Where are the equations for pipe span located in the codes? I'm banging my head on this one. I see the suggested spacing in table 121.5 in B31.1, but not the supporting equations.

Let me add that I know what the equations are based off of max bending stress or E, I and W. I just can not confirm them in the code anywhere...

Mr luf or Mr breen had a reference to an article
from Kelloggs somewhere on the web.

Priceless.

Read it and take note of the reference to 4Hz

I've seen that, but still doesn't reference any code...

Only SUGGESTED in b31
I dont think spans are covered by codes
But MSS-SP69 is a 'Standard Practice'

Been there too, still don't see an equation?

Did you bring your own spoon PJ?

Just looking for some help Kevin, I've got plenty of sarcasm here at the office already. Knowledgeable help is in short supply though.

"I dont think spans are covered by codes"

Yes and no... B31.1 has a recommended i.e., non-mandatory span chart .... B31.3 is silent so your looking for something which does not exist!

PJ,

Your question is a valid one: It is, I think, quite reasonable to assume that a formula must be hidden somewhere in the codes. I agree with others here, however: No formula exists in any of the U.S. codes.

The B31.1 recommendations, as I recall, are simply based on MSS. I was told many years back that MSS was in turn based on spacing recommendation developed by Grinnell. That would explain why the spacing therein is conservative compared to many of the charts used by major petrochem design companies. (The closer the spacing, the more supports you sell. As an aside to that theory, I know of one design firm that employed a major spring support manufacturer to perform the stress analyses on a particular project. The result was Spring Hanger City! Later, a former colleague of mine was asked to re-stress the systems. With a bit of redesign here and there and the application of some common sense he was able to cut out more than 95 percent of the springs.)

Incidentally, the petrochem company charts I referred to are generally derived using beam calcs with a predetermined maximum deflection. One of my old charts, for instance, states, “Deflection is based on a compromise between a beam with fixed ends and a beam free ended.” The chart itself then provides spacings for a range of pipe sizes in various conditions (e.g. empty pipe, pipe with water, pipe with insulation, etc) and, for each condition, also indicates the amount of deflection expected for the given support spacing.

Regards,

Read note c of Table 121.5. Note that the conservatism of the suggested spacing is due to the conservatism of the criteria.

The codes in general due to provide equations for pipe spans. They provide allowable stress criteria and designer must limit the spans to satisfy the allowable sustained stress criteria.

Hi PJ

It is clear that piping is not really your normal field of engineering and you are just looking for a little info on allowable spans. I have an excel sheet for calculating pipe spans based on BS 3974 Pt.1. You can base your calculation on the allowable stress or allowable deflection and. There are two formula. One is the case of simply supported beams which is never really the true boundary condition at the ends but is a conservative estimate. The other is based on a continous beam which reflects the boundary conditions more accurately. Give me your e-mail address and i will forward it on to you.

Actually, I do work in piping and I do know the equations as I have previously stated. I am working on writing an office standard and wanted to be able refer the basis of my calculations to something in the ASME code. I was unable to find anything and thought this would be a good place to ask if anyone knew if it existed somewhere in the code that I was missing. I appreciate everyone's help.

As some body said in a previous post Tsk Tsk.

Which formula should you use. Simple span, encastre, a combination of the 2, what about valves, etc etc. What if you wrote an FE program like Caesar.

On this point the codes are generaly fine. Even so, I would have thought it very dangerous for any code to define a span given the combinations and permuations. Somebody will put a couple of valves at mid-span of thinner pipe and still tick the box that meets the code. The code rightly leaves the judgement and risk of creating "standard" spans to the user.

My opinion precisely! A span chart should be nothing more than a go by to be used with common sense as the situation merits. However common sense seems to be a dwindling resource these days.

I worked with a person once who had zero common sense and some heated discussions resulted in his use or misuse of the "Recomended" Span Chart in B31.1, see the previous thread on burning trains for those people who want to distill our work down to ten simple rules or so.

So exactly what formula??? use common sense!

Hello PJ and all,

First of all, please understand that it is not my intention to be sarcastic or rude in this reply (except of course to John Luf). I sincerely hope that something that I write here will be of value.

