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How tight is your Valve Seat?

When people ask me, how tight our valve seats are, I usually get an increased heart rate and I most would like to say “Oh yes, they are very tight”.

But fun aside ….

Please note that I’m not talking about external tightness (stem sealing and body sealing), as there are already several accepted standards like ISO 15848-1, API 622, API 624, etc. which describe the evaluating of the valve design with respect to fugitive emissions. Those standards where driven by the legal environmental requirements and do not consider the internal tightness of the valve.

When talking about valves seat tightness, we have to differ between just a maximum leak rate of a production test and a seat performance test.

I often read about ANSI FCI 70-2 “Control valve seat leakage” when people try to classify the seat performance of their valves. This standard actually describes six different seat leakage classifications. The most common used are CLASS IV (metal to metal seated) and CLASS VI (soft seated).

But the Fluid Control Institute (FCI) also notes that the standard ANSI FCI 70-2 has been intended to apply to control valve seat leakage. If line isolation and/or absolute tight shut-off is a normal expectation of the valve allocation, FCI recommends specifying another standard, such as API 598, “Valve Test and Inspection” – so forget this standard for shut-off valves.

How to determine a seat performance?

Detailed requirements for seat performance can be seen in several Customer specification like MESC SPE 77/300 (Procedure and technical specification for type acceptance testing (TAT) of industrial valves). Especially MESC SPE 77/300 is an extensive test and hard to achieve. This standard differs between different valve designs, temperature ranges, gaskets materials, etc. and covers external and internal leakage. The seat tests are an evaluation of the valve seat performance during several critical conditions, not only the tightness of the seat when it is new and only tested at room temperature:

  • Test medium: always Nitrogen for seats (Helium only for stem sealing)
  • All tests are done at ambient, elevated and low or cryogenic temperature and mechanical cycles are applied also.
  • low pressure seat test (2 to 7 bar)
  • incremental seat test (35- / 70- / 110% of MAWP [max allowable working pressure])
  • High pressure seat test (100% of MAWP)

After these stressful tests, the valve seat is worn and the valve should not be sold as a new one anymore. This is the reason why such a standard is only useful for type approval, not for serial production testing.

Nevertheless: Only doing a type test like MESC SPE 77/300 can show the real seat tightness performance under field conditions. But these tests are very expensive and so this is not done for standard valves.

How to prove seat tightness for standard valves?

For standard valves, you prove the tight shut off acc. to standards like API 6D, ISO 5208, etc.

Especially for the oil and gas market, the most important ones are API 6A, API 6D and API 598.

A premium valve manufacturer specifies always leak rate “A” (highest level). This means, that there is no visible leakage during a defined testing period.

I want to show the main details of 4 different accepted standards:

API 6D: Specification for Pipeline and Piping Valves

  • Test Medium: Water (Air and gas only if specified by purchaser)
  • Test Duration: 2 – 5 minutes (depends on the size of the valve)
  • Test Pressure: ASME B16.34
  • Leakage Rates: Rate A: Soft seated-ISO 5208 / Rate D: Metal seated-ISO 5208

API 598: Specification for Valve Inspection and Testing

  • Test Medium: Water, Air, Gas
  • Test Duration: 15 sec – 2 minutes (depends on the size of the valve)
  • Test Pressure: Low pressure closure or high pressure closure (depends on the valve type)
  • Leakage Rates: Rate A: Soft seated: Zero / Meatal seated (liquid test): 0-96 drops/minute / Meatal seated (gas test): 0-192 bubbles/minute Metal seated (depends on the size of the valve)

ISO 5208: Specification for Industrial Valves

  • Test Medium: Water, Air, Gas
  • Test Duration: 1 – 2 minutes (depends on the size of the valve)
  • Test Pressure: Low pressure closure or high pressure closure (depends on the size and type of the valve)
  • Leakage Rates: Not defined, as per requirement of the related valve product standard

EN 12266-1: Industrial valves. Testing of metallic valves. Pressure tests, test procedures and acceptance criteria. Mandatory requirements

  • Test Medium: Water, Air, Gas
  • Test Duration: 15 seconds – 2 minutes (depends on the size of the valve)
  • Test Pressure: high pressure closure – 1,1 x PN for liquid, 6 bar max. for gas testing
  • Leakage Rates: Not defined, as per requirement of the related valve product standard

(Image source: © AS-Schneider)