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Stem of a ball valve wetted / pressure-containing part or not ? 2

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balven

balven

Materials
Aug 29, 2011
9
Hi all,

We have some ball valves specified in high H2S service, for which a ball valve supplier has offered 17-4 PH martensitic stainless steel for stem. The supplier's contention is that the stem will not be in contact with the process fluid under normal service conditions, and only in case of seal failure will the stem be exposed to process fluid. On that basis they wish to use Table A.28 of NACE MR0175 / ISO 15156-3 (instead of Table A.27 under which 17-4 PH will be disqualified), using the "not wetted => not pressure-containing" argument. However, 3.14 of NACE MR0175 / ISO 15156-2 indicates that valve stem is a pressure-containing part.

So, do we go with the valve supplier and accept 17-4 PH stem, or reject this material on the basis of Table A.27 ? Furthermore, although my query is specific to ball valves, I would much appreciate if you could share your experience (on whether valve stem is to be considered a wetted / pressure-containing part) with regard to other valve types - gate, globe, butterfly, etc.

Thanks in advance,
Balvendra Pal
 
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If the valve stem is not wetted, then it is not subject to the failure mode caused by exposure to H2S. I agree with your manufacturer, and Table A.28 applies.
 
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Are there seals on the valve stem? Most ball valves have o-ring grooves and seals there. Since the ball seals undergo fairly severe wear over the life of the valve and can thus pass some fluid to the ball cavity, and some balls are purposely vented, the stem and seals are often very much wetted and pressure-containing elements.
 
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As seen from the answers above, both views may be argumented for. As ball-valves needed to be qualified for 'sour- service/high H2S' often are used in very critical positions, perhaps not easily serviceable or even subsea, it is as likely as not that the end-user wants to include the stem within the qualification.

Two ways out: ask the end-user for enlightment, or include the best quality, with an offer of price-reduction if this is above target.

 
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If the stem was not in contact with the fluid, you wouldn't need stem packing. The valve salesman was either lying to himself or to you.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"
 
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When the ball valve is open from the sealing surface, medium filled in cavity.
 
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