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 13 - Safety
The material presented in this chapter addresses the safety aspects of automated valves as they affect the instrument engineer's and/or the designer's portion of the work. Safety requirements and additional recommendations may be found in ISA S84.01-1996 [Ref. 1]. Automated Valve Selection Selecting the body style of automated valve requires an understanding of the fluid, the piping requirements, and the speed of response required to move the plug, ball, disk, or other internal parts to the desired position. The use of globe bodies is not appropriate when liquids must not be trapped in the valve. Rotary valves with hardened trim are appropriate for hard slurries. The seals on rotary valves tend to leak less than do the standard packing on sliding stem valves. Packless valves, bellows seals, environmental seals, and double packing are all available for highly hazardous materials, where zero leakage into the atmosphere is required. "Fire-safe" valves should be used on flammable, lethal, and toxic services to minimize internal leakage and leakage into the atmosphere in case of fire. These valves utilize backup metal and graphite seals that supplement the primary flexible seal when it deteriorates in a fire. "Fire-safe" valves should have flanged connections or preferably welded inlet flanged bottle connections. Butterfly valves should be of the lugged design. Wafer-style valves should be avoided: they make installation with spiral-wound gaskets difficult because they are more subject to leakage during piping temperature changes due to relaxation of longer bolting. Automated valves can be divided into two main categories: throttling control valves and on-off (or block valves). Automated on-off valves are typically manual-type ball, plug, wedge, butterfly, or gate valves with special features (such as a mounting bracket that bolts to the valve body) that allow the addition of a pneumatic, electric, or hydraulic operator. |
