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Control Valve Leakage Test Requirement in Natural Gas and Monoethylene Glycol (MEG) Environment

Published on 18 Apr 2019 by Tadd Pham


Due to the possible presence of toxic gas in an operating facility, and in order to avoid environmental implications, the Fugitive Emission from some valve packing have to be strongly reduced using specific and proper material, particular design, dedicated devices such as bellows seal to achieve low emission levels on lethal and toxic services.

For those control valves the following rules should be applied:

  • Lethal service, bellow type valves exclusively, with pressure gauge and isolation manifold on the bonnet to monitor bellows failure.
  • Toxic service, “low fugitive emission” with ISO 15848 low-emission certificate. For linear motion control valves when the certificate is not available then bellow valves are to be used.

However, when dealing with Natural Gas and MEG, the process is NOT in the lethal service category per ASME VIII-1 section UW-2. Therefore, Fugitive Emission test is not required. Valve Leakage in Natural Gas and MEG should be tested per ANSI FCI 70-2 Class IV or API 598.

All Severe control valves and control valves with a seat leakage class V or Class VI should be individually tested. All valves other than Class V or Class VI  should be tested randomly, with a minimum of valve per valve model, per pressure class and per valve size.

The test medium should be liquid for all classes, except Class VI should be gas tested.

The seat leakage test pressure for self-acting pressure regulators should be the set pressure. The valve supplier should provide the following:

  • Valve tag number
  • Test medium
  • Test differential pressure
  • Flow direction
  • Measured seat leakage flow rate
  • Measured leakage to atmosphere (via vent outlet) for pressure regulators
  • Leakage class and allowable seat leakage flow rate
  • Test results (packing adjustment, other works, failed, passed, etc.)

Per NSI FCI 70-2, there are six seat leakage classifications. The most commonly used in the industry are Class I, Class IV and Class Vl.

  • CLASS I is also know as dust tight and can refer to metal or resilient seated valves.
  • 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.
  • Class III establishes the maximum permissible leakage generally associated with Class II, but with a higher degree of seat and seal tightness.
  • 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 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.
  • 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.

Control Valve Seat Leakage Classifications – ANSI FCI 70-2 superseding ANSI B16.104

Leakage Class Designation
Maximum Leakage Allowable
Test Medium
Test Pressure
Testing Procedures Required for Establishing Rating
Class I XXX XXX XXX No test required provided user and supplier so agree
Class II
0.5% of rated capacity
Air or water at 50-125 F (10-52 C)
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
Class III
0.1% of rated capacity
Air or water at 50-125 F (10-52 C)
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
Class IV
0.01% of rated capacity
Air or water at 50-125 F (10-52 C)
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
Class V
0.0005 ml per minute of water per inch of port diameter per psi differential
Water at 50-125 F (10-52 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
Class VI
Not to exceed amounts shown in following table based on port diameter
Air or nitrogen at 50-125 F (10-52 C)
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 (millimeter) Leak Rate (milliliters per minute) Leak Rate (Bubbles / Minute*)
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.

Per API 598 - Valve Inspection and Testing covers the testing and inspection requirements for gate, globe, check, ball, plug and 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.

API 598 states for shell and Back Seat 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
NPS
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.)

< 2 0 0 (1) 0 (1) (2)
(3)
2 - 6 0 12 24 (2)
(3)
8 - 12 0 20 40 (2)
(3)
> 12 0 28 56 (2)
(3)

Notes:

  • 1 milliliter is considered equivalent to 16 drops.
  • For check valves larger than NPS 24, the allowable leakage rate should be per agreement between Buyer and Supplier.
  • There should 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.
  • The maximum permissible leakage rate should be 0.18 cubic inch (3 cubic centimeters) per minute per inch of nominal pipe size.
  • The maximum permissible leakage rates should be 1.5 standard cubic feet (0.042 cubic meter) of gas per hour per inch of nominal pipe size.



Tags: MEG leak control valve test