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P91 to P11
2

P91 to P11

P91 to P11

(OP)
I am redesigning a superheater vent pipe which vents to the atmosphere through a silencer with 400 psi back pressure.  I require P91 45 elbows due to stress but I can use either P22 or P11 pipe to obtain acceptable code stress ratios. Is there a preference to either the P22 or P11?  

RE: P91 to P11

stm397;
I would try to avoid dissimilar welds in this situation because of the caution and special attention to detail required to weld Grade 91 material, which is a creep strength enhanced ferritic steel. Can't you order extra heavy wall for the fittings using Grade 22 material? If it were me, I would pipe the entire line using Grade 22 because it has higher allowable stresses in comparison to Grade P11.
 

RE: P91 to P11

(OP)
Thanks for the info.  Our stresses are pretty high at the elbows.  Not sure heavier P22 class will do it but will give it a try.  Due to wall thickness variation and transition will be required but we would end up with a more reliable joint.

RE: P91 to P11

Would this be an appropriate use of P23, elbows + straight run, assuming a small quatitiy can be found?   

RE: P91 to P11

(OP)
ASME B31.1 item 137.3.2 Nonboiler External Piping I find the following line: "Lines open to the atmosphere, such as vents or drains downstream of the last shutoff valve, need not be tested."

My question is that since this line is open to the atmosphere at one end but pressurized leading up to the silencer does it still qualify for this exclusion of pressure testing. All welds will be 100% radiographed. I was wondering if after that does the line still need to pressure tested.

RE: P91 to P11

not under B31.1 but per process piping it may. ask the Jurisdiction/Inspector. Some Cities or States do not enforce it.
 

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