Lift Station Lead/Lag Control

What lead/lag decides

A lift station with more than one pump has to answer two questions continuously: does the well need pumping, and does it need more than one pump. Lead/lag control is the logic that answers them. The lead pump handles normal inflow. The lag pump is called when inflow exceeds what the lead can move, and it is also the pump that takes over when the lead is out of service. Alternation, which decides which physical pump holds the lead role, is a separate function and is covered on its own page.

The panel-level view of lead/lag, including how it is built for pressure boosters and other duty-standby applications, is on the pump panel lead/lag page. This page is about the wet well: how the setpoints are chosen, how the lag call is made, and what parallel pumping actually delivers.

The setpoint ladder

Every level setpoint in a lift station is an elevation in the wet well. Listing them from the bottom up keeps the relationships straight.

Sizing the lead-on band

The volume between all-off and lead-on is the cycle volume. It sets how often the lead pump starts, and pump motors are rated for a maximum number of starts per hour, commonly six to ten for submersible wastewater pumps and fewer for large motors. Too small a band and the motor exceeds its rating; too large and wastewater sits in the well long enough to go septic.

For a constant-speed pump, the shortest cycle occurs when inflow is exactly half the pump capacity: the well fills as slowly as it empties. That gives the classic sizing rule.

For a pump rated at six starts per hour, T is ten minutes. With a 500 gpm pump, the cycle volume is 1,250 gallons. In a six-foot-diameter well, which holds about 211 gallons per foot of depth, that is a band of just under six feet. In practice the well geometry is fixed and the calculation runs the other way: given the band the well can provide, the result is the number of starts to expect at the worst-case inflow, which is then compared with the motor rating.

Variable speed pumps change the arithmetic because the pump can slow to match inflow and run continuously. Even then, at very low inflow the pump reaches its minimum speed and the station cycles, so the same check applies using the capacity at minimum speed.

Making the lag call

The lag-on setpoint is the minimum. A station that calls the lag only on level will always call it eventually, but a well with large storage can spend a long time between lead-on and lag-on with the lead pump losing ground slowly. Three conditions are commonly combined.

Level

Level reaches lag-on. This is the baseline and it must exist in every station, hardwired through a float if the station has one.

Lead run time

The lead has run longer than a set time, typically ten to twenty minutes, without level falling. Inflow is at or above one pump capacity, and waiting for level to climb another foot gains nothing.

Rate of rise

Level is rising faster than a set rate with the lead running. This catches storm inflow early. It needs a filtered level signal, or turbulence in the well will call the lag on noise.

Whatever the condition, the lag start is delayed by a few seconds after the lead start so the two inrush currents do not coincide. On a generator this matters more: two motors starting together can exceed what the set can hold, and the transfer switch sequence is often the reason a station is in trouble in the first place.

What two pumps actually deliver

The lag pump is often expected to double the station capacity. It does not. Two pumps in parallel each see a higher discharge head than one pump alone, because the force main friction loss rises with the square of flow. Each pump slides up its curve to a lower flow, and the combined flow lands somewhere between one and two times the single-pump flow. On a short force main with mostly static head the gain is close to double; on a long force main it can be as little as thirty percent.

This has two consequences for control. The lag call must come early enough to matter, because it buys less time than the pump nameplate suggests. And the capacity a station is credited with in a wet weather plan should come from the system curve, not from adding nameplates. A triplex station adds a third pump for redundancy and for the third-pump step on the system curve, which is smaller again.

Stopping

Simple stations stop everything at all-off. A better sequence stops the lag first at an intermediate lag-off level and lets the lead finish the drawdown. This reduces the number of two-pump stops, keeps the discharge velocity in the force main more consistent, and puts the alternation event at a single-pump stop where it is unambiguous. Both stops are subject to the minimum run time.

Do not use a very short minimum run time as a protective device. If level is falling faster than expected the pump is doing its job. The minimum run time exists to stop a pump from being started and stopped repeatedly by a noisy level signal at the all-off point, and a few seconds to a minute is enough.

When the lead fails

A pump is called and does not prove: no run confirmation from the starter auxiliary or the drive, an overload trip, a seal failure, a motor thermal switch, or a current that says the motor is spinning but nothing is being moved. The correct response is immediate. The lag pump takes the lead role now, the failed pump is removed from the rotation, and an alarm is raised. Waiting for level to climb to lag-on means the well rises for no reason while a healthy pump sits idle.

1. 1

   Call the lead

   Energize the lead output. Start a fail-to-prove timer, typically three to ten seconds.

2. 2

   Check the proof

   Run confirmation from the starter auxiliary contact or the drive running bit, and optionally current above a minimum. If the timer expires without proof, the pump is failed.

3. 3

   Promote the lag

   Set the other pump as lead, call it, and repeat the proof check. Latch a pump-failed alarm on the first pump.

4. 4

   Hold the failure

   Keep the failed pump out of rotation until the fault is reset locally or from SCADA by an operator who has looked at it. Automatic retry after a delay is reasonable for an overload that may be thermal, but not for a seal failure.

5. 5

   Alarm the loss of redundancy

   A station running on one pump is one failure from an overflow. That state deserves its own alarm and its own priority.

What to trend

• Wet well level with pump run status overlaid. Every problem on this page shows up here first.
• Starts per hour per pump. Compare with the motor rating and with the original cycle volume calculation.
• Run time per cycle and drawdown rate. A slowing drawdown with the same level band means a fouled impeller, a closing check valve, or a rising force main head.
• Lag calls per day. A rising count with no change in flow means the lead pump is losing capacity.
• Motor current per pump. Rag-bound pumps draw high current and move little; air-bound pumps draw low current.