Cavitation In Depth
Introduction
This document focuses on relatively clear water as found in aquifers, lakes, canals, fountains, chiller systems, and potable water systems. However the principles apply to all Non-Compressible Newtonian (free flowing) fluids.
Pump selection and implementation must account for the properties of the fluid, specifically how that fluid reacts to the pump operations of intake and impartation of energy into that fluid. Cavitation places definite upper and lower limits on what pumps can and cannot do with fluids.
There are three mechanisms at work in centrifugal pumps capable of lowering pressures below the vapor pressure of the pumpage, resulting in cavitation.
LOW INTAKE PRESSURES - Every centrifugal pump can only obtain fluid by one means, by creating a low pressure area at the eye of the impeller, allowing higher pressures at the fluid source to push fluid to the pump. If the pump causes intake pressures to drop below the vapor pressure of the fluid, the fluid changes phases from a liquid into a gas, initiating the cavitation process.
VELOCITY INCREASE PRESSURE DROP - Centrifugal pumps impart energy into the fluid by means of a velocity increase, causing fluid pressure inside the impeller to first DECREASE (Velocity Increase = Pressure Decrease - Bernoulli's Principle of Energy Conservation in Fluids). The low pressures caused by the velocity increase can fall below the vapor pressure of the fluid, causing the fluid to undergo phase change from a liquid into a gas. The pressure increases only when the fluid is slowed down, causing an energy transformation from Velocity Energy into Diffusion Energy, a process called Pressure Recovery. Pressure Recovery occurs in the following places: Impellers, Diffusers, Pump Cases, and in the discharge piping after leaving the pump. But before pressure recovery occurs, cavitation can occur.
TURBULENCE - High velocity flows occurring in opposing fluid flow lines, and through restricted passages, can create highly turbulent vortices with resulting localized areas of low pressure that can result in fluid vaporization, initiating the cavitation process.
Under cavitation conditions water is one of the worst actors regarding equipment damage due to cavitation. Water is hard on equipment in cavitation conditions for at least two reasons: relatively high density (small molecular size and heavy molecular weight), and a sharp well defined phase change behavior. The combination of small heavy molecules and high cavity wall implosion velocities (resulting from the sharp and fast rate of phase change), results in the release of extreme inertial energies as the walls of the cavity strike against each other and against objects in the fluid flow path during the cavity implosion. All pump components exposed to the fluid can be damaged by cavitation, but the most affected components are usually impeller vanes and shrouds, pump cases, diffusers, cutwaters, and wear rings.
Liquids with molecules larger and more complex than water, and non-homogenous liquids such as many petrochemicals, can be much less harmful to equipment in cavitation conditions because they often have a lower density than water, their larger more complex molecular structure and sometimes non-homogenous nature causes a "blurred" or less well defined phase change behavior, both of which reduce the rate of cavity creation and collapse, and the amount of energy released in the implosion, and therefore the amount of damage caused by cavitation is reduced.
The data provided below is from research using various liquids including water. Some of the research also involves experimental methods creating cavitation by means unfamiliar to pump users including the use of sound waves and lasers.
For ease of writing and clarity, this document makes a sharp differentiation between "bubbles" and "cavities". Although both terms refer to accumulations of gas phase molecules in a liquid, commonly called bubbles, there is a huge difference between the two types of "bubbles".
Cavities - Pockets of gas in a pumpage caused by the cavitation process. These pockets of gas originate as pumpage molecules change phases from a liquid into gas phase, followed by the near instantaneous implosion of these same cavities as the molecules change phases from gas back to liquid, the implosion releasing extreme energies in the form of shock wave pressures and heat. The phase change behavior of water is near instantaneous, so the cavities are created very fast. At first, the cavities are microscopic. If conditions are correct, the cavities can coalesce into larger and larger bubbles, eventually becoming macroscopic. When cavities coalesce into large cavities the term "Super Cavities" or "Super Cavitation" may be used to describe the cavities.
Bubbles - Bubbles are defined as pockets of gas that, regardless of their source, for the most part do not involve molecules changing phases, do not involve near instantaneous creation of or the near instantaneous implosion of those bubbles, and therefore bubbles do not release immense pressures and heat that damage equipment. Bubbles may cause some damage to equipment by exposing seal faces or by causing the pump to lose prime. Bubbles can change during the pumping process by compression and by diffusion into or out of solution in the pumpage. But Diffusion is a slow and orderly process that cannot produce the immense energies such as occur during the molecular phase changes involved in cavitation.
Follow the links below in their numbered sequence to learn progressively more about cavitation. There are videos, photographs, and surprising discoveries about cavitation by researchers.
Bibliography and Other Information Below
Written by:
Richard Neff
President of Irrigation Craft
Please email comments and criticisms on this article to:
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 / 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
Cavitation and Bubble Dynamics
Christopher Earls Brennen
Oxford University Press (1995)
Published Research Papers
Young (1989)
Tomita and Shima (1977)
Fujikawa and Akamatsu (1980)
Other Sources
Terry Henshaw (Pumps and Systems Magazine, 2001-2002)
