Proportional Valve Flow Characteristics

Proportional directional valves (PDVs) provide infinitely variable flow control based on an electrical input signal. Unlike on/off valves, they allow precise speed and position control of hydraulic actuators.

The flow through a proportional valve depends on three main factors:

• Spool Opening (%): How far the spool has moved from center, controlled by the input signal
• Pressure Drop (dP): The differential pressure across the valve metering edges
• Valve Size: The rated flow capacity at rated conditions

The Orifice Equation

Proportional valve flow follows the orifice equation, with the orifice area varying with spool position:

Q = Qrated x (Opening/100) x sqrt(dP_actual / dP_rated)

Where:

• Q: Actual flow rate
• Qrated: Rated flow at rated dP and 100% opening
• Opening: Spool opening percentage (0-100%)
• dP_actual: Actual pressure drop across the valve
• dP_rated: Rated pressure drop (typically 5, 7, or 10 bar per edge)

Flow Gain and Linearity

Ideally, flow should be proportional to spool opening (linear characteristic). However, real valves exhibit:

• Deadband: Small spool movement range with no flow (typically 2-5%)
• Hysteresis: Different flow for same signal depending on direction of change
• Gain variation: Flow rate may not be perfectly linear with opening

Pressure Drop Considerations

The rated pressure drop (dP) is critical for valve sizing:

• 5 bar per edge (70 psi): Low pressure drop, energy efficient, requires larger valve
• 7 bar per edge (100 psi): Common industrial standard
• 10 bar per edge (145 psi): Higher pressure drop, allows smaller valve, more heat

Note: Total dP across valve = P to A + B to T = 2x per edge value

Flow Curve Types

• Linear: Flow proportional to opening, standard for most applications
• Progressive: Flow increases more slowly at low openings, good for fine control
• Degressive: Flow increases faster at low openings, quick response