 Throttling valves are used to control the amount or pressure of a fluid allowed to pass through, and are designed to withstand the stress and wear caused by this type of operation.Because they may wear out in this usage, they are often installed alongside isolation valves which can temporarily disconnect a failing throttling valve from the rest of the system, so it can be refurbished or replaced.Non-return or check valves allow free flow of a fluid in one direction, but prevent its flow in a reverse direction.