A check valve, also known as a non-return valve, is a mechanical device designed to allow fluid (liquid or gas) to flow in one direction only, automatically preventing backflow in a piping system. It operates based on differential pressure, meaning it opens when the upstream pressure exceeds the downstream pressure and closes when the flow reverses. Check valves do not require manual operation or external control, making them ideal for systems where automatic flow control is essential. They come in various types—such as swing, lift, piston, ball, and dual-plate wafer designs—each suited to specific flow conditions and installation orientations. The valve ends can be flanged, threaded, welded, or wafer-style, and they are manufactured to meet standards such as ASME, API, ISO, and DIN. Typical applications include water systems, HVAC, oil and gas pipelines, chemical processing, and steam systems. Proper selection of a check valve considers factors like pressure class, media compatibility, flow direction, and installation space, ensuring reliable and efficient system performance.