New Three-Phase Pump Delivers Low Flow at Startup
A new 3 hp, 460 V, three-phase submersible pump is installed at 300 ft in a 6-inch well. The pump curve shows 25 gpm at 320 ft of head. At startup, the pump delivers only 8 gpm. The discharge pressure gauge reads 40 psi, and the pumping water level is 180 ft. The drop pipe is 1-1/4-inch, 300 ft long; friction at 8 gpm is about 4 ft. Motor current is 4.6 A per phase, nameplate is 5.2 A; phase-to-phase voltages are 472, 470, and 471 V. The installer did not check phase rotation. The check valve holds.
1. Calculate the actual total dynamic head at the measured condition and compare it with the pump curve. What does the comparison tell you?
Read the worked answer
Pressure head = 40 psi × 2.31 = 92.4 ft. Lift from the pumping level = 180 ft. Friction at 8 gpm = 4 ft. TDH ≈ 180 + 92.4 + 4 = 276 ft. The curve shows about 320 ft at 25 gpm and would be somewhat higher at 8 gpm, perhaps 335 ft. The pump is therefore producing far less head than its curve predicts at that low flow. If the pump were simply operating at a normal point on its curve, 276 ft of head would correspond to a much higher flow, not 8 gpm. This points to a pump that is not generating rated head, such as reverse rotation, a damaged impeller, a restriction, or a closed valve.
2. What is the most likely cause, and how would you verify and correct it?
Read the worked answer
The most likely cause is reverse rotation, because the installer did not check phase rotation. A three-phase motor can run either direction depending on phase sequence. A centrifugal pump running backward may still pump, but it produces less head and flow and draws lower current, which matches the 4.6 A versus 5.2 A nameplate. Verify phase sequence at the starter and at the motor with a phase-sequence meter, or use a rotation tester. Do not rely only on a momentary bump. If rotation is wrong, swap any two of the three motor leads at the starter and recheck. Then restart and measure flow, pressure, and current. If rotation is correct and the condition persists, pull the pump and inspect the impeller and intake.
3. What startup records and checks should be completed before leaving the site?
Read the worked answer
Record static and pumping water levels, drawdown, discharge pressure, flow, and voltage and current on all three phases. Verify phase rotation and amp balance; a large imbalance suggests a voltage or motor problem. Check that the overloads are set to the motor nameplate current and that the starter and disconnect are correctly sized. Confirm the check valve holds, the pressure tank precharge and switch settings match, and the well seal and cable splices are secure. Verify that the pump is not running below minimum submergence and that low-water protection, if installed, is working. Document the final operating point against the pump curve.