Understanding Pump Fundamentals for an Energy Efficient World

Written by Hydraulic Institute Members and Pump Systems Matter Sponsors

Page 2 of 2

Pump Suction Intake Considerations

Pump Location in the System

The pump's location in the system has a major effect on the net positive suction head available (NPSHA). A change in the pump or suction source elevation directly corresponds to an increase or a decrease in the NPSHA. In a new system, placing the pump at the lowest possible point or elevating the suction source can often be accomplished with minimal cost impact. After the system is built, changing NPSHA-except for changing level set points-is often cost-prohibitive.

Pump Suction Piping

The head loss component of the NPSHA is based on the friction losses in the pump suction piping. These losses can be significant and increase with the square of the increased ratio of flow rate. Pump performance can be limited by the NPSHA. Reducing the piping friction losses may be possible by increasing the diameter of the suction piping, reducing the number of elbows or fittings, or selecting valves with lower losses, i.e., by replacing a globe valve with a gate valve.

Liquid Properties

The temperature-dependent properties of the process liquid can significantly affect NPSHA, head, rate of flow and power requirements. Water at 68-deg F has a vapor pressure head of 0.78-ft, but has a vapor pressure head of 33.9-ft at 203-deg F. The increased water temperature represents a 33-ft reduction in NPSHA if no other changes are made. Changes in liquid temperature affect the liquid viscosity. For Newtonian liquids, raising the temperature tends to reduce viscosity, and lowering the temperature tends to increase viscosity.

Supply Tank and Atmospheric Pressure

The pressure acting on the liquid surface of a supply tank directly affects the NPSHA. It may be possible to increase the NPSHA by increasing the suction tank pressure, but this option should not be selected without verifying the supply tank pressure limitations and related process factors.

Pump Affinity Rules

The pump affinity rules describe how changing the impeller diameter (up to 5 percent change only) and rotational speed affect pump performance. The pump curve is derived from a series of test points connected together forming a smooth line. The discrete flow and head test values can be thought of as belonging to a coordinate point. When using the pump affinity rules, it is important to adjust both the head and flow values for the same coordinate point.

Changes in Rotational Speed

When the rotational speed of a pump is changed, the rate of flow (capacity), head and power for a point on the pump curve vary according to the pump affinity rules.

Figure 3 shows a pump performance curve at the manufacturer's test speed of 1,770-rpm and a speed of 1,500-rpm. As the speed is reduced, the pump curve moves down and shifts to the left. The pump affinity rules do not recommend what should be done to the pump efficiency at the new speed. However, pump efficiency usually follows with the affinity rule adjustment of flow. The values of efficiency do not typically change much with modest speed changes.

The pump affinity rules provide an accurate representation of pump performance change over a range of speeds.

Changes in Impeller Diameter

When the diameter of a pump impeller is trimmed (up to 5 percent change only), the rate of flow, head and power for a point on the pump curve vary approximately with the pump affinity rules.

Pump Operation

A pump must be operated using established procedures to minimize repairs and unexpected downtime. A checklist should be developed to verify that all safety precautions, ancillary equipment and valve settings, manufacturer recommendations, instrumentation hook-ups, etc. are in order before starting a pump.

When shutting down the pump it is important to follow an established shutdown sequence for safety and to prevent hydraulic transient flow-related problems, water hammer, reverse rotation of the pump, unexpected tripping of other equipment in the system and other problems.

"Understanding Pump Fundamentals for an Energy Efficient World" is based on the opening chapter of Optimizing Pumping Systems, A Guide to Improved Energy Efficiency, Reliability, and Profitability written by HI and PSM experts. Subsequent articles from this guidebook will include: Pump and System Interaction; Calculating Cost of Ownership; Improving Performance of Existing Pumping Systems; Building Better Pumping Systems: Optimizing New Designs; Pumping System Economics: Opportunities to Improve Life Cycle Performance and Optimizing Pumping Systems Case Studies.

References

Centrifugal Pump Tests, ANSI/HI 1.6 and Vertical Pump Tests, ANSI/HI 2.6 are available at http://www.pumps.org/

Hydraulic Institute, Inc., 9 Sylvan Way, Parsippany, NJ 07054, 973-267-9700, http://www.pumps.org/.

Pump Systems Matter, 9 Sylvan Way, Parsippany, NJ 07054, 973-267-9700, http://www.pumpsystemsmatter.org/.

Pump Systems Matter (PSM) and the Hydraulic Institute (HI) will soon publish a new guidebook on Optimizing Pumping Systems to improve understanding of the complex task of matching pump performance characteristics to the system requirements. It will also contain explanations to justify such systems improvement projects to senior management who make key capital and budget decisions. It presents practical information for those who have not had broad exposure to pumping systems and those who wish to improve their systems. The material assumes the reader has basic familiarity with engineering principles and practices. It presents the collective knowledge of many industry experts, which is intended 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). See http://www.pumpsystemsmatter.org/ for more information.

Comments (0) Add Comment