1. Problem Description & Scope

This guide addresses symptoms of low flow or no discharge in centrifugal pumps commonly found in industrial applications across the US and UK manufacturing sectors. Affected equipment includes process pumps, boiler feed pumps, and wastewater handling systems. The severity classification is critical, as low or no discharge can lead to process shutdowns, equipment damage, and safety risks. Common root causes include cavitation, impeller wear, air lock, suction problems, and system curve misalignment.

2. Safety Precautions

Lockout/Tagout (LOTO): Always isolate the pump from all energy sources before performing any diagnostic or repair work. Ensure that the pump is de-energized and physically secured.

PPE Requirements: Wear appropriate personal protective equipment including safety glasses, gloves, and non-slip footwear. In environments with hazardous chemicals or high-pressure systems, additional PPE may be required.

Stored Energy: Verify that there is no residual pressure in the pump or system piping before opening any valves. Use pressure relief valves if necessary.

Hazardous Conditions: Avoid working in areas with flammable or explosive substances unless the system is fully depressurized and isolated.

3. Diagnostic Tools Required

4. Initial Assessment Checklist

5. Systematic Diagnosis Flowchart

1. Symptom: No Discharge
   1. Check suction side for blockage or air lock.
   2. Measure suction pressure. If below 0.5 bar, air lock is likely.
   3. Verify pump is running and motor is energized.
   4. If suction pressure is normal, check for cavitation.
2. Symptom: Low Flow
   1. Check for system resistance or restrictions.
   2. Measure discharge pressure and compare with expected values.
   3. If pressure is low, check for impeller wear or misalignment.
   4. Verify pump speed and motor load.
3. Symptom: Unstable Flow
   1. Check for cavitation by measuring vibration and temperature.
   2. Use thermal imaging to locate hot spots.
   3. Measure flow rate and compare with system curve.
   4. Investigate for air ingress or system curve misalignment.
4. Symptom: Excessive Vibration
   1. Use vibration analyzer to measure bearing and shaft vibration.
   2. Check for misalignment or bearing wear.
   3. Inspect impeller for damage or wear.
   4. Verify motor alignment and coupling.

6. Fault-Cause Matrix

7. Root Cause Analysis for Each Fault

1. Air Lock

Why It Happens: Air lock occurs when air enters the suction line, creating a blockage that prevents fluid from entering the pump. This is common in systems with long suction lines, poor piping design, or improper priming procedures.

How to Confirm: Measure suction pressure. If it is below 0.5 bar, air lock is likely. Inspect the suction line for air pockets or leaks.

Damage if Left Unresolved: Air lock can cause the pump to run dry, leading to overheating, bearing damage, and potential motor failure.

2. Cavitation

Why It Happens: Cavitation occurs when the suction pressure drops below the vapor pressure of the fluid, causing vapor bubbles to form

1. Problem Description & Scope

This guide addresses centrifugal pump issues characterized by low flow or no discharge. These symptoms may manifest in various industrial settings including chemical processing, food and beverage, and water treatment. Affected equipment includes centrifugal pumps used in closed-loop and open systems. The severity classification is as follows: Critical — no discharge; Major — significant reduction in flow; Minor — slight flow reduction with no immediate operational impact.

2. Safety Precautions

Lockout/Tagout (LOTO): Ensure all energy sources are disconnected and tagged out before performing any diagnostic or maintenance work on the pump system.
PPE: Use appropriate personal protective equipment including gloves, safety glasses, and a face shield when working with hazardous fluids or high-pressure systems.
Stored Energy: Always verify that the system is fully depressurized and that no residual energy remains in the pump or associated piping.
Hazardous Conditions: Avoid working in environments with flammable or toxic substances without proper ventilation and containment measures.

3. Diagnostic Tools Required

4. Initial Assessment Checklist

5. Systematic Diagnosis Flowchart

Start with the following questions:

1. Is there any flow?
   • Yes: Check for excessive vibration, abnormal noise, or overheating
   • No: Proceed to check for air lock or mechanical blockage
2. Is the pump rotating freely?
   • Yes: Check for cavitation or impeller wear
   • No: Investigate mechanical binding or seized components
3. Is there a pressure drop in the suction line?
   • Yes: Check for clogged filters, blockages, or air pockets
   • No: Proceed to analyze system curve and pump performance
4. Is the pump motor running?
   • Yes: Check for electrical faults or motor overload
   • No: Investigate power supply issues or control system failures

6. Fault-Cause Matrix

7. Root Cause Analysis for Each Fault

7.1 Cavitation

Root Cause: Cavitation occurs when the pressure at the pump inlet drops below the vapor pressure of the liquid, causing vapor bubbles to form and collapse. This leads to noise, vibration, and potential damage to the impeller.

