Tied Expansion Bellow tied on outside only.

[uzairsahmed]

I am given to understand that tied expansion bellows do not take any axial load and only lateral loads. But what about the bellows which have tie rods with nut on the outer side only. On the flange side and not on the bellow, this would mean that the tie rod is engaged only when the bellow expands and apparently seems to allow axial compression. Does such a model allow axial compression while restricting the accompanied pressure thrust force of an expansion bellow?

Some manufacturers have tied bellows with four nuts on each tied rod( nuts on both sides of tie-rod to lug/flange, on the outer side and on the bellow side) and of two nuts on each tie rods. Would the two nut one allow compression? Based on my discussion with an engineer, he said that when the pipe would expand, tie becomes ineffective, and pressure thrust force is engaged by the piping which pushes it back. Wouldnt this restrict/ hinder expansion?(same as the purpose of the bellow?

Also.. the application in which i am trying to use this bellow is to reduce the thrust force on a plate type heat exchanger which is an anchor adjacent to the bellow. I have seen an existing Plate HX with two nut tie rod tied exp bellow installed axially? (right net to nozzle)I havent been able to model this in either SIMFLEX(which doesnt have this option) and in CAESAR( which has this option).

uzair

 

[rgvrider]

Four bolts on a tie rod as you explain make the expansion bellow a rigid member!

Two bolts will theoretically allow compression and not expansion. However the thrust force is usually sufficiently high that you still need to restrain (anchor) the pipe both sides of the expansion joint. Therefore the pipe was never free to expand in the first place. Hence Tie rods serve no real purpose on an axial expansion joint.

Always restrain the pipe through pipe supports, such that the movement is exactly where you want it. Then solving the problem of expansion is very easy.

 

[rmw]

Tie rods DO serve a purpose on axial EJ's. They prevent the pressure thrust from being transmitted to the adjacent piping or equipment.

As described in the OP, an EJ with nuts only outboard of the tie plates are free to compress if there is enough force to overcome the pressure thrust plus the spring rate of the bellows. With piping you might not care, but with pumps and/or vessels, you may not want that kind of force on the nozzles.

An EJ with 4 nuts that are tight against the tie plate can't expand or contract or move laterally, so it is pointless to do this. If you have 4 nuts, there should be some free-board between the nuts in order to allow lateral movement. Remember that in pure lateral the tie plates move closer to each other as the ends move laterally.

I suspect when you have seen 4 nuts, they were limit rods, not tie rods.

rmw

 

[uzairsahmed]

I understand that the 4 nut one,in which the bellow isnt allowed to either compress or expand behaves in pretty like a rigid structure. Such an expansion bellow (tied completely ends) is installed in the direction perpendicular to the direction of expansion and the bellow works 'laterally'.

What I dont understand if is whether the case discussed above does help or absorb axial expansion of the pipe or not? And we are taking about a case where temp is less compared to pressure. (70 degree vs 9 bar) and thrust force is significant. So even when the pipe expands or "tries" to expand the tie rods become ineffective since the bellows would compress and pressure thrust force would come against expansion force.

To explain the exact scenario ( and this is modeling an existing situation to see its affect). the force on the plate HX's nozzle comes out to be 4 kN axially with bellow. with bellow without tie rods its 18kN ( due to pressure thrust), and with completely tied exp bellow it remains 4kN.

My question is, would the model presented above.. the one that allows compression..would the force on HX nozzle reduce below 4kN? or would it not since the pressure thrust force wouldnt let it accomodate the expansion of the pipe which would get transfered to the HX nozzle(my two cents)? I am trying to understand the differnce in the working of both forms of tied exp bellow

 

[uzairsahmed]

and also.. as one can visualize, the higher temperature difference (for expansion) the better effective the joint would be axially to work against the pressure thrust and wrk better. I was told by a senior engineer that there was a graph of linear relationship between temperature and pressure that shows in what cases of temp values vs pressure would a bellow be effective to use.. all points below this line(less pressure) and bellow is safe. if above ( more pressure) the bellw joint wont move.

I havent been able to find such a graph. Is anybody aware of such a graph?Thanks

 

[uzairsahmed]

Can someone please reply?

what is the essential difference between these two forms of tied expansion bellows?

 

[vpl]

Was there something wrong with rmw's response? It seemed clear to me.

Patricia Lougheed

******

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

 

[rgvrider]

-rmw

Tie bars are only there for ease of assembly. To overcome the pressure thrust and spring rate of the bellow, both sides of the pipe has to be anchored. Therefore the bellow could have been installed without tie rods - ie it is free to compress, but the anchorage of the pipe will stop expansion, hence the tie rod is not required.

I've seen tie rods cause many more problems than they've solved, mainly due to people not understanding their function (four bolts is a classic path to failure).

