Pipe burst pressure

I suspect the formula you used assumes a wall thickness that is thin compared to the OD. I derived the formula some time ago but cannot find the calculation now.

This link

Regards Roger Woollett

This is a thin wall approximation for the hoop-stress. Easily derived by considering cutting the pipe through a plane containing the centre line of the pipe.

Force on either side of cut = (pressure) * cross sectional area

So for length L of pipe, cross sectional area = L d

Stress = Force/Pipe thickness = P L d / (2 * L * t) = Pd/(2t)

For a "thick" cylinder you need to integrate to get Lame's equation and

max circumferential stress = p( (OD)^2 + (ID)^2 ) / ((OD)^2 - (ID)^2)

max shear stress = p (OD)^2 / ( (OD)^2 - (ID)^2 )

Since ID = OD - 2t we get for circumferential stress

p_max = 10000 * ( (OD)^2 - (OD-2t)^2 ) /( OD^2 + (OD-2t)^2 ) = 10000 * ( 0.125^2 - (0.125-0.050)^2 )/ (0.125^2 + (0.125-0.050)^2 ) = 4,700 psi

However it is clear for p_max of 10,000psi you would end up with t=OD/2 which is commonly sold as copper rod :-)

Does the material need to be copper?

Alan

intense

derived by

(ID)^2)

(0.125-0.050)^2 )

Strictly no - the original is copper or a copper based alloy, and for easy of bending the run it would be preferable, but if you can suggest any other suitable material I am all ears!

AWEM

The minimum tensile strength of copper alloy as used in brake pipe is at least 45,000 psi (BS figure). Sch 40/80 pipe is 40,000 psi minimum.

-- Peter Fairbrother

John

Car brake pipe, in cunifer (IIRC Spelling) or plain copper. Local motor factors should be able to supply it. I might have a length in the garage if you cant get any.

Dave

Car brake pipe, in cunifer (IIRC Spelling) or plain copper. Local motor factors should be able to supply it. I might have a length in the garage if you cant get any.

Dave

Thanks Dave but I doubt it will be as thin as 1/8" o/d

AWEM

Car brake pipe is 3/16" or the metric equivalent.

John

John many thanks - order placed with Abacus.

Andrew

Having just looked (at the fittings anyway, goblins have hidden the pipe) it is indeed 3/16"

Dave

Andrew (and everyone else),

I know you've found some suitable pipe now but I though I could add a little to the preceeding discussion. If the wall thickness of a pipe is more than about 1/20th of the diameter then the pipe is considered to be 'thick walled' and the equations discussed previously are generally deemed to be invalid as they assume a constant stress distribution through the wall thickness. For the thick walled case the equations by Lame (pronounced Larmay I think) are used. They are trickier to use as they are a pair of simultaneous equations.

Further info can be had here:

Toby