Water
Those who design and implement pumps for water service should learn as much as possible about the properties of the fluid they are pumping. This document provides basic and essential knowledge of the properties of H20.
Density And Temperature
Relatively speaking, water has a high density compared to many other commonly experienced liquids. Liquids we handle every day are typically lighter than water including gasoline, light oils, alcohol, vinegar, and many more.
Moreover, water molecules are small compared to most other liquids. In the process of cavitation, the small and heavy weight water molecules behave differently than other common liquids, such that water can seriously damage pumps and associated piping far faster than those other liquids. Read the article on Cavitation to learn more about this topic.
Those who work with chilled water systems may be particularly interested in the potential for cool water to damage pumps. A specific type of cavitation called Incipient Cavitation, damages chilled water pumps, primarily due to, believe it or not, excessively high NPSHa values.
Polarized Molecule
H20 is a heavily polarized molecule, that is, the positive and negative charges are widely separated in the H20 molecule. This polarized condition of the H20 molecule arises from the fact that the two hydrogen molecules bonded to the oxygen molecule, are close to each other and on one side of the oxygen molecule. This unbalanced charge causes individual H20 molecules to "stick" to each other strongly and explains why water has a large meniscus in a test tube, why needles or paper clips "float" on the surface, and also why water dissolves and holds substances in solution strongly, (water is referred to as the Universal Solvent).
Water strongly holds relatively large amounts of many substances in solution including metals, salts, minerals, gasses, and especially ionic substances. Relatively clear water, including our drinking water, usually contain dissolved substances that change the temperatures and pressures at which H20 changes phases from liquid to gas and back again (cavitation). Also, when there are few or no dissolved substances in the water, such as in water from Reverse Osmosis machines, the water can become aggressive and capable of dissolving iron, steel, bronze, and more. Therefore, dissolved substances have important implications for those who design and work with pumping systems.
Specific Heat
H20 has an unusually high specific heat capacity. Specific Heat is defined as the amount of heat required to raise the temperature of a given mass of the substance by one unit of temperature, at a given temperature or pressure. Water has a specific heat value 9 times greater than iron (the amount of heat required to raise the temperature of 1-pound of water 1 degree is 9 times greater than that required to raise the temperature of 1-pound of iron 1 degree). Water has one of the highest Specific Heat values of all known substances. The huge heat capacity of H20 means that water is an excellent coolant as evidenced by its common use as a coolant in large industry and power plants.
H20 pumping relies upon the water to remove heat generated by inefficiencies in the pumping process. Many of the inherent inefficiencies inside a pump ultimately result in heat, which must be removed from the pump. Most of this heat is removed by the water moving through the pump. Liquids other than water are much less capable of removing this heat.
Those dealing with water pumps take for granted the amazing cooling properties of water, which gives water pumps a wider operating range than pumps moving other types of fluids.
Vapor Pressure
H20 at ambient air temperatures and pressures is often near phase change points, where relatively small changes in temperature and pressure result in large changes in the water. The weather proves the borderline situation of H20 moving through phase changes. In less than one day we can experience all the phases of water with relatively small changes in air pressure and temperature. In the morning we may experience frost and fog, giving way to a sunny and humid late morning, and then a few hours later in the afternoon we may see clouds forming and then liquid water falling from the sky. In one-half of a day we see the solid (frost), we feel the clear gas (humid but clear air), and finally the liquid (rain), and all those changes due only to a few tens of degrees and possibly some air pressure variation. Just think what a pump can achieve, often to our dismay and loss.
Pumps change water temperature and pressure easily, and those changes must be properly understood and accounted for in the design and application of pumps, to achieve predictable pump behavior by reducing excessive cavitation in water pumps. Follow the link below to learn more about Cavitation.
Learn More About Cavitation
Bibliography
Dr. Roger E. A. Arndt
Professor - St. Anthony Falls Laboratory
Follow this link to Professor Arndt's Webpage at the University of Minnesota. Follow this link to go to the St. Anthony Falls Laboratory
Some of the Papers and Books Mr. Arndt has authored, co-authored, or has been an editor for:
Advances in Turbulence
Editors: William K. George, Roger Arndt
Publisher: New York : Hemisphere Pub. Corp., c1989.
ISBN: 0-89116-747-1Hydropower Engineering Handbook
John S. Gulliver, editor in chief, Roger E. A. Arndt, editor in chief Publisher: New York : McGraw-Hill, c1991. ISBN: 0-07025-193-2Aeration Technology: Presented at the 1994 ASME Fluids Engineering Division Summer Meeting, Lake Tahoe, Nevada, June 19-23, 1994
International Symposium on Cavitation Noise and Erosion in Fluid Systems/Fed Vol. 88/H00557: Presented at the Winter Annual Meeting of the American Society of Mechanical Engineers, San Francisco, December 10-15, 1989 (Fed (Series), Vol. 88.) Roger E. A. Arndt, M. L. Billet, William K. Blake, American Society of Mechanical Engineers Winter Meeting
Hydroacoustic Facilities Instrumentation and Experimental Techniques/Nca10/No H00712: Presented at the Winter Annual Meeting of the American Society of Mechanical Engineers, Atlanta, Georgia, December 1-6, 1991 (NCA (Series), V. 10.)
by T. M. Farabee, Roger E. A. Arndt, American Society of Mechanical Engineers Winter Meeting (1991 Atlanta (Cor)/ American Society of Mechanical Engineers Noise Control and Acoustics d
June, 1991
Books and Research Papers
The Pump Handbook
Third Edition, 2001
McGraw Hill
Karassik, Messina, Cooper, Heald
Centrifugal and Axial Flow Pumps
Second Edition, 1957, 1993 Reprint
A. J. Stepanoff, Ph.D.
Physical Review Letters 81, No. 23 (1998)
Joachim Holzfuss, Matthias Rüggeberg, Andreas Billo August 7, 1998,
Paper Presented at the Cavitation Conference 2001, session 4.006
K.M. Kalumuck and G.L. Chahine
Dynaflo, Inc.
Swiss National Fund, Project No 2100-057253.99/1
Swiss Polytechnic Institute (2001)
Dr. Mohamed Farhat, Professor Francois Avellan, and Philippe Couty
Other Published Research Papers
Young (1989)
Tomita and Shima (1977)
Fujikawa and Akamatsu (1980)
Other Sources
Terry Henshaw (Pumps and Systems Magazine, 2001-2002)
