Hi,everybody!
there is some trouble in designing and even modeling an expansion joint for me.

An expansion joint with some tie rods which be used to balance the pressure thrust in technique manual,but ,in my opinion,these tie rods can't carry the pressure thrust ,beause when the pipe system is start-up and running and the temperature is increasing ,themal growth,now the pipe will expand and the bellow become short,but the tie rod is the same length as before,how can it carry the pressure thrust.
how does the tie rod work truely?

i have the similar question in the lateral expansion joint with tie rod,when the movement present , how the tie rod work to meet with the bellow which is able to be offset or rotational,does the tie rod keep the same length as before?

thanks!

Hi Lanxuedream
1)Pressure thrust try to open up the bellow,
2)Thermal movement try to open up or compress the bellow depending on piping configuration.

Both above activity will be fully restricted by Tie rod of bellow. Bellow vendor will deisgn tie rod according to force expected. Don't worry about it, just pass the proper information to bellow vendor.

For your second question check bellow vendor catalogue, you may need for tied bellow, hinge bellow, Gimple bellow or universal bellow.
All the bellow should specify maximum permissible offset,angular rotation.


Regards

Habib

Lanxu,

You are on the right path. I wouldn't pay attention to SHR's comment to "[not] worry about it." Expansion joints, in my opinion, are not commodity items to be glossed over.

Assuming the pressure thrust exceeds the forces developed from thermal expansion, the restrained expansion joint will only "absorb" the thermal movement from the length within the limit rods. The limit rods will be in tension and will restrain the pressure thrust. Ignoring failure of the tie rods, the piping system does not need additional restraint for pressure thrust because of this joint.

If thermal expansion overcame pressure thrust, the tie rods would no longer be in tension. This becomes, effectively, an unrestrained expansion joint until the axial compression returns to 0. Here you do need restraint on the piping system to control pressure thrust.

Peng covers this well in Chapter 7 of Pipe Stress Engineering.

Hi Steven
Limit Rod & Tie rod you mention above is entirely two different items serve different purpose
1) Limit Rod --use in untied expansion bellow just to protect bellow from overstretching it, It is not meant to take care of pressure thrust, Bellow still work as untied bellow. Seperate restraint need to use in piping system in case thrust force is huge

2) Tie Rod-- Use in Tied expansion bellow to take care of pressure thrust,No seperate special restraint required in piping system because of pressure thrust. Generally nut is tighten in both way to restrict both compression & tension of expansion bellow.So pressure thrust & thermal load balancing concept will not be much useful in this case. Tied bellow is not to allow any axial tension or compresson.


Some time people try to go with concept to restrict bellow in tension for pressure thrust but allowing thermal compression by removing inner nut of tie rod. This a very dengerous option need very careful thought & expertise to do go for that option.

Tie rod design ---Leave it to venor ( Number of rods & diameter)


Regards

Habib

I did carelessly interchange limit and tie rods.

But, I think that we agree that traditional bellows expansion joint cannot take both axial growth and pressure thrust simultaneously. That is the first question lanxuedream asked.

hi shr
thank you for your interpreting!
but i still can not understand that the tie rod as you say can carry both pressure thrust and thermal movement.

as you say ,one tie rod have two pair nuts which are located at two ends,and even one pair nuts which are tightened at outside and inside,so the tie rod , the vetical plate and the pipe will become together which is as sufficient rigid as pipe ,how can it have the flexibility to absorb the thermal movement?

the EJAM code is clear for this tie rod and limit rod, but proballly not clear for control rod:
contorl rod: To distribute the movement between the two bellows of a universal expansion joint, Not designed to restrain bellows pressure thrust
limit ord: To restrict the bellow movement range (axial, lateral and angular) during normal operation; In the event of a main anchor failure, they are designed to prevent bellows over-extension or over-compression while restraining the full pressure loading and dynamic forces generated by the anchor failure .
tie rod: To continuously restrain the full bellows pressure thrust during normal operation while permitting only lateral deflection .Angular rotation can be accommodated only if two tie rods are used and located 90 o opposed to the direction of rotation
my idea is contorl rod and tie rod need defined clear by vendor

Hi Lanxuedream

See the attachment.

Regards

Habib

I am not too fond of any rods being under compression. OK "vacuum design" is an exception. In most cases where rods are used it is best to have lateral movement in the bellows and make sure that the compression doesn't exceed the pressure trust.

I have seen a design where there were 4 about 8 m long rods and the pipe designer had specified 2 rods on compression and two on tension...

Tied Expansion Joints required to accept large amounts of lateral movements. Because tie rods are provided to resist pressure thrusts, these joints cannot accept axial movements. Also, if more than two tie rods are used, angular movements are not accepted. The tie rods are usually at or near ambient temperatures and therefore do not expand and contract as a function of the temperature of the media within the pipe. As a result, the thermal expansion of the length between the tie rods occurs within the expansion joint and is not applied to the rest of the piping system. The bellows design must accommodate this axial thermal expansion as well as the lateral movement for which the expansion joint is needed.

Hi Sca

Your statement is contradictory
1)***Tied Expansion Joints required to accept large amounts of lateral movements****

Disagree, Tied bellow is applicable for some specific ( As mention by vendor catalogue) amount of lateral movement. In case you required large amount of lateral movement you need to go for Tied Universal bellow or Two hinge/tied bellow in a line.

2)***Because tie rods are provided to resist pressure thrusts, these joints cannot accept axial movements.****

Yes tie rod design to take pressure thrust. It tighten properly not to allow any tension/compression movement from thermal effect of piping or pressure trust. Two affect is not related as you stated above

3) ****As a result, the thermal expansion of the length between the tie rods occurs within the expansion joint and is not applied to the rest of the piping system***

We suppose to use bellow to improve piping system. Not piping system to improve bellow.

4) ***The bellows design must accommodate this axial thermal expansion as well as the lateral movement***

Tied bellow design to accommodate lateral movement, not axial thermal expansion/compression


Regards

Habib