Woodward VariStroke-I electro-hydraulic actuator: an innovative solution for steam turbine valve control

Woodward VariStroke-I electro-hydraulic actuator: an innovative solution for steam turbine valve control

Introduction

In the field of industrial turbine machinery control, precise and reliable valve actuators are crucial for ensuring the safe and efficient operation of the system. Woodward's VariStroke-I (VS-I) electro-hydraulic actuator is designed specifically for harsh steam turbine applications, integrating advanced servo technology, redundant feedback systems, and intelligent diagnostic functions, representing the latest developments in linear electro-hydraulic execution technology. This article is based on the "VariStroke-I (VS-I) Electro hydraulic Actuator Installation and Operation Manual" (manual number 26727, revised version R, January 2020) for in-depth analysis, aiming to provide comprehensive technical references and application guidelines for engineers, maintenance technicians, and system integrators.

1、 Overview of VariStroke-I actuator

1.1 Core Functions and Design Philosophy

VariStroke-I is a linear electro-hydraulic actuator used for precise control of steam turbine valves. Its core design concept lies in high reliability, strong anti pollution ability, and excellent dynamic performance. The actuator adopts a double acting or spring assisted power cylinder, integrating an electronic drive module, servo valve, and redundant position feedback system based on magnetostrictive linear displacement sensor (MLDT).

1.2 Main components

The actuator is mainly composed of the following components (see Figure 1-1):

Hydraulic power cylinder: provides driving force, and the stroke range can be precisely adjusted on site.

Rotary servo valve: controls the flow of hydraulic oil to achieve precise modulation of the position of the power cylinder.

Feedback sensor: Redundant MLDT sensor provides high-precision cylinder position feedback.

Integrated electronic driver module (PCB): executes control algorithms, processes input/output signals, and provides advanced diagnostic functions.

1.3 Product Form

To adapt to different installation spaces and application requirements, VS-I offers three configurations:

Integrated: The servo valve, power cylinder, and electronic drive module are integrated into a compact unit.

Remote servo kit: The hydraulic power cylinder can be installed up to 3 meters away from the servo valve.

Only servo version: allows users to use their own hydraulic cylinders, providing only servo valves and control systems.

2、 Key performance and specification parameters

2.1 Physical and Performance Specifications

Cylinder and Rod Diameters: Multiple cylinder diameter options are available, including 4 inches (101.6 mm), 5 inches (127 mm), 6 inches (152.4 mm), 8 inches (203.2 mm), and 10 inches (254 mm), to meet different force requirements.

Spring assisted version: Provides four different spring force levels, S, T, U, and V, to assist the valve in reaching a safe position when oil pressure is lost.

Position accuracy and repeatability: The full stroke position accuracy is ± 1%, and the repeatability is ± 0.5%.

Fault safe operation: The internal servo valve return spring ensures that the actuator can reliably move to the preset fault safe position (extended or retracted) in the event of power, sensor, or processor failure.

2.2 Environmental and Electrical Specifications

Environmental temperature: -40 ° C to+85 ° C. Under specific hydraulic oil temperature conditions, the allowable ambient temperature for servo valves and remote cylinder bodies can reach up to 105 ° C each (see Table 2-7).

Protection level: IP66, suitable for harsh industrial environments.

Power supply voltage: 18-32 Vdc, rated at 24 Vdc. Steady state current 2.3 A, transient peak current 10 A (up to 100 ms).

Input/output signal:

Requirement signal: Two redundant 4-20 mA analog inputs.

Position feedback signal: Two redundant 4-20 mA analog inputs (remote servo requires external sensors).

Analog output: A 4-20 mA output used to indicate valve position.

Discrete input/output: used for reset, operation enable, alarm, and shutdown status indication.

2.3 Hydraulic specifications

Working fluid: Suitable for petroleum based hydraulic oil and fire-resistant hydraulic fluids such as Fyrquel EHC.

Oil supply pressure: 5.5 bar (80 psi) to 34.5 bar (500 psi).

Fluid cleanliness: ISO 4406 20/18/16 or cleaner is required.

Flow demand: Sufficient hydraulic flow should be provided based on the full speed movement (Slew) and steady-state operation requirements of the actuator (see Figure 2-1 and Figure 2-2).

2.4 Performance Index

The performance of VS-I depends on the matching of servo valve size, oil supply pressure, and cylinder volume used. To ensure smooth operation of the actuator without excessive overshoot or limit cycles, the following performance index formula must be met:

VS_Constant * sqrt(P_supply) / ( (π * D_cyl^2) / 4 * L_stroke) ≤ 1

Among them, VS_Cstant=180. Not meeting this relationship may result in performance degradation and accelerated wear, and trigger a 'Performance Index Warning' alarm.

3、 Safety, Compliance, and Installation Points

3.1 Safety Warning

The manual emphasizes multi-level safety warnings that must be strictly followed: