Sizing of Hydropneumatic Tank

[vsv101]

This is the first time I am sizing a hydropneumatic tank with multiple pumps. I will highly appreciate any help from our experienced members who have executed similar jobs.

I am sizing a hydropneumatic tank for a distribution system with multiple pumps. The pump capacities are 2 ea.x 70 gpm @ 75 psi TDH, 1 ea. x 285 gpm @ 75 psi TDH and 1 ea. x 425 gpm @ 75 psi TDH.

I used the formula Vt = (P1 + 14.7)/(P1-P2)* 15(Qp)(MF)/Nc

Where : Vt = Volume of Hydropneumatic tank, U.S.gallons
P1 = Max. system Pressure @75 psig
P2 = Minimum system pressure @ 50 psig
Nc = Desired no.of pump start/stop per hr.@10X
(Note: the recommended no. was 6X, however, I had
used 10X in dozens of installations without any
problem)
MF = 1.1
Qp = Pump capacity, USGPM

My problem is which pump capacity shall I use for Qp. I am inclined to assume the biggest pump capacity, 425 GPM since the smallest pump can act as jockey pumps. Am I correct in my assumption.

Thanks a lot for any help from our members who are expert on this field.

VSV

 

[bhwtam]

vsv101,

The function of the hypropneumatic tank is to smooth out the fluctuation of the system pressure thus reduce the frequency of start/stop of the pumps.

When you size your tank capacity, you should use the highest pumping rate as Qp. (i.e., if only 1 pump is used at one time, then it is the capacity of your biggest pump, if combined pumps will be in operation at the same time, then it will be the combined flowrate of the duty pumps)

Regarding the 6X/10X assumption.

6 times/hour starting is the most common recommended as higher than that will cause overheating of motor. Going to 10X will not burn down the motor immediately but it will shorten it's life.

On the other hand, when you size your pumps, you will taken the maximum flow requirement as the criteria, which may only happened 10% of the time. So, in normal operation, the starting frequency may be much lower than 10X even you used it during your calculations.

Hope it helps

bhwtam

 

[vsv101]

bhptam,

Thank you very much for the input/help. I used the combined flow of 770 gpm (485gpm + 285gpm)for Qp which is the maximum flow requirement.

Reiterating again my thanks for the input.

VSV...