A 15 mm spring-loaded non-return valve, also known as a check valve, is a type of plumbing valve that allows water or other fluids to flow in one direction while preventing backflow in the opposite direction. It is designed to automatically close and prevent backflow when the pressure of the fluid in the system drops, and to open and allow flow when the pressure is sufficient.

The "15 mm" refers to the size of the valve, which is designed to fit on a 15 mm pipe. The valve includes a spring that is loaded to keep the valve closed and prevent backflow. When water or fluid is flowing in the correct direction, the spring is compressed, allowing the valve to remain open. However, when the flow reverses or the pressure drops, the spring is released, and the valve is quickly closed by the spring's force to prevent backflow.

Spring-loaded non-return valves are commonly used in applications where backflow prevention is essential, such as in water supply systems, swimming pools, and irrigation systems. They are easy to install and require minimal maintenance.