Rating of Atmospheric tank

[kryanl]

In the coming months, we are going to get an existing tank re-rated. The tank currently operates at pretty much atmospheric pressure. I was assigned the duty of digging out the original files for the tank, and I notices something that I can't seem to reason to my self. The tank was designed to be rated for "+/- 10 in. w.c" either a very slight vacuum or pressure. The original specs also state the design was for fluid of 1.23 S.G. Now, the tank is 13' tall, and when it is full (elev. 11') that means there is approx. 5.8 psi pressure in the tank, and since the tank is open to atmosphere, that means 5.8 psig. So, then, I would assume the tank is rated for the +/- 10" above the fluid in the tank, but then how can it ever acheive a vacuum? Also, why even bother to rate a tank for +10" when all that equals is about foot of fluid? Why not just rate it for 6 psi, then you don't have to worry about the liquid level? Am I over-analyzing this? Please help.

Thanks,
Kay

 

[matay]

Kryanl.
As far as I understood, even the tank open to the atmosphere, while draining the tank, if any procoution did not taken against vacuum or if the vent pipe to atmosphere is not designed to meet all possible vacuum requirements, as a protection, tanks designed to withstand some vacuum.This might be the reason you should take into consider.

 

[solinar]

Do not think about the pressure due to static head as being a numerical value for rating purposes. The tank will most likely have been designed to hold a full level of liquid +/- the 10" water column.

This tank was likely originally designed to hold as worst case scenarios 1) full of liquid + 10" w.c. (0.36 psi) and 2) empty with 10 w.c. (0.36 psi) of external pressure. Depending on the size of the tank, and how much original engineering was performed, it is also possible there is a required liquid level for the external design pressure to keep the floor from sucking up.

If the tank is open to atmosphere (vent through an open nozzle or some similar relief) the only way I can see external pressure being applied is if you have some kind of gas evacuation system that pulls a vacuum on the tank faster than the relief will allow atmospheric air back in.

Also, why even bother to rate a tank for +10" when all that equals is about foot of fluid? Why not just rate it for 6 psi, then you don't have to worry about the liquid level?

The design pressure is on top of a full level of liquid. If this is a flat bottom storage tank of any notable size, it will not come close to supporting a 6 psi design. 6 psi of pressure would over stress the roof to shell joint and require some massive anchor chairs to keep the tank flat and on the ground.

 

[JStephen]

The design pressure on a flat-bottom tank is always the pressure in the vapor space at the top, not the pressure down through the liquid (which varies with depth). The 10" water column p/v may have been a design requirement, or someone might have just calculated what the as-furnished tank could handle.

Refer to API-650, Appendix F, and possibly to API-620 for design methods for a tank of these dimensions.

When you fill or drain a tank, air will move in or out through the vent. In order for that air to move, there has to be some differential pressure, and so you can have some nominal pressure or vacuum acting on a tank that is open to the atmosphere. If the tank has a conservation-type vent, that only opens at a specific pressure or vacuum, it will develop that pressure or vacuum each time it is filled or emptied.

Normally, a tank like this would not be designed for more than about 1" of water column, partly for the floor-liftup problem mentioned above. You'd need additional information to know how they actually figured a 10" vacuum.