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 19 - Maintenance
Philosophy The philosophy of control valve maintenance is similar to that for any piece of mechanical equipment. Repairs should be conducted only by those personnel who have been trained. Before repairing, everyone involved should be aware of the materials and design as well as the requirements of the service. Several manufacturers and technical societies (including ISA) offer training classes for maintenance personnel who are responsible for control valves and actuators. This chapter will discuss the causes of failures and provide other comments specific to control valve maintenance. Control valves are in service in almost every piping system, are required to absorb pressure drop, are sacrificial because their parts wear, are frequently called upon to fail safe, and must be in good working condition to control flow. For these reasons, proper and timely maintenance is vital. Control valves consist of closely fitted parts with carefully engineered tolerances and surface finishes. Other devices in the loop sometimes may be fixed or adjusted without shutting down the loop. Warranty Be aware that the standard warranty offered by most manufacturers covers two areas: materials and workmanship. It is unusual to offer operational guarantees; if a guarantee for something other than materials and workmanship is required, it must be asked for in purchase negotiations and received in writing. Warranties are usually offered for one year after the valve is shipped. Some manufacturers will offer longer warranties when asked. These are useful for projects that may not start up for an extended period of time. After a manufactured item leaves the possession of the manufacturer or his authorized distributor, its well-being is out of control of the manufacturer. Any servicing, repairing, or modifying that is done should be done strictly in accordance with the manufacturer's instructions to avoid voiding the warranty. None of the servicing suggestions offered in this chapter are intended to replace the manufacturer's service instructions. To assure that the warranty is not nullified, all servicing should be documented, and all repair parts used must be from the OEM (original equipment manufacturer) during the term of the warranty. |  The control valve is often the most costly device in a control loop, and it is the most likely to cause process downtime. | |
