I agree with R.K. Technically the decision is often based on the type of facility it is located in. If it is at a power station or connected to a boiler, B31.1 would be the most appropriate code. However, if the user is a process facility like a chemical plant, they could say it falls under B31.3. In general, if it deals with steam I prefer B31.1 but in reality the two codes are quite similar. Based on my discussions with those that know the codes more than I, a good determination could well be based on how many cycles the pipe will see. B31.1 is designed for power stations that have long runs and few cycles over all. B31.3 is more cognizant of the frequent cycling that process systems have.
This is definitely something you need to agree with your client on, preferably in writing.
Thanks,
Duncan
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Duncan McEwen, P.E., P. Eng.
SSOE
Senior Mechanical Process Engineer