In this post, we will cover duplex pump control for lift stations. In the weeks prior, we gave a good overview of lift stations, and covered simplex pump control. Check out those posts for more detail.
Duplex Lift Stations
Duplex lift stations have two pumps:
- the lead pump, which is the first pump to be activated
- and the lag pump which is used as the levels rise higher than the lead pump can manage
With two pumps, duplex lift stations have some critical benefits that make them popular. A second pump greatly increases the capacity that a lift station can manage. In addition, the lag pump provides a backup solution if something goes wrong with the lead pump, or when one of the two pumps must be removed for service. However, controlling a second pump adds complexity to the lift station, which usually results in additional sensors and control logic.
Keep in mind that hysteresis plays a critical role in protecting pumps, and should be part of the control plans for a duplex lift station. If you plan to use normally open cable suspended float switches, you will now need 3 float switches to handle the pump control. Each pump will need a sensor for turning the pump on, and they will share a sensor installed deeper in the tank to turn the pump off. Add a high level alarm to this mix, and now you have 4 float switches in the lift station.
While this solution can get the job done, it certainly has its challenges and limitations. We discussed the problem of float switches getting tangled and causing sanitary sewer overflows (SSOs) in our post about simplex pump control. If 2 sensors can have tangling problems, surely 4 sensors will only increase the likelihood of tangles and the dreaded SSO. In addition, assuming the sensor life is 4 years, you will average one sensor failure a year, which could cause SSO’s or pump damage.
Sensors That Work
Traditionally, people have turned to a continuous level measurement to avoid tangled sensors. This is an excellent solution that provides some critical advantages that we will discuss in the coming weeks, but there are still float switch options that provide a good solution at a lower cost.
Last week we discussed a couple of float switch products that we offer. These unique sensors provide the functionality of multiple float switches all within one float body. These sensors are extremely durable. They consistently outlast other float switches and eliminate the problems caused by tangled cables.
Two of our float switches are designed specifically for Duplex pump control:
- The KA-4Y39 is a perfect replacement for a lift station that would normally utilize 4 normally open float switches. There is no need to change anything within the controller. It simply has 4 different internal switches that close at four different tank levels, in the same way that 4 separate floats would work. When wired to the appropriate controller, this will provide hysteresis control for both pumps, as well as a high level alarm.
- The KA-4H5E float switch is great for new lift stations. This level switch – like the KA-4Y39 – has four internal switches, but with hysteresis built right into the float. It can provide hysteresis pump control and a high level alarm on a duplex lift station, all with one sensor.
By switching to a float switch that includes multiple internal switches, you can effectively control a duplex pump on a lift station – but with lower costs and greater stability. Why risk tangled floats or put up with frequent replacements? Contact our Measurement Experts if you have any questions about the float switch models mentioned above.
We will wrap up our discussion of float switches in lift stations by discussing options for providing override backup. When float switches or controllers fail, what back-up tools do you have in place? We'd love to hear what is working for you.
3D rendering provided by Romtec Utilities, a lift station solutions provider.