Control Valve Leakage Rates & Seat Leakage Class
Types of Leakage from Valves
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.
Causes of Valve Leakage
Common causes of valve seat leakage include:
- Valve is not fully closed. This can be due to various reasons, with the most common being that the valve seat is prevented from closing fully due to dirt, rust, or line debris, or that the actuator has insufficient travel
- The seat is damaged, e.g. scored
- The valve seal is damaged.
Valve Leakage Class Standards & Acceptable Leakage Rates
There are many standards for leakage rates, or as it is often called; Shutoff Classification 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. See below for further details.
American Petroleum Institute
The American 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 valve seat 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 Leakage Class
ANSI FCI 70-2 supercedes ANSI B16.104 and specifies six different seat leakage classifications; class I to class VI.
Leakage Class I is also know as dust tight and can refer to metal or resilient seated valves.
Leakage Class II establishes the maximum permissible leakage generally associated with commercial double-seat control valves or balanced single-seat control valves with a piston ring seal and metal-to-metal seats.
Leakage Class III establishes the maximum permissible leakage generally associated with Class II, but with a higher degree of seat and seal tightness.
Leakage 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.
Leakage Class V is usually specified for critical applications where the control valve may be required to be closed, without a blocking valve, for long periods of time with high differential pressure across the seating surfaces. It requires special manufacturing, assembly and testing techniques. This class is generally associated with metal seat, unbalanced single-seat control valves or balanced single-seat designs with exceptional seat and seal tightness.
Leakage Class VI 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.
The following tables detail for each valve seat leakage class the allowable leakage rate, and the seat leakage test procedure:
Bubbles per minute as tabulated are a suggested alternative based on a suitable calibrated measuring device, in this case a 0.25-inch OD X 0.032-inch wall tube submerged in water to a depth of from 1/8 to 1/4 inch. The tube end shall be cut square and smooth with no chamfers or burrs. The tube axis shall be perpendicular to the surface of the water. Other measuring devices may be constructed and the number of bubbles per minute may differ from those shown as long as they correctly indicate the flow in milliliters per minute.
The following pages on Control and Instrumentation.com give more detail on valves and specifying valves:
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