AWWA M11 Pipe Buckling Question 1

[couplandboy1]

I'm reviewing a shop drawing from a tank builder which includes a vertical overflow pipe internal to the tank. The pipe will be empty and subjected to varying levels of external hydrostatic pressure varying from 0 feet to, say 20 feet at the bottom of the pipe as the water reaches the overflow weir.

I employed the calculation to calculate at what point the pipe would buckle from external pressure (equation 4-2 from M11). My calculations showed that buckling (for the pipe thickness they propose) could be a problem and I asked for justification.

The issue is what is "uniform and radial". I figured that the maximum "uniform and radial" pressure would occur at the bottom of the pipe. The tank guy contends that "uniform" means applying the average external pressure = (0 lb/sf at the top + max pressure at bottom)/2. Essentially, he's saying that the pipe would need to be horizontal and subjected to that max pressure at every point of the pipe in question.

They provided a justification and ultimately they are responsible so this is a relatively low risk question to answer. There are some flanges and bends that will stiffen the pipe that I acknowledged but didn't account for so I think we'll be fine either way. I am very curious about others' interpretation of "uniform and radial" and would appreciate some thoughts.

 

[bimr]

Uniform means the pressure is constant around the entire pipe. Radial means that the force is acting perpendicular to the pipe.

You are correct in your reasoning. You are determining the forces that the pipe will exposed to at 20 feet depth, so you should use the pressure of 8.67 psi (20 feet * ( 62.5/144)).

Having said that, one would expect that the standard pipe should not be affected by 8.67 psi external pressure.

Not sure what diameter of pipe that you have, but schedule 40 pipe should have a collapse pressure of several thousand psi.

http://www.engineeringtoolbox.com/stainless-steel-pipes-bursting-pressures-d_463.html

 

[couplandboy1]

I pulled the 20 feet as an example. The conditions are in that general range though.

It's a 48-inch pipe. Whether or not schedule 40 is applicable to that size is one of the contractual issues and 1/4" thick pipe is proposed in the absence of a defendable "schedule 40 standard".

You answered my question with the interpretation of uniform and radial.

 

[bimr]

With such a large pipe, it was prudent to check.

Further information is in the steel handbook:

http://www.steeltank.com/Portals/0/pubs/Welded Steel Pipe 10.10.07.pdf

 

[rconner]

For some background, I believe some of the most detailed work on collapse due to external pressures/vacuum on relatively thin steel and aluminum tubes and pipes was done at the University of Illinois at Champaign-Urbana in the early to mid 1900's. While there were/are clearly applications in e.g. the water field, I suspect the eventually great interest also in aluminum may have been for a fledgling aeronautical industry, where the air tended to get more thin and unlike much of the water field weight and cost of same was (unlike at least the early water field) was at an absolute and more critical premium (lest that industry and its products get off the ground, so to speak!)
I think there are multiple publications from this research, and perhaps some more applicable to your inquiry, but you may be interested e.g. in some of the results, relationships and factors involved e.g. testing on pages say 56 on in the report at

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CB4QFjAA&url=https://

http://www.ideals.illinois.edu/bits...9E4YjgKJ2u2EMInRbqXPw&bvm=bv.90237346,d.eXY​.

 

[coloeng]

If a 48" pipe is so thin walled that it will buckle(or do you mean collapse?) with only 8.67 psi of pressure at the maximum point, then it is probably too thin walled to even be installed. It would probably collapse if it is laying horizontal before installation. Are you sure you were using the correct units for your equation?

 

[couplandboy1]

It's a bit more than 20 feet (that was just an example). However, with a 48-inch pipe that's quite a bit of area. 8.67 psi is 1,250 lbs per square foot. I had a pipe manufacturer verify and I was using the correct formula. It surprised me too.

 

[rconner]

For reference a few years ago we happened to conduct a special test wherein two minimum Pressure Class 150 (0.46 nom. MT), 48" push-on joint ductile iron pipes, each 20 feet long, were joined together horizontally aboveground ​on timbers. This mini-pipeline was closed with a similarly joined cap and plug, and thereafter a 440 volt vacuum pump was used to pull a strong vacuum on the assembly. With a little less than a full vacuum on the line, this pipe assembly and level of vacuum was held for 24 hours, sustaining incidentally throughout an aggregate external/radial load (just from the "head" of air outside the pipes) of more than a million pounds.

 

[BigInch]

Installation limit for normal handling and shipping of steel pipe without excessive headaches is Diameter / wall thickness <= 100

48" and 0.25" walls and you're at 160
Good luck with that.

 

[couplandboy1]

Maybe that's the case for you knuckle-dragging petroleum types.

 

[BigInch]

We'd lose too much money farting around with kid gloving buckling pipe. Couldn't even stack um more than knee hi to a horny toad. 3/4" wall is the only way to go.

 

[rconner]

Hey, some of 'em are pretty darn smart (see attached, and what this one has in his right hand) - the only thing that is missing is the even bigger smile on his face when he chomps down on them! All have a good weekend.