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 10.1 - Control Valve Features: Process Conditions that Influence Valve Design
| Process Conditions that Influence Valve Design Process conditions, such as pressure and temperature or abrasive, viscous, slurries, erosive, toxic, flammable or explosive fluids, have their own characteristic problems and hazards. High pressures and temperatures create stresses that must be accommodated for by design. A combination of extreme conditions results in increased plant cost due to the need for material with high mechanical strength and corrosion resistance. High pressure increases the amount of potential energy available in the process plant. For these plants, in addition to the energy of compressed gases and of fluids kept under pressure in the liquid state, there may also be a concern of chemical reactivity under pressure or an adverse reaction from rapid de-pressurization. Leakage is much more pronounced in high pressure operations. Because of the large pressure difference, the amount of fluid that can discharge through a given area is greater. This has a considerable impact on the consequences of a release, as the hazard zone extends to a larger area. High temperature also poses material failure problems, most frequently due to metal creep and hydrogen embrittlement. Service in high temperature conditions usually increases valve cost, not only due to the materials of construction, but also due to the requirement for special supports to handle the stresses generated by dynamic fluid forces on a plug (globe valve), disk (butterfly valve) or ball (ball valves). In fact, for rotary type valves (ball, plug and butterfly), the matter of instability is more complex since they have "torque" instead of "force." Pressure cycling caused by rapid pressure fluctuations of large magnitude will fatigue the bonnet bolting and gaskets. Cumulative pressure cycles over the expected life of the valve should be reported to the manufacturer promptly so that provisions can be made to neutralize or damp high pressure fluctuations. | ||||
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