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 9 - Control Valve Trim
The heart of the control valve is the trim, especially the mating parts that throttle the stream to the demands of the controller. These valve parts are subjected to the most taxing service, and selecting the best type for an application involves many considerations. Because the trim in rotary motion valves is vastly different in design from that in linear motion valves, body choice is also affected. Aside from being the right size, the most important consideration is probably serviceability - will the trim do a proper job for an acceptable length of time? The corrosive and erosive nature of the fluid are important elements in the analysis, as are extremes of temperature or pressure differential. The risk of cavitation or excessive aerodynamic noise requires appraisal, as does the importance of flow characteristics on the loop stability. The trim design affects the forces that are demanded of the actuator. Hence, dynamic and actuator cost are involved. Special considerations for valve trim include a need for tight shutoff and regulatory requirements, such as those governing sanitary service or the desirability of having a quick-change facility for easy cleaning or replacement. The following are the main considerations for choosing a valve trim:
Selection of Valve Trim Material The overwhelming majority of valve trim material is AISI type 316 stainless steel. This is a good all-around choice for general service for about -320°F (-195°C) to 750°F (+400°C) and moderately corrosive fluids. More specialized material choices are dictated by special erosive, corrosive, or temperature requirements. However, such trim is usually of the custom variety, and increased cost and delivery cycles are to be expected. While there are no hard-and-fast rules governing what trim material to specify, FCI standard 65-1 may be consulted [Ref. 1]. Guidelines published by various manufacturers are of use, but often contradictory and vague. Table 9-1 is typical of such guidelines. In the selection process, one has to make a reasonable compromise between initial cost and expected service life. For example, if a solid Stellite trim will extend the service life by only 20% but doubles the cost of the valve, then it may be a poor choice. Reliability, downtime, and labor costs must be considered. Pressure drop alone is not necessarily an indication of wear. A valve operating at choked flow (?P=FkXTPPI) on gases experiences the same velocity whether the pressure drop is 15 or 5000 psi. Associated vibration caused by high throttling noise, on the other hand, may cause substantial wear on guide surfaces.  Hardened Trim Hardened trim is usually specified for high pressure drop service for other than clean, dry gases. Although solid hard plugs are the economical choice for valves below 1 inch in size (type 440-C or 17-4 PH stainless steel, alloy 6, and tungsten carbide are typical), a hard material overlay (usually a cobalt alloy of 38 to 60 RC hardness) is more economical for larger sizes. For abrasive or slurry surfaces, it is wise to use a fully coated or solid, hardened closure member and seat ring. But hard surfacing of the seating surfaces may be sufficient to improve the life expectancy of the seat tightness on services such as superheated steam, two-phase flow, or high temperatures over 600°F (315°C). An alert manufacturer will select a material combination that offers at least 150 Brinell hardness difference between the closure member and the seat ring whenever severe service is encountered. For more details on materials refer to Chapter 11. |
