A type of valve used in a pipeline to ensure that the flow of a fluid is in one direction only and therefore prevent the inadvertent change of direction of flow. Also known as a check valve or one-way valve, there are various types and designs used for a wide variety of applications. They all consist of a mechanical mechanism that prevents the (p. 257) reversing of flow such as using a spring-loaded ball that seats and blocks flow in the event of a reversal of pressure. Flexible diaphragms are also used which flex and open in one flow direction but seal in the other. Others consist of a hinged disc that opens freely with flow but seals shut when the flow is reversed.