Valve Leakage Classifications

Types of Valve 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;

Standards for Acceptable Rates of Valve Leakage

There are many standards for classification of valve 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.

American Petroleum Institute (API)

The Americam Petroleum Institute standard API-598 covers the testing and inspection requirements for gate, globe, check, ball, plug & butterfly valves. It has acceptable leakage rates for liquids as well as gases. 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:

where
(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.