How often should pumps alternate?

For a duplex station, every cycle is the usual default and it produces the most even wear. Where cycles vary greatly in length, alternating on accumulated run hours balances wear better.

Should alternation survive a power failure?

Yes. Store the alternation state in retentive memory. Otherwise every power blip resets the station to pump 1 as lead, and over years that pump accumulates significantly more wear.

How many starts per hour is too many?

Check the motor nameplate and manufacturer data. For the submersible motors typical at municipal lift stations, six to ten starts per hour is a common limit. If your station exceeds that, widen the level band rather than accepting the cycling.

Should lead/lag logic live in the PLC or in a pump controller?

In the PLC for any station on SCADA. A dedicated pump controller is fine for a standalone station, but it puts the alternation state, the run hours, and the failure logic somewhere the utility cannot see or change.

Lead/Lag Pump Control | CurryControls.com

The short answer

Lead/lag control designates one pump as lead, which starts first, and another as lag, which starts when the lead cannot keep up. Alternation swaps those roles periodically so wear is shared. The core design decisions are what triggers the lag pump, how alternation is scheduled, and what happens when a pump fails, and all three should be written into the control narrative before any code is written.

Key points

Lead starts first; lag starts on a higher demand threshold or on a failure of the lead.
Alternate on each cycle, on run hours, or on a schedule — each has different consequences.
A failed pump must be removed from the rotation automatically, not just alarmed.
Minimum run and minimum off timers protect motors from short cycling.
Operators need a manual override that survives a controller restart.

The basic scheme

A duplex station has two pumps sized so that one can handle normal flow. In level-based control, the lead pump starts when the wet well reaches its start level and stops at the stop level. If inflow exceeds what the lead pump can move, the level continues to rise, and at a higher setpoint the lag pump joins it. Both run until the level falls to the stop point.

Setpoint	Action	Notes
Low level / dry run	Stop all pumps, alarm	Protects pumps from running dry
All stop	Stop pumps	Above the low level cutoff and above the pump suction
Lead start	Start lead pump	Set so the pump runs long enough to satisfy minimum run time
Lag start	Start second pump	Inflow exceeds one pump capacity
High level alarm	Alarm, notify	Independent float is good practice here
High high / overflow	Alarm, emergency notification	Hardwired float, independent of the transmitter

Typical duplex wet well setpoints, bottom to top

Alternation strategies

Alternate every cycle: Roles swap each time the station finishes a pumping cycle. Simple, predictable, and gives near-equal starts. The most common choice for a duplex station.
Alternate on run hours: The pump with fewer accumulated hours becomes lead. Balances run time rather than starts, which suits stations where cycles are very unequal in length. Requires the hour meters to be maintained and to survive a restart.
Alternate on a timer: Roles swap on a fixed schedule, such as weekly. Predictable for maintenance planning but can leave one pump idle for long stretches at a low-flow station.
Manual selection: An operator picks lead. Necessary as an override, and it should always be available, but it should not be the normal mode or one pump will accumulate all the wear.

Failure handling is the part that gets skipped

A control scheme that alternates cleanly and then hands the lead role to a pump that has been faulted for two days is worse than no alternation at all, because the station will not pump and the alarm looks like a level problem.

01Detect failure to start: if a pump is called and run confirmation does not appear within a few seconds, declare it failed.
02Remove a failed pump from the rotation immediately and promote the healthy one to lead.
03Alarm the failure distinctly. "Pump 1 failed to start" is actionable; "station high level" is a consequence.
04Do not auto-reset a motor protection trip. Require an operator acknowledgement.
05When only one pump remains available, say so on the display. Operators need to know the station has no redundancy tonight.
06If both pumps are unavailable, escalate immediately rather than waiting for a high level.

Failure-aware lead selection, in outline

Available_1 = NOT Fault_1 AND NOT Maint_Lockout_1 AND HOA_1_in_Auto
Available_2 = NOT Fault_2 AND NOT Maint_Lockout_2 AND HOA_2_in_Auto

IF   Available_1 AND Available_2 THEN  Lead = Alternation_Selection
ELIF Available_1                 THEN  Lead = Pump_1,  Alarm "No redundancy"
ELIF Available_2                 THEN  Lead = Pump_2,  Alarm "No redundancy"
ELSE                                   Alarm "Station has no available pumps"

Lag = the other available pump, or none.

Protecting the motors

Short cycling destroys pump motors. Each start draws heavy inrush current and heats the windings, and motors are rated for a limited number of starts per hour, commonly in the range of six to ten for the sizes used at municipal lift stations. Check the motor nameplate and the manufacturer data for the actual figure.

Minimum run timer: once started, a pump runs for a set period even if the level reaches the stop point, unless a protective interlock intervenes.
Minimum off timer: a pump that stops cannot restart for a set period, allowing the motor to cool and the check valve to seat.
Adequate level differential: the gap between start and stop levels must give a run long enough to satisfy minimum run time at the highest expected inflow.
Stagger lag start: do not start both pumps within the same second. Sequence them so the electrical service does not see two inrush events together.

HOA and where the PLC fits

The Hand-Off-Auto selector is the operator interface of last resort and it must work when the controller does not. Hand should start the pump through the starter regardless of PLC state. Auto passes control to the PLC. Off should positively prevent operation.

Bring the Auto position back to the PLC as an input. Without it, the controller does not know a pump has been taken out of automatic, will keep calling it, and will report a failure to start for a pump that is running perfectly well in hand. This single input eliminates a large share of nuisance alarms at pump stations.

Variable speed changes the shape of the problem

With variable frequency drives, lead/lag becomes a capacity question rather than a purely discrete one. The lead pump modulates to hold a level or a flow, and the lag pump is added when the lead reaches maximum speed and cannot keep up. Both then typically run at a matched, reduced speed rather than one at full and one modulating.

The additional design points are minimum speed, which must stay above the point where the pump cannot overcome static head, and the delay before adding or dropping a pump, which prevents the station from cycling a pump on and off around the transition point.

Frequently asked questions

How often should pumps alternate?: For a duplex station, every cycle is the usual default and it produces the most even wear. Where cycles vary greatly in length, alternating on accumulated run hours balances wear better.
What triggers the lag pump?: Most commonly a higher wet well level, meaning the lead pump is not keeping up. It should also start on lead pump failure, and it may start on a high-level condition regardless of the normal sequence.
Should alternation survive a power failure?: Yes. Store the alternation state in retentive memory. Otherwise every power blip resets the station to pump 1 as lead, and over years that pump accumulates significantly more wear.
How many starts per hour is too many?: Check the motor nameplate and manufacturer data. For the submersible motors typical at municipal lift stations, six to ten starts per hour is a common limit. If your station exceeds that, widen the level band rather than accepting the cycling.
Should lead/lag logic live in the PLC or in a pump controller?: In the PLC for any station on SCADA. A dedicated pump controller is fine for a standalone station, but it puts the alternation state, the run hours, and the failure logic somewhere the utility cannot see or change.

Direct contact

Have a controls question?

Reach Eric Sullivan directly about anything on this site, a controls or automation topic, or one of his personal projects.

Call Eric · 863-698-8266 Contact Eric