Sometimes we come across support comments on piping isometrics like zero gap guide, +/- 0.75" gap line stop etc. But, without any tolerances specified for these gaps of 0" or 0.75", what tolerance piping fabricators are expected to achieve for hot process piping connected with sensitive rotating equipments ?

regards,

sam

In general, pipe fabrication tolernaces are given by the Pipe Fabrication INstitute (PFI) document ES-3.

Supports, being structural steel, are generally fabricated under the rules of the AISC Code. Consult your project's structural engineer for more details.

Dear,

Thanks for the answer.

But, you might have misunderstood me.

The background of my query was that, our arm-chair digital simulators very often design such sensitive gap restraints in Caesar-II, it becomes impossible to erect in reality & any changes in gap plays havoc with resulting equipment nozzle loads.

But, being engineers, they are expected to have hands-on knowledge of the items they simulate! Instinct, intution & morality can guard against this type of mistake.

regards,

sam

In our practice when we do some very sensitive equipment like compressor or turbine when zero mm gap or 3 mm gap of support make really huge difference, nozzle allowable load point of view we generally use rigid strut to avoid that kind of problem.
To ensure exact zero mm guide of limit stop gap at site.

Hi MR. shr, I believe that 3 mm for Mathematical Execution really "make huge difference", but for Site performance, is not complete accuracy, this is the Tolerance for the support (Avoid interference, weldings, corrosion, craps, etc..) because in construction is very hard to apply it like the sensitive analisys require, I think that we should not confuse, Gap with the Tolerance. 3 mm is = 0 gap with 3 mm tolerance.

Regards

Dear Adan
I agree with you. It is really difficult to ensure at site (with a conventional guide or limit stop support) to make exact zero mm or 3 mm gap as marked by stress engineer in stress isometric.

Most of the support we are not bother of tolerance of 3 mm. So site tolerance is absolutely acceptable.
But we can not take that tolerance for very sensitive equipment ( like compressor or turbine). In that case we recommend to use rigid strut instead to have exact zero guide or limit stop closer to equipment nozzle.

I also encountered a problem with regards to 3 mm gap. I have a Control Valve assembly with size of 36". When i checked the guide loads on the trunnion, the load is 160,000 N. But when i remove the guide, The displacement on the trunnion is only 2mm. So meaning if there is 3 mm tolerance, in actual, there will be zero guide loads. Your thoughts?

Thanks

Have you checked the forces resulting from friction. If the displacement is only 2mm and you have a 3mm gap, the forces in both situations result from friction only and should be the same in both cases.

JR park,

The mathematical model is an approximation of real world problem ( however I do admit that we are maybe sometimes legally or contractually obligated to stick to the results of an analysis).I would advise, use your engineering judgement here.In my humble opinion qualification or disqualification for systems ( or increase or reduction in load) with change in gaps like 1 mm , 2mm etc. is meaningless and that is where the analysts judgement and experience should play an important role.One has to understand the limitations of the mathematical model ( say the limitations of a a 6DOf based beam element),supplement with finer analysis ( shell elements , beam elements with warping degree of freedoms etc)wherever necessary and at the end use your judgement, experience and advise from more experienced persons.We are fortunate enough to have strses engineers like John Luf,John Breen, Dave Diehl , Richard Ay( to name a few)in this forum and I would personally trust their view in contrast to some numbers from the program, any day.

Regards