Woodward Gas Stop/Ratio Valve

General Information

The Woodward Gas Stop/Ratio Valve performs a dual function for industrial or utility gas turbines. One

function rapidly shuts off fuel to the turbine fuel control system. Another function provides accurate control

of gas fuel pressure at the outlet of the stop/ratio valve. This pressure is applied to the inlets of the gas

fuel control valve.

The Gas Stop/Ratio Valve features a modular design and meets critical control characteristics while

allowing the same valve design to accommodate a variety of stroke, force output, and mechanical

interface arrangements. The electrical and mechanical interfaces have been designed for quick and easy

assembly or removal of the valve, at the factory or in the field. The components include an

on-board hydraulic filter, electrohydraulic servo valve, trip valve, single-acting hydraulic cylinder, and dual

LVDTs (the 6” and 8” valves also have the option for triple LVDTs).

Optimum control of the gas turbine requires that the actuator and valve accurately and quickly track the

demand signals transmitted by the control. The stop/ratio valve has been designed to provide output

forces that exceed the opening and closing requirements with some margin. The additional margin helps

ensure that the system moves rapidly even under service conditions where the valve has been

contaminated or worn. The hydraulic trip relay valve has been selected to provide high operating force

margins, high flow capacity, and to ensure the desired closure rate of the valve under trip conditions.

By using a long actuation rod between the hydraulic cylinder and the valve lever arm, the side-loading

forces on the actuator shaft and seals are greatly reduced, decreasing the wear between sliding parts,

and increasing the useful service life of the system. The ample distance between the wetted heavy-duty

linear slide rings within the stop/ratio valve accommodates any remaining side load. These provisions

provide extended service life even in severe service conditions.

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.