Dear all
Some time back I was working with client who makes typical plants. There were 3 water pump discharge line ( see attach sketch ). I tried with one tied rubber bellow at pump discharge nozzle but could not manage pump nozzle allowable load . Line design temperature was 45 deg C. So I suggest one more tied rubber bellow at vertical portion of line ( see attached sketch , note additional rubber bellow ).
My client was reluctant to accept my design since they continuously installed typical pump line with one tied bellow each at pump discharge nozzle. We force them to accept new proposal since we could not produce a nice stress report with only one tied bellow per each discharge line.

Now question do we really require that additional bellow ?
I have worked with another very established design company who put ( rest +
Guide + axial stop ) support just after tied bellow same location where I presently marked rest +guide support in attach sketch.
I know we can not put pump nozzle , tied bellow reducer & then axial stop support in same line.
Caesar report also show tremendous high axial load in this case.
But new client they are quiet confident that pump will wotk fine with same configuration since so many typical line they already installed is working fine.

One point I am thinking now. It is a simple cooling water discharge line. No practical problem I am also expecting with that configuration even though I can not make a good caesar output report . I definitely will not do that thing for pump which is working with high temperature.

Now to prepare stress report I am making following assumption.
1) Very small axial deflection bellow can absorbed , may be by laterally deflection. ( no much theoretical )
2) While modelling tie rod in caesar I am putting X restraint with gap 1 mm. ( pump nozzle is in X direction )
3) Since 1 mm restraint gap caesar will show pressure thrust I remove effective Id of bellow.

That means I am considering a tied rubber bellow which can absorbed very small amount of axial movement also. Calculated axial movement is also less than 0.3 mm between pump discharge nozzle & first axial stop support.

So I like to know what is the opinion of Expert . Specially I request “JOUKO “ to put some light on the issue.

I will leave the answer to this question with JUOKO. I only have one general observation.CAESAR II will use PXA load at both ends of the bellow irrespective of whether it is tied , untied , hinged, gimbal etc, unless the analyst makes the effective ID equal to zero. The PXA load should be applied at all change in directions ( elbows, pump casing etc) and depending on the relative stiffnesses of the different components of the piping system,the actual PXA loading on the different elements can be checked. In case of tied bellows which nuts locked the PXA should not be of any concern as the nozzle loading will see the same PXA loading as with a rigid pipe. In case of untied bellows, the nozzle load will see minimum PXA load withy max. load going to pump foundation . CAESAR II however will show high axial load( caused by PXA ) in all cases unless the effective id is zero and the loading ie PXA is applied at correct locations.I find many analysts overlook this point.



Regards

Dear shr

Out of curiousity only. Would it be of any help for the pump nozzles to put guides on remaining three rests beside the gate valves and taking out the dead weight of the upper piping by two spring hangers on the header?

regards
Siddharth.

Yes Siddharth
adding spring naturally improve flexibility. But do we required any more flexibility or not that is the question.

First thing with all bellows is to make sure that the modelling is correct. Especially if these theoretical calcs are taken to extreme. Couple items to look at:
- for rubber bellows you have to enter all stiffnesses. If you do not do so CAESAR II calculates one or the other and it is not correct as the buil in formulas are for metal bellows.
- model tie rods using correct diameter, temperature and material. CAESAR II may calculate even temperature expansion etc. I have not done too many tests but better to be save than sorry. Include into the model pump stiffness. Nothing is rigid. Typical case of hair splitting can be seen on my site worksamples - www.jat.co.za. There is one metal bellows with 2 tie rods side by side. Plant owner's designer figured out elongation of the rods was too much for the pump's nozzles. Only way was to add rods. Bellows were added in a first place because they could not build big enough concrete foundations to hold the pipe forces. My limited brain tells me that if the foundations cannot hold the pump then pump cannot be modelled as rigid point and therefore minor elongation of a rod is absorbed easily.
- double check what CAESAR II is doing with the pressure thrust. Nozzle should see as maximum the bellows pressure thrust less pressure thrust calculated using pipe ID.

In your case you have tie rods so the pump doesn't see bellows pressure thrust "directly". In your sketch you have horizontal section. If it expands you will see increasing axial force as the expansion is resisted by the vertical pipe (no compensators). This force is increasing until it reaches the full pressure thrust and then the rods are loose. Now you have untied bellows. See above for the nozzle load. If you have rod nuts on both sides of the flange/bracket situation is different.

If a pump manufacturer says that his pump is made out of paper and can take only small forces then ask them to give pump stiffnesses including foundations. Enter into CAESAR II and you will get different results.

If a calculation says that pump fails but there is an existing installation that works I scrap the calcs as I consider them wrong. Somebody calculated that a bee cannot fly. Last time I saw one it was happily flying.

Dear Jouko
Thanks a lot. I was eagerly waiting for a suggestion & advice from you.
Your comments give me a fresh thought & confidence.
Regards
Habib