Question

Hi The Reich 19L Submersible Pump with two motors has two variations, with and without a non-return valve. Practically, what is the difference?

Answer

If the pump is used in an internal water tank then to fill the tank it is normally required that water from an outside source be pumped into the tank via the submersible pump. That is, water is required to flow in a reverse direction through the pump and the pump without the non-return valve is fitted.
Where the pump is simply dropped into an external water supply tank then the pump with non return valve is preferred as it prevents water running back into the tank.