Final Control Elements
Core concepts
Control valves include body, trim, actuator, and often a positioner. Size with Cv, the flow coefficient, and consider inherent versus installed characteristic. Linear, equal-percentage, and quick-opening trims behave differently once system pressure drop changes; equal percentage is common where most drop is elsewhere in the system. Rangeability and turndown describe usable capacity. Actuator action matters: air-to-open usually fails closed, while air-to-close usually fails open; spring range and bench set define stroke. Positioners improve response but cannot correct an undersized valve.
Traps
Cavitation occurs when the vena contracta falls below vapor pressure and the downstream pressure recovers above it; flashing occurs when downstream pressure remains below vapor pressure. Cavitation damages trim and causes noise; flashing causes erosion downstream. Do not confuse fail-safe action with instrument air failure. Avoid sizing a valve to operate below about 10–20% or above about 80–90% at normal flow. Remember Cv is not dimensionless and depends on the published unit convention.
Worked example
For water, Cv = Q × sqrt(SG/ΔP), with Q in gpm and ΔP in psi. At 100 gpm, SG = 1.0, and ΔP = 25 psi: Cv = 100 × sqrt(1/25) = 100 × 0.2 = 20. Select a valve whose required Cv falls in a controllable opening range at normal flow, then check maximum and minimum cases.