RWT ( Remaining wall Thickness ) assessment of Pressure vessel

[gasoperations]

Here is a question / clarification required for API 510 ( Pressure Vessel Insp. Code )

Section 7.4.2.1 states Quote " For a corroded area of considerable size the wall thicknesses may be averaged over a length not exceeding
the following" Unquote

During a recent inspection a corroded area ( 30 mm x 120 mm ) is noted on a pressure vessel with nominal thickness of 9.5 mm. Required wall thickness is 5.2 mm at design press. & Temp. Wall thickness recorded in UT inspection is 3.5 mm to to 7 mm in the corroded area at various locations.

If average thickness is calculated as per sec. 7.4.2.1, for all the thickness readings in the corroded area, wall thickness will be 6.5 mm that is well above the minimum required thickness.

Does it mean the vessel is acceptable for operation? What will be the assessment if minimum thickness is down to say 1.5 mm but average thickness is still above 5.2 mm?

Any expert comments will be appriciated

 

[BJI]

You need to comply with all the requirements / limitations of 7.4.2. It is not all readings that are averaged, you need to use a grid with equal spacing and determine which line (CTP) through the flaw produces the lowest average thickness. Generally this will be the longitudinal plane unless you have high supplemental loads. There are limitations on vessel/flaw size, minimum number of averaged points and there is also a limit on the minimum thickness reading, so a point at 1.5mm wouldn't be acceptable. Refer also to API 579 Part 4 and 5 for more detail.

 

[BJI]

Update: In the 2022 version of API 510, this has been removed and reference is made to API 579 for locally corroded areas.

 

[gasoperations]

@BJI
I have not seen API 2022, need to check.
Otherwise why 1.5 mm wouldn't be acceptable if UT readings along the CTP are averaged out as per the section 7.4.2. ( May be this is the reason that it has been removed in 2022 edition. ) let me try to check there

 

[BJI]

Under 7.4.2.2 one of the requirements for thickness averaging was that the lowest individual reading cannot be less than 50% of t required. The limiting thickness in API 579 is MAX(0.2tnom, 2.5mm) for pressure vessels, but you should review the appropriate FFS procedure outlined.

 

[gasoperations]

@ BJI. Thanks for update. I found API Ballot draft for 11 edition. Could you please confirm if these are the final changes in 2022 edition.

 

[BJI]

It has been revised, this is 7.4.2:

7.4.2 Evaluation of Locally Corroded Areas
For locally corroded areas, refer to API 579-1/ ASME FFS-1, which provides assessment methodology.

 

[gasoperations]

Thanks BJI