Which type of valves must be opened by hand?

Control valves are designed to regulate the flow of fluids in a system, adjusting and modulating the flow based on specific parameters. These valves often require manual operation to adjust flow levels, whereas other types of valves like safety valves or emergency valves are typically designed to react automatically to changes in pressure or specific emergency conditions.

Control valves allow for precise management of gas or liquid flows, which is essential in many applications where operational efficiency and safety are priorities. Manual operation ensures that the user can make adjustments as needed, which is critical in processes that require continual monitoring and adaptation to specific requirements. In contrast, safety valves are generally designed to open automatically to prevent dangerous pressure build-up, and emergency valves respond to specific fail-safe conditions without human intervention.