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.

2nd Moment,
You wrote:
"there's always a data sheet(s) on minimum pipe spans"
You are by far not the only one who has done this.

Don't you mean "Maximum" pipe spans?

Jop,

Too true!

It is maximum pipe span or conversely minimum number of supports per length of pipe.

Thanks

Thanks for the post 2nd moment. B31.1 is more stringent than B31.3. Most of the span charts that flood the web is for Oil and Gas Industry. For Power the allowable sagging is more stringent and the allowable sustained deflection is less. The factor that governs is the sustained stress. More the span more is the sustained stress that in turn increase the pipe stress under operating condition. So if the thermal stress is not too much ie the pipe operates under less temperture and pressure there is a room to increase the pipe span.
I dont know how the vibration frequency affects the sustained stress. I would appreciate if you throw more light on this part of the subject.

Perhaps more stringent deflection in 31.1 than 31.3 also has to do with the life expectancy of the power plants which is significantly higher than process / chemical plants.

On the subject of piping spans.
The petroleum industry has a standard of 1/2" to 3/4" maximum deflection, depending on the owner.

All efforts to route a line considering Safety/Operation/Maintenance/Cost etc is a waste if you don not take care of supporting that line effectively.
Such an huge importance for pipe supports but I feel very bad when I see pipers not considering pipe supporting seriously.

Pipe supporting is called more oftnly as an art & I feel that, it is very very true when you are actually dealing with it...!

Let us talk about only dead load & let us not consider pressure / temp conditions etc initially to understand it.
- Make use of the pipe support span charts available on your project. (Engineering from macro level is no more done now a days. The well accepted things have been documented already to make use of them in similar cases & normally called as company standards.) You will definately have these support span charts in the company.
- Pls take care of the dead loads of the line considering the fluid (liquid/gas) flowing inside the pipes for all the lines. Higher is the dead load lower will be the span between the two supports. (Support standard must have been already developed by considering this logic)
- For stress critical lines you will get advised from stress engineer for supporting location changes needed to the line submitted line.

Simple logics to keep in mind while you are dealing with pipe supporting-
- Route a line in such a way that you will make use of the main steel (building steel, rack steel) etc. for pipe supporting. by just taking care of this simple rule you will save lots of money by avoiding new steel in the plant for pipe supports.
- Supports from equipment are also possible but we need to think of it very early so that we can take care of the pipe loads at the equipment design stage itself. Communication to the equipment vendor is also very crucial till the end of the project.
- Ideally, every point load acting in the pipeline need to be supported for ex- Valves/control valves etc.

Pipe supporting is very difficult to explain & one will learn it through his experience..& there is lot to learn in Piping.
Thnx