Forum,
I have a line that normally operates at 150 degrees F. During an emergency situation, they could use the pipe as a drain for 500 degree fluid. The higher temperature is rare, but does not fit the definition of the B31.3 occasional load since it is a strained based load case. The client wants the line to be able to handle the 500 degree case. Do I get any added benefit due to the fact that this is a rare case, sort of like how seismic activities receive an extra 33% for allowable stress?

If I treated this as an occasional load in CAESARII, the allowable limit would be 133% of the sustained allowable, which is not near the allowable for the EXP case.

Could this be a discussion I have with the client and ask them if they are comfortable with reaching a value of 125% allowable stress, or does this violate law/code? Thanks,
Devin

You can not treat as an occasional load. It does not matter that the higher temperature is rare, you will have to consider it.
But you may use f=1.2 .

Regards,

danb is definitely pointing you in the right direction. The key is indeed to consider how many cycles the system will see at that temperature. The base B31.3 equation with F = 1.0 is intended to give you 7000 cycles.

You should discuss with the client how often they really expect the 500°F case to occur. If it is going to happen once every five years, such that there may be a total of ten times in the life of the unit that this cycle happens, you may be able to work with the client to justify a high stress value that still keeps you below the S-N curve.

You want to avoid having to excessively spaghettify the piping layout with so much flexiblity for a rare case that you end up with stability problems the rest of the time.