This volume is part of the Practical Guide Series developed and published by the ISA, the International Society for Measurement and Control. The Practical Guides were conceived because of a shortage of published material in the field of measurement and control that bridges the gap between theory and actual industrial practice. Many books in the field have catered to the needs of technical students, who need to be oriented to basic control theory and concepts, or college-level readers, who are interested in engineering mainly from a classroom perspective. There are handbooks for practicing engineers that cover measurement and control, but these handbooks often devote only a chapter or two to topics that merit more attention. Within the Practical Guides Series, separate volumes address each of the important topics and give them comprehensive, book-length treatments. Each book in the series can be understood and used by technical students, sales engineers, sales personnel, and managers, and relied upon by those who have "real-live" industrial concerns such as correct application, safety, installation, and maintenance. Another unique feature of the Practical Guides is the stress placed on the actual experience of measurement and control practitioners. The Practical Guides are overseen by various Volume Editors and a Series Technical Editor, who have extensive experience in measurement and control. The Volume Editors have been selected for their specific expertise in the volume topics, and bring together numerous Contributing Writers with even more specialized knowledge. The Series Technical Editor, who is responsible for general technical consistency within each volume and across all volumes, helps guide the Volume Editors. The Practical Guides capture the hard-earned experience of the writers and, by employing examples and recording anecdotal observations, make that experience as applicable for the reader as possible. Case studies, either hypothetical or based on real case histories, are used to illustrate typical situations and show how good planning and practical applications made the difference between success and failure. Some of this information has never been documented before. This volume is designed to be at home in a library, in a classroom, or on the plant floor. The comfortable reading style, large pages, and frequent illustrations will contribute to ease of use. The page design uses graphics to "call out" some of the major points of the text, such as crucial safety checks and important examples. Each Practical Guide gathers widely scattered information in a single text, with bibliographies directing the reader to other sources. |
TABLE OF CONTENTS Chapter 7 - Cavitation and Flashing
To avoid dangerous and costly problems in their process systems industrial valve users should understand the basics of the ways in which cavitation and flashing affect control valves. Cavitation occurs in liquid systems when local pressure fluctuations near the liquid's vapor pressure result in the sudden growth and collapse of vapor bubbles (cavities) within the liquid. The cavity collapse produces localized shock waves and liquid microjets. If these impact on the adjacent surfaces of pumps, valves, or pipe, severe pitting and erosion damage can occur, which can reduce critical wall thickness. Cavitation often produces high levels of noise and vibration across a broad range of frequencies. Excessive vibration can loosen flange bolting, damage piping support structures, and destroy process equipment. These hazards and excessive noise create dangerous conditions for people and their environment. The pressure-reducing characteristics that make control valves useful also make them susceptible to cavitation. However, an effective combination of system design, valve selection and design, and material selection can minimize or eliminate the unwanted effects of cavitation. Flashing is a vaporizing process similar to cavitation. However, flashing differs from cavitation in that the vapor phase persists and continues downstream because the downstream pressure remains at or below the vapor pressure of the liquid. High velocities and mixed-phase flow are generated by the expansion of the liquid * into vapor, which can cause erosion and thinning of pressure boundary walls. Although with flashing noise and vibration are usually much less than with severe cavitation, flashing can generate excessive vibration associated with high-velocity flow. Reducing velocity and using erosion-resistant materials are effective design strategies that minimize the damage from flashing. This chapter discusses the fundamentals of cavitation and the related phenomenon of flashing as they apply to control valves. Methods for evaluating cavitation in valves are discussed, including how to test valves and establish cavitation coefficients. The applications of cavitation coefficients, materials, and cavitation-resistant valve designs are shown in various examples. This chapter introduces definitions and nomenclature from the ISA recommended practice RP75.23, "Considerations for Evaluating Control Valve Cavitation." Additional information may be found in the references at the end of the chapter. A glossary of special terms is also found in the nomenclature and glossary section at the end of the chapter. |
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