Why Is a Deep Well Submersible Pump Delivering Less Water?

Why has the discharge rate of the deep-well submersible pump decreased?

A deep-well submersible pump may continue to operate even if its flow rate is far below expectations. The cause is not necessarily damage to the pump itself; insufficient flow can stem from issues with well conditions, power supply, piping, valves, pump selection, or internal wear.

Troubleshooting should be based on measured data rather than blind component replacement. Record the flow rate, pressure, voltage, current, and dynamic water level, and compare these figures against initial commissioning data and the approved pump performance curve.

Do not judge flow rate solely by the visual appearance of the discharge stream or the time required to fill a tank of unknown volume.

Check the following:

• Flow meter accuracy
• Pressure gauge accuracy
• Pump speed or Variable Frequency Drive (VFD) frequency
• Valve opening position
• Tank volume and fill time
• Units used on the pump performance curve

Pressure readings alone do not indicate pump flow rate. High pressure combined with low flow may indicate a pipeline obstruction, whereas low pressure combined with low flow may signal issues with the water supply, rotation direction, or the pump itself.

The static water level is measured when the well is at rest. Once the pump starts, the water level typically drops to the dynamic water level (i.e., the pumping water level).

If the dynamic water level is lower than expected, the pump faces a higher vertical head. As the required head increases, the flow rate may decrease accordingly.

Possible causes include:

• Seasonal decline in the groundwater table
• Drought
• Pumping activities in the surrounding area
• Clogged well screen
• Excessive pump flow rate
• Reduced aquifer recharge

Measure the water level while the pump is operating at a known flow rate. If the water level continues to drop, reduce the pumping rate and re-evaluate the well's sustainable yield.

Do not lower the pump installation depth without first verifying the well depth, screen location, motor cooling conditions, and the status of the sediment zone.

The pump's actual flow rate is determined by the intersection of its performance curve and the system curve. The actual head may be higher than expected due to the following reasons:

• Low dynamic water level
• High discharge elevation
• High required discharge pressure
• Undersized water delivery pipe (riser)
• Excessive length of surface piping
• Addition of pipe fittings or filters
• Valve in a partially closed position
• Excessive pressure loss across the check valve

Recalculate the total dynamic head based on the current water level, actual internal pipe diameter, and measured flow rate. Do not use the total depth of the well as the pump head.

If the required operating point falls outside the pump's performance curve range, simply changing electrical settings will not resolve the hydraulic mismatch.

Incorrect phase sequence may cause the three-phase motor to rotate in reverse.

A pump rotating in reverse may still deliver water, but flow rate and pressure are typically lower. Current readings may also deviate from normal operating values.

Perform a rotation test using a method approved by the manufacturer. This usually involves comparing flow rate, pressure, and current. If the direction of rotation needs to be changed, disconnect the power supply and have qualified personnel swap any two phase connections.

Never alter electrical connections while the system is energized.

Low voltage reduces motor torque and may prevent the pump from reaching its normal operating speed.

Possible causes include:

• Insufficient cable conductor cross-sectional area
• Excessive cable length
• Loose or damaged connections
• Inadequate transformer or generator capacity
• Supply voltage imbalance
• Incorrect starter wiring

Measure the voltage before startup, during acceleration, and under stable load conditions. A normal no-load reading does not guarantee that the motor will receive sufficient voltage during operation.

For three-phase motors, compare the current in each phase. Electrical testing should be performed by qualified personnel.

Inspect the entire discharge piping system, including:

• Pump check valves
• Conveyance piping (riser pipes)
• Gate valves or butterfly valves
• Elbows and reducers
• Flow meters
• Strainers/filters
• Tank inlets

Valves may be partially closed, installed in the wrong orientation, or stuck. Strainers may be clogged, and sediment buildup can reduce the internal diameter of older pipes.

Check valve springs and internal flow paths can also cause head loss. Even if a valve's nominal size matches the pipe, its actual flow area may be smaller. Where conditions permit, compare pressure readings upstream and downstream of accessible components.

