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                                          Chapter 9
                                                   Cross-Connection Control
                                                                                       resisted, will produce motion. Weight is a type of force result-
                                                            Roof-mounted solar panels
                                                                                       ing from gravitational attraction. Pressure (P) is a force-per-
                                                                                                         2
                                                                                                                      2
                                                                                       unit area, such as lb/in. (psi) or kN/m (kPa). Atmospheric
                                                                                       pressure is the pressure exerted by the weight of the atmo-
                                                               Heat exchanger
                                                                                       sphere above Earth.
                                                        Utility
                                                                                          Pressure may be referred to using an absolute scale,
                                                         sink
                                                                                                              2
                                                                                           2
                                                                                       lb/in. absolute (psia) or kN/m absolute (kPa absolute). Pres-
                                                                                                                                2
                                                                                       sure may also be referred to using a gauge scale, lb/in. gauge
                                                                                                   2
                                                                                       (psig) or kN/m gauge (kPa gauge). Absolute pressure and
                                                                                       gauge pressure are related. Absolute pressure is equal to the
                                                                                       gauge pressure plus the atmospheric pressure. At sea level
                                                                                       the atmospheric pressure is 14.7 psia using the US customary
                                                                                       units. Thus,
                                                                     Sink
                                                                                       P      = P    + 14.7 psi  (US customary units)  (9.1)
                                                                                        absolute  gauge
                                                                                       or
                                                      Sink
                                                                                       P gauge  = P absolute  − 14.7 psi  (US customary units)  (9.2)
                                                                                          In the SI units where P   and P     areinkPa
                                                                     Coffee                                   absolute   gauge
                                                                                                                       2
                                                                                                              2
                                                                    machine            (1 kPa = 1000 Pa = 1000 N/m = 1kN/m ) their relationship
                                                                                       can be given as follows:
                                                                                               P absolute  = P gauge  + 102 kPa  (SI units)  (9.3)
                                                                                       or
                                                      Sink
                                                                                               P    = P      − 102 kPa  (SI units)  (9.4)
                                                                                                gauge  absolute
                                                                                          In essence then, absolute pressure is the total pressure.
                                                                                       Gauge pressure is simply the pressure read on a gauge. If
                                                                             Chemical
                                                     Water                    feeder   there is no pressure on the gauge other than atmospheric, the
                                                    fountatin                          gauge would read zero. Then the absolute pressure would
                                                                                       be equal to 14.7 psi (102 kPa), which is the atmospheric
                                                  Booster pump                         pressure.
                                                                                          The term vacuum indicates that the absolute pressure is
                                                  Meter                                less than the atmospheric pressure and that the gauge pres-
                                       Water main                                      sure is negative. A complete or total vacuum would mean a
                                                                            Backpressure
                                                       Recommended installation  backflow  pressure of 0 psia or −14.7 psig (−102 kPa gauge). Because
                                                         of backflow preventers        it is impossible to produce a total vacuum, the term vacuum,
                                                                                       as used in the text, will mean all degrees of partial vacuum.
                                    Figure 9.10 Cross-connection in a modern seven-story office
                                                                                       In a partial vacuum, the pressure would range from slightly
                                    building in New Hampshire.
                                                                                       less than 14.7 psia (0 psig) to slightly greater than 0 psia
                                                                                       (−14.7 psig).
                                                                                          Backsiphonage results in fluid flow in an undesirable
                                    9.3 THEORY OF BACKFLOW AND
                                                                                       or reverse direction. It is caused by atmospheric pressure
                                    BACKSIPHONAGE
                                                                                       exerted on a pollutant liquid such that it forces the pollutant
                                    A cross-connection is the link or channel connecting a source  toward a potable water supply system that is under a vacuum.
                                    of pollution with a potable water supply. The polluting sub-  Backflow, although literally meaning any type of reversed
                                    stance, in most cases a liquid, tends to enter the potable  flow, refers to the flow produced by the differential pressure
                                    supply if the net force acting on the liquid acts in the direc-  existing between two systems, both of which are at pressures
                                    tion of the potable supply. Two factors are therefore essential  greater than atmospheric.
                                    for backflow. First, there must be a link between the two sys-
                                    tems. Second, the resultant force must be toward the potable
                                                                                       9.3.1 Water Pressure
                                    supply.
                                        An understanding of the principles of backflow and  For an understanding of the nature of pressure and its rela-
                                    backsiphonage requires an understanding of the terms fre-  tionship to water depth, consider the pressure exerted on the
                                    quently used in their discussion. Force, unless completely  base of a cubic foot of water at sea level (see Fig. 9.11a).
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