Diagnosis and Resolution Guide: Low Flow or No Discharge in Industrial Centrifugal Pumps

1. Problem Description and Scope

This guide addresses the critical symptoms of low flow or complete absence of discharge in industrial centrifugal pumps, failures that can lead to production interruption, serious equipment damage and high maintenance costs. The goal is to provide a systematic methodology for maintenance technicians and reliability engineers to identify the root cause and apply the appropriate correction. Centrifugal pumps are essential components in several sectors such as chemical, petrochemical, sanitation, food and manufacturing in general, with their performance failure being an indicator of a serious problem that requires immediate attention.

The most common symptoms include:

• Flow rate significantly below specified.
• Reduced discharge pressure.
• Complete absence of flow.
• Abnormal noises, such as "gravel" or "bubbles".
• Excessive vibration.
• Overheating of the motor or pump casing.

This guide focuses on the most frequent and complex causes, such as cavitation, impeller wear, air lock, suction line problems and system curve mismatches. The methodology presented allows classifying the severity of the problem as:

• Critical: Total pump stop, imminent risk of severe damage or plant shutdown. Requires immediate intervention.
• Major: Pump operation with very reduced performance, affecting process efficiency. Requires planned and urgent correction.
• Minor: Performance slightly below expectations, with the potential to worsen. Requires monitoring and remediation planning.

2. Safety Precautions

DANGER: BEFORE ANY INTERVENTION ON THE PUMP OR SYSTEM, IT IS ABSOLUTELY CRITICAL TO IMPLEMENT LOCKOUT AND TAGOUT PROCEDURES (LOTO - LOCKOUT/TAGOUT) IN ACCORDANCE WITH STANDARDS NR-10 AND NR-12. CHECK THE ABSENCE OF ELECTRICAL ENERGY AND RELEASE ANY ENERGY STORED IN THE SYSTEM (RESIDUAL PRESSURE, VOLTAGE ON CAPACITORS ON THE ENGINE PANEL). THE FLUID MAY BE HOT, UNDER HIGH PRESSURE, CORROSIVE OR TOXIC. ALWAYS USE PERSONAL PROTECTIVE EQUIPMENT (PPE) APPROPRIATE FOR THE FLUID AND WORK ENVIRONMENT, INCLUDING SAFETY GLASSES, CHEMICAL OR MECHANICAL PROTECTIVE GLOVES, HEARING PROTECTION AND SAFETY FOOTWEAR. ENSURE THAT THE PLACE IS VENTILATED AND FREE OF FLAMMABLE OR TOXIC GASES BEFORE OPENING ANY COMPONENT OF THE SYSTEM.

3. Required Diagnostic Tools

4. Initial Assessment Checklist

Before beginning any detailed diagnosis, perform an initial assessment to collect critical data and identify clear information.

