Nominal tank diameter in 5.6.1 API 650

[TheEverest]

Dear all,
What does the nominal tank diameter in 5.6.1 represent? IS it INSIDE Diameter OR OUTSIDE Diameter of tank?

 

[JStephen]

Usually, the tank diameter will be a rounded number (120 feet or 18 meters or something) and that is the nominal diameter. It may be the ID or OD or at centerline of plates. See Annex L.4 and Section 5.1.5.3(b). This may be addressed somewhere else, but I'm not seeing it at the moment.

 

[TheEverest]

I found out in section 5.6.1.1 that unless otherwise noted it is the centerline diameter of the bottom shell courses plate. But in Annex F, figure F.2, Rc is given as inside radius of tank shell, can we use nominal radius(nominal diameter/2) in this case?

 

[Geoff13]

For design it's a meaningless difference between 100' centerline diameter and 100' inside diameter. On a 100' tank with a 1/2" wall the difference is D=100.00' vs D=100.04'. This won't affect ordered thickness, App F required area, etc. I'd use D=100.0' for both cases.

However for detailing and construction it can be important. On the same tank if you detail it for 100' centerline, but build it 100' inside you'll have a 1.5" gap at the final vertical weld.

 

[TheEverest]

Thank you, Geoff13.