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Class Breaks at Flanges

Class Breaks at Flanges

Class Breaks at Flanges

(OP)
I have seen that "Class changes should never occur between flange sets, even if it is possible to align the mating flange bolt holes. Always change classes at welds (e.g. at a flange weld neck to pipe weld). This usually isn't clearly shown on P&IDs, but should be on your physical piping dwgs."

Can anyone cite a specific code requirement for this, or is it just a best practice?

RE: Class Breaks at Flanges

Apart from the fact it would look very odd, in reality, very few different rated flanges physically match up.

So it's really a practical issue.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

RE: Class Breaks at Flanges

For one, ASME B31.4 Class breaks, to which the code actually refers to as "piping of different pressure ratings" must be made with a valve in between, such valve and any piping on the high pressure side thereto, shall be rated to the highest pressure of the two line specs. A pressure relief on the lowest pressure side, set to no higher than 10% over design pressure, would also be very highly recommended and is required in many situations to prevent overpressure of that piping.

Independent events are seldomly independent.

RE: Class Breaks at Flanges

This was a bit fuzzy in my head until I realized that I can rate a pipe, vessel, system to whatever pressure I want to (up to the MAWP calculated by the code algorithms). So if I have an ANSI 150 system connected to an ANSI 300 system, then the last flange leaving the ANSI 150 part is still rated at ANSI 150 pressure (call it 280 psig MAWP) even though it was physically designed to ANSI 300 specs (call it 600 psig MAWP). I don't know if that helps you, but things got much clearer to me once I realized that the pressure rating is a "not to exceed" number not a "minimum" number.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"

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