Water Main Sizing and Fire Flow in a Looping Distribution System
A new subdivision is served by a 12-in. ductile iron water main (Hazen-Williams C = 130) that loops around a block and feeds 40 homes. Each home is assumed to draw 250 gpd on a peak day, with a peak-hour factor of 2.5. The fire flow requirement from the insurance rating is 1,000 gpm at 20 psi residual for 2 hours. The system's source provides 65 psi at the connection, and the ground is flat. The designer proposes to size the loop at 8 in. to save cost, arguing that the 12-in. main is oversized because the average demand is only about 70 gpm.
1. Compute the peak-hour domestic demand and compare it to the fire flow.
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Average day demand = 40 homes × 250 gpd = 10,000 gpd = 10,000 / 1,440 = 6.94 gpm. Peak hour = 6.94 × 2.5 = 17.4 gpm. That is tiny compared to the 1,000-gpm fire flow - roughly 1.7% of it. So the designer is correct that the domestic demand is small, but the sizing driver is fire flow, not average demand. A system sized for 70 gpm (or 17 gpm) would be grossly undersized for a 1,000-gpm fire. The 12-in. main is not oversized; it is likely sized to deliver fire flow with acceptable head loss and residual pressure.
2. Estimate whether an 8-in. loop can deliver 1,000 gpm at 20 psi residual, given 65 psi at the source.
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Use Hazen-Williams head loss: hL = 4.727 L Q^1.852 / (C^1.852 D^4.871) in consistent units, or the common form hL (ft/1000 ft) = 4.727 Q^1.852 / (C^1.852 D^4.871) with Q in cfs and D in ft. Convert 1,000 gpm = 2.228 cfs. For an 8-in. (0.667-ft) pipe, C = 130: compute the friction slope and multiply by the loop length. Even for a short loop (say 1,000 ft), the head loss at 1,000 gpm in 8-in. DI is on the order of tens of psi - well beyond the 45 psi available (65 − 20). For a 12-in. pipe the loss is far smaller. The 8-in. loop will likely fail to deliver 1,000 gpm at 20 psi residual unless the loop is very short and the source pressure is higher. The correct check is a hydraulic network analysis, not a rule of thumb, but the direction of the answer is clear: 8 in. is too small for this fire flow.
3. Explain the role of the loop and why a single-pipe analysis is insufficient.
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In a looped system, flow splits between the two paths around the block, so each leg carries less than the total and head loss is lower than a single pipe of the same length. A single-pipe (series) analysis assumes all 1,000 gpm travels one path, which overstates head loss and would wrongly reject the design. Conversely, sizing only the average-demand case ignores the fire-flow case entirely. The defensible approach is a network analysis (Hardy Cross or a solver) that: (1) applies the fire flow at the hydrant node, (2) enforces continuity at each node, (3) enforces energy balance around each loop, and (4) checks that residual pressure at the fire-flow node is ≥ 20 psi. The loop is a benefit, but it does not rescue an undersized pipe - both legs must still carry the split flow within the pressure budget.