 

[rmw]

rgvrider,

Shipping bars (braces) are there for ease of assembly, tie bars are there to resist pressure thrust. Get yourself a current version of EJMA (Expansion Joint Manufacturers Association) and read it for yourself. It is their definition, not mine. But I do agree with you that the basic misunderstanding of the function of what type of rod that is on the joint causes problems.

What you say otherwise is not wrong. If you have an EJ in an anchored pipe system, then the pipe anchors react all forces, pressure thrust and spring rate if it is not tied.

Unfortunately, all piping systems are not anchored (sliding supports, etc.) and not all devices connected to EJ's are anchors. PHE and pumps are two things that come to mind real quickly that don't want to be acting as pipe anchors and reacting pipe expansion or EJ thrust forces. Pressure vessels are another. I just finished a project where the EJ's that attached to a pressure vessel would have imparted the equivalent load of an 18 wheeler standing on the vessel if the EJ's hadn't been tied so that the tie rods took the pressure thrust off the vessel.

The rods on EJ's are sometimes (a) tie rods, (b) limit rods, and a couple or more uses that I can't remember right now. (My EJMA is at the office and I am home.)

Go to a reputable metal EJ's website. They usually have a good tutorial on all the different types and styles of EJ's.

Here is one to start with.

http://www.hosemaster.com/products/expansionjoints/options.php

And another - pay attention to the column that denotes whether or not pressure thrust is imposed on attached piping;

http://www.hosemaster.com/products/expansionjoints/comparisonchart.php

And another one with a good explanation;

http://sfpathway.com/Products/MetalExpJoints/tabid/60/Default.aspx

And to the OP, I am not familiar with such a chart as you describe it. Sorry.

rmw

 

[rgvrider]

rmw,

I believe I understand exactly what you are saying. I agree that tie rods will resist thrust movements.

My question is; in what system can you require the use of a thrust restrained expansion bellow, that also has a requirement for axial compression?

Tied bellows are useful for taking lateral and angular movements. However I would much prefer to see a hinged bellow or a bellow specifically designed for this. Too often a tied compression bellow is installed because they allow some angular and lateral deflection.

I have seen tied bellows installed that were designed to take axial compression (too many cooks, below installed for compression, header not restrained so better tie the bellows).

 

[rgvrider]

no idea if I can edit posts....

http://www.hosemaster.com/products/expansionjoints/comparisonchart.php

is a very useful table.

I see the tied bellow takes lateral movement. I doubt they can really take much lateral movement though (length/diameter dependent).

As a general mental note, I think I'll stop referring to bellows as 'expansion bellows'. This may well be their purpose in some instances, but I think it adds to the confusion.

-rmv
Do you think it would generally be cost effective to place tied bellows on pumps or pressure vessels with large diameter pipes (say DN300 and up). My thoughts are that for any reasonable lateral movement (say 5mm) the bellow tries to accommodate, it would have to be significantly long which would increase the price. Would it be not better to design the pipe to move else where?

 

[rmw]

rgvrider,

I can honestly answer your question - I don't know. The only case I can think of is where the rod is not actually a tie rod (there to restrain pressure thrust) but a limit rod - there to limit over-expansion of the EJ (Or over compression if there are nuts to the inside of the tie plate as well and I think that type of rod is defined as a control rod by EJMA.)

Unfortunately, the term "tie rod" is often used to refer to any and all of the above.

If you have the situation you asked the hypothetical question about, then I think you are into the category of a pressure balanced EJ where the pressure thrust is offset by the balance piston and all the expanding equipment or pipe is subjected to is the spring rate of the bellows. The tie rods there only take the pressure thrust forces and distribute them accordingly but the joint will still compress without the equipment seeing all that pressure thrust.

The equipment my company makes uses several varieties of balanced EJ's (as well as tied EJ's, tied expressly for the purpose of keeping the 18 wheeler's worth of weight off of the vessel nozzles) and gimbals, hinged only and slotted (plus several fabric versions.) Some of them I can walk through standing straight up (and I am talking about the metallic ones.)

Regarding your comment about a hinged (or a gimbal joint for that matter) they only allow angular movement (ignoring slotted versions), so if any significant lateral is needed, then you are into a universal, tied or untied as appropriate.

And back to the OP's last question, yes, if you had a tied bellow, and either the PHE or the pipe expanded into it so that it moved away from the outer nuts on the rod, (toward the inner nuts assuming that they are far enough apart) then the forces imparted onto the devices connected to the bellow have to react both the pressure thrust and spring rates.

Depending on how you design your anchor away from the PHE on the other side of the bellow will determine whether or not the forces are imposed on the PHE nozzles. And I would add PHE nozzles to my list of equipment above that doesn't like pipe loads imposed on it.

I have to design piping for a PHE soon, and my EJ's will be tied and the anchors of the pipe will be such that the support on the other side of the EJ from the PHE will slide and be anchored somewhere else away from the PHE.

