..?..non-return valves..? I will assume that your 'non-return valves', are what is also known as check valves. Check valves allows flow in one direction only. These are used wherever you need to prevent backflow into the system, or to maintain a charged circuit with fluid, until you choose to unload(depressurize ) it. Now, why are they used in discharged lines of salt water, basically to prevent backflow....reason not known, here , requires usage info.
