Sizing a Submersible for Peak Demand and Limited Well Yield
A 6-inch well has a static water level of 50 ft and yields 15 gpm. At 12 gpm the pumping level stabilizes at 130 ft. The pump is set at 200 ft on 1-1/4-inch polyethylene drop pipe, 350 ft long, discharging into a pressure tank 15 ft above grade. The pressure switch is 40/60 psi. Peak demand is 14 gpm: 6 gpm for the house and 8 gpm for irrigation. The homeowner wants to run both at once. The existing 1 hp pump is rated 12 gpm at 250 ft. Friction loss in the 1-1/4-inch drop pipe is about 3 ft per 100 ft at 14 gpm, and fittings add about 3 ft.
1. Compute the total dynamic head at 14 gpm and decide whether the existing pump is suitable.
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At 14 gpm, use the 130-ft pumping level as a lower bound, but allow roughly 10 ft additional drawdown, so lift ≈ 140 ft to the pump level. Add the vertical rise to the tank: 140 + 15 = 155 ft. Pressure head at 60 psi cut-out = 60 × 2.31 = 138.6 ft. Friction = 350/100 × 3 = 10.5 ft, plus 3 ft fittings = 13.5 ft. TDH ≈ 155 + 138.6 + 13.5 ≈ 307 ft. The existing pump is rated 12 gpm at 250 ft, so at 14 gpm it will fall well short of 307 ft and will operate off the curve, delivering less flow and lower pressure. It is not suitable for simultaneous house and irrigation demand.
2. Evaluate the pump setting depth. Is 200 ft enough submergence, and what would you recommend?
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At 14 gpm the pumping level may be 140–150 ft, leaving only 50–60 ft of submergence above the pump intake. That is usually enough for normal operation, but it leaves little margin for seasonal decline, additional drawdown, or a lower-yield test. The well yields 15 gpm, and the peak demand is 14 gpm, so the pump could pull the water level down close to the pump if the well performance degrades. I would recommend setting the pump deeper, such as 220–240 ft if the well allows, while maintaining at least 10–20 ft above the bottom, and adding low-water protection. The pump should not be sized to match the well yield exactly; a slightly smaller pump plus storage is safer.
3. Compare 1-inch and 1-1/4-inch drop pipe at 14 gpm. What is the practical effect on head and velocity?
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At 14 gpm, 1-inch pipe has a much higher velocity and friction loss than 1-1/4-inch pipe. Approximate velocities are about 5 ft/s in 1-inch and about 3 ft/s in 1-1/4-inch. Friction in 1-inch pipe could be roughly 8–10 ft per 100 ft, versus about 3 ft per 100 ft in 1-1/4-inch. Over 350 ft, that adds about 20–30 ft of head loss, raising TDH and reducing pressure at the house. The 1-1/4-inch pipe is the better choice and is already installed. If the pump is replaced, confirm the drop-pipe size against the pump manufacturer's recommendation and keep velocity below about 5 ft/s.