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9.3 Theory of Backflow and Backsiphonage 313

Valve open

Submerged inlet

Valve open

Closed supply
Figure 9.14 Backsiphonage in a
Plumbing System

can be calculated. The pressure at the corresponding level in the right tube above the free
surface in the right container can also be calculated.
As shown in Fig. 9.13b, the pressure at all levels in the left tube would be less than at
corresponding levels in the right tube. In this case, a static condition cannot exist because
fluid will flow from the higher pressure to the lower pressure; the flow would be from the
right tank to the left tank. This arrangement is referred to as a siphon. The crest of a siphon
cannot be higher than 33.9 ft (10.3 m) above the upper liquid level, because the atmosphere
cannot support a column of water greater in height than 33.9 ft (10.3 m).
Figure 9.14 illustrates how this siphon principle can be hazardous in a plumbing sys-
tem. 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. This siphon action has been produced by
reduced pressures resulting from a difference in the water levels at two separated points
within a continuous fluid 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 on this principle, it can be shown that as a fluid accelerates (velocity head
increases), as shown in Fig. 9.15 and the following expression, the pressure head (P> ) is
reduced to maintain the same total head:
P v 2
H = z + + (9.5)
g 2g
where
H total head, ft (m)
Z elevation, ft (m)
P
pressure head, ft (m)
g
v 2
velocity head, ft (m).
2g