Dear all,

In CII calculation (B31.3), I found the displacement of spring supports slightly changes as the design pressure changes. In my humble knowledge, I just presume this is because of the pressure stiffening effect as described in Appendix C.(7) of B31.3. But I am not quite sure.

Does anybody can kindly explain this to me?

You are probably seeing the results of either the pressure stiffening effect or the Bernoulli forces on the elbows causing them to open up.

Also, just like a long balloon, a pipe will extend axially (and radially) with increasing pressure, usually called pressure elongation. Generally axial elongation is a much more prominent effect than opening up of elbows, which is usually called the Bourdon effect.

Craig, are you sure about Bernoulli forces on elbows ??

Yeah, you're right - Bourdon forces. Duh!

Thank you, CraigB and MoverZ. I should have searched the CII manual more.

Then is this right if I turn off this option(bourdon effect) there shouldn't be any elongation of pipe (no FRP) due to pressure effect in CII?

Not sure what you mean by 'no FRP'. Under 'Activate Bourdon Effects' you have the translation and rotation choices. I would recommend always apply translations since this is a real effect, particularly noticable in large D/t ratio piping. Rotation occurs in elbows which are not truly circular cross section. So if you have forged elbows, no rotation. If you have pulled bends, include the option.

I think Grant is not using FRP ( Since for FRP bourdon effect is included irrespective of Caesar setting, that elongation value is reasonably high)& want to check spring movement due to pressure variation by switching off Bourdon effect.
In my openion spring movement will be constant irrespective of pressure variation when Bourdon effect & pressure elongation on bend both are switched off.

SHR, I don't agree with your second para. Spring movement is typically a function of thermal expansion. Pressure elongation is exactly the same in effect and is frequently expressed as an equivalent thermal. In fact that was essential in the old days of ADLPIPE etc., software that did not include pressure end effects.

Similarly, the shape a complex 3D pipe system may adopt when supported on springs could be affected by elbow deformation due to pressure (Bourdon effect). Consequently spring position would be affected.

Hi MoverZ

pressure elongation on bend can change flexibility of bend as per appendix D B31.3/B31.1.
So overall system flexibility is affected, it can have consequence on spring movement , how much negligible it may be.

Regards

Habib

SHR,

You are confused. The effect of presure elongation is primarily on straight pipe. The dominant effect of pressure on an elbow with a slightly oval cross section, is to open it out. Thus the effect of pressure on a piping system might be, that opening of an elbow has a marked effect on, say, a long piece of connected pipe and its end displacement.

Do a little test for yourself with a simple geometry and change the 'Activate Bourdon Effects' settings.

Dear MoverZ
I am sorry to differ.
Please have a look in attached file.I made two file with two different pressure.
To get same spring load we have to change Toll setting pressure stiffening "No"
For default setting or if set "Yes" caesar will change elbow flexibility factor as ( pressure dependant) per Appendix D B31.1/B31.3 note-7.

Spring work as per stiffness property , So if piping stiffness property changes spring must react on changes.

Regards

Habib

SHR,

Try to stick to your story. Above you said and I quote ...

"In my openion spring movement will be constant irrespective of pressure variation when Bourdon effect & pressure elongation on bend both are switched off."

I said you are wrong, spring MOVEMENT may be affected by pressure.

Now you are talking about spring LOAD.

You are WRONG about spring MOVEMENT. I agree that spring LOAD may not change (assuming it's a constant spring).

Hi MoverZ
Yes I made certain error on the statement.
I was trying to respond Grand query to get unique spring load( load or movement ) result by changing caesar setting.( for different pressure value)
Instead of pressure elongation I had to tell "Pressure Stiffening" (at caesar setting Tool-Computation control).


Spring load is directly related to spring movement (vertical) so there should not be any issue with referring movement or load.


Regards

Habib

Hi Grant

If you wish to see no change in spring load/movement for changing pressure
you have to set "Pressure Stiffening" (at caesar setting Tool-Computation control) to "NO" in case you are using metallic piping with caesar default setting.

By default caesar consider
1)Bourdon force off for metallic piping
2)Pressure Stiffening as per Appendix D B31.1/B31.3 note-7


Regards

Habib

Good grief SHR, don't you read anything ?

You are may be wrong in stating .....
"Spring load is directly related to spring movement (vertical) so there should not be any issue with referring movement or load." We all know what Hookew's law is. But what about constant force springs as I mentioned ?

Furthermore, saying ...
"If you wish to see no change in spring load/movement for changing pressure" ....

In creating a stress analysis model, you should be looking for realistic representation of what might actually occur in a piping system, not finding ways to hide or ignore effects.

Hi MoverZ
Yes I could have mention that load , movement relation applicable for variable spring only. constant spring as it state always constant load.

Regarding "realistic representation" I like to say realistic approximation representation.

Caesar default do not consider bourdon effect, does not mean that effect is not present there for metallic piping, I like to consider it under approximation.

Code (B31.1/B31.3) ask Appendix D note-7 for Large bore thin wall thickness pipe , but I think Caesar consider it irrespective of pipe OD & thickness. I don't see we need to be very serious of it.

I have asked to set "Pressure Stiffening" -"No" just to cross check how variation of load came into picture, not to apply in real analysis.


Even if somebody use that, still can argue for pipe diameter & thickness limit because Code has not specify clearly the limit.
I already cross check, it is not going to create huge error in calculation.

You are right to say we should not "finding ways to hide or ignore effects." but still can go for realistic approximation , as long as we understand what assumption we are considering & what could the possible impact.

SHR,

I have dealt with re-analysis of large bore gas transmission piping, where the design temperature was 50 deg C and a stupid engineer thought it ok to ignore pressure elongation. It was thin wall, high yield pipe and the thermal equivalent of pressure was about 25 deg C. Consequently expansion and thrusts were grossly under-estimated. Clearly pressure elongation and thermal expansion are not the same, but ignore pressure effects at your peril.

Returning to your latest missive ... Deliberately introducing an error is ok provided it's not a huge one ? That is a bold judgement. How big is huge in that sense ? To paraphrase a famous WW2 air vice marshal, there are old stressmen and bold stressmen, but no old, bold stressmen. Think about it.

Hi MoverZ

Thanks For sharing your practical experience.
I appreciate if you share more information on that gas piping regarding diameter , thickness , pressure & what exactly happen at site (accident?).
I think it could help our forum member.

Thanks & Regards

Habib