Stop/Ratio Valve Operation
The Gas Stop/Ratio Valve actuator is controlled by an electronic servo-control system (not included),
which compares the demanded and actual valve positions. The control system modulates the input
current signal to the electrohydraulic servo valve to minimize the positioning system error. See Figure 1-2
for a functional schematic of the single acting actuator.
Hydraulic oil enters the actuator via a removable element filter with integral high P indicator and is
directed to a four way, electrohydraulic servo valve used in a three-way configuration. The PC1 control
pressure output from the servo valve is directed to the top of the hydraulic piston. When the force
exerted by the hydraulic pressure exceeds the force of the opposing loading springs, the output piston
extends, rotating the valve in the opening direction.
A trip relay valve assembly is interposed between the electrohydraulic servo control valve and the servo
output stage. Loss or reduction of the externally supplied trip signal pressure causes the trip relay valve to
shift position. This connects the upper cavity of the actuator piston to the hydraulic drain. The force
supplied by the return springs pushes the actuation rod up, rotating the valve to the closed position.
Two redundant LVDT position feedback transducers are also mounted within each actuator. An optional
third LVDT is available on the 6” and 8” stop/ratio actuators only. The LVDT sensor cores and support
rods are connected to the main actuator output rod by a coupling arrangement guided on a bushing. This
guide bushing maintains LVDT alignment to minimize core damage due to sliding wear and the
associated loss of sensing accuracy.