When a piping line specification requires 6mm of corrosion allowance, do you consider necessary to indicate this requirement also for valve description?Considering that valve's body thickness (according to main standard: API, BS, ASME) is greater than the pipe wall thickness (including 6mm of c.a.)it would seem unuseful.Anyway sometimes this is required by Client/Licensors in Oil and Gas Downstream Business.Does anybody know why?Furthermore, according to ASME B.16.34 (Ed.'96)ANNEX F, it's stated that the minimum wall thickness indicated is 2.54mm more than that calculated according a pressure based formula; which should i say? ASME B.16.34 considers as minimum a C.A. of 2,5 mm, so i could ask for plus 3.5 mm of C.A.? Besides, if i compare minimum body thickness according to std such as API, BS, etc. they are much more greater than those indicated in ASME B.16.34, so if i specify a gate valve according to API 600 i don't think that requiring 6mm of c.a. on body valve is necessary.Your statement about overall valve body thickness against standards and calculation is generally right since valve makers take into consideration other parameters such as tools, machining and standard parts to design and built their products.However, you MUST advise corrosion allowance required by your piping system. Never forget that valves are not simple items to carry the fluid such as pipe, fittings and other bulk items. They have a function and contain moving parts. Some sealing designs/arrangements do not tolerate any corrosion or dimension variation. Also, if corrosion allowance is expected, valve maker shall consider the same in its design and this is on top of body wall thickness calculation.To make it short, there are two types of sealing arrangements in valves, static sealing and dynamic sealing.

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