Control Valve Sizing

Record minimum, normal and maximum flow with corresponding inlet and outlet pressures, temperature and fluid properties. Include pipe geometry, required shutoff, materials, pressure class and actuator constraints. Emerson's sizing overview identifies the process and valve information needed for selection.

The liquid Kv and choking calculator addresses turbulent liquid service without attached-fitting corrections. It accepts density, absolute pressures, vapor pressure, critical pressure and the candidate valve's pressure recovery factor FL. Gas, steam, viscous and inlet two-phase service require other methods.

For the basic non-choked liquid relation, Kv = Q √[(ρ/1000)/ΔP], using Q in m³/h, density in kg/m³ and pressure drop in bar. Bürkert's liquid-flow reference gives this density-dependent equation. The coefficient is referenced to water flow at 1 bar; it is not the actual service flow or a pipe diameter.

For example, 5 L/s is 18 m³/h. With density 1000 kg/m³ and 50 kPa pressure drop, the basic requirement is 25.46. That number alone does not establish DN50, 75% travel, or any particular valve type.

At fixed inlet pressure and valve travel, reducing outlet pressure eventually ceases to increase flow. The calculator uses the documented liquid choking limit and caps the sizing differential at that limit. FL must come from the manufacturer for the relevant trim and travel, rather than from a generic valve-type assumption.

Constructed comparison: retain the example flow and density, and assume inlet pressure 300 kPa absolute, vapor pressure 3 kPa absolute, critical pressure 22.064 MPa and FL = 0.9. These are teaching inputs, not a verified property state or product selection.

Recalculate with your inputs. The reported requirement assumes a piping geometry factor of one and no viscosity correction. Attached reducers or other fittings require their own corrections.

Pressure falls through the valve restriction and then partly recovers. Vapor can form internally even when outlet pressure is above vapor pressure. If bubbles subsequently collapse, the process is cavitation. If outlet pressure remains below vapor pressure, flashing is indicated.

Being below the calculated choking limit does not prove cavitation-free operation. Likewise, a choking result does not quantify damage or noise. Review the selected trim, pressure recovery, service conditions and manufacturer limits; the calculator is not a cavitation-damage predictor.

Compare the required coefficient at each operating case with manufacturer capacity-versus-travel data. Evaluate the installed response as system pressure losses change. Rated capacity alone does not establish controllable minimum flow or installed rangeability.

This guide does not prescribe a universal opening percentage, fixed fraction of system pressure drop, or blanket Kv multiplier. Check travel limits, shutoff differential, actuator force or torque, leakage requirements and material compatibility for the actual application.

The pump-duty calculator can help establish a steady system head requirement. It does not solve the installed control loop or choose the valve and actuator.