Woodward SonicFlo™ Gas Triple Coil Electrohydraulic Servo Valve Assembly

Introduction

The SonicFlo™ valve controls the flow of gas fuel to the combustion system of an industrial or utility gas

turbine. The unique design yields a linear flow characteristic unaffected by discharge pressure up to a

pressure ratio (P2/P1) of at least 0.8. The design also integrates the valve and actuator into a compact

assembly. The key characteristics of this valve are a highly linear critical gas flow versus stroke

relationship at constant upstream pressure. The integral actuator is a single-acting spring-loaded design

for failsafe operation. The actuator includes an onboard hydraulic filter for last chance filtration of the fluid

to ensure reliability of the servo valve and actuator. The servo valve is electrically redundant with triple

coil design. Feedback for the actuator is provided by a dual coil, dual rod LVDT (linear variable differential

transformer) directly coupled to the hydraulic piston.

Triple Coil Electrohydraulic Servo Valve Assembly

The hydraulic actuator assembly uses a two-stage hydraulic servo valve to modulate the position of the

actuator output shaft and thereby control the gas fuel valves. The first stage torque motor utilizes a triple

wound coil, which controls the position of the first and second stage valves in proportion to the total

electric current applied to the three coils.

If the control system requires a rapid movement of the valve to send more fuel to the turbine, total current

is increased well above the null current. In such a condition, control port PC1 is connected to supply

pressure. The flow rate delivered to the piston cavity of the actuator is proportional to the total current

applied to the three coils. Thus, the opening velocity is also proportional to the current (above null)

supplied to the torque motor.

If the control system requires a rapid movement to close the gas fuel valve, the total current is reduced

well below the null current. In such a condition, port PC1 is connected to the hydraulic drain circuit. The

flow rate from the piston cavity to drain is proportional to the magnitude of the total current below the null

value. Thus, the closing velocity is also proportional to the current (below null) supplied to the torque

motor.

Near the null current, the four-landed valve isolates the control port from the hydraulic supply and drain,

balancing the piston pressure against the spring to maintain a constant position. The control system,

which regulates the amount of current delivered to the coils, modulates the current supplied to the coil to

obtain proper closed loop position of the valve.