API 650 5.7a table 1

[Maxvolonte]

Hello colleagues, I have a question about the Shell cut out for an API 650 tank. The contractor wants to use the reinforcement pad as a backing for one of the connections on the Shell. Shell is 33 mm thick, nozzle is 8" diameter. Engineering drawings call for a 10 mm gap between neck and Shell bevel. On most of the Jobs I have been in the past, this is supposed to be a full penetration Weld with no backing, having the welder search for the root from the outside and installing reinforcement pad once the outside and inside is completed and NDT. Table 5.7a or B, are not clear about this, since they call for a plus on the cout out. Can someone tell me if I am right or wrong, or explain to me the interpretation of these tables? It will be much appreciated.
Please ignore the bevel preparation since it has been rejected.
Best regards

 

[JStephen]

So far as I know, the normal procedure, and the intent of the API standard, is that the repad and neck are put into place, and the groove weld between the shell and the neck is made entirely from the inside of the tank using the repad as backing.

 

[IFRs]

I don't think the order of welding matters if you get CJP between the neck and the shell. Ihe weld symbol for the neck-to-shell welds in figure 5.7 clearly only calls for weld from one side only. Note however, if you are working on an existing tank using API 653 and welding for hydro-exempt, the root pass of the nozzle attachment weld is back-gouged which practically means you can't have the repad there.

 

[JStephen]

I assumed that the reason for specifying a minimum gap between the pipe and the tank shell was to achieve some welding of the repad at that point as well.

 

[Maxvolonte]

Thanks for the replies colleagues, but let's say, I have a 3 to 4 mm gap all the way around, and I place the repad on the backside, what is it going to guarantee the penetration of the root to thru the shell thickness and fusion to the reinforce pad. PT on that case won't work. Don't you think is better to leave just 3-4 mm gap, PT the root, then place the pad with a bevel on its outer face in order to accomplish the total adherence?
Best regards

 

[IFRs]

Nothing prevents you from doing that, and if you are the owner feel free to require (and pay) for the additional work. The figure 5-7 in API 650 does not show a 3-way tie, none is required. Per API 650 figure 5-7 and table 5-7, the gap between the shell and nozzle is at a minimum 6.5mm and a max of 9.5mm. Your gap of 3 to 4 mm does not meet the standard. Note also that if your shell material is API Group IV or higher, PWHT is required.