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 10.18 - Control Valve Features: Flow Straightening Devices
Flow Straightening Devices Straightening Vanes Upstream Most of the major control instabilities due to inadequate approach limits, result from swirls or vortices produced by two close-coupled elbows in different planes. Straightening vanes furnish inexpensive insurance for good flow control. This is especially true for butterfly and segmented ball valve installations or any rotary valve, where space limitations in the pipe run necessitate a rotary valve and swirl-producing disturbances precede the valve. Examples are shown in Figure 10-34.  In installations where it is impossible to provide a sufficient length of straight pipe (to remove swirl and restore an acceptable reference profile geometry), flow conditioners are used in combination with specific pipe length. The use of vanes is often forbidden in high velocity steam lines. This is especially true when the valve is followed by a turbine or other high speed piece of equipment that might be ruined by ingestion of a disintegrated vane. The straightening vanes consist of a number of parallel passages of relatively small transverse dimensions installed in a pipe parallel to the line of flow. For pipe sizes less than 2", straightening vanes are not practical. Straightening Vanes Cast into Body There should be at least four partitions across any diameter. The length of the passages should be at least ten times the maximum transverse dimension of the passages. For maximum efficiency, no passage through the vane should have an area more than 1/16th the size of the inside diameter of the pipe. |
