Which of the following statements about fuel gas systems is true?

The statement that the three most common fuel gas systems are air-propane, air-acetylene, and oxyacetylene is accurate because these systems represent widely utilized methods for generating heat in various applications, including welding and cutting.

Air-propane systems use propane mixed with air for combustion, making them suitable for heating and certain welding tasks. Air-acetylene systems operate similarly but utilize acetylene gas, which produces a hotter flame, ideal for tasks requiring high temperatures. Oxyacetylene systems combine oxygen with acetylene, generating an even more intense flame suited for welding metals. Recognizing these three systems is crucial for understanding how different gases affect heating and combustion processes.

The options signalling the incorrectness of the other choices stem from misconceptions about the systems in question. For example, oxyacetylene indeed has heating applications, and both acetylene and propane systems are commonly used. This knowledge is essential for professionals in pipe fitting and related fields to effectively work with fuel gas systems.