Can anyone share a sketch or drawing how this "specially designed plug" looks like?What Went Wrong? 6th ed. by Trevor Kletz said:18.1.7.a ... Plugs installed to facilitate pressure testing should be welded in position. However, it is bad practice to seal weld over an ordinary screwed plug. If the thread corrodes, the full pressure is applied to the seal. A specially designed plug with full-strength weld should be used.