Valve control can be electric actuator, why use hydraulic control?

Hydraulic valves are generally used in large diameter valves, if the electric valve is not easy to drive, need a large torque. For example, the valve of cooling water tower flow control in the power plant, the diameter of the general choice of large power plants is about 2 meters, 2 meters, how much force is needed, so the electric valve can not drive.

Another is the power plant in front of the main steam door quickly shut off the valve, listen to the name also know, fast, or a principle, hydraulic pressure has strength. So fast. Another reason is that the electric valve is driven by a screw, want to switch the screw slowly turn. And the hydraulic valve transmission rod is directly lifting, must be fast.