Sand, scale, biological growth, or debris may obstruct water flow into the well or pump.

Warning signs include:

• Gradual decrease in flow rate
• Increased drawdown
• Presence of sand or turbidity
• Unstable pressure
• Fluctuations in motor current

If the pump strainer is clogged, the unit may need to be removed for inspection. If the well screen is clogged, the issue lies with the well itself and should be assessed by a qualified well contractor.

Do not increase the pump speed to compensate for restricted intake; doing so may exacerbate drawdown and lead to sand pumping.

Damaged pipes, loose threaded joints, or failed flanges can result in some the volume of water delivered to the wellhead.

The pump may be operating normally, yet the volume of water delivered to the surface is reduced.

Possible signs include:

• Reduced surface discharge flow
• Abnormal pressure readings
• Gas in the discharged fluid or unstable flow
• Sound of flowing water in the well after shutdown
• Rapid loss of residual pressure

It may be necessary to pull the pump to pressure-test and inspect the discharge piping. The cause of the fault should be confirmed before replacing the pump's hydraulic components.

Sand and prolonged operation can cause wear on impellers, bowls, wear rings, bearings, and bushings. Increased internal clearances lead to internal water recirculation, thereby reducing head and flow rate.

Wear is likely if the system exhibits the following:

• Gradual performance decline
• Increased vibration
• Abnormal noise
• Fluctuations in motor current
• Persistent sand presence
• Recurring bearing issues

The SLAPK troubleshooting guide indicates that impeller and wear ring wear, loose or clogged impellers, and excessive sand accumulation can all cause performance degradation or operational irregularities.

After ruling out electrical, well, and piping issues, inspect the unit to confirm the presence of internal wear.

Follow this sequence to minimize unnecessary pump removal:

1. Check flow meters and pressure gauges.
2. Record voltage and current for each phase.
3. Verify VFD frequency and valve positions.
4. Measure the dynamic water level.
5. Compare the operating point against the pump performance curve.
6. Verify the direction of rotation.
7. Inspect strainers, valves, and accessible piping.
8. Check for well screen clogging or sand issues.
9. Test the discharge piping for leaks.
10. Pull the pump for inspection only when necessary.

Retain all readings alongside the original commissioning records. Trends in data are often more valuable for reference than isolated measurements.

Avoid selecting a higher-power pump without measuring the dynamic water level; avoid increasing the VFD speed without checking the pump's operating limits; avoid loosening protection settings simply to prevent nuisance tripping; avoid assuming pump wear solely based on low flow; and avoid ignoring inaccurate instrument readings.

Yes. A drop in the pumping water level increases the required head and may shift the operating point toward a lower flow rate.

Yes. A three-phase centrifugal pump can still deliver water when rotating in reverse, though flow and pressure are typically reduced.

Not necessarily. Current depends on the pump's performance curve, voltage, rotational speed, and the presence of any faults. A comprehensive analysis should consider both flow and pressure data.

Increasing speed should only be considered after verifying the allowable speed range, well yield, motor load, and system head. Blindly increasing speed can lead to pump overload or further lower the water level.

The pump should be pulled for maintenance when measurements indicate blockage, wear, mechanical damage, or leakage in downhole components (which cannot be inspected from the surface).

Low flow is a systemic symptom and does not necessarily indicate a pump failure. Accurate measurements of flow, pressure, voltage, current, and water level should be taken first.

Before pulling the pump, inspect the well condition, required head, rotation direction, power supply, valves, piping, and discharge column. This approach minimizes unnecessary maintenance and helps identify the root cause more quickly.

Please provide the following information to SLAPK: pump model, design flow and head, measured flow and pressure, water level, installation depth, voltage, current per phase, cable length, and piping details. Our engineers will compare operating data against the performance curves of QJ or SP series pumps to help you identify potential causes of performance degradation.

Contact SLAPK for a deep-well pump performance assessment