A dewatering pump VSD (variable speed drive, which runs the motor at the speed the process needs rather than flat out) matches pumping rate to the water actually entering the pit or tank. Most quarry pumps are instead controlled in one of two ways: switched on and off by a float, or run continuously with a valve throttled to limit flow. Both waste energy, and both shorten the life of pumps, motors and pipework.
The problem with float and valve control
On a float-controlled pit pump, the pump starts at full speed when the level reaches the high float and runs until the low float, then stops. In wet weather it may cycle many times an hour. Every start draws a high current, applies a mechanical shock to the pump and coupling, and sends a pressure surge through the delivery line.
Wash plant pumps face the opposite problem. They are sized for maximum demand and run continuously, with a valve partly closed to reduce flow when less water is needed. The motor still works against the closed valve, converting electrical energy into pressure that is then wasted across it.
Typical symptoms include:
- High numbers of starts per hour in wet periods, with frequent motor and starter failures
- Pressure surges and water hammer causing pipe and joint failures
- Throttled valves that wear and need regular replacement
- Electricity costs for pumping that are higher than the duty requires
Why speed control saves so much energy
Centrifugal pumps follow the affinity laws: flow is proportional to speed, pressure to the square of speed, and power to the cube of speed. The Australian Government’s energy guidance notes that for pumps and fans, reducing motor speed by 20% can reduce the power required by 50%. Running a pump slightly slower for longer therefore uses far less energy than running it at full speed and stopping, or throttling it with a valve.
The saving is smaller where a pump must lift water a large fixed height, because that static head must be overcome at any speed. This is assessed for each pump before projecting a benefit.
Dewatering pump VSD control: the system
The pump motor is supplied from a VSD, and a controller adjusts speed to hold the water level, or the delivery pressure, at a setpoint.
How it works
- 1
Measure
A submersible pressure or radar level sensor measures the sump level. For wash plant supply, a pressure transmitter measures delivery pressure.
- 2
Control
The controller increases pump speed as level rises and reduces it as level falls, holding a steady level instead of cycling.
- 3
Protect
Minimum speed, dry-run detection and soft starting protect the pump, motor and pipework.
- 4
Sequence
Where there are several pumps, duty and standby pumps are rotated and added as inflow increases.
What variable speed control is worth
The benefit combines energy, maintenance and reliability. Energy is the most predictable to estimate, provided the pump’s current operation is known.
Alternatives to a VSD
- Retain float control. No cost, but starts, surges and energy use continue.
- Soft starters. Reduce starting current and shock, but the pump still runs at full speed whenever it runs.
- A smaller pump. Can suit a steady duty, but quarries rarely have one, and wet weather inflows still need capacity.
- Pump supplier drive packages. Can be effective for a single pump. Integration with level control, other pumps and plant interlocks is where an independent control design adds value.
The same principles apply to food plant transfer and process pumps, covered in process pump VSD control.