Barbs grip onto tubing, providing a secure hold. These valves open to allow flow in one direction and close when flow stops or reverses. Nylon valves are more impact resistant than PVC or polypropylene valves. Flow coefficient (Cv) is the amount of water (in gallons per minute) at 60° F that will flow through a fully open valve with a difference of 1 psi between the inlet and the outlet.