SIZING A PRESSURE TANK

[johnhan76]

I am sizing a pressure tanks for a small subdivision.
Here is the data:

Instantaneous peak demand = 60 gpm
avg pump discharge = 200 gpm

I have had some people suggest that I provide for 8 minutes of pumping time. I have had others say that it doesnt matter how long the pump stays on as long as the "number of pump starts" is not excessive. Since the peak demand and avg pumping rate are so different it is much more economical to size the tank to provide 8 minutes of storage rather than keeping the pump on 8 minutes. any insight will be helpful. thx.

 

[jproj]

johnhan76:

I don't work in this field and probably can't help, but I have a couple questions for you:

1) Shouldn't the instantaneous peak demand be higher than the average demand, or is the peak demand the highest required flow with the pumps being capable of supplying 200 gpm?
2) What is the average flow into the tank (is is relatively constant)? This may be more of a sizing criteria than the outlet flow.

jproj

 

[johnhan76]

yes it should. the 200 gpm is the average pumping rate from the wells. the average flow into the tank is approx. 200 gpm.

 

[JTBuer]

Probably best to talk to the pump supplier but for small systems (4 to 6 hp) I've sized the pressure tank for 2 minute running time.

 

[semo]

Ten States Standards call for the wells and pumps in a hydropneumatic system to be at least 10 times the average daily consumption rate. The gross volume of the pressure tank should be 10 times the capacity of the largest pump.

If you are chlorinating, you should also consider detention time separate from the above.

You might check with your state to see what their standard requirements are before under/oversizing the tank.

 

[nuke2]

Semo had a good idea - your local health department (or State water authority) should have sizing guidelines available for your use. The answer will depend on whether you will be installing a conventional pressure tank or one with a precharged bladder.

 

[stanier]

Check out

http://www.haestad.com/library/books/awdm/agree.asp

Look into Advance Water Distribution Modelling handbook for sizing your systems.

Do Civilquiz on the

http://www.haested.com

site to get a discount on the book.

 

[cvg]

assume 4 - 6 pumping cycles per hour, less cycles where frequent starting and stopping may be annoying or where greater reserve is desired for fire protection or emergency use. More cycles will result in a smaller, cheaper tank.