Is an essential and necessary part of a commercial grease trap installation, the flow restrictor allows air to enter the pipe and restricts the amount of water going into the trap so as not to overflow it. If the water flows too quickly and the trap gets overwhelmed, grease can escape the trap. When considering a flow restrictor look for options that provide a more extensive air vent, equal to the size of the drain pipe. Also, be sure the actual restrictor (the inner wall fitted with the hole) is made of solid metal. Plastic ones are not strong enough and can break or deteriorate quickly.