Pump Horsepower Characteristics
The Cycle Stop Valve or CSV is designed to work on pumps with centrifugal impellers, pumping fairly clean and fairly cool water, delivered at a constant pressure, within a certain pressure differential. Although this may be a small niche of the entire pumping industry, it encompasses a large number of pumping applications.
Pumps with centrifugal impellers work best with Cycle Stop Valves. These include Submersibles, Turbines, Jet Pumps, as well as multistage, split case, and end suction centrifugal pumps. Some centrifugal impellers have better brake horsepower characteristics than others. Choosing the right pump can make a big difference in power requirements at low flow rates. You should no longer look only at the Best Efficiency Point or BEP when choosing a pump. It can be beneficial to give a point or two in efficiency at max flow, to have a pump that has better HP characteristics at low flow. Some pumps will only reduce in HP or amperage by 10% or 20%. Others will decrease in amperage by 50% to 60%. See the pump curves in Figure #1 and Figure #2.
In Figure #1, the pump is reduced from 10 HP to 7 HP at low flow. In Figure #2, the pump reduces from 10 HP to 3.25 HP. Obviously the pump in Figure #2 will use less energy at lower flow rates.
Along with the style of impeller, the design of the pump can also affect efficiency at low flow. Submersible pumps can have floating stack or fixed stack impellers. Floating impellers are loose to move up and down on the pump shaft. Some floating designs are not held up by the pump shaft and motor bearing, and allow each impeller to drag and carry its own thrust load. Other floating designs allow the pump shaft to hold the impeller up during down thrust. This puts all the down thrust on the motor thrust bearing. A fixed stack impeller pump has all the impellers locked to the pump shaft. This allows the whole stack and shaft to float up and down to a point. However, the impellers are locked to the shaft at a point and held up by the motor thrust bearing.
Most of these type pumps will reduce in amperage when the flow is restricted. However, fixed stack impellers have the least drag, and usually have the best horsepower characteristics at low flow. Again, choosing the right pump can offer better energy efficiencies at low flow rates. Some 2 HP submersibles with floating stack impellers may only reduce from 14 amps to 12 amps at low flow. Others with fixed stack impellers can reduce from 14 amps to 5 amps, and would be considerably more efficient at low flow rates.
Most pump curves for smaller submersibles do not show a BHP or power curve. If you have the efficiency curve, you can figure the horsepower. The standard BHP formula is flow X head X 100 divided by 3960 X Efficiency. Our Horsepower calculator at this link will make the math easy for you.
For instance a submersible that will produce 40 GPM at 310' has efficiency of 50%. The HP calculator shows the HP required at 6.26 HP.
The same pump restricted to 5 GPM, delivers 420' of head with an efficiency of 20%. The HP calculator shows the power required has been reduced to 2.65 HP.
Everything you need to know is in the pump curve. If you don't find one that drops by 40% or 50%, look for another pump. Different brands and designs of pumps have different power characteristics.
Positive displacement pumps, pumps with axial or mixed flow impellers are not recommended to use with Cycle Stop Valves. The power required with these type pumps will increase when the flow is restricted with a valve. See Figure #3
In Figure #3, the BHP or Brake Horse Power continues to climb with the head or pressure produced. Therefore as the flow is decreased, the horsepower increases. These type pumps are not recommended for use with Cycle Stop Valves.
The pump manufacturer offered the curves in Figure #1 and Figure #2 to show that varying the pumps speed with a VFD, would be more efficient than restricting with a CSV. As you can see, when the right pump is chosen, there is very little if any difference in power requirements at low flow rates between VFD to CSV controls.
However, with VFD controls, other uses of power are not shown on the pump curve. Parasitic lose, is the energy used by the VFD itself, as well as energy lost by line and load filters, and any other electronic equipment. The unstable voltage waveform or harmonic content of the power created by a VFD, can add 15% to the heat in motor windings, and decreases the efficiency of the motor by about 5%. Pump curves also do not factor in the loss of motor efficiency for running at partial load. VFD's also need protection from the environment. This requires air conditioning or heating the building containing the VFD. Fan cooled motors may also require additional cooling at low RPM, because their cooling fan is also spinning at low RPM. All these things combined can add considerably to the energy used by a VFD controlled pump system.
The CSV does not require any electrical power, affect the quality of power used by the motor, reduce the efficiency of the motor, reduce the RPM of the cooling fan, or require an air-conditioned environment. Choosing the right pump means there is usually very little if any difference in power consumption between CSV and VFD controls.
The fact that HP of a pump decreases when a valve restricts the flow rate, is counter intuitive. CSV control is far from being "like driving a car with one foot on the brake and one foot on the gas", as many VFD and pump manufacturers would like you to believe.
The pump curve is still a good way to choose a pump with the best brake horsepower characteristics. When using a pump curve to compare the difference between power requirements of a CSV and VFD controlled system, all other VFD losses not shown on the pump curve should be added back in. Choosing the right pump for a CSV, and adding back in the additional losses for VFD controls, means a CSV controlled system can easily be the most efficient.