Control Valves: Sizing, Design, Characteristics
| C ontrol valves are devices with movable, variable, and controlled internal elements for modulating fluid flow in a conduit. The valve restricts flow in response to the command signal from a process measurement control system. Basically, a control valve consists of a pressure containment enclosure body and various internal elements–fixed and movable–commonly called the valve trim.
While there are uncommon exceptions, control valves are designed to function in either a push-pull or linear sliding-stem manner, or in a rotary-stem manner. The former is epitomized by the traditional globe body design, while the latter is most commonly seen in the butterfly vane type–although also found in the ball, partial ball, plug, and rotary plug types.
Performance, cost set materials
Control valves can be built in sizes to handle liquid flow, from tiny fractions of a cubic centimeter to thousands of gallons per minute, or gases from bubbles to millions of cubic feet per hour. Body size will range from about one-eighth inch to five feet or larger. Despite these extremes, a given valve can exhibit a considerable degree of flow control turndown over its operating range depending upon the process application and factors involved. Numerous ISA (Research Triangle Park, N.C.) and ANSI (New York) standards detail various control valve considerations.
Proper application of control valves requires a combination of engineering knowledge, broad experience, and sensitivity to the aspects of the art involved. It isn’t always cut and dried, doesn’t always follow a formula or book or rules, and certainly doesn’t benefit from being a casual afterthought to the system design. Good control-valve application requires broad knowledge of control valve types, design details and operating characteristics.
Beyond that, the application engineer requires a good understanding of aspects of control dynamics, thermodynamics, hydraulics, fluid dynamics, fluid physical properties, metallurgy, codes and standards, seals and gaskets, and finally for those really tough service applications, a good dose of ingenuity and imagination for a solution.
Final control element
Even worse, there is a tendency to continue this backward way of thinking and settle for the lower cost of outmoded control valve designs to avoid the expense of redesign.