Valtek uses a systematic method for selecting body types, sizes, materials, pressure ratings and trim sizes based on flow characteristics.

Valtek control valve flow capacity (Cv) is based upon the industry standard, ANSI/ISA S75.01. This standard and the corresponding measuring standards contain Equations used to predict the flow of compressible and incompressible fluids in control valves. Slightly differ- ent forms of the basic Equation are used for liquids and gases.
Basic steps for sizing and selecting the correct valve include calculating the required Cv. Equations for calcu- lating Cv for both gases and liquids are found in this section.

Valtek has programmed the ANSI/ISA sizing Equations and procedures, making computer-aided sizing avail- able on IBM-PC or compatible computers. These programs permit rapid control valve flow capacity calculations and valve selection with minimal effort. The programs also include exit velocity, noise prediction and actuator sizing calculations. See Section 22 for more details on computer-aided valve selection.

These instructions are designed to expose the user to the different aspects of valve sizing. The step-by-step method outlined in this section is the most common method of sizing.