Water seal of traps in multi-storeyed building or house may sometimes get broken due to siphonic action.

When the waste water is suddenly discharge from a sanitary fixture A of the upper floor, it moves down rapidly through the soil pipe and in it is movement it may suck some air from the lateral pipe connecting the soil pipe with the fixture at the lower floor.

This sucked air causes siphonic action resulting in the flow of water from the trap of the fixture to the soil pipe and thus breaking it is water seal.

To overcome this difficulty a separate pipe of smaller diameter is attached to the trap which connects the trap with the vent pipe, this is known as anti-siphonage pipe.