A non-return valve is fitted to ensure that a medium flows through a pipe in the right direction, where pressure conditions may otherwise cause reversed flow. Non return valves allow the fluid to flow only in one desired direction. How to make a check valve - Non return valve working principle. Non return valve basically comprise housing and piston, which is pushed onto the seal seat by. The function of non-return valve is to limit the flow of fluid through it one direction. A check valve, clack valve, non-return valve, reflux valve, retention valve or one- way valve is a valve that normally allows fluid (liquid or gas) to flow through it in.