5. Systematic Diagnosis Flowchart

1. Initial Symptom: Low Flow or Absence of Discharge
   1. Pump does not discharge ANYTHING (zero flow) or extremely low flow?
      1. Electrical and Drive Check:
         1. The motor is spinning?
            • If NO: Investigate electrical fault in the motor, fuses, circuit breakers, thermal relay, frequency inverter.
            • If YES: Continue.
         2. Is the direction of rotation correct? (Check the arrow on the pump housing).
            • If NO: Invert two phases when connecting the three-phase motor or consult the inverter manual.
            • If YES: Continue.
      2. Basic Hydraulic Check:
         1. Are the suction and discharge valves completely open?
            • If NO: Open valves. Test. If the problem persists, inspect valves for obstructions or mechanical failure.
            • If YES: Continue.
         2. Is the pump primed? (Are the pump body and suction line full of fluid and free of air?)
            • If NO: Prime. Purge all air. Test. If the problem persists, suspect a leak in the suction line or inability to prime.
            • If YES: Continue.
      3. Pressure Measurement:
         1. Measure suction pressure with a vacuum gauge:
            • If the vacuum is too high (close to -0.8 bar): Serious suction problems (obstruction, excessive suction height, low tank level, severe cavitation). Go to Subsection 5.1.a.iv.1.
            • If low vacuum or positive pressure close to zero: Pump is not "pulling". Suspect air lock or impeller problem. Go to Subsection 5.1.a.iii.1.
         2. Measure discharge pressure with a manometer:
            • If zero or very low pressure: The pump is not generating pressure or there is a complete obstruction in the discharge. Go to Subsection 5.1.a.iii.2.
            • If normal pressure but no flow: Complete obstruction in the discharge line after the measuring point or valve closed.
      4. Investigating Common Problems:
         1. Cavitation:
            • Symptoms: "Gravel" or "bubble" noise, vibration, reduced pressure and flow, overheating.
            • Diagnostic Test: Check NPSHa. If NPSHa < NPSHr, then it is cavitation.
            • Action: Go to Section 7 (Root Cause - Cavitation).
         2. Air Lock:
            • Symptoms: Pump runs "dry", air suction noise, lack of pressure.
            • Diagnostic Test: Inadequate priming or suction leak (soapy water, smoke).
            • Action: Go to Section 7 (Root Cause - Air Lock).
         3. Impeller Wear or Blockage:
            • Symptoms: Gradual drop in performance, abnormal noise, vibration.
            • Diagnostic Test: Disassemble and inspect impeller.
            • Action: Go to Section 7 (Root Cause - Impeller Wear/Blockage).
         4. Problems in the Suction Line:
            • Symptoms: Excessive vacuum in suction, noise, cavitation.
            • Diagnostic Test: Measure vacuum, inspect filter, suction height.
            • Action: Go to Section 7 (Root Cause - Suction Problems).

   6. Failure-Cause Matrix

   Symptom	Probable Causes (Probability)	Diagnostic Test	Expected Result if Cause Confirmed
   Pump doesn't discharge anything.	1. Air Lock (High) 2. Discharge valve closed or obstructed (High) 3. Priming failure (Medium) 4. Incorrect engine speed (Average) 5. Impeller completely blocked/damaged (Low)	1. Check fluid level and bleed air. Vacuum gauge on suction. 2. Inspect valve position and obstructions. 3. Try priming and purging air. 4. Measure engine RPM with tachometer. Check rotation direction. 5. Disassemble and inspect impeller.	1. Lack of flow, "dry" pump, low suction vacuum. 2. Discharge pressure increases rapidly, but without flow. 3. Absence of flow, air suction noise. 4. RPM too low or reverse direction. 5. Impeller visibly clogged or broken.
   Very low discharge flow.	1. Cavitation (High) 2. Impeller wear (High) 3. Restriction in the suction or discharge line (Medium) 4. Suction problems (e.g. high suction height, low level) (Medium) 5. System Curve Misfit (Low) 6. Internal leak in pump or seal (Medium)	1. Measure pressures, temperature. Stethoscope. Check NPSHa. 2. Measure vibration. Disassemble for visual inspection of the impeller. 3. Measure suction and discharge pressures. Check filters, valves. 4. Measure suction vacuum. Inspect suction line. 5. Compare operational data with pump and system curve. 6. Inspect mechanical seal, measure sealing flow (if applicable).	1. Gravel noise, vibration, low suction pressure. 2. Excessive vibration, loss of efficiency, visible wear. 3. Large pressure drop at the restriction point. 4. High suction vacuum, difficulty in priming. 5. Operating point outside the best efficiency curve. 6. Visible leak, noise or overheating in the seal.
   "Gravel" or "bubbling" noise.	1. Cavitation (High) 2. Air entrained in suction (Medium) 3. Impeller with foreign object (Low)	1. Measure pressures, fluid temperature. Stethoscope. Check NPSHa. 2. Inspect suction line for air leaks. Check priming. 3. Disassemble and inspect impeller with endoscope or visually.	1. Low suction pressure, vibration, heated fluid. 2. Foam formation in the fluid, difficulty in priming. 3. Metallic noise, impeller damage.
   Excessive vibration.	1. Cavitation (High) 2. Damaged or unbalanced impeller (Medium) 3. Coupling misalignment (Medium) 4. Damaged bearings (Average)	1. Measure pressures. Stethoscope. 2. Vibration analysis (FFT) to identify impeller/balance failure frequencies. Disassemble. 3. Use laser alignment kit. 4. Vibration analysis (FFT) to identify bearing failure frequencies.	1. Gravel noise, low pressure. 2. Vibration spikes in rotation frequency, visible damage to the impeller. 3. Angular or parallel misalignment above 0.05 mm. 4. Characteristic noise, heating, vibration peaks at rolling frequencies.
   Excessive heating (housing, seal, bearings).	1. Operation without flow or very low flow (High) 2. Cavitation (Medium) 3. Impeller blocked or with friction (Medium) 4. Damaged bearings (Average) 5. Mechanical seal failing (Average)	1. Check discharge flow. Inspect discharge line. 2. Measure pressures, temperatures. Stethoscope. 3. Measure motor current. Disassemble. 4. Infrared thermometer. Vibration analysis. 5. Inspect seal, check leaks, temperature.	1. Pump operating at or very close to the shut-off curve. 2. Gravel noise, low suction pressure. 3. High motor current, evidence of friction. 4. Temperature above 80°C in bearings, vibration. 5. Fluid leak, seal temperature > 90°C.
   Abnormal motor current consumption (too high or too low).	1. Obstruction/friction in impeller (High) 2. Severe cavitation (Low) 3. Corroded/worn impeller (Low) 4. Coupling failure/misalignment (Medium) 5. Damaged motor/pump bearings (Medium)	1. Measure electrical current. Disassemble and inspect impeller. 2. Measure electrical current. Check noise, suction pressure. 3. Measure electrical current. Disassemble and inspect impeller. 4. Measure electrical current. Align coupling. 5. Measure electrical current. Vibration analysis.	1. High current. Impeller blocked or chafing. 2. Low or unstable current. Gravel noise. 3. Low current. Impeller with visible wear. 4. High current, vibration. Misalignment > 0.05 mm. 5. High current, heating, vibration.

