6-7.
SIPHON PRINCIPLE
Many of the situations involved in cross connections are based upon the principle
of siphonage, as illustrated in figure 6-8. If an inverted "u" tube is filled with water and
placed in two open containers at exactly the same level (see figure 6-8 A), a static
condition wiII exist, and the water wiII not flow in either direction. The atmospheric
pressure on the water in the open containers and the vacuum, or negative pressure,
caused by the absence of air in the tube wiII support the column of water in the inverted
tube up to a height of 33.9 feet at sea level (other conditions being standard). If,
however, we raise one container to a higher level (see figure 6-8 B), we create a
pressure differential, or head (H), due " to the weight of the water in the tube. This head
causes the water to flow from the higher to the lower container. In this situation, water
actually "flows uphill." Figure 6-9 illustrates how this siphon principle can be hazardous
in a plumbing system. As long as sufficient pressure is maintained in the supply Iine,
the system remains in equiIibrium; however, if the supply Iine is closed or the pressure
is reduced to less than the head at the second floor level (plus atmospheric pressure),
water wiII flow by siphonage from the water closet to the open faucet at the lower level
(provided certain other conditions exist, which wiII be discussed in paragraph 6-9b).
Figure 6-8. Siphon principle.
MD0160
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