One of the most efficient ways to supply water to small housing developments that do not have public water supply available is to use two pumps. First, provide a well to pump to ground storage, then use a second pump to boost pressure to the desired system pressure.
The advantages of a lower producing well, along with pumping against only the tank elevation, is a monetary savings. It can be an energy savings also since the well can be pumped at a constant flow against a fairly constant head. This allows the well pump to be sized efficiently for the consistent conditions.
A booster system is then designed to furnish a constant pressure to the development against varied flows. This is accomplished by adding pressurized tanks to balance the pressure or by using a variable speed device to maintain the pressure.
When the booster system is designed with pressure tanks, a pressure range will exist between the pressure at which the pump starts and the pressure that it shuts off as the case when no water is needed. Typically, this is 20 psi. A ground storage tank would commonly be 20 to 30 feet tall to provide the storage with modest pressure for the development.
One of the problems with this design is that the range of flow on the booster pump is quite large. First of all, the booster will be designed for worst case. This would be when the storage tank is low and when demand is high causing the system to boost to the desired system pressure. When this occurs, the pump will run to the far-right side of the curve.
Centrifugal pumps running more than 110% of flow at the BEP (Best Efficiency Point) will have an unbalanced load. This will cause damages to bearings shafts, and mechanical seals which can wear or more often become damaged by cavitation.
With a 30-foot full tank and a 20 psi lower start pressure, there is a 76 ft less head on the system (30 ft + 20 psi). On a single stage centrifugal, that will have a huge effect on the flow range and the reduced head will often move the flow past the ideal 110% limit of flow. Care must be taken to limit the booster to a smaller flow range.
Booster pumps can also be operated by a variable frequency drive, or VFD. This device will operate the pump at a consistent pressure regardless of the storage tank level. The system pressure can be limited to 3 psi or less which will eliminate the 20-psi change in head between starting and stopping the booster pump. In addition, when the demand flow is reduced, the variable frequency drive will reduce speed to create a different performance curve which will keep the pump from running out past the BEP.
An additional benefit is the power savings of the reduced speed. By reducing the motor speed by 16% the required kilowatt usage is reduced by 42%. World agencies such as the U.S. Dept. of Energy, EuroPump Booster and HVAC have issued recommendations and minimum efficiencies and these agencies recommend using variable speed for energy efficiencies. If a system curve is generated based on percentage of max flow at various periods of the day, and the average usages are applied, the booster will be operating at much less power requirements for most of the time.
Keep in mind that in either case, pumps are usually designed for the maximum condition that can be anticipated. A pump curve and system curve based on regulatory statistics shows a pump typically running with a 5 hp motor would only require about 1 horsepower to provide water at the lowest demand during a day. Additionally, since the pump changes curves as the demand decreases, the BEP also decreases, and the pump avoids running out into the far right. This reduces repair costs as well. Lower costs, consistent pressure, and longer pump life are all advantages of the variable speed drive.