Pipe wall stress calculation

[Aquapark]

Does anyone know of a simple method of calculating the required radial load needed to circumefentially expand a steel pipe ?

I want to force open the pipe ID by 1% and simply wish to calculate how much rdail force will be required using a pair of wedges and a hydraukic ram.

Is there a software package I could use ?

 

[JohnBreen]

You should try Googling "Auto-Frettage". That may get you what you want. It is a typical process used in the construction of very high pressure piping systems and very high pressure vessels.

Auto-Frettage
A cold-working process, chiefly applied to cylinders and tubes of heavy wall thickness, e.g., guns. Auto- frettage may be affected by expanding the bore by hydraulic pressure until practically all the metal has been stressed beyond its elastic limit. On removing the applied pressure the cylinders are left in a state of internal stress with compressive stress at the bore and tensile stress at the outside. Thus, before failure can occur, any bursting force acting from the bore must overcome this internal compressive stress before the steel is subject to tension.

Regards, John

 

[BigInch]

Isn't that only valid for thick walled cylinders?

I'd do it with a "hydrotest".

Estimate the yield force just using Barlow's formula
F x D = Pressure = SMYS * 2 * t /D. After you reach yield the rate of water volume added/change in diameter will decrease. Watch that rate closely and stop adding water at the calculated volume that yields a 1.01% diameter increase. Don't forget to consider the compressibility of water to accurately calculate the volume of water required to get there.


BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[JohnBreen]

Isn't that only valid for thick walled cylinders?

Yes.

I was hoping that Aquapark would find a healthy dose of CAUTION in a discussion of autofrettage. The same can be said for the hydrotest. There is so much potential for injury here that if it is attempted, extreme caution and protection from potential pipe rupture is indicated. Autofrettage is done behind blast shields and about 30 percent of the attempts result in rupture of the thick wall pipe. Remember that there is much variation in the pipe wall thickness. The vagaries of materials from heat to heat should also be considered. One can never be too cautious when using pressure to take a cylinder past its yield point.

 

[BigInch]

110% agreed.

By suggesting hydrostatic testing method, I did not mean that adequate safety measures should not be taken, and to accurately premeasure the volume of water required, such that overexpansion to ultimate strength is not possible. That should make it relatively "safe", given that the pipe is well inspected beforehand for no defects. The ultimate strength is well above the yield strength, so considreable plastic expansion (much more than 1%) would be possible without a reasonable danger of reaching ultimate stress and bursting.

But you're right, its nothing to play around with. I've had a 30" diameter pipe blow on a 500 MAOP - 625 psig hydrotest, under 3 ft of ground cover and a 6" concrete parking lot pavement on top. Left a 15' wide crater with dirt and concrete chunks scattered well past the "safety" barrier distance.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com