jedax81He was asking if it is possible to use WN(Weld Neck) FF(Flat Face) Flange to mate it with FF(Flat Face) Valve Flange with different pressure ratings.
My answer is “Yes” that is possible. Taking into account the possible leakage, you have to look into the area of the gasket. The larger the area the better. When you are going to use RF(Raised Face) to FF(Flat Face), the area of the gasket will be small so the stress of the gasket goes up until it reaches to its maximum level where it fails and leak occurs. FF-RF & RF-RF will have the same load distribution towards the gaskets.
Considering its pressure rating, you have to make sure that the working pressure shoul not exceed on the lowest pressure rating of the flange since you are using class 125 & 250 mating together.
There will be only price difference between RF & FF. The only thing I’m not sure about is the dimensions of flange with 8″ pipe. Just make sure they have the same number of bolts.
There are clients that will give whole specification of the material you have to use and there are some that will only give the rating and size and it is up to you to look what kind of flange you will use.