Woodward VariStroke DX hydraulic servo dual redundant sled

Woodward VariStroke DX hydraulic servo dual redundant sled

Introduction

The continuous and reliable operation of steam turbines (steam turbines) is directly related to the production efficiency and safety of factories in key fields such as power generation, petrochemicals, and industrial drives. As the core control component of a steam turbine, the reliability of valve actuators is crucial. Any unexpected shutdown can result in significant economic losses. The VariStroke DX Dual Redundant Hydraulic Servo Skid launched by Woodward is a revolutionary product designed to address this challenge. It provides unprecedented operational reliability and availability for critical API612 turbine applications through its three core functions of dual servo redundancy, online maintenance, and fast disturbance free switching.

Overview of VariStroke DX sled system

1.1 System positioning and application scenarios

The VariStroke DX sled is a complete, factory tested dual redundant hydraulic servo system package. Its main design objectives are:

Maximizing turbine operating time: Allowing maintenance or replacement of servo valves during normal operation (online) of the turbine, thereby eliminating unplanned shutdowns caused by actuator failures.

Meeting critical application standards: Designed specifically for critical steam turbine applications that comply with API 612 (7th edition) standards, it is an ideal choice for situations requiring extremely high availability.

Simplified system integration: As an independent sled type component, it integrates all necessary hydraulic, electrical, and logic control components, greatly reducing the complexity of on-site installation and commissioning.

As shown in Figure 1, the VariStroke DX sled serves as the interface between the turbine control system and the VariStroke hydraulic power cylinder, receiving control commands and driving the power cylinder to precisely control the steam valve.

1.2 System composition and core components

The core of the sled consists of the following components:

Dual VariStroke GI servo valve: Based on Woodward's mature VariStroke GI linear electro-hydraulic actuator technology, each valve is a complete servo unit, including an electronic drive module, rotary servo valve, and internal position feedback.

Quick switching hydraulic shuttle valve: The specially designed shuttle valve is the key to achieving fast and smooth switching. It can ensure seamless transfer of control from one servo valve to another within milliseconds, while blocking the output pressure of non master servo valves to avoid system disturbances.

Integrated isolation valve and instrument:

Manual isolation valve: allows the operator to isolate any servo valve for online maintenance, replacement, or calibration without affecting another servo valve that is controlling the valve.

Large pressure gauge: It intuitively displays which servo valve is in the main control state and the output oil pressure, making it easy to monitor the status.

Electrical junction box (Zone 2 model): a terminal block that integrates all control interfaces and interconnecting lines, making it easy to connect and maintain.

Master/Slave Control Logic: The sled is equipped with intelligent logic, which can manage the cooperation of dual servo valves without external control. One of them is designated as the 'Master' unit, responsible for the actual control of valves; The other serves as a 'slave' unit that tracks the status of the master unit in real-time. Logic supports automatic switching (when the main unit fails) and manual switching (through local switches or external signals).

Core technological advantages

2.1 True online maintenance and high availability

This is the most prominent advantage of VariStroke DX. Once a traditional single servo system malfunctions, it must be shut down for maintenance. The redundant design of the DX sled allows for:

Isolate it from the hydraulic circuit through an isolation valve.

The system automatically or manually switches control to the backup servo valve without disturbance.

Disassemble, repair, replace or calibrate the isolated servo valve while the steam turbine is running continuously.

After the repair is completed, it can be put back into the system and used as a backup unit or switched back to the main control unit.

This capability transforms unplanned downtime into planned maintenance, significantly improving the overall availability of the system.

2.2 Fast and interference free switching

One of the key indicators of system performance is switching time. The indicators of VariStroke DX sled are impressive:

Manual (command) switching time: ≤ 60 milliseconds

Automatic (main unit failure) switching time: ≤ 70 milliseconds

The rapid switching is thanks to its dedicated fast shuttle valve and optimized control algorithm, ensuring that the control of the steam valve is almost unaffected, avoiding pressure fluctuations or impacts during the switching process, and ensuring the stable operation of the steam turbine.

2.3 Comprehensive signal redundancy

To match its hardware redundancy architecture, the sled also supports comprehensive redundancy in electrical interfaces, further enhancing system reliability: