In control valve there are two type of valve seat we can use metal to metal seat & soft seat.On this valve seat i have following doubt. 1)I have never seen practically the internal assembly of valve so my doubt is that basically weather the seat is soft seat or metal its basic MOC is same as valve body(SS or CS) only in soft seat we are using PTFE ring & place on valve seat & in metal this PTFE ring is not threr am i correct??????? 2)what are the common general rule for using metal to metal seat & soft seat,means its depend on process parameter like liquid,gas, steam & on valve leakage class.

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