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 6 - Sizing
This chapter consists of an introduction to the ANSI/ISA S75.01 standard, "Flow Equations for Sizing Control Valves," followed by the latest version of this standard. Few changes are expected in later versions. However, refinements for calculating flow coefficients for very small flows and for viscous fluids are expected. Moreover, according to Dr. Hans Baumann [Ref. 1], changes are probable for the valve-style modifier Fd . Chapter 22 of this book addresses sizing by a computer program. Required Calculations There are several calculations used in sizing. Among them are the following:
It takes an energy drop of the fluid through the valve for the control valve to "control." A control valve should have available at least 10% of the system pressure drop for acceptable control and up to 30% of the system drop for good control. The "loss" of energy through the valve is loss of available energy to the " fluid, but it is accounted for by the heat generated, the noise production, the material removal, vibration, and sometimes by the increase in kinetic energy. Control valves are not "sized"; at least a user does not calculate a size. They are chosen from a fixed series of body sizes (the same as standard pipe sizes) and types to satisfy the needs of the process conditions. Aside from rules of thumb, the calculations for the flow coefficient from process conditions are detailed in the ISA standard ANSI/ISA S75.01. The scope of the standard indicates that the equations are not intended to be used for mixed-phase fluids, non-Newtonian fluids, slurries, and dry solids. Noise, choked flow, and actuator calculations are discussed in this book (Chapters 8 and 9). Flow Coefficient From process flow requirements, a calculated flow coefficient (Cv from customary U.S. units; Avand Kvin SI or metric units) is compared to manufacturers' capacity charts. While the dimensions and units on the three flow coefficients are different, they are related numerically. The relationship is explained in the international standard IEC-534-1 "Control Valve Terminology and General Considerations" [Ref. 2]. Often several models of valves as well as several different sizes from many manufacturers can be chosen for each application. Chapter 14, which discusses valve selection, will assist in narrowing the choice. The Cv concept for a valve originated in the 1940s as a sizing technique (see Chapter 2 for historical background). It was defined as the number of gallons of water that would pass through a valve with a one pound per square inch (psi) pressure drop:  Hence, the sizing standard S75.01. In metric units, with q in cubic meters per hour and pressure drop in bar,  In order to calculate the flow coefficient for various types of valves, various types of fluids, and in pipes that are different sizes than the valves, various factors are applied to the simple equation. As more data become available, the sizing equations, or at least the factors, are being refined. The ISA Control Valve Standards Committee SP75 and its Control Valve Sizing Subcommittee meet at least once a year and consider changes.
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