Old Pipe Spec
Old Pipe Spec
(OP)
Hello Members,
Would like your responses to the following:-
Consider an existing Pipe Spec for a plant which was developed in 1992 to cover A335 P11 pipework to ASME B31.1
The selected pipe schedules were Sched 120 which were determined to be acceptable for the design conditions at the time according the the wall thickness calcs. However if you check out the required wall thickness to the 1998 or later editions of the Codes then Sched 120 is not thick enough. The reason being the reduced allowable stress in the later Codes ( 9300psi vs 11000psi originally used)
My concern is that the original Pipe Spec cannot be used for any changes to piping under that pipe spec and a new pipe spec must be developed for the plant for any modifications/extensions which were performed from when the allowable stress was reduced in the Code. Is this approach correct?
Regards
Would like your responses to the following:-
Consider an existing Pipe Spec for a plant which was developed in 1992 to cover A335 P11 pipework to ASME B31.1
The selected pipe schedules were Sched 120 which were determined to be acceptable for the design conditions at the time according the the wall thickness calcs. However if you check out the required wall thickness to the 1998 or later editions of the Codes then Sched 120 is not thick enough. The reason being the reduced allowable stress in the later Codes ( 9300psi vs 11000psi originally used)
My concern is that the original Pipe Spec cannot be used for any changes to piping under that pipe spec and a new pipe spec must be developed for the plant for any modifications/extensions which were performed from when the allowable stress was reduced in the Code. Is this approach correct?
Regards





RE: Old Pipe Spec
Yes. We found many piping failures in the field with 1 1/4 Cr, 1/2 Mo material, especially at branch connections, due to creep-fatigue interaction and so we recalculated the allowable stress (at temperature) curve for the next Code update.
With that understanding it would not be good practice to use the old allowable stresses from the original Code of record. Modifications, repairs and replacements should incorporate the new data or simply go to 2 1/4 Cr, 1 Mo material as the replacement.
The above is my opinion only and does not necessarily reflect the opinion of any Code body or its sponsor.
Regards, John.
RE: Old Pipe Spec
Again many thanks for your "solid" response.
Regards
DSB123
RE: Old Pipe Spec
From what I was able to find a lot of documented failures were at welds and HAZs (notably the failure of seam welded piping). We had smaller diameter seamless piping in our plants so I was never sure we were in the same risk category as the seam piping. We adopted the lower stress values though.