Understanding Pump Fundamentals for an Energy Efficient World (Part Two)

Written by Hydraulic Institute Members and Pump Systems Matter Sponsors

Page 2 of 2

Friction Dominated Systems

Closed systems are always purely frictional-all the fluid in the system circulates from the pump suction, through the pump, to the discharge pipe run, then back to the pump suction. Because all the fluid in the system returns to the same point, the only resistance in the system comes from the friction loss in the pipe runs and the components. Open systems that coincidentally have the same pressure and elevation at all supplies and discharges are purely frictional, as well. For systems with pure friction, the system head is zero at zero flow (see Figure 2).

Figure 2. System curve resulting from only frictional losses

Static Head Dominated Systems

A system dominated by static head is one in which the primary function of the pump is to overcome static head (i.e., gravity, a difference in liquid elevation or pressure difference). Figure 3 shows a system curve dominated by static head. In an open system, the fluid is pumped from a source to a destination. Often the fluid source and destination are at different elevations and/or different pressures, which introduces a static head element in the system total head.

Figure 3. Static head dominated system curve

Pump Efficiency and the Best Efficiency Point

Figure 4 shows the system curve, the pump head curve and the pump efficiency curve plotted as a function of flow. Every centrifugal pump has a maximum efficiency point (also called the BEP). Figure 4 also shows the actual operating point. In this example, the pump is operating to the right of its BEP. For most pumps, the peak value of pump efficiency does not change significantly with speed.

Figure 4. Pump and system curves with the pump efficiency curve shown

Multiple Pump Operation

Multiple pumps can operate in series or parallel. Pumps placed in parallel provide additional flexibility in the range of flow rates. Pumps placed in series provide greater head without having to increase the impeller diameter.

Pumps should not be operated in series or parallel unless specifically procured for this purpose, since serious equipment damage may occur.

For parallel operation, the pumps must have approximately matching head characteristics, or the system operating head may exceed the shut-off head of one or more pumps and result in zero output flow. This result would have the same effect as operating against a closed discharge valve.

In series operation, the pumps must have approximately the same flow characteristics. Since each pump will take suction from the preceding pumps, the stuffing boxes and casing must be designed for the higher pressure, and the thrust bearing requirements may also increase.

Clearly understanding how the constituent parts of the system interact and function together-whether the system is a new design or an existing one undergoing a revamp-must be foremost in the design team's minds. In addition, having a thorough understanding of the fundamentals of each part of the system and how they operate together is the important first step. These fundamentals will be explored in greater detail in Part Three of this series of articles based on Optimizing Pumping Systems: A Guide to Improved Energy Efficiency, Reliability, and Profitability, now available for purchase at http://www.pumpsystemsmatter.org/ or http://www.pumps.org/ or call 973-267-9700 ext. 10.

New Pump Systems Optimization Guidebook is Now Available

Pump Systems Matter (PSM) and the Hydraulic Institute (HI) have published a new guidebook, Optimizing Pumping Systems: A Guide to Improved Energy Efficiency, Reliability, and Profitability. Pump engineers have long known that the highest level of pumping efficiency and equipment reliability is achieved by matching the pump performance characteristics to the system requirements.

Optimizing Pumping Systems has the details that improve understanding of this complex task, and the details to justify such systems improvement projects to senior management who make key capital and budget decisions. It presents practical information in a single source both for those who have not had broad exposure to pumping systems and for those who wish to improve the systems for which they have responsibility. Material presented assumes the reader has basic familiarity with engineering principles and practices. It presents the collective knowledge of many industry experts who intend to empower the reader to optimize systems efficiently, reliably and economically.

The guidebook is one of an evolving range of services offered by Pump Systems Matter (PSM) and Hydraulic Institute (HI). PSM is a market transformation effort that emphasizes pump systems education and outreach focused on energy savings and total cost of ownership. HI is the largest association of pump producers in North America, and a global authority on pumps and pumping systems. Its mission includes the development and delivery of comprehensive industry standards as well as the expansion of knowledge that effectively advances the application, testing, installation, operation and maintenance of pumps and pumping systems.

For more information, visit http://www.pumps.org/ or http://www.pumpsystemsmatter.org/.

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