API-650 Table J-1b

[gdubyuh]

Referencing API-650, 11th Ed., Addendum 1, Table J-1(b) lists the 'Maximum Roof Depths for Shop Assembled Dome Roof Tanks'. J.3.5.3 indicates this table is specifically the maximum roof depth for a dome or umbrella roof that has a straight flange to allow for butt-welding of the head to the top of the shell.

The listing in the table for the maximum depth, to me, seem very shallow. A Standard F&D (Flanged and Dished) head typically has a Dish Radius equal to the O.D. of the head. So, taking a head with a 96" O.D., and drawing out a dish radius of 96", would give you a head depth (exclusive of any straight flange) of about 12 3/4". So, for the table to list the maximum head depth as 3 1/2" for an 8 foot diameter tank, seems really shallow. Am I missing something?

 

[JStephen]

It looks to me like this should be the minimum depth, not the maximum. It is stated that rather than limiting the maximum dish radius (the 1.2D limitation) on standard roofs, this criteria is to be used instead, which would limit how flat a roof could be, not how domed it could be. I can't think why you'd require F&D formed roofs to be shallower than segmented umbrellas.

 

[gdubyuh]

I was thinking the same thing. I rarely design with a flanged roof..maybe 1 out of 200 API-650 tanks. I went back to API-650 10th Ed., Add. 4 and confirmed that those editions also refer to the J-1 table as the 'Maximum Depth'. Guess I've just never noticed it since it rarely comes in to play for me. Thanks.