how to calculate the stiffness of pre-insulated pipe

[818what]

The pipelines I am working on are refrigeration pipe with rigid PU foam insulation. At the location of support we use prefabricated high density ring block (cut into two pieces) between pipe clamp and the pipe, and in other place the insulation is with lower density and injected on site. The compressive strength & modulus of PU foam are obviously much lower than steel, therefore using rigid support in pipe stress analysis is kind of conservative, which yield very high thermal stress. Therefore it is better to enter the stiffness of the support. The tension/compression stiffness formula is E.A/L, can I calculate the stiffness of pre-insulated support in the same way? eg.
Stiffness = E*(D*W1)/L
where E is the compressive modulus of insulation material (~10Mpa)
D is the outside diameter of pipe
W1 is the width of prefabricated ring block (or should I take the width of pipe clamp, which is smaller than the ring block width?)
L is the insulation thickness of insulation

 

[KevinNZ]

If you are relying on compression of the insulation at the supports to adsorb thermal expansion you might be close to compressing the insulation and damaging it. You might just end up with guides with gaps, which what you should use for thermal expansion in the first place.

 

[818what]

Thanks KevinNZ. Yes I agree that the insulation block may be damaged and leave some gaps instead, but it is hard to predict. So I'd like to include the stiffness as well. Just not quite sure if my stiffness calculation is right as I couldn't find any reference on the stiffness of this type.

 

[RVAmeche]

I wouldn't attempt to take any credit for additional stiffness of the insulation; I'd be surprised if the manufacturer didn't have some guidelines as well.

That being said, I'd assume the growth/deflection is based upon the pipe material and go from there. If the piping is small bore and contains many changes in direction, you can do the old school table/chart method if you don't want to do stress analysis.

Ask the manufacturer.

 

[LittleInch]

I'm confused.

Are you trying to add the stiffness of the pipe in flexing sense or are you looking at the bearing loads on the PUF at the support locations?

PUF is great stuff, but you will crush the foam quite easily and then it doesn't re appear like steel does.

For me far better to insert the PUF as a rigid block and determine the force / load / pressure on the PUF and then see if you're going to crush it.

It is pretty brittle stuff so won't deform very much before you crush it.

your calc though is difficult when applied to a circular support as the forces are not really 90 degrees to the pipe over the entire area of the support.

A drawing always helps

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