Valve Leakage - A Lesson in Leakage- Written by the Valve Pipeline Staff
Valve Leakage - A Lesson In Leakage
All valves leak. Valves may be said to be "bubble tight" or zero leakage; but in actuality that is just a term that specifies the allowable leakage at that classification. There are six seat leakage classifications defined by ANSI/FCI 70-2-1976 (supersedes ANSI B16.104). The six valve leakage classifications are as follows:
ANSI/FCI 70-2-1976 VALVE LEAKAGE CLASSIFICATIONS
|Class I.Identical to Class II, III, and IV in construction and design intent, but no actual shop test is made.|
|Class II.Intended for double-port or balanced singe-port valves with a metal piston ring seal and metal-to-metal seats. Air or water at 45 to 60 psig is the test fluid. Allowable leakage is 0.5% of the rated full open capacity.|
|Class III.Intended for the same types of valves as in Class II. Allowable leakage is limited to 0.1% of rated valve capacity.|
|Class IV.Intended for single-port and balanced single-port valves with extra-tight piston seals and metal-to-metal seats. Leakage rate is limited to 0.01% of rated valve capacity. (known as metal to metal)|
|Class V.Intended for the same types of valves as Class IV. The test fluid is water at 100 psig or operating pressure. Leakage allowed is limited to 5 X 10 ml per minute per inch of orifice diameter per psi differential.|
|Class VI.Intended for resilient-seating valves. The test fluid is air or nitrogen. Pressure is the lesser of 50 psig or operating pressure. The leakage limit depends on valve size and ranges from 0.15 to 6.75 ml per minute for valve sizes 1 through 8 inches. (known as soft seat classification)|
Nominal Port Diameter
API-598 - from the American Petroleum Institute also covers testing/inspection requirements for gate, globe, check, ball, plug and butterfly valves. API 598 indicates that for shell and backseat tests no visible leakage is permitted through body, body liner, and joints; i.e. no visible evidence of drops or wetting of the external surfaces.
API 598 does have an allowable leakage rate for test fluids past the seats. API 598's allowable leakage rates are as follows:
|Valve Size||All Resilient Seated Valves||All Metal-Seated Valves (except Check Valves)||Metal-Seated Check Valves|
|Liquid Test (drops/min)||Gas Test (bubbles/min)||Liquid Test (drops/min)||Gas Test (bubbles/min)|
|0||0 (b)||0 (b)||(c)||(d)|
|2.5" - 6""||0||12||24||(c)||(d)|
|8" - 12""||0||20||40||(c)||(d)|
(a) 1 millilitre is considered equivalent to 16 drops (b) There shall be no leakage for the minimum specified test duration. For liquid test, 0 drop means no visible leakage per minimum specified test duration. For gas test, 0 bubbles means less than 1 bubble per minimum specified test duration. (c) The maximum permissible leakage rate shall be 0.18 cubic inch (3 cubic centimetres) per minute per inch of nominal pipe size. (d) The maximum permissible leakage rates shall be 1.5 standard cubic feet (0.042 cubic meters) of gas per hour per inch of nominal pipe size. (e) For check valves larger than NPS 24, the allowable leakage rate shall be per agreement between purchaser and manufacturer.
MSS-SP-61-1999 - The Manufacturer's Standardisation Society also has a section on valve leakage in Section 5 of their "Pressure Testing of Steel Valves - MSS-SP-61-1999", The following leakage rates are deemed acceptable by this testing standard:
- GATE, GLOBE, BALL VALVES: 10 cc/hr per inch of nominal pipe diameter. (e.g. A 6" globe valve is allowed to leak 60 cc/hr in a test)
- CHECK VALVES: 40 cc/hr per inch of nominal pipe diameter All shutoff or isolation valves specified to MSS-SP-61 must pass the above standards. The seat closure test must be performed at a fluid (liquid or gas) pressure no less than 1.1 times the 1000°F (380°C) rating rounded to the next 5 psi (0.5 bar).
Contact - The Valve Pipeline
If a valve comes back from service, always cycle (open and close) the valve a few times outside before putting it back in stock or trying to do any repairs- it is possible for pressure to get stuck in the cavity of the valve.