Control Valves

Cv is the flow coefficient of a control valve - defined as the flow rate in US gallons per minute of 60°F water that produces exactly 1 psi pressure drop across the valve. It is the universal sizing parameter for control valves. To size a valve, calculate the required Cv from your process data (flow rate, fluid density, inlet and outlet pressures) using the IEC 60534-2 equations. Then select a valve whose Cv at 70–80% travel matches the calculated Cv - this keeps the valve operating in its most controllable mid-range rather than near the ends of travel where the flow characteristic is non-linear.

Equal-percentage trim (flow increases exponentially with travel - small incremental changes at low travel, large incremental changes at high travel) is the correct choice when the control valve is one of several pressure drops in a liquid system - the pressure split between valve and pipe changes with flow, and equal-percentage characteristic compensates for this non-linearity, providing a more linear installed characteristic. Linear trim (flow proportional to travel) is correct when the control valve is the dominant pressure drop in the system, for gas pressure control, or for liquid level control where a proportional installed response is required.

Cavitation occurs when local pressure in the liquid stream drops below the fluid's vapour pressure, forming vapour bubbles that collapse violently near the valve plug and seat, causing erosion, noise (up to 120 dB), and vibration. Anti-cavitation trim stages the pressure drop across multiple restriction stages (perforated cage or stacked discs), keeping local pressure above vapour pressure at every stage. Specify anti-cavitation trim when your sizing calculation shows σ (sigma, the cavitation index) below the valve's critical σ value, typically when the valve pressure drop exceeds 40–60% of the inlet absolute pressure.

ANSI/FCI 70-2 and IEC 60534-4 define six seat leakage classes. Class I: no test required. Class II: 0.5% of rated Cv - standard metal seat. Class III: 0.1% - tighter metal seat. Class IV: 0.01% Cv - typical metal seat in good condition. Class V: 0.0005 ml/min/inch bore/psi ΔP - hard seat, lapped metal-to-metal. Class VI: bubble-tight (< defined bubble count in 1 min) - soft seat (PTFE, elastomer). Specify Class IV for general isolation after control, Class VI for tight shut-off where leakage to atmosphere or across the seat is not permitted.