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 10.8 - Control Valve Features: Freeze Protection (Cryogenic Service)
Freeze Protection (Cryogenic Service) The design of valves in cryogenic service must be adapted for this duty. The major characteristic in the design of this type of service valve is the coefficient of thermal contraction of the seat ring material. The coefficient is normally higher than that of the stainless steel of the ball and valve body. The seat rings shrinks on the ball at low temperatures and therefore causes the operating torque to increase. In extreme cases, the seat ring may be overstressed, causing it to split. The effect of differential thermal contraction between the seats and the ball may be offset by reducing the installed prestress between the seats and the ball by an amount which would ensure undue stress at the cryogenic operating temperatures. However the sealing capacity of these valves may not be satisfactory at low fluid pressures, if they must also perform at ambient temperatures. There are several other means of overcoming the effects of differential thermal contraction including supporting the seats on flexible metal diaphragms or choosing the seat-ring materials with considerably lower coefficients of contraction than virgin PTFE (PolyTetraFluoroEthylene), such as graphite or carbon filled PTFE. Another option is to make the seat rings of stainless steel with PTFE inserts where the PTFE contents are kept to a minimum. At cryogenic temperatures, the plastic seat ring materials become rigid. To ensure a high degree of seat tightness, the surface finish of the seating and the sphericality of the ball must be made to exacting standards. Steam Tracing This is an arrangement for heating a process line or valve to keep liquids from freezing or condensing. Often a pipe run or piece of tubing carrying live steam is simply placed alongside or coiled around the line to be heated. Heat Tracing This technique adds heat to a process line or instrument measurement line by placing a steam line or electric heating element adjacent to the line or valve. |
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