Water Supply Analysis and Hydraulic Calculations
Water supply basics
A flow test gives static, residual, and pitot-derived flow. The results locate the supply curve. Hydraulic calculations then compare the system demand-sprinkler flow plus hose allowance-at the base of the riser against the available pressure at that same point. Hazen-Williams friction loss uses the C factor for the pipe (commonly 120 for steel, 150 for cement-lined cast iron, and values for copper or plastic per listings). Elevation adds 0.433 psi per foot of rise.
Worked example
A flow test yields 70 psi static and 50 psi residual at 1,000 gpm. Estimate the pressure at 1,500 gpm with P = Ps − (Ps − Pr)(Q/Qr)^1.85: P = 70 − 20(1,500/1,000)^1.85 = 70 − 20(2.118) = 27.6 psi. If the calculated demand at the riser base is 1,500 gpm at 35 psi, the supply is short at that flow. The designer must find a larger main, a pump, a storage tank, or a reduced-demand design that the code permits. NFPA 13 does not mandate a universal safety margin, but many authorities require 10 psi or 10 percent; always check the local amendment.
Traps
Using static pressure alone; mixing flow-test points; forgetting the hose allowance; omitting elevation when the riser serves upper floors; and using a C factor that does not match the listed pipe. Also watch units: gpm and psi must correspond to the same elevation datum.
Documentation
Record test location, time, who conducted it, gauge calibration, hydrant flow, and whether the test was conducted at a representative demand. A calculation is only as good as its water supply basis.