In summation, there is no equation provided by the ASME B31 Codes for calculating the data shown in Table 121.1.4. And there never will be. The table is there in the B31.1 Code on a "take it or leave it (please)" basis and it is a little less than a "rule of thumb". What most of the replies above are saying is we understand what you are looking for but neither the table nor any underlying equation should be placed in any company's book of standards (even if it did appear in a Code book). Why? Because there are too many "except for(s)" that must be added to such a table to keep the design honest (the notes under the table at issue are but a few of the "except for(s)"). At best, such a table can be a starting point but a real analysis will eventually need to be done and the risk is that that the need for that analysis may be ignored simply because a "standard" was used. If you want to use the B31.1 table as a starting point, fine, but why do you need an equation? Most companies today are interested in limiting their liabilities and if the legal types were to understand the potential ramifications of providing design guidance based upon oversimplifications they would likely withdraw the entire company design standard from further use. I think it is not enough to think that design legitimacy of company standards can be established by simply saying "well it is in the Code".

I would further suggest that you visit the "Forward" in the B31.1 Codes and read the "the Code is not a handbook" paragraph. It would be folly to place Table 121.1.4 in your standard without a similar caveat.

There is a discussion over on Engineering-Tips (thread378-152615) in which Ed Klein posted the Kellogg equation that has been alluded to above by SUPERPIPER:

http://www.eng-tips.com/viewthread.cfm?qid=152615&page=2

But (although I suspect that this is the equation that was used by B31 MDC (circa 1950's) to create the table at issue) I don't think that is what PJ is really looking for. The Kellogg equation calculates maximum deflection ("sag"), but it is for a limited set of boundary conditions. PJ would have to have a set of three or four equations from say, ”Roark's Formulas”, to cover the set of all possible varying boundary conditions in his standard just to reduce the number of "except for(s)".

http://www.spreadsheetworld.com/ItemDetails.asp?ItemID=1000-00-0001-00

As Chuck and John (the elder) have alluded to, any span tables that the B31 Codes give you MUST be used in concert with healthy applications of common sense. That is to say, there are not many times in actual piping design where the tables will be directly applicable. You look at the span table and you can tell what the longest "typical" span between supports might be but if you have concentrated loads like valves and or flanges, the spans will be shorter. And, what material are you using? Carbon steel, stainless, steel, unobtanium,…….? Each would have its own table.

Of course, the only EQUATION (per se) that applies (and this has virtually nothing to do with the span tables) is the equation that the Codes provide for calculating stresses due to sustained pressure plus sustained weight – this will determine maximum spans. That calculated stress must be less than Sh as provided by Appendix "A" of the Code. So from a practical point of view, forget the span tables and let CAESAR II do the stress calculation for you.

Per the Code the stresses are calculated using the modulus of elasticity of the material at the "cold" temperature (the pipe will be stiffer than it really will be at the "hot" temperature). So, the REAL (“hot”) deflections will not be calculated by your sustained stress analysis. And conversely, since the span table is based upon 750 degrees F. I guess that means the hot elastic modulus was used in the table’s development so using the span table will not always assure you that (in every possible case) the pipe stresses would necessarily comply with the Code.

Having said all of that. If you want to play around with "an equation" as an academic exercise, you can look at Table 121.1.4, note 3, and see that the B31.1 table is based (in part) upon a maximum acceptable "sag" of 0.1 inch. With that understanding, you can play with (borrow John Luf's trusty slide rule) the Kellogg equation (and others to be found in Roark) and back-calculate your own span tables. You can alternately use the hot and cold modulus of elasticity and get some feel for how that affects the allowable span. AND, your span table will be as useless as the one in paragraph 121.1.4.

I have enjoyed reading this discussion. However, I hope that nobody ever asks us to justify the inequality (equation) presented in paragraph 119.7, regarding "method of analysis". But these “points of interest” are just part of the things that make piping engineering fun for us all.

Regards, John (the younger).

I have a spreadsheet which with a minimum input will give out "allowable spans" according to span and deflection criteria with various boundary conditions. This is just in the way of a handy look-up. For new plant I limit the sress to 1/2 the code allowable. This I consider prudent not only for those valve installations but also to allow room for back fits in later plant life. [Old plants seem to double the original piping installations as the fertile process imaginations are indulged.] Naturally if I come up on a back fit myself I can't use 1/2 but allow the sress to 85% of code allowable.
Yet often the criterion is deflection. 0.1" maximum sag (mentioned by John Breen after B31.1 he says) is ludicrous. If it is my call I allow 10mm but I know Foster Wheeler for one like to use 6mm. However over a 3.0m span 10mm is 1/300th, if the pipe is to fall at a steady 1 in 200 10mm sag is not permissible.
Finally I always consider racking spans. All different size pipes have to travel in the same racking spans which are 6-8m with intermediate beams for the smaller pipes. Some really big pipes can be found to span 30m agreeably except for the massive support loads. So again we revert to those 6-8m spans. Finally vary those racking spans a little to break up vibration resonances.

Michael