VariStroke‐I

Applications

The VariStroke-I is a linear electro- hydraulic actuator that is designed to provide the linear actuation force to operate steam turbine control valves or valve racks. This integrated actuator is intended for use on mechanical-drive or generator-drive steam turbines, and uses a low-pressure hydraulic oil source (typically turbine lube oil) to provide its output shaft force.

The VariStroke-I’s superb accuracy and resolution make it ideal for steam turbine valve control and related turbine speed and load control. The VariStroke-I electronics module accepts one or two (redundant) 4–20 mA position demand signals to position steam control valves. Precise and stable steam valve control directly relates to improved steam turbine speed and load control and reduced system mechanical wear. The actuator’s fast slew rates (ranging from 200 to 500 mm/s) allows applied generator or compressor drive turbines to quickly respond to loss-of-load or emergency shutdown events.

The VariStroke-I actuator’s robust design (corrosion resistant materials, single moving rotary valve, 50 lb chip shear force, and self-cleaning port design) make it ideal for challenging applications where dirty or contaminated oil may be present.

This actuator can be purchased in the configuration which best meets the specific application’s requirements (Integrated, Remote Servo, or Servo Only models are available). Refer to Figures 1 to 3 below.

The VariStroke-I’s redundant features make it ideal for critical steam turbine applications, where turbine up-time and availability are essential. This linear actuator can be configured to accept simplex or dual-redundant position demand inputs, then select the correct input to follow. This linear actuator’s dual-redundant 4–20 mA demand inputs and dual-redundant MLDT (magnetostrictive linear displacement transducer) shaft position sensors allow it to continue to operate even with demand input or feedback sensor failures, ensuring extend run-times between forced outages.

In turbine retrofit applications, the VariStroke-I can be used to directly replace the existing servo system. This saves costly repairs to existing and aging servos, eliminates the difficulty of obtaining spare parts for obsolete equipment, and reduces calibration time and difficulty.

Description

The VariStroke-I is a linear electro-hydraulic actuator that utilizes a double-acting or single-acting power cylinder (part number dependent) with integrated electronic driver module, servo valve, and MLDT-based position feedback sensors to precisely control steam turbine valves. The actuator’s driver module accepts one or two (redundant) 4–20 mA demand setpoints and compares these setpoints to the sensed actuator shaft position to accurately control output shaft position.

The actuator’s output shaft position is controlled by a digital controller with an internal rotary servo valve that ports supply oil to and from its power cylinder piston. This actuator’s special digital controller architecture allows it to perform very stable position control during normal conditions, and also respond in milliseconds to desired valve step changes during system or plant transients. As a means of protecting the turbine, an internal valve-return spring forces the actuator to a failsafe position to safely close turbine control valves upon any internal unit failure (electrical input power failure, position sensor failure, processor failure, etc.).

The VariStroke-I actuator is a product family with different models available for purchase depending on the force, stroke, and redundancy required. This actuator is available with standard bore diameters, standard stroke ranges, and with single-acting (spring return) or double-acting (pressure return) operation. The VariStroke’s unique “variable stroke” capability also allows users to customize/set the actuator’s exact stroke length in the field to meet their required application.

The VariStroke-I is factory and/or field configurable via a computer-based service tool. The actuator’s PCI Service Tool uses a simple user-friendly format to allow users to easily configure, calibrate, and adjust all internal functions and response settings. The VariStroke-I also includes a 4–20 mA output channel to indicate output shaft (control valve) position, and unit alarm and shutdown relay outputs for use as unit health and status indications.

The total installed cost for this fully integrated actuator is low because it has been completely assembled and tested at the factory. This greatly reduces OEM and end-user fabrication time, testing time, and site assembly time.