3/4" x 3/4" fpt Plastic Check Valve, This helps prevents back flow of water in air lines if the unit happens to power off or run in reverse. this check valve can be used in both high and low pressure situation due to it being non restrictive. If a check valve is not used the pressure of the water can force water back up through the airline which would cause damage to the compressor. For example the second picture shows a RVC, which is set up on the intake side since when rotary vane compressors turn off the rotor rotates freely.