Tables for pipe support spacing 12

[Tankman650]

Are there published tables for the maximum unsupported run of pipe per given pipe size??? I can calculate the maximun stress values but was wondering if there are standard tables for support apacing based on stress and deflection.

 

[DSB123]

JohnBreen,
Good observations and post. Another thing a lot of people forget is that the pipe span tables are created in many instances for "ambient" piping systems and are then applied irrespective of the pipe temperature. Spans should be modified if high temperature lines are to be considered. I know that high temp lines are nearly always stressed out but the designer needs to account for the temperature in assessing spans. As you say a lot of the span tables are based on simple supports but in reality the end conditions are never simple and there is always some rotational restraint.

 

[StressEng]

Anyone know the formulae to calculate these spans?
So we can make a excel table for all diameters, thickness, materials, temperatures, etc.

Thanks to all.

 

[JohnBreen]

Hi All,

Ed, don't I remember that Pete developed that spread sheet already? How 'bout it Pete, did you ever get around to doing that?

Regards, John.

 

[StressGuy]

I have a 500kb PDF file that has six pages scanned from the old Kellogg book on the span calcs. I'll see about finding a place to put it and then I'll post a link to it. Obviously, I can't scan the whole book, but this much ought to at least be able to spread the understanding on this topic. Judging from the interest this thread keeps getting, it is quite obviously needed. Stay tuned.

Edward L. Klein
Pipe Stress Engineer
Houston, Texas

"All the world is a Spring"

All opinions expressed here are my own and not my company's.

 

[sterl]

Lot of good stuff in this thread...

For my own $1./50 :

Certain pipe industries utilize normalized tabulations that yield support spacings "closer than" the ANSI B31 or Kellog spans due to the cumulative effects of low points in specific pipes that are deliberately sloped. Normally these slopes are effected to ensure that flow variances do not result in unantipated flow phenomena, for example: Condensate induced shock when re-applying steam to what should be a dry header; pumped recirculaion return lines on many central refrigeration systems, where not only could there be a charge management problem due to sags accumulating liquid but also a temperature induced stress when the pressure is reduced on a pipeline carrying relative warm vapor and pockets of liquid at low points.

The practicalities of supporting even that slope over 1000 or so feet of pipe leads the refrigeration industry to use 1/240 as a practical, and manageable, design slope....With the Deflection in a 10-foot support span headed for 5/8" per some of the mentioned tabular data, the accumulation problem, with its inclemencies, is not solved by the "field applied" slope.

As well, above and beyond the "stress" considerations: At the lower and lower temperatures that "normal" end-user industries are requiring of large compression systems: the Rigidity and (nil) Ductility Transition Temperatures become a very real concern and the incorporation of conventinal "expansion loops" exacerbates the problem...

So investigations of the rupture of a low temperature 2-phase pipe often establish the immediate conditions as: Bottom of pipe broke while the pressure was reducing; the flow was near its lowest; in the midst of a long stright run; halfway between supports; where its most proximate area of support is at the immediate point of contact, a deformable or otherwise, flexible insulation material. Conclusion: Minimal localized stress at the supports, minimal hoop and axial stress, minimal momentum forces due to changes in flow direction, minmal bending moment, on tensile side of pipe...Pipe still broke.

 

[1974vet]

Here goes my efforts to explain (hope isn’t too long)-
A little more info on the “span charts”:
Looking at Navco, p150, the Navco suggested support spacing is based on 2300 psi bending stress. MSS-SP-69, p8, is based on 1,500 psi, although I can’t seem to quickly find it in the text. There are many “company” standard span charts also. Anyway, most tables have notes and qualifications although I’ve seen some “company” charts copy other sources and leave this critical information out. These notes may include items like the following:
*The pipe is assumed to have standard wall thickness with insulation,
*No concentrated loads present such as valves,
*There are no changes of direction in the spans,
*Spans are assumed to run in the horizontal plane,
*The maximum deflection of the span under load is limited to 0.1”, and
*The stress intensification factors of components are not considered.

