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 12 - Control Valve Applications
The purpose of this chapter is to present the information that will help process engineers specify control valves properly. It will show that not all of the control valves' attributes are uniquely required for the process design. There are many common requirements for the categories of applications, regardless of the industry or the process. By recognizing the common requirements and their importance, the designer can specify the most economical design that meets performance needs. Many books on control valves concentrate on the design aspects of the valve, and little information is included on the subject of applications. An exception to this is Reference 1, which provides the practical design input associated with a good valve application. This reference provides many examples of control valve applications and illustrates the importance of knowing the control characteristics dictated by the process. Another source of good application knowledge is the control valve manufacturers. Many have prepared specific application brochures and guidelines for experiences they have frequently encountered. These brochures have a wealth of background information because they integrate the experience of many customers involving the same applications and the experience gained across different industries encountering similar applications. Many practical guidelines regarding selection and installation practices are also provided in Reference 2. In the discussion in this chapter all applications of control valves are classified into four categories. In describing the applications, the attributes that will be emphasized are those important in each of the four categories, independent of the many different application names used from industry to industry. Valve types or materials will not be discussed unless they are key to a successful application. Selection among valve types is covered in Chapter 14; selection among materials was discussed in Chapter 11. There are exceptions within every category because of the needs unique to a specific process within an industry. An example of this would be a pump recirculation valve that would normally fail open to protect the pump but, in some nuclear power plants, fails closed by design. It is assumed in the following discussion that the valves have been sized correctly. A correctly sized valve is a major factor in a successful application. The use of the ISA data sheet, Reference 3, ensures that all the pertinent information is available for the sizing decisions. It is recognized that many companies have their own valve data sheets, but many contain omissions that result in important information not being passed onto the valve designer. The ISA data sheet standard provides a good checklist of key information that may impact the users' specific application. Chapter 6 of this volume discusses sizing; Chapter 22 describes a valve sizing computer program. In the discussion in this chapter emphasis is placed on the attributes that are unique to the application and not so much on problems that occur due to poor selection. Of course, being aware of these attributes can help assure that you make a proper valve design choice. The application categories are as follows:
In addition to these four categories, a separate discussion is provided for valves that are installed in parallel or series applications. These configurations occur in many processes so a specific section is devoted to them. Also discussed in this chapter are common valve application problems caused by improper specification or installation practices. The definitions of terms used in this chapter are discussed in Chapter 3 and are based on the ISA standard S75.05, titled "Control Valve Terminology." |
