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# Fluid dynamics hydraulic grade line practice problem #3

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I am working the fluid dynamics portion of Michael Lindeburg's practice problems. I do not understand the answer to problem 3 of the fluid dynamics section, which is a hydraulic grade problem. The answer to portion "b" of this question is "A", 330 psig. This answer uses the pressure at point A, 31.1 psig, not the differential pressure, 11.1 psig, between points A & D, as I expected. What am I missing here? If the hydraulic grade line is the sum of the pressure & potential energies at each point in the piping system, then it would seem that the "elevation of the hydraulic grade line at point A referenced to point D" would be the difference in the sums of pressure & potential energies between these two points. The differential pressure between points A & D is 31.1-20 = 11.1 psig. Thanks in advance for any replies to this one!!!

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Scan the problem for those of us that have retired the sample problem books...