Seal Welded Seat Rings

My guess would be to prevent crevice corrosion and potential leakage.

Tack welding fails in-line, causing failure of seat, and downstream equipment. tack welds are basically never of quality, and should be considered no better than a feel good weld. Furthermore, a gate valve's full pressure against a seat, combined with other body stresses, can easily crack a tack weld with some corrosion help..

finally, threaded connection are highly prone to failure, which is why we require seal welding. way too many steam failure are past threaded connection.