Which of the following is NOT a type of valve used in gas systems?

In gas systems, various types of valves are used to control the flow of gas, each serving specific functions. Check valves, ball valves, and globe valves are all recognized and commonly utilized types within these systems.

Check valves are designed to allow gas to flow in one direction only, preventing backflow, which is essential for maintaining proper system function and safety. Ball valves feature a rotating ball mechanism that provides a quick and efficient shut-off capability, making them a popular choice due to their reliability and ease of operation. Globe valves are typically used for regulating flow, thanks to their design which allows for fine adjustments.

On the other hand, a piston valve is not traditionally categorized as a type of valve in gas systems. While piston valves can be found in various applications, they are more common in hydraulic systems or similar setups rather than in gas control applications. Hence, identifying piston valve as the option that does not belong in the context of valve types for gas systems is accurate.