Overpressured during Hydrotest of boiler

[kumar88]

After 1 nos of boiler superheater tube replacement, we conducted hydrotest. the test pressure is 1.5 x MAWP= 71 Bar. But due to mistake in chart recorder, we overpressured it till 122 bar.

We depressured it and performed visual inspection to see if the overpressure caused any deformation. But there was non found. So we redo the hydrotest with correct test pressure of 71 bar, and we found that 1nos of tube (different from the 1 we replaced) is leaking at the superheater header to tube weldment.

My question is, could the tube leak due to the overpressure or the tube is already defect before that. Is there a way to know what caused the tube leak. The original tube we replaced also because of the defect in the weldment.
All the other tubes are fine with no trace of leak at 71bar.

Thank you

 

[davefitz]

I have trouble believng that the low temperature components ( whose allowable stress is a fraction of yield or UTS) had been overpressured to 2.57 times allowable stress . In particular any pressure vessel or flange or forged valve would have experienced some failure at 1.25 times allowable .

"Whom the gods would destroy, they first make mad "

 

[racookpe1978]

I could accept the single-case over-pressure incident with a burst (complete failure) of the assembly, and in truth, the writer did admit there was the single failure. We really do "over-design" and "under stress" components sometimes with our choices. And, sometimes, the tolerances and cooler metal (higher stress resistance) and slow pressurization rate and wall tolerances and weld strengths do all add up on the "stronger than expected" side.

But I am surprised there was no noticeable deformation or stretching or ballooning of components, particularly those with the larger diameters. Do your inspection again looking not at the small diameter tubes, but the large diameter headers and pipes.

 

[GenB]

I do not believe the test at higher pressure rupture the welds, it may have been already crack or traces. but it looks that you wish a scape that the owner will not accept...

 

[weeeds]

You mentioned that this is a boiler and if so then the rules for pressure vessels don't apply. If built to the ASME Code then this is a Sec. 1 boiler and per PG-99.1 "No part of the boiler shall be subjected to a general membrane stress greater than 90% of its yield strength ...". This is what you need to ensure has not been exceeded otherwise you have a piece of scrap.