Bisected AWWA Storage Tank?

[keithwwalker]

We have a 3 million gallon potable storage tank in our system which is too large, leading to water quality issues.

Even with the installation of a mixing system, I believe the tank to be too large.

One of our maintenance managers has the idea that the potable welded steel storage tanks can be bisected down the middle with a vertical bulkhead.

In my opinion it would introduce significant stress concentration factors at the corners. That is why you don't see square steel tanks in service.

Any other opinions?

Thanks,
kww

 

[bimr]

A round shape is more inherently strong than a square shape. Because of that, round tanks are fabricated of lighter gauge, less costly steel than square tanks and are generally less expensive. The circle is a strong structural shape. The reason being is that stress is distributed equally along the arc instead of concentrating at any one point.

If an engineer where to choose a rectangle shape; they would then need to incorporate additional structural support in areas, thereby adding unnecessary additional structural weight to the overall design.

If you put a wall in the middle of the tank, the wall would have to be designed to withstand the force of a full tank on one side and an empty tank on the other. The wall would have to be stiffened with reinforcement.

You would probably be better off if you put smaller concentric tanks within the larger tank.

 

[cvg]

if you are going to erect another smaller tank, why would you put it inside the existing one? Just decommission the old tank and use the new smaller one. Or use the old, large tank for emergency fire water storage.

And how would either idea (bulkhead or concentric tanks) improve water quality? Either you need to mix better with a mixing system or multiple tank inlets / outlets; change your operational procedures to drain the large tank more frequently; or install a new smaller tank.

 

[RWF7437]

Never heard of "water quality issues" with a 3 MG tank. There are thousands of larger tanks in service with no water quality problems. If usage is so small that water becomes stagnant why not just install an altitude valve and only fill the tank half full ? This would certainly be cheaper than the structural changes suggested. It would also be easier to change later if/when demand increases.

 

[JedClampett]

I agree with everything said above. If you try to design a flat plate to split the tank in half, you're going to deal with some huge stresses and not just in the corners.
Tanks are pretty cheap now. Put a smaller one in your system. Consider it as helping the economy.

 

[nackra]

Another point to justify new smaller tank is foundation design. Seismic may be another factor depending on location.