Hi all,

I'm modelling a tied rubber bellow near a pump nozzle; I modelled the bellow as for Application Guide (tied bellow simple model), with restrained cnodes in axial and along the two bending moment direction.

When I look at the restraint summary I see that in the sustained case all the load from pressure thrust (effective area x pressure) is absorbed from the modeled rigid element with the restrained cnodes.

When I look at the operating case I see that the expansion of the pipe causes large shear load at nozzle: it seems that the model of the tied bellow it's not capable to absorb the laterl movement of the pipe: it's this a realistic behavior of the EJ?

The tie rods, in the reality, do not permit relative lateral movement between the two pipe connected from the EJ???

TY.

Define your tie-rod cnodes with 'guide' restraints. That will ensure the joint componenets remain axial.

You could model a gap in the guides if there is a small degree of allowed lateral movement.

OK MoverZ.

Thanks.

I’m going to contradict MoverZ suggestion here. If you are modeling a tied rubber joint it should be capable of lateral movement and NO axial or angular displacements. A “guide” restraint will prevent the lateral movement. From your description, it sounded like you modeled the joint correctly. The high shear force at the nozzle should be the result of the joints lateral displacement. You can check your model by multiplying the lateral displacement in the operating case times the joints lateral spring rate.

I modeled the rubber EJ as for apllication guide; lateral displacement is allowed as for reality.

Dan, can you tell me in wich way the TIED EJ is capable to absorb lateral movement? maybe some link to images or explanations will be valued.

TY.

A tied expansion joint will deflect laterally due to shear as depicted by the attached sketch. While the sketch was developed for a metal joint the principals are the same. You may also want to download a catalog from a rubber EJ manufacturer.

I mean, how a Tied EJ (with 3 tie rods) can permit lateral displacement?

The tie rods will pivot at the lugs. The illustration I like to use is to take 3 or 4 pencils of the same length and press them together with the palms of your hands. You will see that you can only move your hands laterally.

Thanks Dan.

Best regards.

Daer Dan

When you model tied expansion bellow in caesar there is absolut zero axial flexibility hence huge thermal axial load will be there.
In practical purpose cooling water line will not have much thermal expansion.
One of the well known engineering company we use to put tied bellow in pump nozzle then put limit stop support after the bellow.
Concept was isolate pump nozzle by limit stop. End caesar model at limit stop Considering the fact that very minor expansion in between pump nozzle & limit stop will be taken care by bellow & flange gaskets.
Mind it this kind of concert to be applied only for cooling water line or low temperature line. High temperature line can not be conceptually isolated like this.

Regards

Habib

Dear Sir,

The system you are describing is NOT a tied EJ, it is an unrestrained EJ with limit stops which will absorb movement in any of the three axis. It will also expose the system to the longitudinal pressure thrust. So, it would be acceptable in low pressure systems.