During the pressure test of a piping system, 3 out of 20 3/4" #800 gate valves had a minor leakage,(according to customer inspector about 1 drop per minute). These valves were SW/NPT and the leak was detected once removed the sealing plug inserted in the threaded side of the valve.
For what it is worth, from the practical side:
Welding procedure / practices on the SW connection can greatly help prevent this type leakage.  We had this problem until we started removing the bonnets / stem / seat from the gate valve body until body is welded into place.  Also, once valve body is tacked in place it is evenly pre-heated by torch before the SW is completed.
A 1.5mm (1/16") internal gap between the male and female should be in place before welding.  Random RT can be utilized for verification.

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