Pipe Span and Required Deflection

Hi All,

I'm in the process of sorting out all the piping classes and material specifications. My problem is this:

Within several companies' standard piping details (which I've come across)there's always a datasheet(s) on minimum pipe spans for particular piping runs/configurations based on pipe full and pipe empty, etc.

Now, the acceptable span between pipe supports is on average 8m for laden carbon steel pipe. I stress "average" because there does not seem to be a definitive method of working out minimum pipe span. Basing the recommended span on induced stress, either for simply supported or built-in beams, yields acceptable spans considerably more than 8m, even taking into account derating for joints such as flanges, threaded joints, etc.

Therefore, designers in the past have apparently based the required spans on maximum defelection. This seems reasonable as it considers factors like drainage gradient and natural frequency better than stress-based evaluation. But what is an acceptable deflection? Data suggests that 25mm (1") is preferred, but this seems so arbitrary!

And I'm sick of hearing "but that's what we've always used"!

Any feedback would be appreciated, thanks

As you mentioned yourself, an important driver in determining an acceptable deflection is the pipe gradient. If you are developing a generic allowable pipe span table for all services. You will need to look at the minimum gradient of piping that is required to be free draining . Then ensure the deflection at midpoint of the pipe span will not allow service commodity to gather.

Regards
Paul

Table S-1, Spans of Horizontal Pipe from The Piping Guide by Sherwood for a starter. There are few more and some table can be even be downloaded from the web.

1" deflection is quite a deflection. Do we really allow so much of sagging? It is standard using 5mm as deflection for Power Piping. Any comment?

Since my original post, further issues have come to light. First of all the original figure of 25mm deflection was a decimal place out – apologies for the finger trouble. According to an older copy of B31.1 code of practice 2.5mm is an acceptable deflection.

But B31.1, supported by MSS 69, also reveals that that this deflection criterion results in a distance between supports / hangers of 4.3m for a 100 NB pipe. This results in far more steel and support gantries than say pipingdesigners.com’s pipe span chart of 8m interval – which incidentally states acceptable deflection of 12.7mm. The stated 41 MPa allowable stress also differs with ASME power piping limit of 16 MPa.

What I have also discovered, from our structural engineer, is that very often a design for an exposed structure will aim for a natural frequency of greater than 3 to 5 Hz range for wind loaded structures. This would then tie in with a deflection criteria controlling the pipe span and not a stress value i.e. deflection is an inherent factor of damped / undamped resonance.

This is all very circular. Ultimately, and in accordance with what ASME B31 explicitly states, what has worked in the past is acceptable for the present and future. The aim of the piping designer is establish a point of reference and then continue using that point of reference – whether it is in-house, a published handbook, vendor recommendation or a listed code.

As it happens, “that’s the way we’ve always done it!” is, after all, congruent with generally accepted engineering practice.