Calculation of overpressure 4

[robin72]

Hi all,
Could you tell me how I can calculate the overpressure in 2" pipe API 5L X52 which cause blown up of the pipe by 50%.

 

[C2it]

Are you seriously saying that the pipe has expanded by 50% in diameter ?

If so, don't calculate .... run.

 

[DSB123]

C2it,
Surely it's too late to run!!!

 

[BigInch]

Oh. I thought he wanted to make 3" pipe.

"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward CEO BP
"Being GREEN isn't easy." Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.liv

 

[C2it]

ok, here is the proper answer.

Apply Newton's second law to velocity of the metal fragments flying past your head. If your head is still there, relate the initial velocity and particle mass to a driving force, loaded area and thus a pressure.

Yeah baby.

 

[robin72]

hi all,
thanks for the answers. I am doing accident analysis this why I need to calculate the overpressure of the explosion.
I thought I can calculate the pressure using the proprieties of the material but I don't know what is the relationship.

 

[BigInch]

If it was a hoop stress failure,

P = S * 2*t / D
P = Pressure, psig
t = wall thickness of pipe, inches
D = Pipe outside diameter, inches

S = ultimate strength of the pipe material, psi
or
S = yield strength, if it only expanded and didn't actually burst


"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward CEO BP
"Being GREEN isn't easy." Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.liv

 

[zdas04]

The hoop stress calc seems to be a way to start, and it may be the way to the end. Let me ramble for a minute to explain.

Any place that the pipe is still pipe-like (i.e., round and intact), was subject to forces equal to the yield point, and that is what that equation shows you. So I guess you could solve it for the pressure using the pre-yield diameter to get the pressure when it probably started to yield (the published number is the minimum specified yield stress, actual yield point is always higher than that, it is not possible to generalize how much higher or to go back to a piece of pipe that has yielded and test it since the act of yielding changes the mechanical properties).

That pressure will be the highest that the pipe saw since the tensile strength is less than the yield strength and the yield point is a maximum. If the pipe stretched to 150% or 500% it doesn't matter since if you redo the equation above for the larger pipe diameter you'll get a lower pressure (i.e., the diameter tends to change a larger percent than the wall thickness change). The new diameter is just a measure of the pipe's elasticity, not a measure of the pressure it was subjected to.

I'm thinking that the hoop stress calcualtion will give you the overpressure, but it will be very conservative and I would present it as an "at least" number.

David

 

[racookpe1978]

If the pipe ruptured - after expanding the diameter a given percent - could the tear-through force (determined from the shear stress and for the final thickness of the pipe wall at the place where it yielded) be used to calculate the internal pressure?

 

[robin72]

The pipe didn't ruptured but blow up.
Could you tell me the equation to calculate the internal pressure.
Our lab has measured the hardness and found HV 253, is there any relation with internal pressure.
Thanks for help

 

[rmw]

Are you sure this is a pipe and not a sausage?

Blowing up 50% and not rupturing is pretty remarkable. How gradual (or not) was the pressure build up?

rmw

 

[robin72]

Yes, but the valve ball has flowen away and the valve body pretty ruptured.

 

[zdas04]

I've seen 2-inch ball valves on vents come apart at the center thread a couple of times during overpressure situations. The ball makes quite a projectile. On some valves those threads can be the weakest part of the system.

I would use the pressure when the pipe started to yield as the probable max pressure and assume that everything that happened after that was at a lower pressure.

David

 

[BigInch]

That's why plug valves work better on vents. The plug doesn't make such a good musket.

"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward CEO BP
"Being GREEN isn't easy." Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.liv

 

[zdas04]

You can also buy top entry 2-inch ball valves that don't have those body threads.

David