This question comes to mind? As to Restriction Orifice can have 150# but Orifice flange (Flow meter) is not recommended to have 150# rating.

The answer in one sentence is :- DUE TO FLANGE INTEGRITY

The answer in one sentence is :- DUE TO FLANGE INTEGRITY

It means in simple words :- Orifice that are flange tapped have reduced strength

Due to this drilling in between them their pressure-temperature retaining capacity reduces

So to ensure flanges have Strength to withstand Design pressure and temperature a higher rating than 150# i.e 300# is used .

Now There has to be some proof supporting It !!!

Due to this drilling in between them their pressure-temperature retaining capacity reduces

So to ensure flanges have Strength to withstand Design pressure and temperature a higher rating than 150# i.e 300# is used .

Now There has to be some proof supporting It !!!

## The Proofs

The standard we use for Normal flanges below 24” is

But For Orifice the flange standard is

This standard starts with 300# Flange rating .

In restriction Orifice as there are no tapping hence we go with

As always ! Thanks a lot for Reading !!

**ASME B16.5**But For Orifice the flange standard is

**ASME B16.36**This standard starts with 300# Flange rating .

In restriction Orifice as there are no tapping hence we go with

**ASME B16.5**flanges which Start with**150# Rating**.As always ! Thanks a lot for Reading !!

PS:- It is observed that for very large line sizes ( Depends few say above 10” , few say above 20”) 150# can be used as these flanges are stronger at higher line sizes but I have not found concrete proof in standards yet.

I would be very grateful if somebody knows the standard that has this documented.

I would be very grateful if somebody knows the standard that has this documented.