you are referring to a pressure vessel standard. it does not cover thermwells, but it does refer you to the requirements for vessel pressure test. suspect that the specifying engineer is taking the easy way out and telling you to sort out what the max pressure is supposed to be.
typically the weakest part of a thermowell is not the well itself, but the connection. there are thermowell standards of a sort: ASME B40 and various connection standards that you also must comply with. None specifically deal with pressure testing in a specific way.
Where you typically run into problems is when the vessel is being pressure tested above the thermowell rating. This is a result of the vessel designer and the instrument specifier using or referring to separate standards i.e. the instrument standard typically does not refer to the hydro test requirements for the vessel and adjacent piping, and vice versa. the specifier put the question to you. so call him...he'll pass you along to the vessel designer or refer to the name plate on the vessel. that is where the confusion starts, some times they use the wrong connection rating for the instrument.
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