   7. Root Cause Analysis for Each Failure

   7.1. Cavitation

   Why it happens: Cavitation occurs when the absolute pressure at the pump suction (NPSHa - Net Positive Suction Head available) falls below the vapor pressure of the liquid at the pumping temperature, or below that required by the pump (NPSHr - Net Positive Suction Head required). This causes the formation of steam bubbles which, upon reaching a higher pressure zone in the impeller, implode violently. This implosion generates localized shock waves that can exceed 7000 bar.

   How to confirm: The most characteristic symptom is a noise similar to "gravel" or "exploding bubbles" inside the pump. Other indicators include excessive vibration (especially at higher frequencies), drops in flow and pressure, increased fluid temperatures and, in advanced cases, pitting (cavitation erosion) on the surface of the impeller and casing. The diagnosis is confirmed by calculating the NPSHa and comparing it with the NPSHr provided by the manufacturer. If NPSHa < NPSHr, cavitation is inevitable.

   Damage if left unresolved: Cavitation is extremely destructive. Bubble implosions cause:

   • Erosion (Pitting): Significant superficial damage to the impeller, casing and mechanical seal, drastically reducing useful life.
   • Loss of Efficiency: The formation and collapse of bubbles disturbs the flow, reducing pumping capacity.
   • Excessive Vibration and Noise: Lead to premature failure of bearings, mechanical seals and other components.
   • Structural Damage: In extreme cases, cavitation can compromise the integrity of the pump casing.

   7.2. Impeller Wear or Blockage

   Why it happens:

   • Wear: Results from abrasion by solid particles in the fluid (e.g. sand, debris), corrosion by chemically aggressive fluids or erosion due to prolonged cavitation. Wear of the vanes and the gap between the impeller and casing (or wear plate) reduces the pump's ability to generate pressure and flow.
   • Blockage: Occurs when foreign objects (fibers, plastics, large solids) enter the pump and become trapped between the impeller vanes or between the impeller and the casing.

