LCC Quick Notes, Turbine Electric Actuators
Electric actuators are ideal steam turbine governor valve positioners, but care must be taken in specification to avoid pitfalls. Unless accompanied by independent rapid trip closure devices, electric actuators should not be used on steam turbine stop valve applications. This Quick Note addresses the key Electric Actuator specification issues and respective LCC links for solutions.
◘ FORCE at VELOCITY... Electric Actuators are rated for a given steady state and temporary peak force (thrust) at a stroke velocity and operating temperature, and are not immediately comparable in force capability to hydraulic cylinders. A rough comparison is possible using the stall force (velocity = 0) of the electric actuator vs. the internal hydraulic feed pressure times the cylinder area. A more exacting calculation summing all forces at all operating conditions is needed prior to final specification.
◘ STROKE... Electric Actuators come in standardized stroke lengths, with the same or longer stroke of the original hydraulic operator specified. In operation the stroke is limited by the servo drive setup.
◘ ENVIRONMENT... At temperatures which affect motor magnetic strength Electric Actuators need supportive cooling. This can be internal in the form of oil circulation or external in the form of circulated air box enclosures. Also, where Class 1 atmospheres are encountered actuators should be specified with compatible motors, encoders, and electrical connections.
◘ POWER OFF RESPONSE... The ideal steam turbine governor valve application power off response is a slow closure in five to ten seconds from full open. This provides a reasonable position buffer for short term power loss while closing a failed system in total loss. Specification details to achieve power off failure is correct roller pitch and screw fiction combining with static closure forces. Special consideration to valve closure spring systems is necessary. Huge closing forces which were formally needed to "pump" hydraulic cylinders filled with oil are unnecessary, but sufficient closure force is needed to backwind the actuator upon power loss.
◘ SERVO DRIVE TUNING... Unlike hydraulic positioning, the Electric Actuator is modulated by a sophisticated controller, and can be tuned to vary it's response velocity with the magnitude of the position error signal. When properly specified, this can convert a previously instable system into an ultra-stable critically damped system.
LCC Electric Actuator Links:
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