Valve leakage refers to flow through 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 slurries) by opening, closing, or partially obstructing various passageways. Valves are technically fittings ...

which is set in the 'off' state. The importance of valve leakage depends on what the valve is controlling. For example, a dripping tap is less significant than a leak from a six-inch pipe carrying high-pressure radioactive steam. In the United States, the

American National Standards Institute The American National Standards Institute (ANSI ) is a private non-profit organization that oversees the development of voluntary consensus standards for products, services, processes, systems, and personnel in the United States. The organi ...

specifies six different leakage classes, with "leakage" defined in terms of the full open valve capacity: * Class I, or 'dust-tight' valves, are intended to work but have not been tested * Class II valves have no more than 0.5% leakage with (or less if operating pressure is less) of

air pressure Atmospheric pressure, also known as barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The Standard atmosphere (unit), standard atmosphere (symbol: atm) is a unit of pressure defined as , which is equival ...

at the

operating temperature An operating temperature is the allowable temperature range of the local ambient environment at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the de ...

* Class III valves have no more than 0.1% leakage under those conditions; this may require soft valve seats, or

lapped Lapping is a machining process in which two surfaces are rubbed together with an abrasive between them, by hand movement or using a machine. Lapping often follows other subtractive processes with more aggressive material removal as a first step ...

metal surfaces * Class IV valves have no more than 0.01% leakage under those conditions; this tends to require multiple

graphite Graphite () is a crystalline form of the element carbon. It consists of stacked layers of graphene. Graphite occurs naturally and is the most stable form of carbon under standard conditions. Synthetic and natural graphite are consumed on large ...

piston ring A piston ring is a metallic split ring that is attached to the outer diameter of a piston in an internal combustion engine or steam engine. The main functions of piston rings in engines are: # Sealing the combustion chamber so that there is min ...

s or a single

Teflon Polytetrafluoroethylene (PTFE) is a synthetic fluoropolymer of tetrafluoroethylene that has numerous applications. It is one of the best-known and widely applied PFAS. The commonly known brand name of PTFE-based composition is Teflon by Chemou ...

piston ring, and lapped metal seats. * Class V valves leak less than cubic metres, per second, per bar of pressure differential, per millimetre of port diameter, of water when tested at the service pressure. Class VI valves are slightly different in that they are required (at or operating pressure, whichever is less) to have less than a specified leakage rate in millilitres of air per minute:

References

* Valves {{engineering-stub