Storage Tank Wall thickness (API 650)

[msshah17]

Hello,

I am working with the client regarding storage tank (API 650 code)- build 1960 - 304 SS Material, ID = 14', H = 20'

Per fabrication drawing, the shell thickness was = 3/16" = 0.1875" so per specification I build the tank model in TANK software. No problem.

last month, they did the inspection at couple of places and the Ave. wall thickness is = 0.201"

My question - How come the value get higher. I thought because of the Corrosion (If any for 304 SS0,it will be lower?

Please let me know your expertise.

I appreciate your help.

Regards,
Mayur

 

[bimr]

The 3/16" is a "nominal" thickness. While steel is manufactured to a nominal thickness, the actual steel thickness may vary slightly. The tolerance is specified by the codes. Refer to the link for examples of the steel manufacturing tolerances.

http://www.bssa.org.uk/topics.php?article=320

 

[zelgar]

The minimum shell thickness should be based upon calculations or the standard's minimum + corrosion allowance. As long as the thickness is above 3/16" the tank is still meeting the requirements of API 650 (which is a tank construction standard). A lot of tanks constructed in accordance with API 650 are operated & maintained in accordance with API 653 (which typically has a lower minimum shell thickness requirement, but does require routine inspections).

 

[msshah17]

I appreciate your input.
Thanks - Mayur