Pipe support lift off

[KevinNZ]

Here's a question for B31.1 and B31.3 piping designers.

Under some load cases (usually thermal) a pipe can lift of a support and be left hanging in the air.

B31.3 has rules (Appendix P, example S302) for dealing with this situation but not B31.1.

Should prudent B31.1 piping designers..
1. ignore the lift off and accept the calculations for thermal range and cold gravity loads.
2. apply B31.3 like rules and remove the lifted off support from the gravity case.

Does the B31.1 committee lag behind the B31.3 committee or does B31.1 not have as many rules because of the more conservative allowable stresses etc.

 

[XL83NL]

You should not ignore lift-off for B31.1, obviously. That needs no explanation. As a rule of thumb, I accept all lift-offs where the displacement is less then 1/16" (approx 3 mm.).

 

[DSB123]

As XL83NL says if the lift-off is small you can ignore it generally but if it is local to equipment then the nozzles may be overloaded due to weight. Any lift-off larger than say 3mm should never be ignored unless you can ensure that the sustained stress and sustained + occasional stress meet the Code allowables and also the equipment loads are acceptable.

 

[pennpiper]

KevinNZ,
It seems to me that if you design the pipe configuration properly the first time for the correct support and flexibility you would not have "lift-off".

Sometimes its possible to do all the right things and still get bad results

 

[MJCronin]

Agree with Pennpiper ....

How much is the liftoff and are you sure that the support should not be a spring hanger ??



MJCronin
Sr. Process Engineer

 

[KevinNZ]

Thanks for your replies

I agree lift off should not be ignored. I was interested in why one code tells us this and how to design for it and other does not.

 

[curtis2004]

Kevin,
It is because of two separate committees who oversee B31.1 and B31.3.
Thanks,
Curtis

 

[lilliput1]

If there is danger of overstressing the pipe make the run more flexible by adding U shaped bend.