Screwed connections are prohibited where threads provide the only seal of the pressure boundary." I want to know why this requirement is applied. And, this requirement shall be applied, is there any code/standard that this requirement is addressed. The context is necessary to really evaluate the phrase. Assuming that this pertains to piping or valves, the requirement includes flanges or welded end connections for the process connections. Threaded ends are not acceptable. The requirement would apply to the bonnet, another pressure containing connection. It is common for the first valve off of the pipe to be either welded or flanged (especially in services where there is H2S present). If you need an isolation valve, for example for a PT, the first ball valve may be welded or flanged, then your gauge valve, then then the PT.

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