I am almost certain that SP-69 was generated using allowable stress values from the power piping code. The following is an attempt to show how the span tables might have been generated:

Select a low allowable stress value, e.g., 1,500 psi (Sall(weight)/i), for the combined bending and shear value, such that a sufficient factor of safety is provided, and allowing the standard span chart to be applied to a wide range of piping systems:

Beginning with the generalized primary stress equation,
PD/4t + iM/Z <= kSh
where iM/Z values are code dependent, and
k values are code dependent and occurrence of load (assumed = 1.0 here), and
Sh is code dependent

Using beam formulas for span:
M = 1/2[(wl^2)/8 + (wl^2)/12] is the average of a uniformly loaded simple beam and a uniformly loaded beam fixed at both ends.
This reduces to: M = (wl^2)/10

Substituting this expression of M into the generalized stress equation and rearranging results:
L<=SQRT[(Sh-PD/4t)(10Z/iw)]

So then, an example of a non-critical type system might look like:
Assuming B31.1 and A106 Gr B material
P = 150 psig
T = 350F
Sh = 15,000 psi
Let PD/4t = 3000 psi (approx)
This leaves 12,000 psi. A system design of this type will utilize higher SIFs, say in the range of 6-8, so,
12,000/8 = 1,500 psi

An example of a critical type system might look like:
Assuming B31.1 and A181 material
P = 2600 psig
T = 1060F
Sh = 6,000 psi
Let PD/4t = 3000 psi (approx)
This leaves 3,000 psi. A system design of this type will utilize lower SIFs, say in the range of 1-2, so,
3,000/2 = 1,500 psi
Note that the wall thicknesses will be greater and the fabrication control improved resulting in the lower SIFs.

I’ve left out several steps to save some space, because I think that the most important thing to know when using a standard span table is generally how it was constructed, and what simplifying assumptions were made.

Also related is a calculation (ref. Hick’s) to determine the minimum slope of a pipe to ensure complete draining. On the one hand it is a little conservative since emptying is not considered, but there are other factors that can negate its conservatism. If interest, I can post that relation also. . .

 

[StressGuy]

Alright,

I found a spot to post the excerpt from the out of print Kellogg book.

http://home.houston.rr.com/edwardklein/Support_Spans.PDF

Edward L. Klein
Pipe Stress Engineer
Houston, Texas

"All the world is a Spring"

All opinions expressed here are my own and not my company's.

 

[pipingdesigner]

You can find an excel pipespan chart here:

http://www.pipingdesigners.com/downloads/pipe span chart.xls

pipingdesigner

http://www.pipingdesigners.com

 

[superpiper]

To mr Klein,
I learnt more from reading those six pages than from spending 4 years in our stress department.thanks.


To 1974vet

I am after an equation to calculate pipe spans for sloping lines in the format of span being a function of slope,
so that no pockets occur. please post the hicks equation as commented.

 

[UW1981]

By the way, another good source for span charts is from Chapter B-4, Stress Analysis of Piping Systems, Piping Handbook, 7th edition. This has charts for various sizes of pipe (standard schedules), both empty and water full. It also has a neat deflection chart.

As most everybody has observed, these charts are good "estimates" or guidelines - given the assumptions they are based on......

For slope considerations, there is a good article available on the web titled "DETERMINATION OF THE OPTIMAL PIPE SUPPORT SPANS FOR GEOTHERMAL PIPELINES. If you just type in the first four words of the title in GOOGLE, you'll find it. Sectin 5.3 talks about sag and drainage.

Good luck!>

 

[1974vet]

(please post the hicks equation as commented)

Sorry for delay . . .

From Hick's
1. Compute the allowable span between hangers
for a pipe filled with water, S = WL^2/8m, where S = bending stress in pipe, lb/in^2; W = weight of pipe and water lb/lin in; L - maximum allowable distance between hangers, in; m = section modulous of pipe, in^3. by using a table of pipe properties, as in Crocker and King-Piping Handbook, L = (8mS/W)^0.5

2. Compute the pipe slope required by the span
to prevent pocketing of water of condensate at the low point in the pipe, the pipe must be pitched so that the outlet is lower than the lowest point in the span. when the pipe has no concentrated lods-such as valves, cross connections, or meters-the deflection of the pipe is y in = 22.5wl^4/(EI), wehre w = weight of the pipe and its contents, lb/ft; l = distance between hangers, ft; E = modulous of elasticity of pipe, lb/in^2 = 30 x 10^6 for steel; I = moment of inertia of the pipe, in^4.
with the deflection y known, the pipe slope, expressed as 1 in per G ft of pipe length, is 1 in per G ft = l(lower case L)/4y. Thus, a pipe slope of 1 in in 4.53 ft is necessary to prevent pocketing of the water when the hanger span is 47.4 ft. with this slope, the outlet of teh pipe would be 47.4/4.53 = 10.45 in below the outlet.

Personally, on the one hand I think this is a bit conservative since the deflection decreases as the pipe empties. However, it may not be so conservative when some construction practices, creep, and so on are considered.

 

[XELR8]

First of all, each of the above response above are very accurate and well taught.

The best rule of thumb for hanger spacing for pipe is the pipe diameter in feet plus 10. Example: a 3" pipe would require a support every 3'+10" = 13'-0. Use that and you should be fine for small diameter pipes 1/2" - 12".