Radiographs in API 650 tank roof built to API620 1

[monirri]

Hi. We are building a tank designed according to API 650 with 1,5psi internal pressure from nitrogen blanketing, except the roof, that is API620. The roof´s radius is 0,54D outside any type of roof considered in API 650, so is made according to API 620. Roof plates are 1/2 thick. Welds are full penetration butt-welded joints. Roof joint efficiency is 1 by design. The tank is going to have a mix 96% water and a skim of oil.

Question is: the designer says that the roof should be 100% radiographed because it is API 620 and joint efficiency is 1. However, my gut says that it should not be that way, because joint efficiency for the shell is 1 by default in the design, and they don´t ask for the shell to be 100% radiographed.

I mean, if the roof radius is 0.8D it is an umbrella and RT percent is 0%. Let´s suppose that is 0.79. That´s instant 100% RT? It doesn´t seem logical. I recognize I don´t fully understand table 5-2 of API 620.

And since API 650 doesn´t ask for the roof to have more END than hydro-pneumatic testing, I can´t see why the roof should be fully radiographed. And when speaking about risk, the most risk you can have is having a gas leak and the hydroneumatic test does that. I would like to know more opinions on this, because radiographs amount to more than U$30.000

Thanks

Carlos Molina from

http://www.apiexam.com

 

[monirri]

Any ideas?

Carlos Molina from

http://www.apiexam.com

 

[JStephen]

The extent of radiography required for a particular joint efficiency is mandated by the code being used. Where you're not in strict compliance with a code, it would be a matter of engineering judgment as to how that ought to work.

For many tank roofs subject to internal pressure, the required thickness due to pressure will be very small and roof thickness is not affected by choice of joint efficiency. You might check on the roof in question and see how thickness varies as joint efficiency is adjusted.

 

[DekDee]

monirri,
Not sure if this is any help,
Regards,
DD

APPENDIX A—TECHNICAL INQUIRY RESPONSES
SECTION 1.3 LIMITATIONS
620-I-04/98
Question: Can the rules in API Std 620 and API Std 650 be combined to design a tank?
Reply: No, unless so stated in either document for specific applications.

 

[Duwe6]

Off the top of my head, Spot RT and a Joint Eff of 0.85 for your 0.5-inch full-penn roof will yield a LOT more than an MAWP of 1.5 or 2.0 psi. And Spot only requires a few shots, one per seam max. If you use the rules of ASME UW-52, and only one or two welders, you could be RT-ing as few as 2 Spots.