why metal seated when you can use soft seated

[Mous1747]

the service is ammonia and the max temp is 200°F but the set pressure 3900 psi
not sure why this PSV is using metal seats? why can't just use soft seats ?
Is there any other deciding factors beside temp and chemical compatibility?

 

[Compositepro]

Because soft seats would be destroyed by that pressure when the valve opens the first time.