WOODWARD VariStroke DX Skid Dual Redundant Hydraulic Servo System

The core positioning is a high availability dual redundant hydraulic servo system designed specifically for key API612 steam turbines, supporting online maintenance and reducing unplanned downtime

Application scenarios and adaptation relationships

（1） Core applications

Adaptation equipment: Mechanical or generator driven steam turbine

Control objects: steam turbine control valves, valve racks, Trip&Throttle Valves

Applicable scenarios: API612 standard steam turbine applications that require extremely high system uptime (such as key equipment in the petrochemical and power industries)

（2） Adaptation components

Must be paired with Woodward VariStroke hydraulic power cylinder (VHPC), to be ordered separately

Adaptation basis: Select the aperture, stroke length, reset spring force and other parameters of VHPC according to application requirements

Reference documents: VHPC Product Specification (No. 03465), VariStroke GI Product Specification (No. 03464)

Core composition and working principle of the product

（1） Core components

Core components: 2 independent VariStroke GI linear electro-hydraulic actuators (integrated electronic drive module, servo valve, MLDT position feedback sensor)

Auxiliary components: integrated manifold, manual isolation valve (with tag locking function), hydraulic pipeline, pressure gauge, electrical wiring terminal (Zone2 model including junction box)

Switching component: specially designed fast switching shuttle valve, achieving disturbance free switching between master/slave servo valves

（2） Work logic

Control mode: Master/Slave logic, with one servo valve as the master and the other as the slave by default

Slave state: The slave servo valve tracks the health status of the master servo valve in real time and does not participate in the control output

Switching mechanism: Automatically switch to slave control in case of master control failure (≤ 70ms), or manually switch through local switch or external relay (≤ 60ms)

Isolation function: The integrated isolation valve can isolate any servo valve separately without affecting the normal operation of another servo valve

Core Features

（1） Redundancy and high availability

Full dimensional redundancy: dual servo valves, dual redundant power input, dual 4-20mA demand signal input, dual power cylinder position feedback signal

Online maintenance: supports online repair, replacement, testing, and calibration of servo valves without the need for downtime

Fault transfer: The automatic fault transfer logic is built-in and does not require external control logic or interfaces. It switches quickly (≤ 70ms) and minimizes system disturbances

（2） Convenience and Monitoring

Status indication: Integrated with a large pressure gauge, visually displaying the current pressure of the main control servo valve and output oil main pipe; Discrete output relays can be connected to DCS or indicator lights for remote monitoring of master/slave status

Easy installation: Factory pre assembled and tested, significantly reducing manufacturing, installation, and testing time for OEMs and end-users

Convenient operation and maintenance: equipped with isolation switch type electrical wiring terminals, easy to isolate and replace servo valves; Isolation valve with tag locking protection, in compliance with safety regulations

（3） Environmental adaptability

Adaptation to hazardous environments: Two types of hazardous environment certification models, Zone1 and Zone2, are available

Model difference: Zone2 model includes a top electrical junction box (integrating all control interfaces and connecting wires); Zone1 models do not have junction boxes and require users to install Zone1 certified junction boxes separately or directly connect servo valves

Key specification parameters

（1） Performance parameters

Parameter values

The maximum manual switching time is 60 milliseconds

The maximum automatic failover time is 70 milliseconds

Maximum distance from the power cylinder is 10 meters (32.8 feet)

（2） Physical parameters

Model size (length x width x height) Weight Installation method

Zone2 model 629.9mm × 968.2mm × 724.9mm (24.8 inches × 38.12 inches × 28.54 inches) 200kg (400lbs) horizontal installation

Zone1 model 401.3mm x 968.2mm x 432.6mm (15.8 inches x 38.12 inches x 17.03 inches) 200kg (400lbs) horizontal installation

（3） Environmental parameters

Parameter Range

Working environment temperature -40 ℃~+85 ℃ (-40 ℉~+185 ℉)

Working oil temperature+15 ℃~+70 ℃ (+59 ℉~+158 ℉)

The impact resistance performance complies with US MIL-STD-810C method 516.2, procedure 1 (10G peak, 11 millisecond duration)

（4） Hydraulic parameters

Parameter requirements

Working pressure range 3.44 bar~35 bar (50 psi~500 psi)

Oil type: mineral oil or synthetic oil

Recommended maximum oil cleanliness ISO 4406 code 20/18/16 level (nominal 24-40 μ m, β 75)

Recommended oil viscosity is between 20 and 100 centimeters per liter

Interface standard hydraulic ports comply with SAE J518 Code 61

（5） Compliance certification

Regional/Standard Certification Content

European (CE) EMC Directive 2014/30/EU; ATEX Directive 2014/34/EU (Zone1: II 2 G Ex db IIB T4 Gb; Zone2: II 3 G Ex nA IIC T4 Gc IP66)

International IECEx certification (Zone1: II 2 G Ex db IIB T4 Gb; Zone2: II 3 G Ex nA IIC T4 Gc IP66)

North American CSA certification (Class I, Div.1 Groups C&D T4; Class I, Div.2 Groups A-D T4), applicable to the United States and Canada

Product Value and Advantages

High availability: redundant design+online maintenance, significantly reducing the risk of unplanned downtime and improving the stability of turbine operation

Low cost: Factory pre integration testing reduces on-site construction and debugging costs; Built in switching logic, no additional external control module required

Easy operation and maintenance: Isolation valves and dedicated wiring terminals simplify the maintenance process; Clear status indication for easy monitoring and troubleshooting

Strong adaptation: Supports VHPC of different specifications and adapts to various steam turbine application needs; Compliant with API612 standard, strong compatibility