Water Supply Analysis and Hydraulic Calculations
This domain checks your ability to build a demand, test a supply, and run the math that proves one meets the other.
Assembling the demand
- Read the density from the occupancy curve at the remote area size, then multiply density by area to get sprinkler flow.
- Add hose allowance at the point where it applies: 100 gpm light hazard, 250 gpm ordinary hazard, 500 gpm extra hazard.
- Work from the most remote sprinkler back to the base of the riser, then out to the supply connection.
Testing the supply
A flow test records static pressure, residual pressure, and the flow measured at the hydrant. Flow from a pitot reading is Q = 29.83 C d squared times the square root of the pitot pressure, with C about 0.9 for a well-rounded outlet and 0.7 for square-edged. Available flow at 20 psi residual is Q20 = Qtest times the square root of (static minus 20) divided by (static minus residual).
Friction loss
Hazen-Williams in psi per foot: p = 4.52 Q to the 1.85, divided by C to the 1.85 and d to the 4.87, with Q in gpm and d in inches. Use C equal to 120 for wet black steel, 100 for dry steel, and 150 for copper or CPVC.
Worked example
Static 72 psi, residual 48 psi, measured flow 950 gpm. Q20 = 950 times the square root of 52 divided by 24 = 950 times 1.47 = about 1,398 gpm at 20 psi. Compare that to the demand plus a safety margin before you commit to the design.
Traps
- Using static pressure as if it were residual.
- Omitting the hose allowance or placing it at the wrong node.
- Applying the dry-pipe area increase after the calculation instead of before it.
- Accepting a pitot reading below the accuracy threshold or a test that did not stay near the static condition.