University:
California State University, Northridge
Course:
EOH 356B | Environmental Health II
Academic year:

2024
Views:

18

Pages:

2
Author:

gewoekerdodx

A cross connection is any physical connection between wastewater and potable water, with the potential for backflow. Backflow is any force pushing the wastewater towards potable water. In other words, a cross connection can result in contamination of drinking water, and it is a fundamental concern for environmental health professionals. In order to understand backflow, we start with an equation that should be familiar to all physics students: P = F/A where P = pressure, F = force, and A = area Pressure is typically measured in psi (pounds per square inch) with two scales: psi a (absolute scale) and psi g (gage scale) Psig is what we see on pressure gauges, and psia is the larger, absolute pressure in pounds per square inch. This may seem confusing at first, so consider two examples: 1. If we open a gauge at sea level (which releases the pressure), it may read zero, but we know that gravity is pushing down at about 14.7 pounds per square inch. Thus, psig = 0, but psia = 14.7 2. If a pump sucks all the air from a closed container (i.e., a perfect vacuum), the gauge will start reading negative numbers. Thus, we know that in a perfect vacuum at sea level, psia = 0 (by definition), but the gauge will read psig = -14.7 In each example, psig = (psia - 14.7). Using these units, we can now define backsiphonage as backflow that occurs in a negative psig (i.e., backflow caused by sucking liquids). In contrast, backpressure occurs in a positive psig (i.e., backflow caused by a higher pressure fluid forcing itself against a weaker pressure fluid). We can study backsiphonage by deriving an equation for the pressure of a water column at any height. Consider the pressure for one cubic foot of water. Since once cubic foot of water weighs 62.4 pounds, and this cube of water rests on an area that is 1 sq. foot, we can calculate the psi reading with the equation P = F/A: P = F/A = 62.4 lbs. / 1 sq. ft. = 62.4 lbs. / 144 sq. in. = .433 psi Next, for two cubic feet (one on top of the other), we double the weight resting on the same square foot: P = (62.4 lbs. / 1 sq. ft.) * 2 = .866 psi Finally, for any number of cubic feet: P = (62.4 lbs/sq. ft.) * H (where H is the height in feet) To generalize this equation for any water column (Pw): 1) divide 62.4 lbs. by 144 square inches (62.4 / 144 = .433), 2) add any other existing pressure in the system (P), and we then get the general equation: Pw = (.433*H) + P VARIABLES: P = pressure (psi) psig = lbs./square inch (gage) F = force (lbs.) water = 62.4 lbs./cubic ft. A = area (square inches) Pw = pressure in a water column (psi) psia = lbs./square inch (absolute) H = height of water column (in ft.)

Cross Connections

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