Well Pump, Storage, and Fire-Flow Adequacy
A community has one well pump rated 500 gpm and a 250,000-gal elevated tank with 200,000 gal usable storage. Average-day demand is 450,000 gal. On the peak day, demand is 900 gpm for 4 consecutive hours; the rest of the day averages 300 gpm. The state requires the system to maintain 1,000 gpm fire flow for 2 hours plus normal demand, and to keep a 50,000-gal emergency reserve. The pump can run continuously but its motor is 20 years old. The operator must decide whether to add a second pump, increase tank size, or manage demand.
1. Calculate the peak-period drawdown and the storage available above the required reserve.
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Usable storage = 200,000 gal. Required reserve = 50,000 gal, so storage available above reserve = 150,000 gal.
During the 4-hour peak, the pump supplies 500 gpm and demand is 900 gpm, so the tank supplies 400 gpm: 400 gpm × 240 min = 96,000 gal.
After the peak, the tank would contain 200,000 − 96,000 = 104,000 gal, which is 54,000 gal above the 50,000-gal reserve. The peak alone is therefore within usable storage.
2. Calculate whether the tank can meet the required fire flow plus normal demand at the worst time.
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Fire flow plus normal demand = 1,000 + 300 = 1,300 gpm. With the pump at 500 gpm, the tank must supply 800 gpm for 2 hours: 800 gpm × 120 min = 96,000 gal.
The worst normal condition is after the 4-hour peak, when the tank is at 104,000 gal. After the fire flow: 104,000 − 96,000 = 8,000 gal, which is below the required 50,000-gal reserve. So the system fails the fire-flow requirement at the worst time. If the fire occurred with a full tank, 200,000 − 96,000 = 104,000 gal would remain and the reserve would be met, but operators cannot rely on a full tank at the worst moment.
3. What recommendation would you make, and what supporting checks are needed?
Read the worked answer
Recommend additional supply or storage. A second pump of about 400 gpm or more would reduce the fire-flow drawdown: with 900 gpm total pumping, the tank would supply only 400 gpm × 120 min = 48,000 gal, leaving 56,000 gal after the post-peak low level, just above the 50,000-gal reserve. Additional tank capacity would also work. Before choosing, verify the well's sustainable yield, the actual fire-flow requirement, normal demand patterns, tank mixing and turnover, and the condition of the 20-year-old motor. Demand management can reduce normal demand but should not be counted on to solve a fire-flow deficiency.