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.