Calculating Sump Volume

[robmclellan]

How do you calculate the minimum sump volume in a wetwell for a duty assist system? I am able to work out the duty requirements, but not the assist....

The plan area of the wet well is 14.79m2, and the bench is under the proposed OFF levels for the pump, and therefore does not need to be considered.

The Duty pump is rated at 25 l/sec, and the assist pump is rated at 4 l/sec

The average inflow into the wetwell is 6 l/sec.

I would like to achieve 10 starts per hour.

If anyone knows how to calculate this, I would greatly appreciate it.

Thanks

Rob

 

[CivEnvEng98]

The standard cycle time equation is T = (V/Q-S) + (V/S), where:
T = cycle time (min)
V = L
Q = Flow out (L/min)
S = Flow in (L/min)

However, if you know T, Q and S but not V, the equation can be rearranged as follows:

V = 1/(1/(Q*T - S*T)+1/(S*T))

Factoring in your data I obtained V = 1,642 L

That will give you a cycle time of 6 minutes (10 starts/hour) under average daily flow conditions. You need to reevaluate under peak flow conditions and see if your cycling too much, which I suspect may be the case here. Generally most pump manufacturers don't like more than 8 - 10 starts/hour/pump under peak flow conditions.

I've not factored in your assist pump and feel it's not a good idea to do so anyway.

 

[rcooper]

For Duty/assist/assist/... situations, I work out the volume required for the single duty pump and design my layout. I then set the start and stop levels for the first assist pump 200mm higher than the start and stop levels for the duty pump. The second assist start and stop levels are 200mm higher than the first assist pump and so on. This gives approximately the same start and stop times for each of the assist pumps as for the duty pump.