Maximum gap between flanges before bolt up

[valero]

Where would I find the maximum allowable distance between piping flanges prior to bolt up? This is assuming no spacer will be installed between the flanges. I know this is a matter of piping fit-up, but I need to know in what code and where I can find the maximum allowable distance. I first thought B16.5 then B31.3, but I can’t seem to locate the answer. This is in boundary petro refinery piping.

 

[weldtek]

I'm not sure if this is what you're getting at but you might look at B31.3, Par. 335.1.1 (c.) This paragraph deals with alignment prior to flange boltup.

 

[valero]

Thanks for the reply. I have seen that section in B31.1 it is pertaining to alignment of flanges and alignment of bolt holes. The measurement I am looking for is the allowable distance between the flange faces. At a minimum you would have to allow for the thickness of the gasket, and be assured you will get the proper amount of crush on the gasket to seal at the desired operating pressure and temp. If you have too much distance between the flanges you will not be able to get a seal because the bolts will only tighten and bring the flanges together to the point of yield where they begin to stretch.

 

[deanc]

If your piping/application is that rigid it many not be the best place for a flanged joint.

 

[valero]

Let’s assume we are talking about a specified flanged connection. If the dimensions of piping assemblies are not correct (say one of the straight runs of pipe was cut a little too short) there could be an excessive gap between the faces of the two flanges. My question is what would be the maximum allowable gap between the flange faces. When you design a piping system there is a specified gap left between the flanges to accept a gasket I need to know where to find that dimension.

 

[TGS4]

If you knew the stiffness of your piping system, then you could calculate the axial force that it would take to "pull" the flanges together. Then, knowing the bolt loads to seat the gasket and maintain appropriate gasket pressure throughout the operating pressure range, you could calculate the margin that the bolts have to handle this additional axial force (although, don't forget to include the operating bending moment on the flange). Throw in an allowable stresson the bolts, and there you go.

Cheer,
TGS4

 

[NozzleTwister]

Valero,

With reference to the specified gap between flanges used when designing a piping system, if your pipe specs call for the spiral-wound gaskets used in most refinery services, this gap is 1/8". The gap used is normally the nominal gasket thickness and should be noted in the Piping Materail Specifications for the line in question.

The answer to what is the maximum allowed gap is not clear cut and requires good engineering judgement. A lot of factures come into play such as line size, system flexibility, operating temperature and equipment connections.

1. If you think you may not be able to seat the gasket, then the gap is to large.
2. If your line is connected to rotating or other sensitive equipment, closing an excessive gap might cause your equipment loading to exceed the allowable, not just on bolt up but also when the system is in operation.

A GOOD Field Piping Engineer should be able to resolve most of these issues as they come up during construction, if not, shoot it back to the Engineering Contractor for a quick answer.

Good luck,

NozzleTwister
Houston, Texas

 

[robsalv]

The answer could be a bit more complex... if you need to allow for some cold spring, then the gap between flanges equals the amount of spring (gap) you need to create (without overstressing the flange during bolt up of course).

If you know what cold spring is, then don't read on. If you don't, then here's a quick description: If you have a piping system that gets hot, it makes sense to make the pipe a little short so that when it thermally grows, it grows to the right length and doesn't impose unnecessary forces on the rest of the system. That "shortness" is often set at flange faces.

Cheers
Rob