   How to confirm:

   • Wear: A gradual decline in performance (flow and pressure) over time. Internal visual inspection (via endoscope or after disassembly) will reveal thin reeds, rounded edges and increased gaps. Vibration analysis may indicate unbalance if wear is uneven.
   • Blocking: A sudden and drastic drop in performance, accompanied by abnormal noise (scratching, scraping) and sometimes increased motor current (if the motor is overloaded trying to turn the blocked impeller) or circuit breaker tripping.

   Damage if unresolved:

   • Wear: Progressive loss of efficiency and capacity, increased energy consumption, overheating of the fluid and pump due to recirculation.
   • Blocking: Overheating, damage to the shaft, mechanical seal, bearings and, in serious cases, the engine. It can lead to impeller or casing breakage.

   7.3. Air Lock

   Why it happens: An air lock occurs when there is a significant amount of air or gas trapped inside the pump or suction line, preventing fluid flow. Centrifugal pumps are not designed to pump gases efficiently. This may be caused by:

   • Inadequate Priming: Pump was not completely filled with liquid before starting.
   • Suction Line Leak: Air is drawn into the line through loose connections, poorly sealed flanges or cracks.
   • Gases Released from the Fluid: The liquid can release gases (e.g. CO2 in carbonated drinks, putrefactive gases in sewage) or have dissolved air that separates at low suction pressure.

   How to confirm: The pump works, the motor rotates, but there is no discharge flow or the discharge pressure is zero. The pump may operate "dry", generating abnormal noises (sucking air) and heating up quickly, especially the mechanical seal. Checking the fluid level in the pump body (if there is a sight glass or bleed plug) will reveal the presence of air.

   Damage if left unresolved: Dry operation can quickly damage the mechanical seal (due to lack of lubrication and cooling) and overheat the bearings. In pumps with seals that rely on fluid for cooling, failure is imminent.

   7.4. Suction Line Problems

   Why it happens: The suction line is the most critical point for the pump's performance, as any restriction or adverse condition directly affects the NPSHa, which can lead to cavitation. Common problems include:

   • Excessive Suction Height: The vertical distance between the liquid level in the tank and the center of the impeller exceeds the design limit.
   • Inadequate Suction Pipe Diameter: Piping that is too small generates high speed and large pressure losses due to friction.
   • Clogging of Filters or Sieves: Reduces the flow area, increasing pressure loss.
   • Valve Restriction: Valves partially closed or clogged in the suction line.
   • Low Suction Tank Level: Reduces available NPSHa by exposing the suction inlet to air.
   • Vortices in the Suction Tank: Vortices form that draw air into the suction line when the fluid level is low or the inlet is poorly designed.

   How to confirm: A vacuum gauge in the suction line will indicate an excessively high vacuum (close to the theoretical maximum, -1 bar or -30 inHg). Visual inspection will reveal blockages in filters, the position of valves, the tank level and the presence of vortices. Calculating pressure losses in the suction line will confirm whether there is an inappropriate restriction on the pipe diameter.

   Damage if not resolved: All suction problems leading to insufficient NPSHa result in cavitation, with the associated damage described in section 7.1. Furthermore, they can cause operational instability and noise.

   7.5. Misadjustment or Change in the System Curve

   Why it happens: The system curve represents the total head loss (static and dynamic) that the fluid encounters as it travels through the piping from suction to discharge. The pump operates at the point of intersection between its characteristic curve and the system curve. A mismatch occurs when:

   • Physical Changes: Modifications to the piping (diameter, length, number of bends, valves), installation of new equipment downstream or upstream, increase in static height.
   • Process Changes: Change in the desired flow rate, in the viscosity or density of the fluid, in the pressure of the discharge tank.
   • Incorrect Valves: Control valves operating outside their ideal range, check valves stuck or obstructed.

   How to confirm: The measured operating point (flow and pressure) differs significantly from the pump's Best Efficiency Point (BEP). Calculating the pressure losses in the system under the new conditions will confirm the change in the curve. Measuring pressures at strategic points in the pipeline will help identify abnormal pressure losses.

