Specifier’s Notebook: Selecting Optimum Pump Configurations

Even though the design and operation of pumps has rapidly advanced over the years, the fundamental factors of flow and resistance have remained constant.

Centrifugal pumps are designed to provide a required flow against an imposed resistance (pressure). Flow requirements are met by the speed and size of the impeller. The work required (horsepower) is a result of both flow and pressure.

Pump curves were developed as an aide for proper pump selection given two basic criteria: flow and pressure. Using the pump curves and affinity laws, a designer can select a pump and predict the performance throughout a specified operating range with reasonable accuracy. The success or failure of this selection relies on understanding some basic rules regarding system requirements and pumping arrangements. Back to basics

The two basic pumping systems are constant flow and variable flow. Variable-flow systems—which are becoming more prevalent—are divided into three basic categories: constant-primary flow with variable-flow secondary, variable-flow primary systems and distributive-variable flow. The configuration selected is based on the desired plant operation.

The pump arrangements for each system type can be a single operating pump with standby, multiple pumps operating in parallel or multiple pumps operating in series. Single pump applications for constant-flow systems are less complicated; however, these systems sacrifice overall system efficiency and operating costs.

Pumps placed in a serial configuration are typically used for low-flow and high-pressure drop applications. This is more common in industrial processes. Basic selection criteria require the flow for each pump to be identical with the overall system pressure divided equally between the pumps.

More common to the heating, ventilation and air-conditioning industry is the use of pumps in a parallel configuration. Using this scenario, the flow is divided equally between the pumps while the overall system pressure remains constant for each pump. This arrangement has several benefits. First, the pumps selected will be smaller than using a single pump with full standby. Second, if properly selected and matched, should one pump fail, the remaining pump will provide approximately 70 percent of the total system flow, thereby maintaining a level of redundancy. This can be observed by plotting a two-pump parallel-pumping system curve and noting the operating point of a single pump. Successful pumping

Some design suggestions for successful pump applications:

• Plot system curves . A system curve for single- or multiple-pump operations should always be plotted. This is a more critical issue when selecting pumps to operate in a parallel or series configuration. Care must be given to ensure that the impeller curve extends beyond the system curve. If the impeller curve drops off to the left of the system curve, operational problems will occur.

• Provide proper controls . If a constant flow system is designed, system over-pressurization should be controlled by the use of three-way automatic valves at the terminal device. If two-way automatic valves are used, then a system bypass should be installed about two-thirds of the total distance downstream of the pumps. A pressure-relief or system-bypass valve should be controlled by sensing differential pressure across the pumps.

Variable-speed pumping systems require a higher level of control using differential pressure transmitters throughout the system to control pump speed. Further, system optimization and energy efficiency will be obtained by using wire-to-water efficiency strategies, which sequence pumps operating in a parallel configuration.

• Don’t forget net positive suction head required (NPSHR). This is the inlet pressure required to prevent cavitation. Water must be pushed into the pump casing; it cannot be pulled in.

Other critical considerations are: atmospheric pressure at the installation altitude; static head at the impeller (positive if above the pump, negative if below); vapor pressure for the operating water temperature; and friction losses for pipes, fittings and valves.

Pump Design Tips

Plot system curves

Provide proper controls

Don’t forget net positive head suction required (NPHSR)

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