When an oxyacetylene torch is extinguished, which valve should be turned off first?

When extinguishing an oxyacetylene torch, the acetylene gas valve should be turned off first. This practice is crucial for safety reasons. Acetylene is more flammable and dangerous than oxygen, and by shutting off the acetylene first, you reduce the risk of flashback, which can occur if the oxyacetylene mixture remains present while oxygen continues to flow. After the acetylene is safely turned off, the oxygen valve can then be closed. This procedure minimizes the chance of having unburned acetylene mixed with the remaining oxygen, which could ignite if any spark or flame is present.

In contrast, turning off the oxygen first could leave acetylene gas in the system while still allowing oxygen to flow, potentially creating a hazardous situation. Therefore, it's critical to maintain this order to ensure safe operation and reduce the likelihood of dangerous incidents when working with oxyacetylene equipment.