Duplex Pumps – How They Work and Where You Use Them

In Wisconsin, duplex pump systems are required by code in commercial buildings when there are three or more water closets or more than 20DFU’s draining into a sewage basin. Duplex systems are also used in large volume stormwater pump stations, as well as when redundancy is desired.

The duplex pump system is one of the most common configurations in a pump station design, providing efficiency and redundancy. The duplex design is often referred to as lead/lag systems because the standby pump (lag) is activated as demand exceeds the capacity of the lead pump or if the lead pump were simply to fail.

For example, a typical design requirement for pumping wastewater is that a single pump will discharge more than the peak inflow. In wastewater and sewer applications, the peak flow needs to be able to be pumped by the lift station even if it never occurs.

An alternator controlling the duplex system allows the pumps to cycle between starts, resulting in even wear on the pumps. If one pump needs to be serviced, the system remains automatic by utilizing the lag pump.

The alternating control panel is typically operated by three control float switches: stop float, lead float, and lag/alarm float. There are some systems that do use four floats, separating the lag and the alarm function.

The stop float is the float that turns off whichever pump was called on to run. The lead float is the float that runs whichever pump happens to be called on for at that cycle. In a three float system, the third float is the lag/alarm float. If the liquid reaches this float, the lag pump will turn on and the alarm will sound, warning there is a system problem. At this time, the pump(s) will continue to run until the stop float is lowered to the stop position. The next time the lead float is triggered, the other pumps will assume the “lead” role.

The alarm must be installed on a separate electrical circuit, separate from the circuits feeding the pumps. If one breaker is being used for the incoming pump power for both pumps, it needs to be sized for the possibility of both pumps starting and running at the same time. It is highly recommended to have separate circuits for both pumps and the alarm, totaling three circuits.

The controls can be purpose-built, a PLC or programable I/O. PLC controls can be enhanced with an HMI (Human Machine Interface) Touch Display to directly interact with sensors, valves, pumps, motors, and more.

JMI Pump Systems also offers an assortment of steel, stainless steel, and custom PVC cover options for pump basins as well as packaged systems with controls. The standard covers we provide will typically maintain separate access for pumps and floats for improved serviceability.

JMI Pump Systems professionals are able to assist you with finding the best duplex system for your application. For more information, Your Partners in Pumping are available at 262-253-1353 or sales@jmipumps.com.