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Valve Leakage

Types 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:

Valve Size	All Resilient Seated Valves	All Metal-Seated Valves (except Check Valves)		Metal-Seated Check Valves
Valve Size	All Resilient Seated Valves	Liquid Test (drops/min.)	Gas Test (bubbles/min.)	Liquid Test (drops/min.)	Gas Test (bubbles/min.)
<= 2"	0	0 (b)	0 (b)	(c)	(d)
2.5" - 6""	0	12	24	(c)	(d)
8" - 12""	0	20	40	(c)	(d)
>= 14""	0	28	56	(c)	(d)

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.

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.

Leakage Class	Maximum Leakage Allowable	Test Medium	Test Pressure	Test Procedure
I	---	---	---	No test required, as long as purchaser and vendor both agree.
II	0.5% of rated capacity	Air or water at 50-125°F (10-52°)	45-60 psig or max. operating differential whichever is lower	Pressure applied to valve inlet with outlet open to atmosphere or connected to a low head loss measuring device full normal closing thrust provided by actuator.
III	0.1% of rated capacity	As above	As above	As above
IV	0.01% of rated capacity	As above	As above	As above
V	0.0005 ml per minute of water per inch of port diameter per psi differential	Water at 50 to 125°F (10 to 52°C)	Max service pressure drop across valve plug, not to exceed ANSI body rating.	Pressure applied to valve inlet after filling entire body cavity and connected piping with water and stroking valve plug closed. Use net specified max actuator thrust, but no more, even if available during test. Allow time for leakage flow to stabilize.
VI	Not to exceed amounts shown in following table based on port diameter.	Air or nitrogen at 50 to 125 F (10to52C)	50 psig or max rated differential pressure across valve plug whichever is lower.	Actuator should be adjusted to operating conditions specified with full normal closing thrust applied to valve plug seat. Allow time for leakage flow to stabilize and use suitable measuring device.

CONTROL VALVE SEAT LEAKAGE CLASSIFICATIONS
Nominal Port Diameter Inches	Nominal Port Diameter mm	Leak Rate ml/min	Leak Rate bubbles/min
3	76	0.9	6
4	102	1.7	11
6	152	4	27
8	203	6.75	45
10	254	9	63
12	305	11.5	81

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.