Water Supply Analysis and Hydraulics
What this domain tests
This domain tests your ability to turn a flow test into an available supply curve, compare it with system demand, and calculate friction and elevation losses. Expect questions on static/residual pressure, Hazen-Williams, elevation pressure, hose allowance, and safety margin.
Core rules and concepts
- A flow test records static pressure, residual pressure, and test flow. Available flow at a lower pressure is estimated from Q_available = Q_test x (P_available / P_residual)^0.54.
- Hazen-Williams friction loss in psi/ft is p = 4.52 Q^1.85 / (C^1.85 d^4.87), with Q in gpm and d in inches. C is commonly 120 for steel pipe.
- Elevation changes pressure by about 0.433 psi per foot of rise.
- System demand includes sprinkler flow, hose allowance, and any other simultaneous demand. Hose allowance is commonly 100 gpm light hazard, 250 gpm ordinary hazard, and 500 gpm extra hazard.
- A safety margin is not always mandated by NFPA 13, but many AHJs require 10-20 psi between supply and demand.
Worked example
A flow test gives 80 psi static and 60 psi residual at 900 gpm. The proposed system needs 1,200 gpm at 50 psi. Estimate available flow at 50 psi: Q = 900 x (50/60)^0.54 = 900 x 0.906 = about 816 gpm. The supply is inadequate at 50 psi because 816 gpm is below the 1,200 gpm demand. The designer must improve the supply, reduce demand, or add a fire pump or storage. Note that the calculation assumes the test was properly conducted and that the supply curve is valid in that range.
Traps that cost marks
- Using static pressure instead of residual pressure in the flow-test formula.
- Forgetting the 0.433 psi/ft elevation loss for a riser or upper floor.
- Omitting hose allowance from the total demand.
- Applying a Hazen-Williams C value that does not match the pipe material and condition.
- Ignoring the AHJ's required safety margin or the effect of a backflow preventer.
Exam approach
Sketch the supply curve and the demand point. Calculate elevation and friction separately, then add hose allowance. Compare at the same pressure and state whether the margin is positive.