The following discussion concerning wet well sizing is based upon the assumption of a low flow system, which is less than 3000 gpm.  Systems with flows larger than 3000 gpm require additional considerations to be taken into account.

The design criteria that needs to be identified first is the total system inflow rate, and the flow at which the pumps will discharge. As discussed in the pump design section, the discharge flow can be found at the intersection of the system curve and the pump performance curve. These flows will be identified as:

Q(in) = Inflow rate into wet well (gpm)

Q(out) = Discharge flow rate out of wet well (gpm)

Our intent is to determine the minimum storage volume which the wet well needs to hold between pump starts.  We typically recommend that the minimum time between pump starts should be between eight to ten minutes, or roughly six starts per hour. However, this can vary from manufacturer to manufacturer, so you should check with the particular pump company you are planning on using. Also, you should verify the minimum run time of the pumps with the manufacturer.  These values will be represented as follows:

T(min) = Minimum cycle time between pump starts (minutes)

V(min) = Minimum storage volume of wet well to hold/gather fluid during pump off (gallons)

We can determine V(min) by starting with the following equation, which relates the inflow, storage volume, and outflow to T(min): Assuming that we have properly estimated the flows entering the wet well and that we have selected an appropriate pump for the demand, the worst case scenario is when the inflow is half the outflow. Or:

Q(in) = Q(out) / 2

(Equation V.1)

Plugging this into the Q(in) component of equation 1 and rearranging for V(min), we get: Therefore: 