VFD Pump Control in Wastewater

The control modes

A single station may combine them: level PID as the normal mode, a scheduled full-speed run each day, and a flow limit that caps the discharge into a plant during wet weather. The control narrative names the modes and the transitions between them.

Minimum speed in wastewater

On a clean-water pump the minimum speed is the point where the pump develops enough head to open the check valve. On a wastewater pump it is usually higher, for two reasons that have nothing to do with the pump curve.

Force main velocity

Solids settle in a force main below about 2 feet per second, and a line that has settled needs 3 to 5 feet per second to scour. A pump running at a speed that gives 1.5 feet per second all day fills the main with grit until the head rises and the pump loses capacity. The minimum speed is chosen so that the velocity stays above the settling velocity, and where that is not possible at low inflow, the periodic full-speed run does the scouring.

Ragging

Rags wrap an impeller more easily at low speed, where the vane velocity is lower and the flow through the volute is gentle. Stations with a wipes problem run a higher minimum speed and use de-ragging routines.

The practical floor is often 60 to 75 percent of full speed on a wastewater pump, against the 40 to 50 percent that a clean-water pump on the same curve might allow. That narrows the usable speed range and, at low inflow, means the station cycles; the level PID page covers the stop and restart logic.

The full-speed run

A wastewater station on a drive should run at full speed on a schedule: once or twice a day, or on every cycle at low inflow, for long enough to turn over the force main volume. The run scours the line, clears grease from the wet well walls at the drawdown level, and gives the impeller a chance to shed rags. It is also the moment to take a drawdown measurement for the capacity trend. The program schedules it, or triggers it on a level cycle, and it is recorded so that a missed run is visible.

Clog detection and de-ragging

A drive reports motor current, torque, and power continuously, and those are the earliest signs of a clog. A pump moving rags draws more current at a given speed than the same pump moving clean water. The program compares the current, or better the power against speed, with a baseline learned from clean operation, and raises a warning when the ratio climbs. Many drives include a pump-cleaning or de-ragging function: on a trigger, the drive runs the pump briefly in reverse, or cycles forward and reverse at low speed, to unwind material from the impeller. Where the pump manufacturer permits reverse rotation, the routine is triggered on the current signature, on a schedule, or on the start of every cycle at stations that clog often. A routine that runs and does not restore the current baseline is an alarm to pull the pump.

The drive and the safety layer

A drive adds capability and it adds a failure mode. The design rule does not change: the floats and the relay logic call the pumps when the transmitter, the PLC, or the drive itself cannot. With a drive, that means either a bypass contactor that the float logic can close, or a second pump on its own drive with its own float path, or a hardwired run command to the drive at a preset speed that the float relay can assert without the PLC. Whichever, the backup is tested with the PLC dead, as on the backup control page.

Settings to record

• Minimum speed and the basis for it: curve, force main velocity, or ragging experience.
• Fixed speed, where used, and the reason it is below 60 Hz.
• Acceleration and deceleration ramps, chosen for the check valve and the force main.
• The full-speed run schedule and duration.
• The current or power baseline for clog detection and the warning threshold.
• De-ragging parameters and the manufacturer authorization.
• The staging setpoints and timers where more than one pump runs.