Short Cycling, Tank Drawdown, and Pump Setting
A 1.5 hp, 10 gpm submersible pump is set at 240 ft in a 320-ft, 6-inch well. Static water level is 80 ft; pumping water level after 30 minutes is 210 ft and still falling slowly. The 82-gal bladder tank has a 30/50 pressure switch and a precharge of 28 psi. The owner reports the pump cycles about every 45 seconds whenever a 4 gpm lawn sprinkler runs; the pump was replaced twice in three years. A pump installer proposes simply replacing the 1.5 hp pump with a 2 hp unit and adding a 20-gal tank. The well driller's log shows a 12 gpm yield at completion, with 40 ft of casing and fractures at 150 and 260 ft. The owner wants to irrigate 0.25 acre and fill a 1,500-gal storage tank.
1. Calculate the expected cycle time and diagnose the short cycling. Is a larger pump the right fix?
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Expected cycle time with a properly functioning 82-gal tank at 30/50 is roughly 20 gal drawdown divided by net fill rate. With a 10 gpm pump and 4 gpm sprinkler, net fill is 6 gpm, so about 3.3 min. Forty-five seconds implies an effective drawdown of only ~4.5 gal, so the tank is waterlogged, the bladder is torn, the precharge is wrong, or the tank is isolated. Check precharge with the tank drained (should be about 28 psi for a 30-psi cut-in), verify the bladder and valve, and measure actual drawdown. A 2 hp pump is the wrong fix: it will cycle faster, draw the water level lower, and increase the risk of running dry. The pump is already at 240 ft with a pumping level of 210 ft - only 30 ft submergence.
2. Recommend a pump setting, control strategy, and storage for the irrigation and tank-filling use.
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Keep a 10 gpm pump or verify with a test; do not upsize. Set the pump lower only if the well can support it - 280 ft gives more submergence but still requires a low-water cutoff because the pumping level was still falling at 210 ft. For irrigation, decouple the well from the sprinkler: fill the 1,500-gal storage tank at the well’s sustainable rate and use a booster pump for the 4 gpm sprinkler. Add a cycle stop valve or a larger tank (e.g., 119-gal) and a low-water protection device. The storage tank provides 1,500 gal, enough to buffer peak demand and let the well recover. Record the pumping level and specific capacity to set the cutoff.
3. What test would you run before changing equipment, and what would you record?
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Before changing equipment, run a 24-hour constant-rate test at 10 gpm with recovery, and measure the tank’s actual drawdown and precharge. Record static level, pumping level at 1, 10, 60, and 240 min, recovery to 90% within a set time, specific capacity, and the pressure switch cut-in/cut-out. Compare with the driller’s 12 gpm completion yield. If specific capacity is low or the pumping level approaches the pump intake, reduce the pumping rate and rely on storage. This distinguishes a tank/control problem from a well-yield problem.