To draw the graph, the static pressure of 86 psi is plotted above 0 gpm as shown on slide #17. The residual pressure is plotted next, directly above the 789 gpm calculated flow. These two points are connected with a straight line, which is generally extended through the second point and down to at least 20 psi. Now, any question about the water supply can be answered.
For example, the flow available at 20 psi can be read immediately below the intersection of the curve and the 20 psi line. It turns out to be about 980 gpm. Also, the pressure available at 500 gpm is read immediately above 500 to be about 67 psi. Suppose a sprinkler demand of 760 gpm at 36 psi is needed. At 750 gpm, this water supply can provide about 46 psi, and therefore would be more than adequate to meet the demand.
A quick way to establish the adequacy of a water supply with respect to various demands is to note the location of the demand point relative to the water supply line. If the point falls on or below the line, the water supply is adequate. If the demand point falls above the line, the water supply is not capable of meeting that demand. The comparison of a demand point to the supply curve is illustrated on slide #17, where the demand point falls below the curve, indicating an adequate supply.
One item to note about the graph paper is that most hydraulic graph paper comes with at least three different scales along the bottom. They are simply multiples of each other to allow plotting a broader range of flows. Ensure the correct scale is used.
FPT 102 - Fire Prevention and Inspection // Instructor: Kevin M. Kolb Slide #16