A clogging indicator (both visual and electrical) indicates the condition of the active strainer.

The Servo Valves used on GE steam turbine EHC control systems are capable of operating with contaminated fluid containing particles up to 200 microns. Up-stream of the torque motor is a replaceable 90 micron strainer. It is this strainer that plugs with dirt causing the servo valve to fail in the closed position.

THE BLOCK II contains two 40 micron servo strainers (one active and one inactive) connected to a selector valve. In this manner, should a strainer become plugged, as indicated by the clogging indicator, it is only necessary to switch servo strainers and continue normal operation. At this time, with the unit runing normally, the plugged strainer can be changed. If switching to the backup strainer does not correct the problem, it can be assumed there are problems with the servo valve or the electronics of the turbine control system.

On early EHC units, the turbine had to be shut down in order to replace a failed servo valve. GE later provided a modification to the hydraulic actuators consisting of a mechanical clamp (as shown at left) over the Fast Acting Solenoid Valve which permitted on-line replacement of the servo.

If the backup clamp should slip after the servo valve was removed, 1600 psi hydraulic fluid could easily injure anyone near by.

THE BLOCK WITH PRESSURE GUAGES
THE BLOCK II is also equipped with an internal Hydraulic Isolation Valve that shuts down all fluid flow to the servo valve. Three quick disconnect pressure taps are provided to confirm the absence of fluid pressure. In the event of a servo valve failure. THE BLOCK II allows the operator to safely remove the servo valve from THE BLOCK II for repair or replacement with the unit on-line.






On turbines that have servo valve isolation capabilities, such as the GE Servo Isolation Scheme, THE BLOCK II is able to be installed on-line.  On all other units the modification can be completed in less than 15 minutes of down time per valve with no cutting, welding, or drilling required.

The Hydraulic Isolation capabilities of THE BLOCK II ensures a safe environment when changing the servo valve on-line (no Fast Acting Solenoid Valve Clamp needed).