Woodward VariStroke-II actuator is specifically designed for steam turbine applications

The VariStroke-II Actuator offers the following benefits to the user in comparison to other electro-hydraulic

actuators:

Dirt Tolerance

The VariStroke-II actuator is specifically designed for steam turbine applications where turbine lube oil is

also used to power the hydraulic turbine control valve actuator(s). Steam turbine applications can be

extremely challenging for hydraulic control valve actuators as dirt, metal shavings, water, and other

contaminants (Babbitt, ammonia, etc.) are common in such oil systems. Also due to the high

temperatures at which steam turbines operate, turbine oil breakdown is common, resulting in the creation

of a sludge-type substance and the varnishing of internal system components. However, the VariStroke-II

actuator is designed to operate reliably within such challenging applications. Its corrosion-resistant

materials, single moving rotary valve, 610 N (137 lbf) of chip shear force, and self-cleaning port design

allow it to operate in such applications without experiencing undesirable sticking or dragging.

Silt Buster

A patented self-cleaning feature that flushes silt and debris from the servo valve. At the interval and

amplitude selected by the user, this function  provides a very rapid motion of the servo valve spool,

allowing any silt to be flushed to the drain passage. This motion is followed immediately by a step of

equal amplitude in the opposite direction. The opposing symmetry of the impulse results in no net change

in fluid volume to the hydraulic power cylinder, and thus does not interrupt the control of the turbine. This

unique feature provides a high degree of stability, reliability, and silt resistance.

Valve Rack Linearization (not currently available)

Because steam flow through single and staged inlet steam valves tends to be non-linear throughout their

flow range, turbine controls must be de-tuned to compensate for instability or sluggish control points

throughout this range. As a way of allowing turbine control optimization without detuning, the VariStroke-II

includes an 11-point linearization table to allow turbine OEMs or users to compensate for poor valve

linearization by digitally linearizing the control-to-valve flow relationship.

Side Load Capability

A common problem with turbine actuators is oil leaking from their output shaft due to connection to valve

rack linkages which have an arc-type of motion. This motion results in side loading of the actuator shaft,

and after long periods may result in shaft-seal wear and resultant oil leakage. Designed for a continuous

side load of up to 10% of actuator output, the VariStroke-II actuator incorporates a high-force precision

bearing and triple-seal technology on its output shaft to solve this typical application problem.

Silt Buster

A patented self-cleaning feature that flushes silt and debris from the servo valve. At the interval and

amplitude selected by the user, this function  provides a very rapid motion of the servo valve spool,

allowing any silt to be flushed to the drain passage. This motion is followed immediately by a step of

equal amplitude in the opposite direction. The opposing symmetry of the impulse results in no net change

in fluid volume to the hydraulic power cylinder, and thus does not interrupt the control of the turbine. This

unique feature provides a high degree of stability, reliability, and silt resistance.