Thick Walled Cylinder Question

[PipesAndStuff]

Hello,

I'm determining the change in length of an 8-inch (DIPS) SDR11 HDPE pipe after pressurizing due the Poisson effect. I'm treating the pipe as a thick walled cylinder since the pipes diameter/thickness < 20.
I'm using the attached equations to compute axial, tangential, and radial stresses. Then computing the axial strain and multiplying by the pipe's length.

Here are the parameters:
Poison Ratio = 0.45
Elastic Modulus = 150,000 psi
Inside Radius = 3.7 inches
Outside Radius = 4.5 Inches
Inside Pressure = 180 psi
Pipe Length = 1000 ft

When I compute the stress for the inside wall of the pipe, I get a change in length of 29.1 Inches.
When I compute the stress for the outside wall of the pipe, I get a change in length of 2.9 inches.

I'm not sure which result to use or if I am approaching this problem correctly. I appreciate any insight people have for this problem.

Thanks,
Tony

 

[LittleInch]

Use mid wall?

Expansion from pressure is dwarfed by thermal effects in PE.

 

[prex]

If you look at your equations, you'll find that sigma_theta+sigma_r=const, so the longitudinal elongation (negative, it is a contraction) is, of course, independent of r and is easily found from the strain eq. for eps_z (sigma_z being, of course, null everywhere for a free ends pipe).
Pls post this kind of questions in the students forum.

 

[PipesAndStuff]

If you look at your equations, you'll find that sigma_theta+sigma_r=const, so the longitudinal elongation (negative, it is a contraction) is, of course, independent of r and is easily found from the strain eq. for eps_z (sigma_z being, of course, null everywhere for a free ends pipe).
Pls post this kind of questions in the students forum.

Thanks, I did post in that forum. This is work related. We need to justify to the utility that contractors need to install thrust collars as called out in the plans, which has happened on a couple of our designs.

 

[KevinNZ]

Thrust Blocks - Pipelines, Piping and Fluid Mechanics engineering

Can somebody explain to me when thrust blocks are required on underground piping? For example, I have a 6" fire water storage tank fill line where the piping drops underground after a control valve and comes back up before it ties into the storage tank. All BW piping. The civil engineer is...

www.eng-tips.com

 

[LittleInch]

Thanks, I did post in that forum. This is work related. We need to justify to the utility that contractors need to install thrust collars as called out in the plans, which has happened on a couple of our designs.

Just use thermal contraction. That will generate much more movement than Poissons effect.

I cannot believe a utility is questioning the need to use collars which prevent pull out on a PE system.

 

[PipesAndStuff]

Just use thermal contraction. That will generate much more movement than Poissons effect.

I cannot believe a utility is questioning the need to use collars which prevent pull out on a PE system.

We are using both. The reasoning being that after being buried at a depth of 5+ feet, the pipe will cool down prior to cold water flowing through the pipe.

The omitting of thrust collars is decided during construction. From what I understand, the contractor is getting the field engineer to agree to it.