Valve leakage
Valve leakage refers to flow through a valve 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] 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 of air pressure at the operating temperature
- Class III valves have no more than 0.1% leakage under those conditions; this may require soft valve seats, or lapped metal surfaces
- Class IV valves have no more than 0.01% leakage under those conditions; this tends to require multiple graphite piston rings or a single Teflon 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.