Written by: Christopher Wright at Piping Design Central

Q: Is there a good, hard place to get a pretty well "set" number on these piping spans? I know spec dictates all this, but I'm looking for a good "nominal" listing, if there is one. All the ones I've found differentiate by like 8-9-10 feet! And it's not like I'm going between Sch.40 and 80 and all that, I'm talking Sch. 40 period. They vary just as much if I look at the Sch. 80's too.

A: Think about it for a minute--you've looked at 3 respected references and found wildly different answers to a problem that people have been solving (usually with success) for years. Seems fairly clear that those references are operating under different assumptions--different assumptions give you different answers. I don't think there's such thing as a 'nominal' listing that's going to do you any good. Using a 'nominal' listing puts you in the position of the statistician who drowned while wading across a river whose average depth was 4 feet.

Figuring this out is not all that tricky. Support spacing is governed first by the dead weight of the pipe and its contents, second by any applicable wind and seismic loading and third by thermal stresses. You assume that the pipe span is a beam and calculate the stresses on that basis. You can make up a spreadsheet real easy that will do this using some of the methodology in the references you're using.

Stress = bending moment/Z; bending moment = distributed load x span2/8 (conservative); distributed load = total load/span; Z = section modulus that you get from tabular piping cross section geometry. My copy of Tube Turns bulletin TT726 wraps it all up with some charts assuming that the external loading uses up only about 1/4 the allowable stress for the pipe. You can put all this into a workable Excel spreadsheet in an afternoon, and gradually tweak it into a very useful tool.

I daresay all your conflicting recommendations come from different assumptions like the fraction of allowable stress used up for external loading, end conditions affecting the bending moment and assumptions about what goes into the distributed load. I just did a worksheet something like what I said, and it turns out that a 27 foot span of 1 inch pipe won't even support its own weight, let alone insulation and contents, but a 6 inch pipe might.

A device like I'm talking about isn't like CAESAR or ADLPIPE and it isn't going to make you into an engineer, but I'm assuming you're not looking to get into any such stuff. What it will let you do is make some sanity checks when someone sends you a layout that that specifies a 27 foot free span of piping. If you see something that doesn't make sense, you can do a few numbers call your customer and ask diplomatically whether they're certain about that span. No need to ram your technical expertise down someone's throat, but if the guy gets a little shirty (and some engineers do) just make a note somewhere that you called and he told you 'piss off and try to follow the drawing for once.' Then you do have something to ram down the guy's throat when he calls back and tells you 27 feet is wrong.

There are other loadings, but they tend to be specialized and tricky to manage and ought to be done with software, although even then it's not rocket science.

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