Flood-Guard operates like a check valve to seal off water backup caused by overloaded sewers. Water flows normally through the drain until the sewer begins to back up. Then the Flood-Guard float rises to seal off the drain opening until the water recedes.
For extreme, extended pressure, use the standpipe models. The 4" standpipe model is threaded to receive 1-1/2" IPS pipe. On the 3" model, the pipe is slipped through the rubber gasket before the screws are fully tightened.