This mod adds a check valve, also known as a non-return valve. It is effectively a one-way pipe. This valve has connections for circuit wires and can output its fluid level.
The valve is crafted from one pipe and one steel plate, and the recipe unlocked by researching Fluid Processing, as with pumps and storage tanks.
The difference between this valve and a pump + storage tank combo is that this valve takes up only a 1×1-tile space, and only holds as much fluid as a regular pipe. It also does not require power to operate. It’s a compact way of ensuring fluid goes where you want it, as well as providing for logical control of pumps.