Water Supply, Hydraulics, and System Sizing
What the exam tests
Domain 3 tests water supply evaluation and hydraulic sizing: interpreting flow tests, building supply curves, calculating pipe friction, accounting for elevation and device losses, and comparing system demand to available pressure. You need to know when a hydraulic calculation is required and how to read a supply curve.
Core rules
- A flow test gives static pressure, residual pressure, and flow. The available flow at a chosen residual pressure can be estimated from Q2 = Q1√((Ps - P2)/(Ps - Pr)).
- Hazen-Williams friction loss depends on flow, pipe inside diameter, length, and C-factor. Elevation changes pressure by about 0.433 psi per foot of rise.
- System demand includes sprinkler flow, hose allowance, and losses through valves, backflow preventers, PRVs, and fittings. Compare demand at the base of the riser to the supply curve with a safety margin.
- Sizing can be pipe-schedule or hydraulic; hydraulic design is common for larger or irregular systems. Pumps and tanks must be evaluated at churn, rated, and maximum flow, not just at one point.
Worked example
A flow test reads 80 psi static and 60 psi residual at 1,000 gpm. Estimate flow at 20 psi residual: Q20 = 1,000 × √((80-20)/(80-60)) = 1,000 × √3 = 1,732 gpm. If the sprinkler-plus-hose demand at the base of the riser is 1,250 gpm at 20 psi, the supply appears adequate, but you still must subtract elevation, backflow preventer, and piping losses. A demand of 1,800 gpm would exceed the estimated supply.
Traps
Using static pressure as if it were available residual pressure; omitting hose allowance; forgetting elevation in a tall building; ignoring a backflow preventer or PRV; and reading a pump curve at churn pressure instead of the demand point.