# Category: Control Valve

1. Select the appropriate sizing equations based on the stated inlet conditions and units of measurement from Fig. 4-35. 2. Calculate the Cv required using the equation for nonvaporizing flow. 3. Calculate the Cv using the equation for vaporizing flow. An initial assumed value of F1 can be taken from Fig. 4-32 or the manufacturer’s […]

The following section is based on ISA-S75.01, “Flow Equations for Sizing Control Valves.” The reader is referred to that standard for more complete discussion of these equations and methods. As that standard points out, these equations are not intended for situations involving mixed-phase fluids, dense slurries, dry solids, or non-Newtonion liquids. In these cases the […]

The first stages of cavitation and flashing are identical; that is, vapor forms as the vena contracta pressure is reduced to the vapor pressure of the liquid. In the second stage of the flashing process, a portion of the vapor formed at the vena contracta remains in the vapor state because the downstream pressure is […]

In a control valve, the fluid stream is accelerated as it flows through the restricted area of the orifice, reaching maximum velocity at the vena contracta. Simultaneously, as the velocity increases, an interchange of energy between the velocity and pressure heads forces a reduction in the pressure. If the velocity increases sufficiently, the pressure at […]

Aerodynamic noise, the most common type of control valve noise, is the result of Reynolds stresses and shear forces that are the results of turbulent flow. Noise from turbulent flow is more common in valves handling compressible gases than in those controlling liquids. The valve manufacturer should provide noise predictions or furnish adequate data to […]

The compressible fluid sizing equations (see Fig. 4-30) can be used to determine the flow of gas or vapor through any style of valve. Absolute units of temperature and pressure must be used in the equation Most commonly the equations are used to calculate the required Cv and thus valve size for a given set […]

Critical flow limitation is a significant problem with sizing valves for gaseous service. Critical flow is a choked flow condition caused by increasing gas velocity at the vena contracta. The vena contracta is the point of minimum cross-sectional area of the flow stream which occurs just downstream of the actual physical restriction. When the velocity […]