Stainless Steel Check Valves Highly Corrosion Resistant with PTFE Seals by Duda Energy Check valves are used to help direct flow in one direction. The stainless steel bodies and PTFE seals are highly corrosion resistant allowing these check valves to be used with a wide variety of fluids including, but not limited to water, oil, gas, air, light alkali’s, acids, biodiesel, fuels and alcohols. Swing check valves are for horizontal use only. When fluid pushes on the stopper in the correct direction, it will “swing” open to allow flow. If flow moves in the opposite direction, it will swing shut.