Short-cycling submersible: waterlogged bladder tank
An 82-year-old farmhouse is served by a 6-inch steel well 280 ft deep, static water level at 62 ft. The 1 HP, 12 gpm submersible is set at 200 ft on 1 1/4-inch poly with a torque arrestor, fed by a 12/3 plus ground drop cable. Above ground sit a 20-gallon bladder tank and a 30/50 pressure switch. The owner complains the pump 'kicks on every few seconds' and the gauge jumps between roughly 32 and 48 psi. With every fixture closed you watch the switch open at 50 psi and close again at 30 psi in about 22 seconds. A clamp meter shows a steady 9.1 A while running, close to nameplate, and the well is known to produce far more than this pump can take. You press the Schrader valve on the tank and get a steady stream of water, not air. Pump and tank were installed together 11 years ago; the tank sits in an unheated crawlspace and shows rust streaks at its base.
1. Diagnose the cycling. What is the most likely cause, and what else must you rule out before you order parts?
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Water out of the Schrader valve means the bladder has ruptured, so the air charge has escaped into the water side and the tank is waterlogged. With no air cushion, the drawdown collapses to almost nothing. Check the numbers: a 20-gallon bladder tank at 30/50 accepts roughly 4 to 5 gallons, and at 12 gpm that is about 22 seconds of run time, which matches exactly what you observed. The motor is not the problem, since amperage is at nameplate.
Before buying anything, rule out the other causes of cycling with no demand: a leaking check valve or foot valve (the gauge would bleed down toward zero after shutoff, and cycling would continue even with a healthy tank), a pressure switch with the differential set too narrow, a fixture or hose leak, and a second check valve or snifter arrangement. Confirm the diagnosis by isolating the tank, draining it, and checking precharge; a ruptured bladder will not hold air.
2. Specify the tank repair, including the exact precharge and the reasoning behind that number.
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Precharge the replacement tank to 28 psi, checked with the tank empty of water and the system at zero pressure. Twenty-eight psi is 2 psi below the 30 psi cut-in: low enough that the pump starts before the diaphragm bottoms out on the tank shell, high enough that the tank is nearly full of water at cut-out so you get maximum drawdown per cycle. Setting it at or above cut-in makes the tank useless at the low end of the range; setting it far below cut-in lets the bladder stretch and fail early.
Also verify the pressure switch is actually opening at 50 and closing at 30 with an accurate gauge, confirm the new tank is rated for potable water (NSF/ANSI 61), and support the tank so the piping is not carrying its weight. If the crawlspace is unheated, discuss freeze protection. After the repair, time a cycle with a fixture open and confirm the run time is longer than before; if the customer still has short cycles, revisit switch settings and tank size.
3. The owner asks whether you can just pump air into the tank through the Schrader valve and skip the cost of a new tank. How do you answer?
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Explain that with a torn bladder the air and water sides are connected, so any air you add migrates to the water side and leaves through the nearest faucet within hours. The pump would return to rapid cycling, and each start puts a heavy current surge through the start winding, burns contactor points, and hammers the thrust bearing. Frequent starts are one of the most common causes of premature submersible motor failure, so a temporary fix risks a much larger bill.
There is also a safety argument: a rusted tank base in a damp crawlspace can rupture or tip. Give the customer the choice in writing between replacing the tank now and accepting the risk, but do not represent an air charge as a repair. If cost is the real driver, price a smaller but correctly sized tank or a cycle-stop device, either of which reduces starts at lower cost than a motor replacement.