Calculating MAWP of API 650 Fixed Roof Tank

[OilBoiler]

Hi everyone,

I need to calculate the MAWP of an existing API 650 tank so I can start designing a vent vapor recovery system. and I'm looking at Appendix F of the API 650 code. I'm a process engineer, so when I got to Figure F-2 I got lost. So if we use the formula shown in section F.4.1, that should give us the MAWP, right? I'm just confused trying to figure out the area resisting the compressive force (as shown in F-2). If anyone can enlighten me on this, I would greatly appreciate it. Thank you!

 

[Tankman650]

OilBoiler,
What design pressure does the name plate indicate?

If no pressure is listed then it probably was not designed for pressure. The design includes more than the roof plates. A small internal pressure on a tank will produce a very large lift force on the foundation. If this force is not accounted in the design, the foundation will lift out or the anchor bolts may rip the shell.

You can safely assume that the internal pressure will balance the roof sheets. For a 7 ga. roof (.179") that results in 7.3 psf (.05 psi).

 

[JoeTank]

Whether designed for an internal pressure or not, every tank can be evaluated for an allowable pressure. Appx F uses three criteria. Rim angle compressive stress, rim angle failure and shell uplift (if unanchored).

Steve Braune
Tank Industry Consultants

http://www.tankindustry.com

 

[JStephen]

In most cases, a tank that isn't specifically intended for pressure will be similar to Fig. F-2(b) and (d) (or a cross between the two). You'd need to determine the actual geometry to calculate the allowable pressure.

Fig. F-2 is a bit confusing. But note that w-h is figured the same way in each of the cases, although the formula is only shown in one place, and similarly for w-c. The R-2 is found by dividing the horizontal radius by the sine of the slope angle theta.

Once you've done that, you also need to check per F.4.2. Then calculate the estimated failure pressure from F.6 and resulting vent setting as in F.4.2.

 

[davidribeiro]

What I normally do is have a wall thickness test done from the outset, and check the required thickness as per Paragraph 3.6.3 (One Foot Method).

If the shell meets this criteria, then:
1. Calculate the total weight of the shell excl roof plates
2. Calculate the area resisting the compressive force (Area roof + Area Shell + Area Rim Angle)
3. Calculate the wind moments at the base.

With these variables now known, you can follow F.4 to determone the MAWP.

Please correct me if I'm wrong guys...

David