API-612 Special Purpose Steam Turbines are most often vital to plant production, with any loss of availability resulting in expensive production losses. However, it is not practical to expect lengthy trouble-free run periods if the turbine is positioning modulated governor control valves with hydraulic servomotors (hydraulic actuators with position feedback pilots).

The nemesis of servomotors is contamination, and shared control oil with lubrication oil systems greatly contribute to this contamination. Even elaborate filtration misses the corrosion contamination in downstream carbon steel piping and carbon steel hydraulic manifolds. Corrosion is accelerated with water intrusion into bearing seals (at negative pressure from use of vapor extractors or vent towers) from gland sealing steam passing seals inches away. Once a hydraulic servomotor pilot valve is contaminated it introduces a time lag in valve response. This time lag increases with added contamination and cylinder scoring to continue to worsen response as shown in the following figure.

The best approach for positioning control valves is no hydraulic dependency whatsoever, which is exactly what roller screw electric actuators provide with the added bonus of far superior performance. The response performance is set to optimize speed and stability and unlike the hydraulic counterpart has diagnostic data to monitor performance. The response diagram for the electric actuator is shown in the following figure.

The precision tuning of the electric actuator positioning function performed in the variable frequency servo drive also enable velocity breaking prior to control valve seating to extend valve life and reduce valve seat stress.