Bent Piping 10

[Joss10]

Dear Piping Engineers,
The line showed in the picture is as CS HP gas line 2" Sch 160 - WP 100 Bar, was discovered bent on that pipe rack because of some of its support were tack welded, root cause of the problem is still under investigation.
The question is whether can leave the line as it is waiting for SD or inmediate repair is advisable.
Thank for any contribution on this matter.

 

[DSB123]

evaristo10,
What you need to do is to perform a fitness for service assessment on the line before you can make a safe decision. Use API 579 as the basis of your FFS analysis.

It looks like there has bee a line stop (either weld or physical stop) where it should not have been and the pipe has essentially buckled to alleviate the stress in the line. As such the stresses to cause the buckle could have been high.

Remember the pressure is 100 bar so not inconsequential.

 

[LittleInch]

The issue is whether the line will be subject to any significant axial stress in operation. If it is then the current buckle could get suddenly much worse and properly buckle. As it is it appears to have strained a few percent based on the radius of curvature of the worst bit and not buckled. The working pressure should be reduced to take account of the damage.

a fully calculated and checked FFS assessment is a must and until you discover why it occurred you shouldn't use it.

Is there a big difficulty in cutting this section out and replacing?



My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[3DDave]

Why the solid axial restraint?

I've seen similar damage due to a few hundred degrees F differential between the supported item and the support over a short span. A longer span with a lower delta would still have trouble. I can't guess how there would be a thermal problem here, but if the pipes don't need to be held rigid in the axial direction, is there some reason not to fix them axially at one place and let the rest of the run float?

 

[racookpe1978]

One problem you have NOT addressed (at least in this forum!) is your attempt to straighten or re-bend that pipe so it can be re-used again.

Forget safety and fitness for duty - as if you can. 8<)
I do not believe there is any way to take that shape and length of bend in the middle of the straight run and get it to "re-straighten" successfully.

Stop your effort to "save" the pipe: Cut it out and re-weld a straight piece. To straighten, you have bend it "backwards" at least 3% past the straight line using a form or bender with a complex radius 6% smaller than the bend you are trying to straighten out. Ans, you MUST stay in the plane of the bend through the whole bend, or you add new bends back into the re-bent bent section.

Ain't gonna happen up in midair with a twisting bend like that.

I play with hot metal as a metal bender/fence straightener/ornamental iron welder in my spare time. This one ain't gonna straighten safely without hundreds of man-hours of work that will almost certainly still fail to give you a safe pipe to operate at that high a pressure.

 

[BigInch]

Many pipelines continue to operate normally with considerable unplanned bending deformations, due to mud slides, subsidence, or displacements from earthquakes, as long as there is no localized damage from local wall buckling. Its worth checking out, if you must remain in operation. With buckling load dissipated, your pipe is probably operating at a lower stress condition than it was designed for, albeit with slightly more displacement than you would like to see. Nothing really to get excited about just yet, as long as the pipe's contents aren't spraying about.

The buckle has alleviated any excessive compressive stress. If there was still any axial stress to speak of and with that deformation having already occurred, column buckling would have either continued until structural collapse, a local buckling failure happened, or it reached a self limiting condition, i.e. no further deformation possible. It appears that local buckling did not occur, therefore it has apparently reached a self-limiting condition. All excessive axial stress (greater than Euler's buckling stress) has essentially been dissipated. Perhaps a small amount of axial load remains which is now holding the bend at its present curvature. That residual axial stress can be calculated (see Palmer's uplift buckling force derivations) as well as the resulting secondary bending stress. Check those combined stresses, ensuring that they are still below allowable combined stress levels. suspect that they are and inspect the pipe to ensure no localized damage. If OK, you could proceed to operate normally. If stresses are higher than allowable, try reducing internal pressure, or temperature, preferably both, until the stresses are within allowable limits.



I hate Windowz 8!!!!

 

[racookpe1978]

"Look" and "appearance" costs are, however, sometimes as important as the "first cost" of replacing the pipe.

A regulator "looks" first, then begins investigating and poking and prying. So do customers.
A clean overhead pipe rack with uniform appearance and little or no rust and destroyed insulation run in straight rows with good-looking isolation valves (not deposited with rust and stalactites of dripping condensate and packing leaks) "sells" to a client.

A bent, twisted and deformed pipe sticking out of a pipe rack clearly caused by bad design, operation, and installation - and NOT corrected since the incident? Why should "I" - the potential customer - trust "you" - the operator who apparently doesn't care enough to fix busted pipes sitting right in front of your eyes - with my company's products?

 

[chicopee]

I would cut out the bent section and weld a new segment. It looks like the pipe was used as an anchor for some strap and ratchet device.

 

[Joss10]

Just to make an update on this issue, as soon the line was depressurized to 0 bar, it recovered its straight position, anyway the bent section is to be replaced and all the restriction points to be removed.

 

[BigInch]

Then apparently it was a self-limiting elastic buckle, where bending stress and displacement replaced axial force as the curve developed.

Out of curiosity what was the operating temperature?

I hate Windowz 8!!!!

 

[Joss10]

Working temperature is around 70 celsius.[pre][/pre]

 

[3DDave]

What happens if pressure is applied at ambient temperature? It occurs to me that a non-uniform wall thickness would also cause this to happen under pressure, regardless of temp. Still, a thermal delta and overconstraints on the ends are the most likely sources. If it came back to straight then it probably did not suffer damage, hence the interest in checking the pressure-only case to make sure it's not a defective tube.

I see you're on schedule to replace it anyway, but I'm thinking of root-cause for bent pipes in general.