   Damage if left unaddressed: Operating the pump outside of the BEP results in:

   • Low Energy Efficiency: Excessive consumption of electrical energy.
   • Reduced Service Life: Increased vibration, noise, heating, and higher failure rate of seals and bearings.
   • Cavitation or Internal Recirculation: Operating too far to the left (low flow) or too far to the right (high flow) of the BEP can induce internal flow and cavitation problems.

   8. Step-by-Step Resolution Procedures

   DANGER: WE REITERATE THE IMPORTANCE OF THE LOTOT PROCEDURE AND THE PROPER USE OF PPE BEFORE CARRYING OUT ANY PHYSICAL INTERVENTION ON THE EQUIPMENT.

   8.1. Resolution for Air Lock

   1. Isolation and Priming:
      • Close the pump discharge valve.
      • Fully open the suction valve.
      • Open the air bleed plug on the pump volute (highest point) or on the suction pipe closest to the pump.
      • Fill the pump body slowly with fluid until a steady, bubble-free stream comes out of the bleed plug.
      • Close the purge plug.
      • Slowly open the discharge valve, monitoring pressure and flow.
   2. Checking for Suction Leaks:
      • With the pump stopped and the suction line pressurized (if possible) or under vacuum (with the pump running), apply soap and water solution to the connections, flanges, plugs and housing. Bubbles will indicate leakage.
      • Tighten connections, replace gaskets or repair cracks.

   8.2. Resolution for Cavitation

   1. NPSHa Increase Available:
      • Increase Suction Tank Level: If the level is low, refill the tank.
      • Reduce Pressure Losses in Suction: Inspect and clean filters, screens and valves in the suction line. Check that the valves are fully open.
      • Reduce Suction Line Length: If possible, optimize the piping layout.
      • Increase Suction Line Diameter: If frictional pressure losses are excessive, consider increasing the pipe diameter.
      • Reduce Fluid Temperature: If applicable, cool the fluid to lower its vapor pressure.
      • Reduce Pump Speed: If it is controlled by a frequency inverter, reducing the speed may help (pay attention to the system curve).
   2. Post-Adjustment Check: Monitor noise, vibration, pressure and flow to confirm elimination of cavitation.

   8.3. Resolution for Impeller Wear or Blockage

   1. Disassembly and Inspection (LOTO CRITICAL):
      • Isolate the pump and apply LOTO.
      • Drain the fluid from the pump and system.
      • Disconnect the pump motor.
      • Remove the volute and front cover to access the impeller.
      • Visually inspect the impeller and housing.
   2. Cleaning (if blocked):
      • Carefully remove foreign objects stuck to the impeller.
      • Check for damage after removal.
   3. Replacement (if worn/damaged):
      • Remove damaged impeller.
      • Install a new impeller (see UNITEC E-Catalog for the correct part).
      • Check and adjust the clearance between the impeller and the casing/wear plate according to the manufacturer's specifications (typically 0.25 mm to 0.5 mm).
      • Rebalance the impeller, if the model allows and is necessary (standard ISO 1940-1, balancing degree G2.5).
      • Assemble the pump, applying tightening torques according to the manufacturer's manual (e.g. volute screws, flanges).
   4. Check: After reassembly, prime the pump and start, monitoring vibration, noise, pressure and flow.

   8.4. Resolution for Suction Line Problems

   1. Physical Inspection:
      • Check filters, sieves and valves in the suction line for clogging and position (fully open). Clean or repair as necessary.
      • Confirm adequate suction tank level. If recurring, evaluate level control automation.
      • Inspect the suction inlet into the tank to avoid the formation of vortices (install an anti-vortex plate, if applicable).
      • Measure the suction height and compare with the project. If it exceeds, reassess the pump installation point.
   2. Calculation and Optimization: Recalculate pressure losses in the suction line with operating conditions and existing pipe diameters. If losses are excessive, an upgrade of the pipe diameter may be necessary.

