing. A second seal ring situated on the upstream side is useful for maintenance purposes, i.e. replacing shaft seals, etc., since this seal works as a blind cover so that the penstock does not need to be dewatered.
The actuation elements are like those for the turbine inlet valve. The high pressure applied from the upstream penstock allows the valve to be operated using the water pressure from the penstock instead of hydraulic oil. Systems controlled using oil or water are also possible.