If there is an unusual surge load, both pumps will operate. Normally the pumps have an alternator that automatically switches which pump acts as the lead pump. They are: Pumps off – Lead pump on – Lag pump on – High-level alarm on. Many specifications we see require four floats. If we carry the same logic into the duplex system, we will require four floats. Once that alarm sounds or makes building management system (BMS) contact, something must happen right now. This simplex system would require three floats: Pump off – Pump on – high level alarm. A high-level alarm would indicate the failure of the pump to keep up with the load. The spacing of these are determined by the required volume to meet the minimum run time of the pump. In a larger simplex pumping system with larger storage, we require an on float and an off float. If the flow is very low, as in a residential sump, we could use a single float with an on-off range of a few inches. Simplex Sump & Sewage Control in Buildings The decision about control is about whether you want to use three or four floats in the basin of a duplex pump. This may result in a specification that is not clear resulting in poor or incorrect selections. I often see specifications that were clearly created by adding different recommendations to existing standard specifications. Sump and sewage specifications should be very simple. When the system is larger, we can still settle on the everyday pump control. In residential systems, a small fractional horsepower pump comes with a mounted on/off float switch and we do not even think about it. Commercial and Institutional control of basin mounted sump and sewage pumps does not have to be complicated.
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