In the selection and specification of control valves, many process variables must be carefully considered. The composition and thermodynamic properties of the fluid being throttled is of paramount importance. Gases must be sized differently than liquids, and of course multi-phase streams require special consideration. Other properties may also place special demands on the control valve, including the fluid’s corrosive nature, the presence of solids and any erosion that may result, or even if the liquid exhibits violent phase change and out-gassing during throttling. All or any one of these characteristics may require changes in the valve sizing and selection methodology as well as the use of specialty valve designs. One simple example is the effect a reduction in process temperature will have upon the well-understood mechanism of cavitation. Although 280° F water throttled from 200 psia to 75 psia may violently cavitate in a standard globe valve, the exact same process at 195° F will not exhibit any cavitation whatsoever! In fact, a simple change in just the valve style or flow direction will often sufficiently alter the pressure recovery characteristics and eliminate the possibility of damage entirely! This simple example demonstrates the need for a multi-variable approach to the selection and sizing of control valves, and should prove the futility of employing an overly simplified approach.

pdf icon MASONEILAN-2002-Fluid-Velocity-Considerations.pdf

MASONEILAN (2002) Fluid Velocity Considerations
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