Piping Support

[ONEPOINT]

Hi,

I was asked to review some data about the Maximum Unsupported Pipe Length(Feet). The data that I was given is a 12 and 16 inches diameter pipe. For a 0.250 wall thickness I was given the following lengths: 80 feet for the 12 inches diameter and 83 feet for the 16 inches diameter.The Pipe SMYS is 52,000, the design factor is 0.54 and density of contents is 62.4lb/ft^3. Distances include a factor of safety to cover any typical corrosion. The lenghth does not include any accessories such as valves or other fittings.The pipe must be adequately supported on a timber or skid truss within 20 feet of each end of excavation.

Based on the above data I have to make a review of the Maximum Unsupported Pipe Length. Is anybody that could help me with some suggestions in regards with the above issue?

Best regards,

OnePoint

 

[ApC2Kp]

Onepoint,
The 80 feet span of unsupported length might be within the SMYS stress for the pipe, however the deflection will be excessive. Deflection is probably feet instead of inches. Usually a deflection criteria of 0.5 to 1.0 inch is applied to spacing of supports. Then I see the requirement of timber or truss within 20 feet of each end of excavation, and wonder is this a temporary bypass line over an excavation? It would be a small cost addder for 0.375" wall thickness pipe, and provide more strength, less deflection.
Typical carbon steel pipe is designed with an allowable stress of 20,000 psi for ambient service in a process plant.
If this line is a temporary bypass, then you might carefully design with a higher allowable stress. The 12" pipe will be 84.5 lbs/ft and the 16" pipe will be 124 lbs/ft when filled with water. That makes the support loads 8400 lbs at each end. (Weight per foot would increase to 99 and 142, with 0.375" wall thk.) Be sure to shore up the excavation.

 

[11echo]

I'm looking at the charts I have both 12" & 16" with 1/4" wall would be Schule 10 pipe. Per my charts MAX. deflection on 12"-sch.10 is 0.112" (water filled) Max. support spacing would be 33'-11.375" (weight 2,592#) and for 16"-sch.10 is 0.120" (again water filled) Max. support spacing would be 39'-4.5" (weight 4,886#).
However normal spacing requirement are normally dicated by the smaller diameter piping in plants I work with. OR even the elcetrical conduit runs (normally running next to piping systems and using the same support(s)) and they dicate 10'-0" spacing. Good Luck! ...Mark

 

[stanier]

Typically this type of analysis only looks at steady state. What happens in a wind or earthquake situation?

If you only do static analysis of a piping system you are only doing part of your job as an engineer.

 

[JStephen]

Pipe that long can vibrate in the wind- found that out once the hard way! Roughly 6" back-and-forth deflection in good stiff breeze. That was empty.

 

[StressGuy]

I'll first point the original poster to this thread that covers some similar issues:

http://www.eng-tips.com/viewthread.cfm?qid=152615&page=1

In addition to the deflection equation I posted in that thread, the Kellog book also has a stress equation that goes like this:

S = 1.2 * (W * (L^2) / Z)

Where:
S = maximum bending stress (psi)
Z = Section Modulus of the Pipe (in^3) (after corrosion and mechanical allowances are removed from the wall thickness)
L = Span between supports (ft)
W = Total weight (lbs) per foot of the run including pipe, fluid, insulation, etc.

I tend to more often find that it is the deflection criteria rather than the stress criteria that limits pipe spans. Again, this comes from the desire to keep the span short enough that you don't get yourself a Tacoma Narrows Bridge.

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.

 

[NozzleTwister]

A lot of good comments here. Let me add one more thing.

Be sure to evaluate the local stresses at your support locations

NozzleTwister
Houston, Texas

 

[DSB123]

NozzleTwister,
Good point about the "local" stresses at the supports. I've found that there are not many designers who consider that point especially bare pipe sat on steel.