Water Supply Evaluation, Hydraulics, and Pipe Sizing
Hydraulic design begins with an accurate water supply. A flow test produces static pressure, residual pressure, and flow, from which the available supply curve is derived. Candidates must understand how to plot the supply curve and compare it to the system demand curve at the point of connection, and why the safety margin (commonly 10–20 psi) is applied. N1.85 friction loss, Hazen-Williams C factors, and equivalent pipe lengths for fittings are core calculation concepts. Understand that the remote area is the hydraulically most demanding area, and that the calculation must balance at the base of the riser with the supply available.
Key concepts: pressure vs. flow, elevation loss (0.433 psi per foot of rise), velocity limits, and the difference between a demand at the base of riser and at the source. Pipe sizing follows from the calculation, not from a table, in a hydraulically calculated system; velocity and friction govern diameter choices.
Traps: forgetting elevation loss; using static pressure as if it were residual; ignoring that a flow test at a nearby hydrant may overstate or understate supply at the building; and applying the safety margin to flow rather than pressure. Another trap is failing to account for backflow preventer and meter losses, which can be substantial.
Worked example: static 80 psi, residual 60 psi at 1,000 gpm. The available pressure at the design flow is interpolated along the supply curve; subtract elevation (say 30 ft = 13 psi) and device losses (meter and backflow, 15 psi), then compare to the calculated demand plus safety margin. If demand plus margin exceeds available, revise the design or the supply.