Why is it recommended that an acetylene cylinder valve never be opened more than 1/2 turn?

The recommendation to open an acetylene cylinder valve no more than 1/2 turn is primarily for safety reasons, particularly to allow for rapid shutoff in case of an accident. In an emergency situation, such as a fire or a gas leak, the ability to quickly close the valve can be critical in preventing the escape of hazardous gas and minimizing the risk of a dangerous situation escalating.

When the valve is opened only halfway, it provides a balance between maintaining the necessary flow of gas for use in applications such as welding or cutting while ensuring that the operator can swiftly shut off the gas supply if needed. This precaution is especially important since acetylene is highly flammable and poses a significant risk if mishandled.

Other options, while relevant to gas usage, do not pertain specifically to the safety aspect emphasized by the 1/2 turn guideline. For instance, continuous flow of gas and maximum pressure are important in specific contexts, but they don't prioritize the essential safety feature that allows for immediate shutoff in emergencies. Preventing gas leakage is a concern as well, but the design of the valve and the proper handling procedures are typically more focused on safety and rapid response than on leakage prevention for a non-fully opened valve.