Water Pressure 5

The siphon principle can be hazardous in a plumbing system. If the supply valve is closed, the pressure in the line supplying the faucet is less than the pressure in the supply line to the bathtub. Flow will occur, therefore, through siphonage, from the bathtub to the open faucet. The siphon actions cited have been produced by reduced pressures resulting from a difference in the water levels at two separated points within a continuous fluid system.

Plumbing repairs needed to be completed which required the potable water supply valve to be closed. A valve downstream was opened by mistake. The closed valve created an imbalance of pressure in the potable water supply line. Flow occurred from the chemical mixing tank to the open valve since the pressure in the chemical mixing tank was greater than the system pressure. The siphon effect occurred because of the difference in elevation between the open chemical tank and the open valve.

Closing the service line valve lowers the internal plumbing system pressure to atmospheric. If unprotected cross-connection exists within the plumbing system and an outlet was opened on the lower level, a siphon situation would occur within the plumbing system thus creating a cross-connection incident within the building.

When an upstream valve in a pipe system is closed and there is a demand downstream of the closed valve, or the pipe is sloped downward, the water downstream of the closed valve continues to flow and a vacuum can be created in the piping system.

Reduced pressure may also be created within a fluid system as a result of fluid motion. One of the basic principles of fluid mechanics is the principle of conservation of energy. Based upon this principle, it may be shown that as a fluid accelerates, the pressure is reduced.