Which of the following is NOT a purpose of a gas shutoff valve?

A gas shutoff valve plays several critical roles in a gas distribution system. The primary purposes include stopping gas flow during emergencies, allowing for safe maintenance work, and controlling pressure within the system to ensure safe operation.

The prevention of backflow of gas is typically managed by devices specifically designed for that function, such as backflow preventers or check valves. These devices ensure that gas flows in one direction, preventing reverse flow that could potentially lead to hazardous situations. The gas shutoff valve itself is not primarily designed for this purpose; its main function is to isolate sections of the gas system when needed. Thus, among the options presented, preventing backflow is not a primary purpose of a gas shutoff valve, supporting the identification of this option as the correct answer.