Pressure containing element within the scope of ASME Pressure Vessel Code must be fabricated of materials according to ASME (materials SA). My question refers to ball valves and the materials to be used: which parts of valves are considered to be under pressure? The simplest explanation I have ever heard from an Engineer is "if you can drill a hole thorugh it and get wet, then it is pressure retaining." Horribly misleading. The pressure vessel codes refer to ASME B16.34. This specification defines allowable materials of construction for valve bodies and trim. If the valve is built in conformance with this specification, then it meets code. The term "pressure retaining" in relation to valve parts is what creates much confusion and excessive documentation. The main issue is if pressure is a significant design factor. The stress in a stem from turning or linearly moving the control element is many times greater than the stress applied to hold the stem against internal pressure. Same holds true for bonnet plates. Usually, the stress applied to compress the packing is many times greater than the stress from internal pressure. Point is that if such parts do not break at assembly or when the valve is operated, they will not break when internal pressure is applied, so added documentation or testing solely to give assurance that the part will safely hold pressure.

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