Radial expansion of PVC pipe under pressure 3

[DPAJR]

I am working on the design of a water main that will be about 500 feet long using 8 inch PVC pipe. I was wondering if there is any information about expansion of PVC pipes under pressure. Is it possible that there is some unrecognized residual storage due to pipe expansion?

Thanks,
DPAJR

 

[BigInch]

There is information that exists on the expansion of PVC pipe under pressure. It is used mostly for water hammer analysis and the equations for pipe expansion under pressure can be found in texts dealing with that subject and mechanics of materials.

You can see some representative properties of various PVC pipe specs, here,

[white black]

http://www.harvel.com/piping-pvc.asp

[/color]

While you can use those properties, and those of the water (or whatever product) inside, to calculate the resultaning volume change of the water + pipe at a specific temperature and specific pressure, you will find that the restraining hoop stress of the PVC pipe as it expands under pressure will not compress the water inside to any significant degree and the volume inside the pipe at will be apx equal to the volume of water at the same temperature and pressure.

In a "long" pipe, some portion of the hoop stress expansion volume is reduced from the effects of longitudinal shortening of the pipe, Hoop Stress x Poisson ratio x length of pipe. Poisson ration for PVC is about 0.4 That shortening occurs only if the pipe is free to move in the axial direction. What is typical for a buried PVC pipe that is not securely anchored at its ends, is that from the beginning of the pipe to a point of about 150 feet away, the pipe will transition from being completely axially free to completely axially restrained, so if the pipe is over about 150 feet long and buried, it will not shorten in the axial direction due to the Poisson effect very much. If less than 150 feet, maybe only half of that thoretical shortening will actually occur.

[COLOR=black yellow] Anyway, what that all means is that it is a very good approximation just to calculate the volume of water in your pipe at ambient temperature as equal to the equivalent volume of water at 1 atm pressure plus the change in that volume of water due to thermal expansion from 60ºF to your ambient temperature. [/color]

If pipe pressure remains high for long periods, a PVC pipe will tend to creep and permanently increase in volume. I think that it could be as much as 15-20% in some cases, but check that if you can.

http://virtualpipeline.spaces.msn.com

 

[JStephen]

As a quick check, assume uniform stress through the pipe wall, so that stress = PR/t, where P is pressure, R is inside radius in inches, t is wall thickness in inches. Presumably you can find the modulus of elasticity somewhere, and calculate strain corresponding to that stress. The change in radius is original radius x strain and you can calculate change in cross sectional area. You'd also have some minor compression of the water. In terms of actual storage, I would think these factors would all be negligible. The exceptions would be for water hammer studies as noted above, or the "what happens if I fill the pipe with water and seal it off" problem.

Minor air bubbles in the line could throw your results off completely.

 

[BigInch]

Not going to make much pressurized water.

INITIAL INITIAL 50 YR 50 YR
MIN MODULUS MIN MODULUS
TENSILE ELASTICITY TENSILE ELASTICITY
STRENGTH PSI STRENGTH PSI
PSI PSI
7000 400000 3700 140000

Above are listed for ribbed PVC, here at CalTrans. Also has some pretty good info and criteria for stress & buckling design, if you're interested.

http://www.dot.ca.gov/hq/esc/techpu...idge-design-specifications/page/section18.pdf

Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[bimr]

When satisfying the requirements of sustained pressure testing, PVC pressure pie must withstand the required pressure without failure -- ballooning, bursting, or weeping.

Much data has been accumulated proving that PVC pipes, held at their pressure ratings continuously for over 100,000 hours, exhibit no perceptible rate of creep at the end of that time.

So it is very unlikely that there is some residual storage due to pipe expansion, especially with the short length of pipe that you are installing.

 

[BigInch]

He will get the temperature expansion "residual", whether he wants it or not, true it won't be much in 500 ft of 8".

Guess its only PE that creeps.

Well, smells like red herring all the way around.

Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[BigInch]

Lots of good info about allowed pressures, thermal expansion and installation details in this PDF.

http://www.harvel.com/downloads/install-pvc-cpvc.pdf

Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[BigInch]

This doc seems to indicate that there is a 0.05 creep to PVC.

Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[BigInch]

You've got to know what pressure you want to operate at. The allowable pressure is given by the manufacturer. depending on PVC Type. Seems to be two types with one allowed .. no don't remember. Check on the link I gave you
[COLOR=white black]

http://www.harvel.com/piping-pvc.asp

[/color] Its in there somewhere. From what I gather, there may be some other PVC types around too, so best to check wherever you buy it.


Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[DPAJR]

Great answers. Thanks everyone. This all started when an engineer I know was questioning the use of pressure tanks in a system I was designing. He said I would be surprised how much residual there was in the system. I believe Red Herring is the correct analysis here.

Thanks,
DPA