Tolerance

[izamil]

Fabrication of Carbon Steel Horizontal vessel

Size of vessel:ý ý55m long X 4.8m ID X 44mm wall thickness.ý

Material:ý SA516-70ý

Code:ý ASME Sec VIII Div 1 (“U” Stamp)ý

As per UG-80 (permissible out-of-roundness of cylindrical, conical, and ýspherical shells).ý

The above vessel after fabrication was found to be 6mm out-of-roundness. ýHowever after PWHT the vessel was found to be 76mm out-of-roundness. ýMeasuring from 12 o’clock – 6 o’clock (4902mm) and 3 o’clock – 9 o’clock ýý(4826mm)ý
The maximum out-of roundness allowed by UG-80 is 1% of the nominal ýdiameter at the cross section under consideration. As there are no openings ýwithin the proximity of the above measurements, the 2% of the inside ýdiameter of any openings does not come in to the equation. ý

The question I would like to ask is:-ý
Does UG-80 mean “PRIOR” to PWHT or “AFTER” PWHT, as there is no ýmention of PWHT in the relevant section.ý

 

[Sharik]

UG-80, Paragraph (a) states: "Internal Pressure. The shell of a completed vessel shall be substantially round..."

The key word is 'completed'. Since the PWHT is a process of fabrication, the 1% out-of-roundness would be applicable after PWHT.

 

[bmoorthy]

1% out of roundness requirement is the final shape requirement.

What about the profile change (If the job is in Vacuum service)?

What about the straightness, does it meet your clients requirement?

Certainly the job requires rework.

How to go about?

If the ovality is restricted to local shell courses (Near the areas where the supporting saddles were provided at the time of Heat treatment) then you may cut them and, flush grind and reroll to bring it to shape. Then weld it back to the shell courses and perform PWHT again with proper supporting.

Now comes the tricky part. Does the shell course require Stress relief after re rolling or is it ok to perform local pwht for the long seam after reroll or is it ok to perform pwht only for the circum seams and leave the long seam as it is?

I think local PWHT of Circum seams will do, Long seams need not be heat treated.

Prepare a detailed rectification procedure and submit it for your AI's review. Take his opinion and follow the same.

Good Luck.

 

[bklauba]

In order to plan an effective rework strategy, it would be helpful to know under what conditions the distortion manifested itself. Was the PWHT done on the entire unit, or piecemeal?? If done on the entire unit, was the temperature monitored at numerous representative points so as to assure uniformity of soak temp and a sufficiently isothermal cool-down? Was the distortion present immediately after PWHT, after removal of in-furnace support, or after transport? Unless you have answers to these Q's, you might end up going in (not quite round)circles.