6-10. THE AIR GAP
The only absolute means of eliminating the physical Iink that may result in
backsiphonage is with the use of the vertical air gap (see figure 6-12). Air gaps should
be used wherever possible and must not be bypassed. The supply inlet to the fixture
should be terminated above the flood level rim of the fixture by a distance. equal to at
least two times the effective opening of the fixture. There should be no provision for
extending the fixture outlet below the flood level rim. If the end of the supply pipe is
threaded or serrated to permit the connection of a hose, a properly installed vacuum
breaker should also be provided.
Figure 6-12. Vertical air gap on lavatory
6-11. VACUUM BREAKERS
A fundamental factor in backsiphonage, as explained in paragraph 6-6 is vacuum
(negative pressure) between the potable water supply and the source of contamination.
If atmospheric pressure is admitted to a piping system between the source of poIIution
and the origin of the vacuum, backsiphonage WiII be prevented. This is the function of
a vacuum breaker. Because a vacuum may be created at numerous places in a piping
system, a vacuum breaker must be located as near as possible to the fixture from which
contamination may be expected. The position of a vacuum breaker must be sufficiently
above the fixture flood level rim so that flooding or submerging of the vacuum breaker is
not possible. There are two types of vacuum breakers:
MD0160
6-17