Example of Reversal of Flow – Backflow Backsiphonage Case 3

A chemical tank has a submerged inlet with no cross-connection control device.

The fire plumb draws suction directly from the city water supply line which is insufficient to serve normal plant requirements.

The plant fire pump draws suction directly from the public water system distribution system. The Public water system cannot provide water for operational purposes and firefighting at the same time. Therefore, a fire pump was installed to provide sufficient water for fire control. As illustrated in the diagram the water supply line to the chemical tank is on the suction side of the pump. A drinking water fountain is also connected to the same water supply line that is connected to the chemical tank. During a fire emergency or if the fire pump is activated for testing or repairs, since there is insufficient water to serve the plant and supply water for firefighting, the water supply line feeding the chemical tank would become sub-atmospheric siphoning water out of the chemical tank and into the plant potable water system.  Negative pressure would be present and someone drinking water from the drinking water fountain would be consuming water from the chemical tank.

An air gap or a backflow prevention device installed on the submerged inlet to the chemical tank would prevent a cross-connection incident when the fire pump was activated.