To obtain neutral preheat flames when lighting a positive-pressure oxyacetylene torch, what action should be taken?

To obtain neutral preheat flames when lighting a positive-pressure oxyacetylene torch, what action should be taken?

• A. Open the acetylene valve to 1/4 turn
• B. Open the cutting oxygen valve and adjust the working pressure
• C. Close the acetylene valve and leave only oxygen
• D. Mix acetylene with air and adjust

Answer: (B)

Neutral preheat flames come from a balanced mix of acetylene and oxygen. For a positive-pressure oxyacetylene torch, that balance is achieved by opening the cutting oxygen valve and adjusting the working pressure until the flame appears as a clean blue cone with a steady inner cone and little or no soot. This tells you the fuel–oxygen ratio is correct for a neutral flame used to preheat the work. Opening only the acetylene valve would create a rich, sooty flame, not neutral. Closing acetylene and using only oxygen won’t produce a flame at all, and mixing acetylene with air yields an incorrect, unsafe flame. So the correct action is to set the oxygen flow (through the cutting oxygen valve) and fine-tune the pressure to reach a neutral preheat flame.

Neutral preheat flames come from a balanced mix of acetylene and oxygen. For a positive-pressure oxyacetylene torch, that balance is achieved by opening the cutting oxygen valve and adjusting the working pressure until the flame appears as a clean blue cone with a steady inner cone and little or no soot. This tells you the fuel–oxygen ratio is correct for a neutral flame used to preheat the work.

Opening only the acetylene valve would create a rich, sooty flame, not neutral. Closing acetylene and using only oxygen won’t produce a flame at all, and mixing acetylene with air yields an incorrect, unsafe flame. So the correct action is to set the oxygen flow (through the cutting oxygen valve) and fine-tune the pressure to reach a neutral preheat flame.