Water Supply and Hydraulic Calculations
Domain 3 tests the arithmetic behind the design: converting a flow test into a supply curve, building the demand point from sprinkler flow plus hose allowance, and proving the supply stays above the demand for the required duration. Expect Hazen-Williams, K-factor and elevation questions with clean numbers.
The core relationships
Sprinkler flow follows Q = K√P, so pressure follows P = (Q/K)². Friction loss in pipe uses the Hazen-Williams form with C values that reflect the material and its condition - roughly 100–120 for steel depending on age and lining, 140–150 for cement-lined steel, copper and CPVC. Elevation changes pressure at 0.433 psi per foot of rise, added to the demand, not subtracted.
Building the demand point
Demand = remote-area sprinkler flow + hose allowance. Common hose allowances are 100 gpm light hazard and 250 gpm ordinary hazard; durations are typically 30 minutes light hazard, 60–90 minutes ordinary, and 90–120 minutes extra hazard. Then add losses for the backflow preventer, meter, riser fittings and any pump, and check the supply curve at that flow.
Reading the supply curve
A flow test gives static pressure, residual pressure and pitot flow. The supply curve is not linear - extrapolate it along the 1.85 power relationship, not by proportion. Where the supply and demand curves cross, the margin at the demand flow is what the reviewer looks for; many reviewers want a cushion of roughly 10 psi or 10 percent above demand rather than a bare pass.
Worked example
K 8.0 sprinklers on the top line of a system, with the highest head 48 ft above the riser and a calculated friction loss of 6.2 psi to the base of the riser. Step 1: flow at 25 psi = 8.0 × √25 = 40 gpm per sprinkler. Step 2: elevation loss = 48 × 0.433 = 20.8 psi. Step 3: pressure required at the riser base = 25 + 20.8 + 6.2 = 52.0 psi. Compare 52.0 psi against the supply at the total system flow, including hose allowance.
Traps that cost marks
Using static pressure as the supply pressure; omitting elevation when heads are above the riser; forgetting the backflow preventer; using new-steel C factors on aged pipe; and quoting the flow-test pitot pressure as if it were the available flow.