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.