Woodward VariStroke DX hydraulic servo dual redundant sled

Dual redundant power input: prevents single power failure.

Dual redundant 4-20 mA demand signal input: receives valve position commands from the turbine controller.

Support redundant power cylinder position feedback signal: can be connected to single redundant or dual redundant position sensors for closed-loop control.

This end-to-end redundant design constitutes a highly reliable control link.

2.4 Flexible Compatibility and Usability

Compatible with multiple power cylinders: The sled itself does not include a power cylinder. Users can choose a suitable VariStroke hydraulic power cylinder separately according to specific requirements such as force, stroke, spring recoil, etc., and remotely connect it through a pipeline of up to 10 meters. This provides great application flexibility.

Compact integrated design: All components are integrated onto one sled, reducing on-site installation of pipelines and wiring work, and lowering overall installation costs.

Clear status indication: The pressure gauge provides local visual indication, and the discrete output relay contacts can be used to connect to DCS or indicator lights for remote master/slave status and system health monitoring.

Product specifications and environmental adaptability

3.1 Physical and Environmental Specifications

Size and weight:

Zone 2 model (with junction box): approximately L629.9 x W968.2 x H724.9 mm, weighing approximately 200 kg.

Zone 1 model (without junction box): more compact, approximately L401.3 x W968.2 x H432.6 mm.

Environmental temperature: -40 ° C to+85 ° C, suitable for extreme weather conditions.

Oil temperature:+15 ° C to+70 ° C.

Installation method: Horizontal installation.

3.2 Hydraulic and Fluid Requirements

Working pressure range: 3.44 to 35 bar (50 to 500 psi), suitable for low-pressure turbine oil systems.

Fluid type: mineral oil or synthetic oil.

Oil cleanliness: Recommended ISO 4406 20/18/16 grade or higher (β ₇₅ ≥ 75).

Port standard: SAE J518 Code 61 flange.

3.3 Hazardous Area Certification

The VariStroke DX sled offers two certified models to meet installation environments with different safety requirements:

Zone 1/Class I, Div. 1 certified model: suitable for areas where explosive gas environments exist for a long time. The sled model itself does not include a junction box, and the wiring needs to be led to a Zone 1 certified junction box or non hazardous area for processing.

Zone 2/Class I, Div. 2 certified models: suitable for areas where explosive gas environments may occasionally exist. This model has integrated a Zone 2 certified junction box for easy installation.

System working principle and functional diagram

As shown in Figure 5, the system workflow is as follows:

The turbine controller sends a valve position command (4-20mA) to the sled.

The Master VariStroke GI servo valve (such as Unit "A") inside the sled controls its output oil pressure through an internal closed-loop according to instructions.

This pressure is directed through the shuttle valve to the control port (C1/C2), which in turn drives the remotely installed hydraulic power cylinder.

The power cylinder drives the steam valve to move, and its position is returned to the sled and turbine controller through feedback sensors, forming a closed loop.

Synchronously receiving the same command from the Slave servo valve (Unit "B") and causing its internal output pressure to follow the main valve, but blocked by the shuttle valve and in a hot standby state.

Once the main valve fails or its output is abnormal, the shuttle valve switches paths within milliseconds under internal logic control, and the slave valve immediately takes over control, achieving disturbance free switching.

Isolation valves S1/S2 and D1/D2 are respectively used to isolate the oil supply and return lines of the servo valve, while N1/N2 are used to isolate its control oil output.

Application Value and Summary

The Woodward VariStroke DX dual redundant hydraulic servo sled is not just a product, but a system level solution to enhance the operational reliability of critical power equipment. Its value is reflected in:

Economic benefits: By almost eliminating unplanned shutdowns caused by actuator failures, continuous process industrial users have saved hundreds or even millions of downtime losses.

Safety benefits: The built-in safety logic and automatic fault switching function reduce the risk of unit failure caused by control failure.

Maintenance benefits: The online maintenance capability has changed the maintenance mode, allowing maintenance work to be scheduled within the planned window, improving maintenance efficiency and reducing the time pressure on maintenance personnel.

Engineering benefits: Its pre integrated, pre tested sled design and built-in intelligent logic greatly simplify control system engineering and on-site installation and commissioning work, shortening project cycles.

For any application that has extreme requirements for the reliability of steam turbine operation, whether it is offshore platforms, isolated power plants, large chemical plants, or critical mechanical drives, the VariStroke DX sled represents the advanced level of hydraulic valve redundancy technology and is a strategic choice to ensure the continuous, efficient, and safe operation of core assets.