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314 Chapter 9 Cross-Connection Control

Figure 9.15 Negative Pressure Created by
10 psig Constricted Flow. Conversion factor: 1 psig
30 psig 30 psig 6.94 kPa gauge pressure

Conversely, it can be shown that as water flows through a constriction (A 2 A 1 ) such
as a converging section of pipe, the velocity of the water increases (v 2 v 1 ):
Q v 1 A 1 v 2 A 2 (9.6)
where
3
3
Q flow rate, ft /s (m /s)
v 1 water velocity at section 1, ft/s (m/s)
v 2 water velocity at section 2, ft/s (m/s)
2
2
A 1 area of section 1, ft (m )
2
2
A 2 area of section 2, ft (m ).
As a result, the pressure is reduced. Under such conditions, negative pressures can de-
velop in a pipe. The simple aspirator is based on this principle. If this point of reduced
pressure is linked to a source of pollution, backsiphonage of the pollutant can occur.
One of the common occurrences of dynamically reduced pipe pressures is found on
the suction side of a pump. In many cases similar to the one illustrated in Fig. 9.16, the line
supplying the booster pump is undersized or does not have sufficient pressure to deliver
water at the rate at which the pump normally operates. The rate of flow in the pipe may be
increased by a further reduction in pressure at the pump intake. This often results in the
creation of negative pressure at the pump intake. This negative pressure may become low
enough in some cases to cause vaporization of the water in the line. Actually, in the
Fig. 9.16 illustration, flow from the source of pollution would occur when pressure on the
suction side of the pump is less than pressure of the pollution source, but this is backflow,
which will be discussed below.
The preceding discussion has described some of the means by which negative pres-
sures may be created and which frequently occur to produce backsiphonage. In addition to

From pollution To fixture
source

50 psig

10
psig

Figure 9.16 Dynamically Reduced Pipe Pressure.
Booster pump Conversion factor: 1 psig 6.94 kPa gauge pressure