And, BTW, EJ's are not good at transmitting pipe weight to the anchors, so don't ask them to do that either unless you have specifically designed them for that purpose. Tied, 2 rods only and in the vertical plane, nuts each side of the tie plates. Such a joint would be good for angular movement only and I recommend spherical washers inside the nuts (finger tight only too). The bellows in this case has to have a stiff lateral spring rate too.

rmw

 

[uzairsahmed]

rmw,

Firstly thankyou for a such a detailed respnse and explanation on the matter. I agree with you both that this topic is fairly misunderstood and thats why many engineers always prefer expansion loops.

1) They way i understand from your discussion, the tied bellow (2 nuts only) wouldnt help reduce the anchor force on our equipment since if the bellow compresses (due to pipes expansion) it would transfer the force to the PHE or pump and this situation would be the same as bellow installed without tie-rod right?

2) When you would use tied bellow for ur PHE would you install them in a direction perpendicular to the nozzles?

3) you mentioned that the loads on the nozzle for 2 nut tied bellow would depend on how the "other anchor , on the other side of the bellow is designed". Could you explain if there is a way i could reduce thrust on PHE by altering the design of the other anchor?

4) In the case where temperature rise.. or expansion force of the pipe is not enough to overcome the pressure thrust and in the spring constant, the tied bellow would behave like a rigid element in the axial direction right?

5) for Tied expansion bellow to work laterally, is it necessary for it to be a two nut tie rod?

6) I understand that for tied bellow (2 nuts) we should provide anchors on both sides caz the pressure thrust can be engaged if the pipe expands enough to engage the thrust (which also makes me ask..if this situation is equivalent to bellow w/ tie rods.. why not jst use bellow w/o tie rod here?), the anchors would be required for 4 nut tied bellow which would not transfer any thrust tothe piping?

 

[rmw]

Uzairsahmed,

I will try to answer by the numbers.

1. Right. Well said.

2. Not necessarily. It depends on your piping arrangement, location of anchor points and the thermal expansion requirements of the PHE itself.

3. By using a sliding support for the pipe connected to the PHE and anchoring away from the nozzles with another EJ in the piping to permit the pipe to slide where the expansion wants to take it. Without knowing your piping, it is difficult to get any more specific than that although I will soon be faced with figuring that out for myself.

4. Correct.

5. Yes and no. Two nut would work as described, or if you had 4 nuts, then the inner nuts would have to be backed away from the tie plate if you wanted the joint to take any compression (against pressure thrust and spring rate as you noted). The inner nuts would serve as limiters if they were needed. If they weren't needed for that purpose, then they would be unnecessary.

6. Sorry, but I just don't understand this question. I have an application with a 2 rod, 2 nut tied universal joint, rods in vertical plane that is taking significant lateral in the horizontal direction, slight lateral in the vertical direction, several degrees of angularity and some torsional movement concurrently, but absolutely no axial. The axial movement of the equipment that it is attached to is taken up in pipe movement (sliding) which is permitted by another 2 rod, 4 nut, snug fit, vertically oriented that does nothing but permit angular motion to account for the pipe spool sliding at the other end while supporting the weight of the spool across it to a pipe support and anchor. I don't know if that answers the situation you described or not.

With a PHE to connect to piping subject to expansion, your goal is to keep piping loads off of the PHE. Either the PHE has nozzles that are not capable of a lot of pipe loading or it has nozzles that are studded into the frame plates and you don't want to put any pipe loads there because it can cause moments which will deflect the PHE frame plates and cause leakage.

So our choice is to anchor the pipe immediately on the other side of the EJ from the PHE nozzle and let the EJ take the vertical and/or horizontal growth of the PHE or anchor the pipe well away from the PHE and let the PHE push the pipe where it wants to go via sliding shoes and combinations of EJ's to permit the movement.

I hope all that is understandable.

rmw

 

[uzairsahmed]

rmw,

thanks for all the help man. Really appreciate it! i will ask a few qs i have had on the points discussed.


3) By this you mean that the other anchor would slide to take the expansion of the pipe into account and reduce the load on the other anchor?

6) what i meant was that a 2 but tied expansion bellow would work as without tie rods so hence this would need anchors on either side of the EJ. but with four nuts on tie rod this wouldnt require anchors on both sides of EJ right?

The application you have explained is an 'L' shaped configuration with tied EJ on bth legs on the L, right?

the first solution you have proposed for PHE would be for tied EJ? This is what i have tried modeling it like. the tied EJ installed in the longer arm of the L where shorter arm ends into the PHE. the expansion of the pipe and PHE would be taken laterally by the tied EJ.

 

[rmw]

Usairshamed,

3. Yes

6. If you are firmly anchored either side of an expansion joint, and the pipe is supported so that it only applies axial expansion to the EJ, then you don't need the rods unless, movements can exceed the allowable expansion of the EJ. In that case, the rods limit the movement of the EJ. So if you don't need that, you don't need the rods.

rmw

 

[uzairsahmed]

rmw


thanks for all the help man.. it has helped clarify many concepts.

Uzair