What must be closed before opening the cylinder valves?

Before opening the cylinder valves, the regulators must be closed to ensure safety and control over the flow of gas. Regulators are designed to control the pressure and flow of gases coming from a cylinder. If the regulators are left open while the cylinder valves are opened, it could result in uncontrolled gas flow, which can lead to dangerous situations such as fire or explosion due to the rapid release of gas.

Closing the regulators before opening the cylinder valves creates a controlled environment, allowing the user to safely manage the gas pressure as needed. Once the cylinder valves are open and the regulators are set properly, the operator can adjust the flow of gas to safe and necessary levels for their specific welding or cutting task.

This practice is a crucial part of safety protocols in welding operations, emphasizing the importance of managing gas pressures and preventing potential hazards.