Internal Pressure

[Leinster]

Dear All,

If you have an open vented tank how would you go about ascertaining the internal design pressure of your tank when calculating the overturning moments? I do not want the maximum design pressure as per Appendix F but the actual design pressure.

I would guess that it is just an assumption as I have seen 5 mbar and 8 mbar stipulated on data sheets quite often. I am designing a tank at the moment and the client does not want to have to use anchor bolts however if I use an internal design pressure of 5 mbar it does require anchor bolts however if I reduce to 2mbar it doesn't.

What would you consider the lowest possible internal design pressure for open vented tanks?

Am I right it assuming that if you have peripheral vents then you can assume an internal design pressure of 0mbar even if the tank does not have an IFR?

I look forward to hearing back.

 

[Duwe6]

API-650 gives the non-Appx F at "not more than the weight of the roof plates". For 3/16" steel, that is 0.7 ounces/sq. inch. [will let you do the units conversion, i'm not very metric]

Unless you make a design arraingement such that there will always be a 'heel' of liquid remaining in the tank to hold the bottom down, anchor bolts are mandatory when an internal pressure gives your flat floor an uplift greater than the weight of an empty tank plus attachments [stairs, piping to the 1st hanger, catwalks, etc].

The tank dictates the protection. If you design to 2mbar, you put on a safety vent set at [or below] 2mbar. Allows "conservation vents" like a Protectoseal 830 series. Reduces evaporation and odors.

If you design to zero pressure, you have to ensure that the tank will never be closed and unvented. It will constantly evaporate contents, and the neighbors will complain about odors, and the Fire Marshal will probably issue a citation for an open tank fire-risk.

 

[Leinster]

Thanks for your reply Duwe6...

The particular tanks we have are Diesel Storage tanks which are to be open vented. Judging by your comments I assume that you can assume 0 mbar internal pressure if the tank is open vented with a simple swan neck breather?

Please confirm

 

[JStephen]

If the tank is atmospheric, then the design pressure used in overturning calculations is zero.

I believe the "not more than the weight of the roof plates" statement is from an outdated API-650 rather than the current standard.

 

[Duwe6]

API-650 11th Edition, Addendum 1 & 2-2009 1.1.1 . . . ."for pressures approximating atmospheric pressure(internal pressures not exceeding the weight of the roof plates)". . . .

 

[JStephen]

Thanks, Duwe, I'll have to look into that.

If I remember correctly, the older standards specifically stated that for pressures not exceeding the roof plate weight, you didn't need to do anything to allow for the pressure. In the current standard, even a very low design pressure is still combined with wind uplift in evaluating overturning, so you don't automatically get a specific pressure allowance.

 

[FTank]

Duwe6 should check his calcs. 3/16" Plate weighs 7.65 PSF which equates to 0.85 oz./sq.in.

Using a "heel" of liquid does not comply with API 650. You must satisfy Par. 5.11.2, both criteria.
The moment of liquid (MF)in formula 2, uses an annulus around the inside, 1/2 the height of the shell. The width of the annulus has to be calculated, but you'll find that it is usually not more than 12".
For the weight of the roof plates, you should use the 7.65 PSF (.053 psi)to try to satisfy F.1.2. This value is so small, so that if you have any interior design pressure to contend with, you'll find you require anchorage (either self-anchoring or anchor chairs).