Lead-Lag Pump Control
Lift stations use a basic concept called lead-lag pump control. The definition of lead-lag pump control is simple: one pump acts as the 'lead pump', and will be the only pump operating until the water level, or demand, gets high enough for the 'lag pump(s)' to engage. When the pumps move enough liquid, they shut off and then alternate roles. A lag pump now becomes the lead and vice versa.
A pump controller reads inputs from float switches or continuous level sensors to trigger the pumps to turn on and off. An alternator switches the pumps' roles to evenly dole out assignments. Hour meters track the total run time for each pump and then displays that information to the lift station operator.
This pumping method does a few things. First, it balances the pumps' run time to reduce maintenance. Second, it boosts reliability since there is always a backup pump in the event that one fails. However, it has the unintended consequence of masking problems. Since the backup pump keeps things moving, problems will sometimes go unnoticed until both pumps are down.
Lead-lag pump control is commonly done with two pumps, in what is known as, a duplex-pump station. This is the most common type of pump station. However, there are many pump stations with three and four pumps as well, known as triplex and quadplex pump stations respectively.
Duplex Pump Control
Most lift stations are duplex pumping stations. These are typically closest to homes and businesses. As the lines from duplex lift stations converge, more pumps are added and triplex and quadplex stations are installed to handle the increased flow.
Duplex pump control is the simplest lead-lag scenario, with one lead and one lag. The control system can be fairly simplistic. Older duplex pump controls may consist of four float switches mounted one above another in a wet well. The bottom float turns the pumps off, the next float turns the lead pump on, the next float turns the lag pump on, and the top float sounds an alarm.
Modern duplex pump control can include the following:
- Continuous level transmitters, flow sensors, and pressure transducers
- Pump controls
- Variable Frequency Drives (VFDs)
- Grinder pumps or screens
- Condition monitoring
- Redundant controls
A duplex control system can be either very simple or quite complex depending on the needs of the particular installation. The controls can be a purpose-built pump control panel, a PLC, or programmable I/O controllers. ControlByWeb's peer-to-peer and expandable I/O features offer lift station controls engineers a few distinct advantages.