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 4 - Valve Bodies
This chapter deals with the parts of a control valve called the body or body subassembly. The body contains the trim components as well as the process fluid itself. Trim is generally defined as the part of the valve that moves and is often the seat or seats. The body is a pressure vessel and as such must meet all of the applicable codes and requirements set forth by industry standard groups such as ASME, ANSI, ISA, and FCI, The choice of materials will be dictated by the process media and other service conditions such as temperature and pressure. There are a host of different body styles, but they can generally be categorized by stem and trim motion; linear and rotary. The choice of body style, as will be seen, is often a function of the application or internal trim considerations. The valve styles described in this chapter are available in configurations designed for control service, and others, such as most gate valves, are meant strictly for on-off operation. The latter category should not be used for throttling control because poor control, excessive vibration, and eventual failure may result. Globe-Style Bodies The most popular of the linear motion type is the globe valve (Figure 4-1). To properly understand this type of valve in all of its variations, we need to take a trip back in history. Globe valves became the standard for throttling service due in most part to their application as pressure regulators on early steam turbines. As their use was expanded to other applications, new body styles were developed to meet a growing list of new services. Double-seated valves for increased flow-capacity, angle bodies for high pressure drop and erosive service, flangeless styles for corrosion-resistant alloy applications, and many other styles are described in this chapter. As a result of refinements to alloys, manufacturing methods, and actuation many of these body styles, such as double-seated and split body types, have become less popular over the years. The term globe valve derives from the shape of the center body section. The wider section at the center allows flow to pass through the trim, which fits in this part of the body. At either side of the center section are the end connections, which may be flanged, butt weld, socket weld, threaded end, or one of several other types. Generally, the body has an opening on the top, through which the trim is installed, that is sealed with a bonnet. The bonnet provides a location for the stem seal and a mounting surface for an actuator. Both bonnet and body are considered pressure-retaining parts and are usually bolted together with studs and nuts or cap screws.  |
