Piping Class and Design Pressure

Pipe specs can be limited by ANY component, including valves. Most people do design pipe specs to be flange rather than pipe limited for a variety of reasons, but there is no code requirement to do so.

Calling a spec "class 600" when it has a MAWP less than class 600 is potentially confusing- but not necessarily dangerous. In reality, your spec is possibly good for class 600 to a certain diameter or temperature. In that case, is it really wrong to call it class 600? But you could call it by any acronym you want if you fear confusion.

Confusion is the big risk. If I call out ANSI 600 flanges, I am specifying a configuration of the flange (flange dimensions, number of bolt holes, etc), and a component (not necessarily a system) MAWP. It seems to me that if you call out "Class 600" next to a pipe (which comes in Schedules instead of Classes) then you seem to be implying that the ambient-temperature MAWP will be around 100 Bar(g). If the actual system MAWP is actually 41.3 bar(g) (600 psig) then you really are courting a major screw-up somewhere down the line.

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.

You can do it. Not a problem. It is done all the time by everybody that commonly design to a pipe wall rating, i.e. virtually all pipeline companies, except for zdas & co. The MAOP will be specified on the PF/PIDs and alignment sheets, if it is a pipeline job.

If it ain't broke, don't fix it. If it's not safe ... make it that way.

Yeah, that is why I don't like the convention of calling class out on the P&ID.

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.

We usually use some arbitray numbering system for piping specs. For example for a system with a 150 psig design pressure, we'll list the piping and fitting, say sometthing like A1, then list out all the applicaple pipe diamters, grade, wall thickness, fitting class, etc. Then, for example, a 900 psig system, we'll name another piping spec B1.

Then special services will have their own specs, say for stainless lube oil piping, etc.

Then all lines on the P&IDs will be labled with something like 10"-A1-XXXX-YYYY, where XXXX is an system designation (with it's own numbering system) and YYYY is the line designation.

There's a gazillion ways to do it, and everyone does it differently.

That sounds like a clear and unambiguous system

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.

Thanks for the help everyone, I appreciate it.