Hello Morten,
Great question that is frequently asked. The application of B16.34 is "under the jursidiction of the ASME boiler and pressure vessel code, Pressure Piping codes, and govenmental regulations." Since the scope of potential valve applications in this arena is very large, B16,34 does not cover all possible inspection requirements for cast pressure parts. As mentioned in a reply above, the inspection requirements for pressure parts retaining potable water are different than those for lethal gas. So in direct reply to your post, you must refer to the governing code or regulations to determine if inspection is required of the valve bodies.
Special class and limited class valves are rarely encountered. Both are limited to only threaded and weld ends. Flanged valves cannot be anything but standard class. In addition, limited class valves can only be 2-1/2 inch and smaller. Special class valves may have a slightly higher pressure rating with some NDT inspection. That is the only benefit. To add to this confusion, valves fabricated by welding (e.g., welding flanges onto a valve body), may require NDT if required by the ASME BPVC (refer to section 2.1.5 in B16.34).
On the practical side, the best check for body soundness is the hydrotest. It is very easy to get false indications or completely miss defects using recognized NDT techniques. And, each technique has its limitation on the type of defect it can reveal. The expression "test it like you use it" best describes this situation. I feel much more comfortable standing next to a valve that has been pressure tested than one that was only RT, PT, or UT inspected. So please don't belief that a body that has been RT, PT, and / or UT inspected is any better than one that has only been pressure tested.
Hope this helps.
