What is used to change and control the opening of valves?

Actuators are devices specifically designed to change and control the opening of valves in various systems, including power engineering applications. They convert energy, often electrical, into mechanical motion to operate a valve. Depending on the type of actuator, this can involve rotary or linear movements that enable precise control over valve positions. This is crucial in processes that require accurate flow regulation, pressure control, or system safety.

Manual levers, while capable of opening and closing valves, rely on human effort and are not automated. Circulation pumps are primarily used to move fluids rather than control valve positions. Thermostatic controls, on the other hand, are used to regulate temperature in systems and do not directly interface with valve movements. Thus, actuators are the most suitable and effective means of automating valve operation in various engineering applications, making them the correct answer to the question.