Water Supply, Hydraulics and System Design Fundamentals
What the exam tests
This domain tests whether you can evaluate a water supply, calculate a demand and decide whether the system will work. Expect flow-test interpretation, Hazen-Williams friction loss, elevation pressure, hose allowance, safety margin and density/area concepts.
Core rules and concepts
- Available supply. A flow test gives static pressure, residual pressure and test flow. Available pressure at another flow is estimated with the 1.85 power relationship: P = P residual + (P static - P residual) x (Q new / Q test)^1.85.
- Demand. Design demand comes from the density/area curve for the hazard, plus hose allowance and any inside or outside hose streams required by the standard.
- Friction loss. Hazen-Williams uses C = 120 for black steel and higher C factors for copper and some plastic pipe. Fittings, valves and backflow preventers add loss.
- Elevation. Each foot of rise costs about 0.433 psi; each foot of drop adds about 0.433 psi.
- Safety margin. Compare supply and demand at the same flow. Many jurisdictions and specifications require a margin, commonly 10 to 20 psi; confirm the project requirement.
- Pumps. A fire pump shifts the supply curve upward; the pump curve, not a single point, must be used.
Worked example
A flow test shows 70 psi static and 30 psi residual at 900 gpm. Estimate the pressure available at 700 gpm. Pressure drop from static = 70 - 30 = 40 psi. Ratio = 700/900 = 0.778. Raise to 1.85: 0.778^1.85 = 0.63. Available pressure = 30 + 40 x 0.63 = 55.2 psi. If the sprinkler demand at 700 gpm is 48 psi and the required hose allowance is included, the margin is about 7 psi. If the specification requires 10 psi, the supply is short and you need a larger main, a pump or a different supply.
Traps that cost marks
- Using static pressure as the available pressure at demand flow.
- Forgetting elevation and backflow-preventer losses.
- Omitting hose allowance from the demand.
- Comparing supply and demand at different flows.