Control Valves: Sizing, Design, Characteristics

03/01/1997



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 and have been fabricated from practically every known metal, metal alloy, and modern engineered plastics or polymers. Essentially, anything that can be cast, forged, molded, or machined can be used for a control valve and its internal parts. Fluids from the most benign to corrosive, with pressures ranging from ultra-high vacuum to ultra-high pressure, and with temperatures from cryogenic cold to extremely hot have been handled. All it takes is full recognition of the needed parameters, engineering ingenuity and, of course, money.

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
Control valves are the final control element. As such, they are just as important to the proper functioning of the control-loop system as the primary measurement device and the controller. It does little good to specify and purchase the most sophisticated and capable measurement elements and controllers if the control valve is given short shrift and viewed as just a simple piece of 'pig iron' hardware. Unfortunately, too many engineers don't seem to understand this.

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.

Control valve standards

— Aerodynamic noise prediction and laboratory measurement of aerodynamic noise generated
— Capacity test procedure
— Cavitation evaluation
— Flow equations for sizing
— Hydrostatic testing
— Inherent flow characteristic and rangeability
— Method of evaluating performance of positioners with analog input signals and pneumatic output
— Process data presentation
— Terminology

Control valve dimension standards

— Buttweld-end globe-style, face-to-face
— Flanged clamp or pinch, installed face-to-face
— Flanged globe-style angle, face-to-centerline
— Flanged globe-style bodies, face-to-face
— Flangeless, face-to-face
— Separable flanged globe-style, face-to-face
— Socket weld-end and screwed-end globe, face-to-face
For more on valve standards,contact: ISA, 919/549-8411, or ANSI, 212/642-4900.





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