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#42395 - 04/14/11 02:02 AM Liftoff.. again...;-)
pipedrifter Offline
Member

Registered: 01/07/08
Posts: 35
Loc: Singapore
Hi everyone. Liftoff has been one of the sticky topic here but I would like to start a new discussion... smile

I have encountered several times in my calculations that pipe support lift off less than 1mm which makes liftoff non-existent and of no concern to hot sustained stress. The problem with that sometimes is that even a 0.05mm would make the support load zero and transfer 100% of loads to adjacent support and it is especially problematic when it is the first support from the nozzle which would result to nozzle overloading. The solution is to move the location of support. Incorporating nozzle stiffness in the model sometimes solve the problem.

I'm wondering what other stress engineer do with such cases. Sometimes it's just playing with numbers. Modifying the layout or support location to pass nozzle loads because of non-existent liftoff (let's say <1mm)is not practical and convincing client to modify it is really difficult.

Any thoughts?

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#42398 - 04/14/11 05:31 AM Re: Liftoff.. again...;-) [Re: pipedrifter]
pooria1978 Offline
Member

Registered: 02/25/08
Posts: 264
Loc: Netherlands

I would "model" the support as some rigid elements with a logical temperature distribution. by this approach, indeed I take the circumfrential expansion of the pipe into consideration. it helps out above mentioned situation most of the times.

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#42399 - 04/14/11 05:36 AM Re: Liftoff.. again...;-) [Re: pooria1978]
MoverZ Offline
Member

Registered: 11/22/06
Posts: 1195
Loc: Hants, UK
To quote Mr. Klein, 'all the world is a spring' That is correct, and applies equally to support locations.

Your calcs assume practically infinite stiffness (1E12 of K units)at a support, such that 0.05mm upward relieves all the load. You could change the support stiffness to something more representative of reality with a fairly simple hand calc.

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