Pipe bends with less thickness than specified.

[lganga]

Dear colleagues

I am back with the thread 378 392289 of August 3rd. Reference Pipe bending.
The construction company purchased, heat induction pipe bends, with a wall thickness less than specified.(higher wall reduction percentage than specified).
After a wall thickness measure, in the extrados of the bends, more than 45% of all the spools are out of the purchase order specification.
The constructor proposed to calculate the thickness, considering the equation 3C from ASME B31.3, with the purpose of having less rejection percentage..
Checking their calculations, we found that, the design pressures for those calculations were taken from the Line List.
As every pipe, bends, flange, valve, branches, etc. belongs to a piping class and its dimensions are according to the WAWP of that class.
So the pressure and temperature for the wall thickness calculation should be the one obtained from the Piping Class and not the one from the Line List.
The piping class requirements (as a design condition), the strress analysis (with all load combinations) and flexibility are basic conditions for accepting the bends.
Let me know your opinion
Luis

 

[BigInch]

You do not have to operate a system at the design pressure specified by the Class Designation of flanges. You have to ask yourself, "What was the purpose of writing design pressures in the line list that were less than the Flange Class Designation, if that design pressure was insufficient for operation."

 

[lganga]

Big Inch

If the bend has a lower design pressure than the Piping Class,then the bend does not belong to that class. In case the bend retains less pressure than the PIPING CLASS then the MAWP has to be the pressure of the bend instead of flange.
As I know, all the elements of every piping class specification are calculated for the temperature, corrosión allowance and pressure, included in that class.
In my case, we are erecting a new plant, and we have to verify the thickness due to a bad fabrication process. So bends has to be able to resist the class pressure as the flange is.
Many times the weakest element of a piping class are flanges, then MAWP of the piping class is defined by it. Usually but not always, the pipes and bends and accesories are no limiting the piping class pressure.
But I am sure that all the list lines, with the same Piping Class assigned, have a design pressure and temperature, lower to that class.

 

[racookpe1978]

No. The bends have to be able to meet (and exceed) the pressure needed FOR THE PLANT. You cannot go backwards and set a new max allowed design pressure just because the pipes or their parts (bends in this case) have a certain max pressure limit.

 

[europipe]

I should reject the bends, maybe later they want to re-rate the system, then there is a problem.

 

[lganga]

Racookpe1978
I agree with you, and as europipe says, the reasonable action is to reject the spools, but it will stop the piping job during several months, as we have spools with the bends.
For me the Construction Company, should have to verify, at least, if the bends satisfy the design conditions of the project documents.
Some of them are:
- P+I, with the lines numbers and the piping class
-List of fluids with design pressure and temperatures requirements, the piping material and the class description.
-Piping class specification, describing the material, size, corrosion allowance and maximun pressure and temperature for pipes, elbows, bends, pipe reductions, flanges, branch connection table, etc. in accordance to every class.
Also to check the stress and flexibility analysis with actual bend dimensions.
Regards
Luis

 

[Gator]

Well, let's all just hope for no spontaneous rapid uncontrolled disassembly...

Piping Design Central

 

[BigInch]

Don't let your presumptions get in the way of what you are supposed to be doing.

What document defines the design requirement for the bends? Read it. Verify it. Do it.