Multiple Check Valves, Tank Sizing, and Water Hammer
A 4-inch, 20 gpm submersible pump is set at 220 ft in a 6-inch steel well. The system uses a 40/60 pressure switch, an 82-gallon bladder tank with 38 psi precharge, 1-1/4-inch Schedule 80 drop pipe, and a pitless adapter. The installer left the pump's built-in check valve in place, added a second check valve 25 ft above the pump, and installed a third check valve at the tank inlet. The homeowner reports a sharp hydraulic bang whenever the pump stops and says the tank bladder failed after 18 months. Static water level is 90 ft; pumping level at 20 gpm is 150 ft. The pump draws 9.5 A at 240 V and the nameplate is 10.0 A.
1. Evaluate the check-valve arrangement. What is causing the water hammer, and what would you change?
Read the worked answer
The pump's built-in check valve is the only one needed at the pump. The second check valve, 25 ft above the pump, creates a trapped pressurized section between two checks. When the pump stops, that trapped section can unload suddenly, producing the bang and repeatedly stressing the drop pipe and tank. The third check valve at the tank inlet is worse: it can isolate the tank from the system, so the tank cannot absorb the pump's shutoff surge or supply water immediately after shutdown. That promotes rapid pressure swings and short cycling. Remove the two added check valves, keep a single check at the pump, and confirm the tank is properly connected with no isolating valve or check at its inlet. Add a pressure-relief valve where required and verify the tank precharge is 2 psi below cut-in pressure.
2. Calculate the tank drawdown and cycle time at 20 gpm. Is the 82-gallon tank adequate?
Read the worked answer
Use absolute pressures: precharge = 38 + 14.7 = 52.7 psia; cut-in = 40 + 14.7 = 54.7 psia; cut-out = 60 + 14.7 = 74.7 psia. For a bladder tank, drawdown ≈ V × P0 × (1/P1 − 1/P2). Drawdown ≈ 82 × 52.7 × (1/54.7 − 1/74.7) ≈ 21 gallons. At 20 gpm, cycle time = 21 ÷ 20 × 60 ≈ 63 seconds. That is marginal; a common target is at least 1–2 minutes between starts. With the tank-inlet check valve restricting tank supply, the effective drawdown would be even less. Recommend a larger tank or a cycle-stop valve, and correct the piping before replacing the bladder.
3. The pump is set at 220 ft with a pumping level of 150 ft. What component or setting would you verify to protect the pump from running dry or cycling, and why?
Read the worked answer
Verify submergence and low-water protection. At 150 ft pumping level, the pump has about 70 ft of submergence, which is generally acceptable, but the well may draw down further during irrigation or drought. A low-water cutoff or motor-protection device should stop the pump if the level approaches the intake. Also verify that the pressure switch is not causing rapid cycling and that the tank precharge matches the switch settings. If the well yield is close to pump capacity, a cycle-stop valve or a smaller pump may be needed so the pump runs continuously at low flow rather than cycling. The 9.5 A draw at 240 V is close to the 10.0 A nameplate, so check voltage and motor protection as well.