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Theory of Backflow and Backsiphonage

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Theory of Backflow and Backsiphonage Chapter Three Pabsolute = Pgage + 14.7psi Water Pressure Theory of Backflow or For an understanding of the Pgage = Pabsolute – 14.7 psi nature of pressure and its and Backsiphonage In essence then, absolute relationship to water depth, pressure is the total pressure. consider the pressure exerted on Gage pressure is simply the the base of a cubic foot of water pressure read on a gage. If there at sea level. (See Fig. 1) The is no pressure on the gage other average weight of a cubic foot than atmospheric, the gage cross-connection1 is the of water is 62.4 pounds per would read zero. Then the link or channel connecting square foot gage. The base may A absolute pressure would be a source of pollution with a be subdivided into 144-square equal to 14.7 psi which is the potable water supply. The inches with each subdivision atmospheric pressure. polluting substance, in most being subjected to a pressure of The term vacuum indicates cases a liquid, tends to enter the 0.433 psig. that the absolute pressure is less potable supply if the net force Suppose another cubic foot than the atmospheric pressure acting upon the liquid acts in of water were placed directly and that the gage pressure is the direction of the potable on top of the first (See Fig. 2). negative. A complete or total supply. Two factors are therefore The pressure on the top surface vacuum would mean a pressure essential for backflow. First, of the first cube which was of 0 psia or -14.7 psig. Since it there must be a link between originally atmospheric, or is impossible to produce a total the two systems. Second, the 0 psig, would now be 0.433 vacuum, the term vacuum, as resultant force must be toward psig as a result of the super- used in the text, will mean all the potable supply. imposed cubic foot of water. degrees of partial vacuum. In a An understanding of the The pressure of the base of partial vacuum, the pressure principles of backflow and the first cube would also be would range from slightly less backsiphonage requires an increased by the same amount than 14.7 psia (0 psig) to understanding of the terms of 0.866 psig, or two times the slightly greater than 0 psia frequently used in their original pressure. (-14.7 psig). discussion. Force, unless com- Backsiphonage1 results in pletely resisted, will produce fluid flow in an undesirable or motion. Weight is a type of reverse direction. It is caused by FIGURE 1. force resulting from the earth’s atmospheric pressure exerted on Pressure exerted by 1 foot of gravitational attraction. water at sea level. a pollutant liquid forcing it Pressure (P) is a force-per-unit toward a potable water supply area, such as pounds per square system that is under a vacuum. inch (psi). Atmospheric pressure is 62.4#/ft3 Backflow, although literally the pressure exerted by the 12" meaning any type of reversed 12" weight of the atmosphere above flow, refers to the flow produced 12" the earth. by the differential pressure Pressure may be referred to existing between two systems using an absolute scale, pounds both of which are at pressures per square inch absolute (psia), greater than atmospheric. Sea level or gage scale, pounds per square inch gage (psig). 0.433 psig Absolute pressure and gage pressure are related. Absolute pressure is equal to the gage pressure plus the atmospheric pressure. At sea level the atmospheric pressure is 14.7 psia. Thus, 1See formal definition in the glossary of the appendix 12 • CROSS-CONNECTION CONTROL MANUAL If this process were Siphon Theory level exactly balances the because of the partial vacuum repeated with a third cubic foot weight of a column of water created by the drop in pressure. of water, the pressures at the Figure 3 depicts the atmo- 33.9 feet in height. The If the faucet were opened, base of each cube would be spheric pressure on a water absolute pressure within the however, the vacuum would be 1,299 psig, 0.866 psig, and surface at sea level. An open column of water in Figure 4 at broken and the water level 0.433 psig, respectively. It is tube is inserted vertically into a height of 11.5 feet is equal to would drop to a height of 77 evident that pressure varies the water; atmospheric pres- 9.7 psia. This is a partial feet above the ground. Thus, with depth below a free water sure, which is 14.7 psia, acts vacuum with an equivalent the atmosphere was supporting surface; in general each foot of equally on the surface of the gage pressure of -5.0 psig. a column of water 23 feet high. elevation change, within a water within the tube and on As a practical example, Figure 5 is a diagram of an liquid, changes the pressure by the outside of the tube. assume the water pressure at a inverted U-tube that has been an amount equal to the weight- closed faucet on the top of a filled with water and placed in per-unit area of 1 foot of the FIGURE 3. 100-foot high building to be 20 two open containers at sea level. Pressure on the free surface of a liquid. The rate of increase for liquid at sea level. psig; the pressure on the If the open containers are water is 0.433 psi per foot of ground floor would then be placed so that the liquid levels depth. 63.3 psig. If the pressure at the in each container are at the Frequently water pressure ground were to drop suddenly same height, a static state will is referred to using the terms due to a heavy fire demand in exist; and the pressure at any “pressure head” or just “head,” the area to 33.3 psig, the specified level in either leg of and is expressed in units of feet pressure at the top would be the U-tube will be the same. of water. One foot of head reduced to -10 psig. If the The equilibrium condition would be equivalent to the building water system were is altered by raising one of the pressure produced at the base airtight, the water would containers so that the liquid of a column of water 1 foot in remain at the level of the faucet level in one container is 5 feet depth. One foot of head or 14.7 14.7 psia psia 1 foot of water is equal to 0.433 FIGURE 5. FIGURE 4. sea level Pressure relationships in a psig. One hundred feet of head Effect of evacuating air from a continuous fluid system at the column. is equal to 43.3 psig. same elevation. FIGURE 2. “Zero” Absolute 4.7 psia Pressure exerted by 2 feet of Pressure water at sea level. If, as shown in Figure 4, the tube is slightly capped and a vacuum pump is used to 0.0 evacuate all the air from the psia sealed tube, a vacuum with a or Vacuum pump -14.7 ' pressure of 0 psia is created psig 10.3 psia 23 within the tube. Because the 24" pressure at any point in a static 0.433 psig fluid is dependent upon the ' height of that point above a 10 ' reference line, such as sea level, 39.9 it follows that the pressure 9.7 within the tube at sea level psia or -5.0 psig 14.7 14.7 must still be 14.7 psia. This is psia psia 0.866 psig equivalent to the pressure at the ' Sea Level base of a column of water 33.9 11.5 feet high and with the column 14.7 14.7 psia or open at the base, water would psia 0.0 psig rise to fill the column to a depth Sea level of 33.9 feet. In other words, the 1See formal definition in the glossary of the appendix weight of the atmosphere at sea CHAPTER THREE • 13 above the level of the other. (See level, since atmosphere cannot shown that as a fluid acceler- FIGURE 9. Dynamically reduced pipe Fig. 6.) Since both containers support a column of water ates, as shown in Figure 8, the pressures. are open to the atmosphere, the greater in height than 33.9 feet. pressure is reduced. As water pressure on the liquid surfaces Figure 7 illustrates how flows through a constriction From pollution To fixture in each container will remain at this siphon principle can be such as a converging section of source 14.7 psia. hazardous in a plumbing pipe, the velocity of the water If it is assumed that a static system. If the supply valve is increases; as a result, the state exists, momentarily, closed, the pressure in the line pressure is reduced. Under such within the system shown in supplying the faucet is less than conditions, negative pressures Figure 6, the pressure in the left the pressure in the supply line may be developed in a pipe. +50 psig tube at any height above the to the bathtub. Flow will occur, The simple aspirator is based free surface in the left container therefore, through siphonage, upon this principle. If this can be calculated. The pressure from the bathtub to the open point of reduced pressure is -10 psig at the corresponding level in the faucet. linked to a source of pollution, right tube above the free surface backsiphonage of the pollutant in the right container may also can occur.
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