Hello,

I already read different post regarding expansion joint thrust force on this site, but still have unclear thoughts regarding the correct way of modeling the thrust force.

I have an axial expansion joint directly connected to the nozzle, upon reading the result, the loads on the nozzle exceeds the allowable due to the added pressure thrust of the expansion joint. As I have read the thrust force will be transmitted directly to the equipment internals/support leaving the nozzle with minimal effect, I would like to ask what would be the correct way to model it?


If we leave the effective ID of the expansion joint to zero, then no thrust force will added to the nozzle load upon nozzle evaluation in Caesar. The thrust force going to the equipment can be submitted to the vendor by manually computing the thrust force, which is Pt=P*pi*Dm²/4, where Dm = mean diameter of bellows.

If we do this (leaving the effective ID of the expansion joint to zero), then on the other side of the expansion joint, opposite the nozzle, the thrust load cannot be considered to the pipe support/anchor design. So in order to compensate for this, should we include an addition force of F2 (which is Pt=P*pi*Dm²/4), directed away from the nozzle towards the support and added to the weight load as W+P1+F2?


Also I have read that the nozzle will not be totally free of pressure thrust load, but will have an equivalent pressure thrust force of F1(which is Pt=P*pi*(Dm²-d²)/4, where Dm=mean diameter of bellows and d=inside diameter of nozzle).

I am hoping can anyone help me understand the correct way of considering these effects.

Would it be correct to input those F1 and F2 forces as an additional force and add it to the weight case as W+P1+F1+F2?

Please see my attached file, F1 would be placed at A and directed towards the equipment (for nozzle force consideration only), and F2 will be placed at B and directed towards the pipe anchor.

Thank you all in advance.

It appears you have the concept correct.
One point, CAESAR II places the full thrust load on either side of the XJ. This is an adequate model for the most part. Your concern here points out that it is not always good enough. But you may want to also consider the point where you apply F2. The surface on which most of the pressure occurs is the elbow in your illustration.
If I say that the pressure thrust is equal to pressure times the effective bellows area (Ftotal), I could say that Ftotal = F(pipeID)+F(differential); where F(differential) uses the difference in area between the bellows effective diameter and the pipe ID. In this case F(differential) would be applied at A and B and the F(pipeID) would be applied at the equipment anchor (a simple increase in your anchor load with no effect on the other results) and also applied at the elbow. [With an anchor on either side of your XJ, I see no reason to go to such detail here.
These pressure thrust forces are everywhere in your system but axial pipe stiffness makes them trivial - except here where the XJ, itself, may allow axial response.]
Also, when calculating and locating pressure thrust you would also find a pressure thrust surface at any ID change along the line.
And, if your F1 and F2 would always be used together, both loads could be identified as the same force set (e.g., F1).

Hello Dave,

Thank you for this information.

Another point of my concern is the nozzle loads.

For example, I have an axial load on the nozzle for Fx = 10kg (without thrust load) and my allowable is 80 kg.

If Caesar II computed a pressure thrust load of 100 kg, then the total loads on the nozzle would be 110kg which now exceeds the allowable value of 80kg.

This is the reason why I go for such details of computing the actual thrust load effect on the nozzle, as I read that the full thrust load will not be experienced by the nozzle but rather go to the equipment's support.

Can you suggest a way to work around this or is there an option for Caesar II to not include the thrust loads on the nozzle loads computation?

Thank you very much.

If you do not wish to have CAESAR II place the P*A(effective) load on either side of the XJ, then do not enter an effective ID in the XJ input. In this case it would be up to you to set the pressure loads where you want them (as we discussed above).

Just to chime in as I have done more expansion joint then I would like...

On one particular job, I calculated the full thrust force in CAESAR II by setting the ID of the EJ so the program would calculate the force. Then in the results via excel I "corrected" the results for pressure thrust by removing the pressure times the open area of the nozzle. This gave very satisfactory results while maintaining all the forces in the pipe.

Thanks,
Duncan