- Oversize the pump: Run the pump at the rated speed but use a larger pump to maximize flow and head. This wastes energy because the pump is providing more flow and head than required.
- Add a throttling valve: Add a discharge valve to the pump to control flow into the pumping system. The valve adds more system resistance (friction), causing the operating point to move up the pump curve, wasting energy.
- Add a bypass valve: This allows water to flow back to the suction of the pump, decreasing system resistance and increasing flow from the pump, causing the operating point to move down the pump curve, wasting energy.
- Trim the pump impeller: This lowers the flow and head produced by the pump and generates a new pump curve. Once the impeller is trimmed, the ability to increase the flow delivered from the pump in the future is lost, and the pump efficiency decreases. The more trimming is done, the more inefficient the pump becomes.
Controlling Flow with a VFD
There is a common misperception that pairing a pump motor with a VFD simply moves the operating point up and down the pump curve. In reality, for every 0.1 hertz (Hz) of speed regulation, a new pump curve is developed, resulting in an infinite number of pump curves generated by varying the speed of a pump. But the system curve stays the same, so as speed is varied the flow in the pumping system is also varied, dropping the amount of frictional head pressure as flow decreases. The pumping system did not change, so the system curve is still valid as speed varies in the pump, with the operating point simply sliding up and down the system curve.
In the majority of applications, pairing a pump motor with a VFD provides a pump curve that exactly matches the pump operating point the system requires at any time, controlling the flow without wasting energy or sacrificing efficiency. In friction dominated systems where the pump is at full speed and operating close to BEP, the pump will continue to operate near the same efficiency point as pump speed is decreased.
The energy required by a pump can be calculated by Equation 1. If flow or head pressure is decreased, energy will be saved. Because the VFD exactly matches desired system design points, power consumed by the pump is minimized, improving overall pumping efficiencies.
Pw = (flow x head) / 3,960 (assuming specific gravity of fluid is 1)
In the case of an oversized pump, using a VFD allows for the use of a larger pump to meet future or periodic high flow requirements without wasting energy during the majority of operation. And unlike the limitations created by a trimmed impeller, the use of a VFD preserves the ability to increase the speed of the pump in the future when expanded flow and pressure are needed.
Protecting an Aging Pumping Infrastructure
As noted, the most prominently regarded benefit of variable speed pumping is energy savings, which is quite significant in many cases. There are additional benefits that positively impact system reliability that justify the use of VFDs, even where the energy savings factor alone may not be enough.