Water Supply and Hydraulic Calculations
Domain 2: Water Supply and Hydraulic Calculations
This domain tests whether you can turn a water supply into a defensible sprinkler demand. You must read flow-test data correctly, apply the density/area method, account for elevation and friction, and add the required hose-stream allowance and safety margin.
Core rules
- A flow test gives static, residual, and pitot pressure at a known flow; plot the supply curve and adjust to the base of the riser.
- Hydraulic demand includes sprinkler flow, hose allowance, elevation pressure, friction loss, and any safety margin required by the AHJ or design basis.
- For a sprinkler, Q = K√P. A larger K-factor delivers more flow at the same pressure.
- Use the Hazen-Williams formula with the correct C-factor for the pipe material; verify the inside diameter, not nominal size.
Worked example
A sprinkler has K = 5.6. At 7 psi, Q = 5.6 × √7 = 5.6 × 2.646 = 14.8 gpm. If the design density requires 20 gpm from that sprinkler, the required pressure is P = (20/5.6)² = 12.8 psi. The reasoning: flow varies with the square root of pressure, so small pressure errors become meaningful flow errors.
Traps
- Using static pressure as the available residual pressure.
- Forgetting hose-stream allowance or elevation pressure.
- Mixing nominal and actual internal pipe diameters.
- Omitting the safety margin or using a flow test that is too old.
- Applying a C-factor for a different pipe material.