API 653 Max Patch Size in Critical Zone

[sajk14]

Hello All,

API 653 Fig 9.13 gives details for "tombstone" shaped patch plates in the critical zone and states the max dimension for these patches along the shell of the tank is 24 inches. It also gives minimum distance requirements between 2 tombstone type patches.

I was just wondering what is the theory behind the above mentioned max and minimum dimensions for the tombstone patch plates in the critical zone? Is it a stiffness issue with regards to the shell to bottom joint?

I know in API 650 section 5.5.2 gives a formula for calculating a minimum radial width requirement for annular plates which is based on the thickness of the annular plate and was wondering if its related to this in some way?

My thoughts being whether we can use a tombstone patch greater than 24 inches along the shell ( say 44 inches ) provided the patch has enough radial width?

Thanks for your thoughts and comments on the matter.

 

[JStephen]

My guess- there is no theory behind it. They're figuring if it's bigger than that, you should go ahead and insert new plate under the shell and the resulting joints will be far enough from each other to be okay.
My guess on where this comes from: You've got a Committee. One guy on that committee thinks you oughtn't do this kind of repair at all. Another guy is perfectly okay with replacing the entire perimeter with this kind of patch. One guy has no opinion on the matter. One guy says the patch should be limited to 27" because that's the rule of thumb that Acme Tank has used for 40 years. And so on. So they all get together and come up with a consensus on what constitutes good practice on that limitation and there it is. If you use 48", I doubt the tank is going to fall down, you just won't comply with the standard.

 

[sajk14]

Thanks J Stephen for your thoughts on the subject.