The Hydraulics of Backflow

Backflow means any type of reversed flow. Backsiphonage is the backflow of non-potable water caused by atmospheric pressure exerted on the non-potable forcing it toward a potable water system that is under a vacuum. Backpressure is the backflow of non-potable water toward the potable water system by the differential pressure existing between two systems both of which are at pressures greater than atmospheric. The prime concern in backflow is pressure.

An understanding of the principles of backflow and backsiphonage requires an understanding of the terms frequently used in their discussion. Force, unless completely resisted, will produce motion. The direction of the motion is the same as that of the force. Weight is a type of force resulting from the earth’s gravitational attraction, which produces a pressure towards the center of the earth. Pressure normally refers to a force per unit area, such as pounds per square inch (PSI). Given the same volume, water has a greater density than air and weighs more than air.

Atmospheric Pressure is the pressure exerted by the weight of the atmosphere above the earth. At sea level, the atmospheric pressure is 14.7 psia. The atmospheric pressure is less as the elevation increases.

Atmospheric pressure changes with elevation. Therefore, gauge readings taken at different altitudes should not be compared and when calibrating a pressure gauge, atmospheric pressure should be taken into consideration.

Water can be a liquid, solid or gas. Water fundamentally cannot be compressed. Water is a complex substance and changes with temperature or pressure. Water freezes at 32⁰ Water boils at 212 ⁰ F. A cubic foot of water weighs about 62.4 pounds. A gallon of water weighs 8.5 pounds at 39.0⁰ F. and weighs 7.996 pounds at 212 ⁰ F. Water density varies with temperature and is densest at 39.0⁰ F. Water temperatures above or below 39.0⁰ F. will expand. Water has “Surface Tension.” Water is the “Universal Solvent” since it will dissolve many substances. Water under pressure may be heated above its boiling point (the greater the temperature, the high pressure of the steam.)

Pressure may be referred to using an absolute scale, pounds per square inch absolute (psia), or gage scale, pounds per square inch gage (psig). Absolute pressure and gage pressure are related. Absolute pressure is equal to the gage pressure plus the atmospheric pressure.

In essence then, absolute pressure is the total pressure. Gage pressure is simply the pressure read on a gage and does not include atmospheric pressure. If there is no pressure on the gage other than atmospheric pressure, the gage would read zero. The absolute pressure would be equal to 14.7 psi, which is the atmospheric pressure.

Vacuum refers to a negative gage pressure, or that amount of absolute, negative, differential pressure existing between a contained fluid and the surrounding atmosphere. Vacuum occurs when the absolute pressure is less than the atmospheric pressure and that the gage pressure is negative.

A  complete or total vacuum would mean a pressure of 0 psia or -14.7 psig. Since it is impossible to produce a total vacuum, the term vacuum, as used in the text, will mean all degrees of partial vacuum. In a partial vacuum, the pressure would range from slightly less than 14.7 psia (0 psig) to slightly greater than 0 psia (-14.7 psig). For backsiphonage to occur, negative pressure would have to be present. Backsiphonage would not occur if the pressure was 0 psi or greater since the absolute pressure is at or above atmospheric pressure.