A guardian valve is a

valve A valve is a device or natural object that regulates, directs or controls the flow of a fluid (gases, liquids, fluidized solids, or Slurry, slurries) by opening, closing, or partially obstructing various passageways. Valves are technically Pip ...

used in marine

steam turbine A steam turbine or steam turbine engine is a machine or heat engine that extracts thermal energy from pressurized steam and uses it to do mechanical work utilising a rotating output shaft. Its modern manifestation was invented by Sir Charles Par ...

propulsion plants to prevent steam from leaking into the astern turbine while the vessel is operating in the ahead mode. It is normally installed between the astern throttle valve and the astern elements of the low pressure (LP) turbine. Typically only the LP turbine of a steam ship's propulsion plant has reversing blade elements. Steam leaking in such a manner would result in a loss of efficiency and possibly overheat and damage the turbine blades. The guardian valve must be opened prior to any maneuvering situation, in order to permit the astern turbine to be used to bring the vessel to a rapid stop or to back down. Any similar arrangement of a stop valve used to protect leaking past a throttling valve could also be referred to as a guardian valve.

References

Safety valves Marine steam engines Steam turbines {{engineering-stub