What is the purpose of the drain in a non-return valve?

The purpose of the drain in a non-return valve is to remove any accumulated condensate. In systems where non-return valves are utilized, particularly in steam systems or processes where heat transfer is involved, condensate can form as steam cools and converts back to liquid. This condensate can cause operational issues, such as water hammer or reduced efficiency, if it is allowed to accumulate.

By incorporating a drain in a non-return valve, it allows for effective management of this condensate, which ensures that the line remains clear and that the valve operates smoothly. This drainage mechanism is crucial for maintaining the overall health of the system, avoiding blockages, and preventing potential damage to equipment.

In this context, a non-return valve by itself is designed to allow fluid to flow in one direction while preventing backflow. The addition of a drain specifically addresses the need to manage liquids that may negatively impact system performance, further validating why option C is the correct choice.