Dear all,

as per 5.10.2.6.a of API 650, i am satisfying all conditions.

I have calculated the maximum limit on the area at roof to shell junction.

now the tank has internal pressure of 1.45 psi(g).

as per appendix f F5.1 i have calculated required area at the roof to shell junction. The required area comes more than the frangibility limit.

What path should i follow?

Also is the calcualted failure pressure (Appendix F-F6) is the frangible pressure so roof to shell joint to fail rather than shell to base plate joint?

Please help.

Regards
ASD23789

"Failure of the roof-to-shell junction can be expected to occur when the stress in the compression ring area reaches the yield point" says F.6, Calculated Failure Pressure.

F.4.1 establishes the maximum design pressure, P, for a tank that has been constructed or that has had its design details established. The basis of that derivation was to include a coefficient of safety of 1.6 addressed to the "failure" pressure.

Theoretically. with the notations of Addendum3- 2011, the failure pressure would be expressed in SI units as:

pf=8*Fy*A*tanθ/D^2+4/PI*DLR/D^2 - units in [Pa]

or

pf=Fy*A*tanθ/(125*D^2)+0.0012732*DLR/D^2 in [kPa]

As I said, F.4.1 considers a safety coeff of 1.6 applied to the pressure effects term (i.e. the first term), so the maximum design pressure, P is:

P=Fy*A*tanθ/(1.6*125*D^2)+0.0012732*DLR/D^2=
=Fy*A*tanθ/(200*D^2)+0.00127*DLR/D^2

whereas F6 prefers to express pf in terms of "P", so would be:

pf=Fy*A*tanθ/(125*D^2)+0.0012732*DLR/D^2=
=1.6*P-0.6*0.0012732*DLR/D^2=
=1.6*P-0.0007639*DLR/D^2

(you can see that they mistyped 0.000746 instead of 0.000764)

In the previous editions, instead of DLR appears a term based on the thickness of roof plates and DLR is calculated with a metal density of 8000 kg/m^3 (8 times the water density).

You may note that according to API pb 937, the target of the frangible roof-to-shell joint should be that the yielding of the top compression ring will occur before uplift of the bottom-base of the shell.


Hope this help,
Best regards.

To answer more directly to your question...

The API 650 frangibility condition is not considered directly as p_failure_roof< p_uplift.
Of course, you can do it in this way- however there is the risk that your Client/ his Consultant to say this is a non-API approach.

F.5.1 is a condition that evaluates the total required compression area at the roof-to-shell junction when the maximum design pressure has already been established. There "required" word would mean "required for safety operations" and you can understand from my previous post that there is a safety factor of 1.6.

I have to recognize that it is not so evident that the condition in 5.10.2.6 (which is "All members in the region of the roof-to-shell joint, including insulation rings, are considered as contributing to the roof-to shell joint cross-sectional area (A) and this area is less than the limit shown....") is the condition that the roof will fail before failure occurs in the tank shell joints or the shell-to-bottom joint.

The background of this formula is the fact the roof force can be separated into vertical and radial components. The radial component leads to the "compression" force in "compression ring". The vertical component is an uplift force for the shell.
At roof failure (corresponding to pf pressure and Fy stress in compression ring), the forces components are radial Hf and vertical Vf .
Vf< DLS (dead load of shell) means "no uplift" condition.
This condition would be interpreted as when the roof fails, this is before uplift.
The condition above written is equivalent with inequality areas required by 5.10.2.6 Frangible Roof.

Best regards.

Thanks Mariog.

Regards
ASD23789