Client specified design pressure and maximum pressure per F.4.2 1

[ismar74]

Hello,
I am dealing with the following issues:
Client has specified design pressure 3,73 kPa for storage tank built acc. to API 650
Calculation acc. to F.4.2 (and pre-designed roof-to-shell area) shows that maximum allowed pressure should be 1,80 kPa. In addition, calculation acc. to F.4.1 show higher design pressure than 3,73 kPa, i.e. 6,80 kPa.
Is ther any way to satisfy Client requested design pressure and in same time max. allowed by F.4.2?
To me, it seams that Client has to re-consider its required design pressure, due to max. pressure allowed by F.4.2?!

Does anyone can confirm that API RP 937A (found on techstreet.com) can clarify design of small storage tanks with frangible roof-to-shell joint for above mentioned issue?

PS: Storage (crude oil) tank, has 7,62m diameter and 8 m height, with ~9° slope supported fixed roof.
Thanks, in advance..

 

[JStephen]

F4.1 and F4.2 are both requirements, not alternative methods. It sounds like pressure is limited by uplift/overturn but not by roof construction. In that case, you can either reduce allowable wind load (see F.1.2) or anchor the tank. Or increase plate thicknesses to increase weight, which is usually impractical.

I'm not sure how helpful API RP 937A is. There have been some methods used to analyze frangible roofs and it may or may not include a tabulation of results/sizes/slopes, etc.

 

[ismar74]

Maybe it could be helpfull to clarify...
Client has specified wind speed to 100 mph, and tank satisfy overturning stability in accordance to 3.11 (API 650). Nevertheless, allowed maximum pressure in acc. to F.4.2 due to high wind overturning moment is quite low.
In addition Client has specified storage tank without anchorage.
I understand that desing pressure could be raised, in acc. to F.4.1 by increasing roof-to-shell area and roof plates thickness, but what is a "value" of that if tank still doesn't satisfy F.4.2?

 

[IFRs]

Without anchors, you really need to check uplift from the pressure.

 

[JStephen]

If the client has specified a design pressure, a design wind speed, and those conditions can't reasonably be met with an unanchored tank, then they need to specify something different. Their requirements cannot be reasonably achieved. (Their requirements can unreasonably be achieved by increasing plate thicknesses to whatever it takes to furnish adequate dead weight, but this is not the solution you want.)

I have run across a number of cases where clients specified an unreasonably high pressure because they did not realize the impact it had on tank design- and they quickly figured out ways to lower that design pressure when they realized it was going to cost thousands and thousands of dollars extra.