What should you immediately do when a flashback occurs?

When a flashback occurs, the immediate action is to shut off the oxygen needle valve. This is crucial because a flashback indicates that the flame has moved back into the torch or even into the gas supply lines. Oxygen supports combustion, so by shutting off the oxygen supply, you effectively stop the burning process, preventing further escalation of the fire hazard. This will help to quickly extinguish the flame and ensure safety.

While turning off the acetylene needle valve or closing the torch pressure valve may also be necessary steps to take in your response to a flashback, the most immediate action should be to address the oxygen supply first since it is essential for combustion to continue. Additionally, shutting off the gas supply might be a more prolonged response and could introduce further hazards by disrupting system pressure. Prioritizing the oxygen needle valve allows for a quicker stabilization of the situation.