Well, quite simply, once the steam finds a leakage point across the seats the wire drawing starts. It only gets worse. I do know of some plant / end user regimes in the past that insisted upon stellited seats in order to prolong maintenance intervals. Sometimes it was not steam service but it was always pointed out on the true disadvantages. However, what most users are unaware of is the fact that on higher temperatures on steam service, the co efficient of expansion between the base metal and stellite (seat) is different. The result is an increase in seat distortion leading to leakage etc., and then wire drawing. You will most probably note that major SV manufacturers now also use modern alloys for the disc and sometimes nozzle components which offer more pliable seating advantages for better designed seats.
Per ISO, only the term Safety Valve is used for all overpressure eventualities regardless of design.
