What are the two types of valves that need to be closed when a flashback occurs?

When a flashback occurs in an oxy-acetylene welding or brazing application, it is crucial to immediately close both the oxygen and acetylene valves. This response is essential because a flashback indicates that the flame has traveled back into the hoses or equipment, which can potentially lead to a fire or explosion if not addressed swiftly.

Closing the oxygen valve stops the supply of oxygen that fuels the combustion, and shutting off the acetylene valve prevents further flow of the fuel gas. Together, shutting both valves helps to eliminate the source of the flame and contains the potential hazard.

While the other options mention different types of valves or systems, they do not specifically address the immediate requirement in response to a flashback, which is to cut off the gas supply to prevent further escalation of the situation. Therefore, focusing on the oxygen and acetylene valves ensures a rapid and safe resolution to the emergency.