Concrete Water Tank Question

[qubitsized]

Hi Everyone,

We have a rectangular concrete potable water tank, per figure below.

Tank is 25mx25m. Depth of water originally at 2m. Free board of 0.6m, manhole, vent, over flow provided. (top image)

We later found out the pump head are not enough. Instead of buying pump with greater head, we decided to adjust water level only by 2.50m and geometry of tank by adding a 'water chimney' (bottom image). This 'chimney' is 5m x 2m and 3m high.

Could you give your comments on this change? Are such tanks normal?

Thanks for any help/advise!

 

[ashtree]

I think it would have been easier to change the pump. Whilst 0.5 m extra will make a difference, unless it is a very low head pump the difference will be pretty small.

Having the water level up the chimney will now mean that any manholes, pipe entries vents etc will have to be well sealed to prevent leakage because you now have the water level above those entry/exit points.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[bimr]

No, tanks like this are not "normal". It is generally required to have 2/3 of the tank above grade so that should the tank leak, the water will not be contaminated.

Raising the height of water may cause the roof to collapse. It depends on how the roof was designed.

Suggest you hire an engineering professional to size the correct pump. It will be more economical.

 

[77JQX]

What happens when the pump draws down the chimney? Aren't you back where you started? Conversely, if you can keep the chimney full, why do you even need the tank?

 

[qubitsized]

Thanks a lot guys!

My higher ups doesn't acknowledge a problem exist and they said it is normal design. I have it all on record.

Construction is on going and pumps already purchased.

I asked for an actual plant reference and currently waiting for feedback.

 

[QualityTime]

This is really an improper design. The roof will be structurally compromised. Usually the roof can take about 6" of surcharge.

 

[MartinLe]

Next problem: Your pump cycles, pumps the small volume from the "chimney" and you have the old, insufficient head again.
Unless inflow and outflow are very well balanced, but then why the big tank?
get a proper pump for the application.

 

[LittleInch]

Every joint and surface in this tank is now under more pressure than it was designed for.

The roof in particular would not be designed for such an upwards thrust force and has a possibility of breaking as soon as you do this.

As noted above, all you have managed to do is reduce your buffer tank volume from the concrete tank size to about 10 m3. To keep it at the height you need for your extra 2.5m head you would need to constantly pump water over the over flow.

To do this for only an extra 2.5m head is crazy and most definitely not normal.

Can't they just modify the pump? or pump speed.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[BRIS]

Raising the water level in the chimney would appear to negate any purpose of the tank - the only effective storage you now have is the volume in the chimney !

 

[hydrae]

I an curious
Is the ‘not enough head’ a discharge pressure problem or a limit on NPSH? (Net Positive Suction Pressure)

As others have pointed out, adding a little in elevation on the suction side will only increase the discharge head by the same amount, so unless this is a high flow-low head pump station the proposed solution does not make sense.

Now if the problem is NPSH, then your solution can avoid all the problems listed, dig the pit deeper
or insert pump cans below the sump shown so the pumps have the correct NPSH.

Hydrae