How to Confirm: Measure suction pressure using a manometer. If the pressure is below the vapor pressure of the liquid, cavitation is likely. Also, listen for a high-pitched, metallic sound during operation.

Damage if Unresolved: Cavitation can cause pitting of the impeller, reduce pump efficiency, and lead to premature failure of the pump.

7.2 Impeller Wear

Root Cause: Impeller wear is caused by erosion, corrosion, or mechanical abrasion. This reduces the impeller’s ability to move fluid, leading to low flow or no discharge.

How to Confirm: Use a flow meter to measure the flow rate and compare with the design specifications. Inspect the impeller visually for signs of wear or damage.

Damage if Unresolved: Continued wear can lead to complete impeller failure, requiring costly replacement and downtime.

7.3 Air Lock

Root Cause: Air lock occurs when air becomes trapped in the suction line, preventing fluid from entering the pump. This results in no discharge or intermittent flow.

How to Confirm: Inspect the suction line for air pockets. Use a flow meter to check for flow rate and observe for air bubbles in the suction line.

Damage if Unresolved: Air lock can cause excessive pump vibration, overheating, and potential mechanical failure.

7.4 Suction Blockage

Root Cause: Blockages in the suction line, such as clogged filters or debris, restrict fluid flow into the pump, resulting in low flow or no discharge.

How to Confirm: Use a manometer to check pressure drop in the suction line. Inspect the suction filter and piping for blockages or debris.

Damage if Unresolved: Prolonged blockage can lead to pump overheating, mechanical damage, and potential system failure.

7.5 System Curve Mismatch

Root Cause: System curve mismatch occurs when the pump’s performance curve does not align with the system’s required flow and pressure. This can result in inefficient operation or no discharge.

How to Confirm: Plot the pump performance curve and the system curve on a graph. If they do not intersect, a mismatch exists.

Damage if Unresolved: Inefficient operation leads to increased energy consumption and potential damage to the pump and associated components.

8. Step-by-Step Resolution Procedures

8.1 Cavitation

1. Measure suction pressure using a manometer. If it is below the vapor pressure of the liquid, adjust the suction pressure by increasing the inlet flow or reducing the suction lift.
2. Install a cavitation inhibitor or use a larger suction line to reduce suction lift.
3. Replace the impeller if severe pitting is observed.
4. Verify pump performance after adjustments using a flow meter and pressure gauge.

8.2 Impeller Wear

1. Inspect the impeller for signs of wear or damage. Replace if necessary.
2. Ensure the impeller is properly aligned with the shaft and free of mechanical binding.
3. Reinstall the impeller and test the pump with a flow meter to confirm performance.
4. Monitor pump efficiency regularly to detect early signs of wear.

8.3 Air Lock

1. Drain the suction line to remove trapped air. Use a vent valve or bleed valve if available.
2. Inspect the suction filter and piping for air pockets or leaks.
3. Install a check valve to prevent air from re-entering the system.
4. Verify pump operation and flow rate after air is removed.

8.4 Suction Blockage

1. Inspect the suction filter and piping for blockages. Clean or replace the filter if necessary.
2. Use a manometer to measure pressure drop in the suction line. If significant, check for debris or clogs.
3. Install a strainer or filter to prevent future blockages.
4. Verify pump performance and flow rate after clearing the blockage.

8.5 System Curve Mismatch

1. Plot the pump performance curve and system curve on a graph. Adjust the system to match the pump’s operating range.
2. Install a variable frequency drive (VFD) to control pump speed and match the system requirements.
3. Adjust pump settings or replace with a higher-capacity pump if necessary.
4. Verify system performance and pump efficiency after adjustments.

9. Preventive Measures

10. Spare Parts & Components

For spare parts and components, visit our e-catalog: https://www.unitecd.com/e-catalog/

11. References

Standards:

• ANSI/ASME B73.1-2011 – Centrifugal Pumps
• NFPA 70 – National Electrical Code (NEC)
• IEEE 1547 – Distributed Energy Resources Interconnection
• ISO 9906 – Centrifugal Pumps – Hydraulic Performance

OEM Manuals:

• Atlas Copco Pump Manual
• Grundfos Pump Operation Guide
• ITT Goulds Pump Specifications

Related UNITEC Maintenance Guides:

• Centrifugal Pump Vibration Analysis
• Pump Shaft Alignment and Coupling Maintenance
• Centrifugal Pump Sealing and Leak Prevention