Not all pump systems benefit from adjustable speed drive
by Johnny Douglass, P.E.

We often hear that installing an electronic, adjustable-speed drive on a pump system that does not need continuous maximum pump flow will result in big energy savings.

Because speed requires power, maintaining minimum speed with an ASD can result in energy savings if the system generally requires a lower flow rate than the pump is designed to deliver. However, applying an ASD can be tricky or inadvisable when the pump is used for open-loop flow.

Open-loop pumping — high static head
In pumping applications, pressure is usually expressed as “head.” For a given pressure, head is the height of a fluid column that will produce that pressure at the bottom. Head is a convenient unit in lift pump calculations because a large part of their work is lifting fluid to a higher elevation. One psi is equivalent to 2.31 feet of water head.

Static head exists in open-loop flow when fluid is either pumped to a higher elevation or pumped into a manifold that is already pressurized by other pumps. Friction head is the additional pressure rise as flow increases. Closed-loop pumps, such as those serving heat exchangers, encounter only friction head, not static head, because the fluid returns to the initial elevation at the starting point each time it cycles through the system.

Motors in high-static applications are designed for worst-case conditions, where if the flow is not reduced during operation, the pumps have to cycle on and off. An ASD can eliminate cycling, but the additional power savings are not as significant as they would be in a closed-loop system where the pump does not have to work against static head. If constant speed cycling is used, look for a pump with high efficiency at the expected combination of flow and pressure.

Match pump to system for best performance
Designers match pumps to systems by overlaying their curves. The constant speed pump curve is a graph of head vs flow. It gradually slopes downward to the right, representing decreasing pressure at higher flows. Manufacturers’ pump curves are overlaid with efficiency lines—concentric rings that look like topographic lines. At the center of these lines is the point where the pump uses the least energy to move the fluid.

The system (piping and reservoirs) can be represented by a similar curve that shows the necessary head at pump discharge to achieve a certain flow. The system curve starts at the elevation head and slopes upward to the right, representing increasing pressure required to overcome flow friction. Flow always occurs where the two curves intersect. The intersection may move when using an ASD because it shifts the whole pump curve down on the pump performance graph.

A challenge with ASD applications in high static head situations is stability. Reduced pump speed results in lower pressures expressed by the entire pump curve sliding downward. On a system with a nearly horizontal curve because of high lift and minimal piping friction, only a small speed decrease will lower the flow, sending the intersection of the pump and system curves far to the left.

That sensitivity can cause wide-ranging cycling or fluctuation in speed and flow as the system “hunts” for equilibrium. For system operation to be stable and efficient with an ASD, the pump should have a steeper downward sloping curve and good efficiency across the range of expected flow variation.

Compare magnetic vs. electrical
If variable speed looks like it might work for your needs, compare a mechanical magnetic variable speed drive to an electronic ASD. Both will reduce the pump shaft energy requirement by the same level for the same flow reduction. However, they will not equally reduce the electrical energy input. The electric ASD converts electrical energy to drive shaft energy more efficiently below 95 percent output speed, while magnetic drives have their greatest efficiency above 95 percent.

Magnetic drive is preferable for lift pump situations because it requires only minimal speed reduction to substantially reduce flow. Also, it may offer greater stability for a pump and system that both operate with high lift and minimal piping friction—where both curves are very nearly horizontal.

An ASD will save energy in a closed-loop flow application where the same fluid is circulated over and over--in a heat exchanger, for example. If it is not necessary for the system to always operate at maximum design flow rate, then an ASD is ideal to reduce the flow during light flow requirements.

Make sure you provide your prospective ASD or magnetic drive vendors with your pump and system curves. This will help determine the efficiency and stability for the range in which you will be operating. Be sure to tell them your operating time if your current constant speed control is based on cycling on and off. Inexpensive battery powered time loggers can be used to actually measure the operating hours over a known span of time.