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Post Fabrication x-ray? 1
- Thread starter Water_Guy
- Start date
Just a caveat upfront: delving into a tank’s MAWP history and attempting certification to a certain code can be like opening Pandora’s Box.
To be pedantic, your tank has an MAWP that isn’t going to change by radiography of the welds - your tank physically remains the same. Any certification only brings assurance that the listed MAWP is actually accurate. It sounds like the tank was never certified to API 650 - how sure are you that the current MAWP is accurate? What joint efficiency was used in the MAWP calcs?
I ask this because your rated MAWP may, or may not, be able to be increased if you perform ALL the QA API 650 requires, including a hydrostat. Using the tank dimensions, thicknesses, etc, you can use the standard joint efficiency given by the code to re-rate the tank. That rating may be higher or lower than what it currently is rated. If the MAWP calc that was originally done used a joint efficiency of 100%, then you may even see a decrease in calculated MAWP unless you do 100% radiography (I believe API 650 uses 0.9 for welds spot-checked with x-ray, but don’t quote me on that).
However, be aware that doing this QA could lead to you needing to grind out and re-weld major portions of the longitudinal welds if you find any defects. Plan for the additional time and expense.
To be pedantic, your tank has an MAWP that isn’t going to change by radiography of the welds - your tank physically remains the same. Any certification only brings assurance that the listed MAWP is actually accurate. It sounds like the tank was never certified to API 650 - how sure are you that the current MAWP is accurate? What joint efficiency was used in the MAWP calcs?
I ask this because your rated MAWP may, or may not, be able to be increased if you perform ALL the QA API 650 requires, including a hydrostat. Using the tank dimensions, thicknesses, etc, you can use the standard joint efficiency given by the code to re-rate the tank. That rating may be higher or lower than what it currently is rated. If the MAWP calc that was originally done used a joint efficiency of 100%, then you may even see a decrease in calculated MAWP unless you do 100% radiography (I believe API 650 uses 0.9 for welds spot-checked with x-ray, but don’t quote me on that).
However, be aware that doing this QA could lead to you needing to grind out and re-weld major portions of the longitudinal welds if you find any defects. Plan for the additional time and expense.
I don't think either API-650 or API-653 specifically allow for this.
I wouldn't expect it to affect the vacuum rating at all.
It would only affect the pressure rating in very specific circumstances. Usually, pressure rating is limited by top angle details, roof slope, or anchorage details and not so often by shell design; may be limited by roof nozzle details, etc., but not so often by shell design.
You could just operate the tank at a lower level to compensate for pressure effects on the shell.
If the tank includes a corrosion allowance, reducing the amount in a re-rate might help. Adjusting specific gravity for a change of service might also help.
Switching from an Annex A design to a basic design is similarly not addressed.
Edit: Just saw the response above. I'm assuming it was built to Annex A with radiography omitted and you now wish to include spot radiography to increase joint efficiency used in the design. If it was built as a non-code tank in the first place, then no amount of re-work or analysis will turn it into an API-650 tank.
I wouldn't expect it to affect the vacuum rating at all.
It would only affect the pressure rating in very specific circumstances. Usually, pressure rating is limited by top angle details, roof slope, or anchorage details and not so often by shell design; may be limited by roof nozzle details, etc., but not so often by shell design.
You could just operate the tank at a lower level to compensate for pressure effects on the shell.
If the tank includes a corrosion allowance, reducing the amount in a re-rate might help. Adjusting specific gravity for a change of service might also help.
Switching from an Annex A design to a basic design is similarly not addressed.
Edit: Just saw the response above. I'm assuming it was built to Annex A with radiography omitted and you now wish to include spot radiography to increase joint efficiency used in the design. If it was built as a non-code tank in the first place, then no amount of re-work or analysis will turn it into an API-650 tank.
JStephen, perhaps I misunderstood the OP - I took him to mean the tank had been built according to the code, but the QA required in the code was not performed, leading to a tank that was "built" according to the API 650 but is not actually a certified API 650 tank.
I took the OP to mean that the tank was built to API 650 and the supplier had chosen to use the joint efficiency applicable to no x-ray testing, essentially an Annex A tank. And that they wanted to RT weld seams to increase the joint efficiency and re-rate the tank accordingly. My thoughts are that the additional NDE will only confirm the original joint efficiency and is unlikely to do much else. You'd have to RT every vert in part because you don't know how many welders there were and also because they are the critical joints.
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1
- #6
HTURKAK
Structural
- Jul 22, 2017
- 3,358
The OP provided very limited info..how old is the tank ? improving the MAWP implies change of service and internal pressure..
My opinion is, this is scope of API 653 and this problem could be considered as reconstructed tank..
This is copy and paste from 653..
IMO, at least the vertical welds shall be removed to uprate the tank....
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