Sump Pump & French Drain

[rlewistx]

I'm helping someone with the design of a french drain system and sump pump. A basement is being put in about 6-12" below the water table. The contractor has installed a french drain system around the basement. It is about 20 ft. x 60 ft. in plan. The gravel is 4 feet below the floor. The perforated pipe is 3 feet below floor. The drain empties into a 4 ft. dia. manhole. The bottom of the manhole is about 40 inches below the french drain pipe. The invert of the french drain into the manhole is about 32" above the bottom.

The flow into the manhole is about 300 gal. per hour. My idea is to fill the manhole up to the french drain invert which is about 40 inches of storage in the manhole, then pump it out.

I have about 18 feet of head to overcome. I used the tables in Graingers for calculating the head based on vertical rise, horizontal run and fittings. I would like to pump out at 25 gal./min. Graingers catolog shows a 1/2 hp pump will do this. The problem I have is the standard pumps don't have the range of on and off that I woul like. I would like a 36 inch range and a teathered switch is only about 9 inches of range rise. I would like this to come on about once an hour to discharge, which is why I want the 36" range.

Any suggestions of how to best tackle this? Should I use a smaller range?

 

[poppeye]

Look for a Crane Deming Distributor in your area. They handle sump pumps that have separate float switches assemblies that can handle the 3' differential you are looking for rather than the attached float switches available through Granger. You would have to install a separate guide rail system for the float switch, shaft and ball assembly. This is wired in series to the pump between the power supply and pump. Some individuals add a separate emergency high level switch in case of switch failure on the primary.