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Pumps & Systems, June 2008

This first in a series is based on the opening chapter of Optimizing Pumping Systems, A Guide to Improved Energy Efficiency, Reliability, and Profitability, written by pump systems experts. This new guidebook continues the mission of Pump Systems Matter (PSM) and the Hydraulic Institute (HI) to advance knowledge on pumping systems.

The U.S. Department of Energy has determined that 25 percent of industrial motor systems energy consumption is currently consumed by pumping systems. Interest in energy efficiency is not a fad; industrial production economics, global energy supply limitations and environmental conservation realities will likely be an enduring theme for decades, if not indefinitely. 

As energy costs continue to increase, pump manufacturers understand that making equipment more efficient will contribute to saving energy. While traditional methods of specifying and purchasing piping, valves, fittings, pumps and drivers often result in lowest first cost, these methods often produce systems with unnecessary, expensive energy consumption and higher maintenance costs. A business entity that incorporates the energy, reliability and economic benefits of optimized pumping systems can enhance profits, gain production efficiency improvement opportunities and initiate necessary capital upgrades for long-term business survival.

Pump Fundamentals

The pumps used in pumping systems fall into two general categories: rotodynamic (centrifugal, mixed flow and axial flow) and positive displacement. Because the majority of pumps and energy usage in industrial and commercial applications are in the rotodynamic pump category, the forthcoming guide from HI and PSM deals exclusively with rotodynamic pumps.

How a Rotodynamic Pump Works

A rotodynamic pump converts kinetic energy to potential or pressure energy. The pumping unit's energy conversion components have three major parts: the driver that turns the rotating element, the impeller and shaft (the rotating element) and the stationary diffusing element. 

Typically connected to the pump's rotating element by a coupling, the driver causes the shaft and connected impeller to spin. With the pump casing primed, the liquid enters the rotating impeller eye, located along the axis of the impeller. The liquid is accelerated into the impeller's vaned passageways, where the continuous transfer of momentum and energy conversion occurs. As liquid flows through the impeller passageways, velocity increases.

When the fluid leaves the impeller, liquid velocity is greatest at the tip of the vanes. The rapidly moving liquid leaves the pump impeller, and the fluid enters the diffusing element of the pump. An increase in cross-sectional area of the flow passage occurs and the fluid slows down. The deceleration of the fluid in the diffusing section converts the kinetic energy of the liquid to potential or pressure energy. The diffusing section of the pump can be either a diffuser or a volute depending on the pump's configuration. 

The shape, size, speed and design of the impeller and diffusing section establish the pump's head and flow characteristics. The pump impeller and diffusing section designs are based on the intended application, the user's specifications for the pump and the pump manufacturer's experience. Once a pump is selected, the casing design envelope cannot readily be changed, but the user can often change the pump impeller diameter and/or adjust the speed to better meet pumping requirements. For certain pumps, the manufacturer may have an alternate impeller, designed for a higher or lower capacity.

Pump Selection Considerations

Selecting a rotodynamic pump requires careful analysis of the system head versus flow requirements; the pump performance characteristics; the pumping application; the footprint available for the pump and driver; applicable specifications, codes, regulations and reliability; maintainability and energy cost considerations. The specifying engineer may need to work closely with the pump manufacturer or distributor to select the optimal pump and its size, speed and power requirements, type of drive, mechanical seal and ancillary equipment.

Understanding the Pump Performance Curve

All pump selections must include matching the operating characteristics of the pump with the system requirements over the expected range of flows.

Types of Curves

There are three basic types of pump curves supplied by the pump manufacturer: the selection chart (also known as the range chart or the family of curves), the published curve and the certified curve. 

Selection Chart

A selection chart shows the performance map for a similar pump family. Figure 1 shows a selection chart for a line of general-purpose end suction pumps. The head and flow scales on the hydraulic coverage range chart are often formatted on semi-log or log-log scales to display a wider range of flow and head values on a single chart.

The selection chart shows the various pump sizes available for a given manufacturer's pump type and speed. The required head and flow rates are plotted on the curve, and the manufacturer evaluates the pumps with a best efficiency point near the specified operating points. 

Published Curves

Once a shortlist of acceptable pumps is developed, the manufacturer's published curves can be referenced to help determine the best pump for the application. Figure 2 is an example of a published curve for a 5x6x11 pump running at 1,770-rpm. Useful operating information can be derived from the manufacturer's pump curve for this application, including the following:

• The impeller diameter falls between 10- and 10.5-in
• The pump is 85 percent efficient at the design point
• The power (at the operating point) is approximately 30 hp
• The net positive suction head required is approximately 10 ft

Contacting the pump manufacturer or sales office to review the suitability of a given pump model for the specified service conditions is recommended when specifying a pump. 

Certified Curve

After a pump has been ordered and released for construction, the manufacturer builds it, and if testing is required, a certified performance curve is supplied. For reliable, consistent test results, it is recommended that the certified curve be based on the testing requirements contained in ANSI/HI 1.6 or 2.6. Unlike the published curve, which is a general curve for a given pump model type and size, the certified curve reflects the test results for the particular pump supplied under the purchase order.