Valve LeakageTypes of Leakage
There are two types of leakage from a valve, namely; fugitive emissions from the valve to atmosphere, and leakage through the valve but contained within the piping system.
Fugitive emissions can both be detrimental to the environment and a potential safety hazard. Valves are considered to be the major contributors to fugitive emission losses.
Leakage through the valve can also be a safety hazard, and can be detrimental to the process.
Reasons that Valves Leak
Common causes of valve leakage include:
- Valve is not fully closed. This can be due to various reasons, including;
- Valve seat is prevented from closing fully due to dirt, rust, or line debris
- Insufficient actuator travel
- The seat is damaged, e.g. scored
- The seal is damaged
Standards for Acceptable Rates of Valve Leakage
There are many standards for leakage rates e.g. DIN EN 917 covers Thermoplastics valves, BS 6364 covers cryogenic valves, however the three standards used most in the oil and gas, and petrochemical industry are API 598, ANSI FCI 70-2 and MSS-SP-61. Seee below for further details.
American Petroleum Institute
The Americam Petroleum Institute standard 598 covers the testing and inspection requirements for gate, globe, check, ball, plug & butterfly valves. It has acceptable leakage rates for liquid as well as gas testing. All valves built to the various API standards are required to meet API-598 leakage criteria prior to shipment from the manufacturer or supplier.
API598 states for shell and backseat tests, no visible leakage is permitted. If the fluid is a liquid, there shall be no visible evidence of drops or wetting of the external surfaces (no visible leakage through the body, body liner, if any, and body-to-bonnet joint and no structural damage). If the test fluid is air or gas, no leakage shall be revealed by the established detection method. For both the low-pressure closure test and the high-pressure closure test, visual evidence of leakage through the disk, behind the seat rings, or past the shaft seals (of valves that have this feature) is not permitted (Plastic deformation of resilient seats and seals is not considered structural damage). The allowable rate for leakage of test fluid past the seats, for the duration of the tests, is listed in the following table:
(a) 1 milliliter 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 bubble means less then 1 bubble per minimum specified test duration.
(c) The maximum permissible leakage rate shall be 0.18 cubic inch (3 cubic centimeters) per minute per inch of nominal pipe size.
(d) The maximum permissible leakage rates shall be 1.5 standard cubic feet (0.042 cubic meter) 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.
Manufacturer's Standardisation Society
The US based Manufacturers Standardization Society (MSS) of the Valve and Fittings Industry is a non-profit technical association organised for development and improvement of industry, national and international codes and standards for, amongst other things, valves.
Section 5 of their Pressure Testing of Steel Valves, MSS-SP-61 1999 relates to seat closure tests, and defines the following leakage rates:
- GATE, GLOBE, BALL VALVES: 10 cc/hr per inch of nominal pipe diameter. (Example: 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).
American National Standards Institute
ANSI FCI 70-2 supercedes ANSI B16.104 and specifies six different seat leakage classifications; class 1 to class VI.
Class I is also know as dust tight and can refer to metal or resilient seated valves.
Class IV is also known as metal to metal. It is the kind of leakage rate you can expect from a valve with a metal shut-off disc and metal seat.
Class Vl is known as a soft seat classification. Soft Seat Valves are those where the seat or shut-off disc or both are made from some kind of resilient material such as Teflon.
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