   8.5. Resolution for System Curve Misfit

   1. System Reassessment:
      • Review the P&ID (Piping and Instrumentation Diagram) to identify undocumented changes.
      • Measure pressures and flows at strategic points in the system to identify where pressure losses are occurring.
      • Check the condition and opening of all control and block valves.
   2. Optimization:
      • Adjust control valves to operate within the optimal range.
      • Consider replacing block valves with types with lower pressure loss (e.g. ball valve instead of globe valve, if applicable).
      • Calculate and, if feasible, modify the piping to reduce pressure losses.
      • If the system curve has changed permanently, it may be necessary to replace the pump impeller or even the pump with one with a characteristic curve more suited to the new process conditions.

   9. Preventive Measures

   Root Cause	Prevention Strategy	Monitoring Method	Recommended Range
   Cavitation	Ensure NPSHa > NPSHr in all operating conditions. Design system with safety margin. Train operators on tank level limits.	Continuous monitoring of suction pressure, fluid temperature and tank level. Vibration analysis.	Daily (operational), Monthly (vibration), Annual (system reassessment).
   Impeller Wear	Adequate fluid filtration. Selection of abrasion/corrosion resistant impeller materials. Optimization of pump BEP.	Vibration analysis (for unbalanced wear). Internal visual inspection via endoscope or during scheduled shutdowns. Performance analysis (flow/pressure).	Quarterly (vibration/performance), Annual (visual inspection).
   Air Lock	Strict priming operating procedures. Preventative maintenance of suction line seals and flanges. Installation of automatic purge valves.	Operational training. Visual inspection of leaks. Priming capacity monitoring.	Monthly (inspection), Semiannual (training/procedures).
   Suction Line Problems	Periodic cleaning of filters and sieves. Valve maintenance. Checking the minimum tank level. Correct piping sizing.	Visual inspection and cleaning of filters (based on Delta P). Vacuum monitoring in suction.	Weekly/Monthly (filters), Semiannual (validation of the suction line).
   System Curve Misfit	Reassessment of the hydraulic system after any change to the process or piping. Updated and accurate P&ID.	Continuous monitoring of flow and pressures (suction/discharge). Comparison with the pump curve.	Annually or after each significant process/piping modification.

   10. Spare Parts and Components

   It is essential to have a stock of critical parts to minimize downtime in the event of a failure. Consult the UNITEC E-Catalog to find the exact parts for your pump model.

   Part Description	UNITEC Specification (Example)	When to Replace	UNITEC Category
   Impeller	Material (316 Stainless Steel, Bronze), Diameter, Number of Reeds.	Wear > 10% of original thickness, visible cavitation damage, irrecoverable unbalance.	Hydraulic Internal Components
   Mechanical Seal	Type (Cartridge, Component), Face Material (SiC/SiC, Carbon/Ceramic), Shaft Diameter.	Excessive visible leakage, overheating, noise, integrity failure.	Sealing and Sealing
   Bearings	Type (Angular Contact Balls, Cylindrical Rollers), Series (6206, 6308), Brand (SKF, FAG, NSK).	Excessive noise, heating (>80°C), abnormal vibration (peaks in bearing frequencies), excessive play.	Transmission and Rotation
   Joints and Gaskets	Material (EPDM, Viton, PTFE), Dimensions (Diameter, Thickness).	Deformation, hardening, leakage after tightening, as part of disassembly/reassembly.	Sealing and Sealing
   Shaft Protective Sleeve	Material (Stainless Steel), Dimensions (Inner/Outer Diameter, Length).	Wear > 0.5 mm, grooves, corrosion.	Hydraulic Internal Components
   Wear Rings	Material (Cast Iron, Bronze), Diameter, Thickness.	Clearance between ring and impeller exceeds manufacturer's limit (e.g. >0.5 mm).	Hydraulic Internal Components
   O-rings	Material (Nitrile, Viton), Dimensions (CS, ID).	Deformation, hardening, leakage, during each disassembly.	Sealing and Sealing
   Coupling	Type (Flexible, Rigid), Size, Material (Rubber, Polyurethane).	Visible damage, cracks, distortion, excessive wear of flexible elements.	Transmission and Rotation

   For consultation and purchase of parts, visit: https://www.unitecd.com/e-catalog/

   11. References

   • ABNT NBR 14611: Centrifugal pumps – Terminology.
   • ABNT NBR 15510: Hydraulic pumps – Performance test.
   • NR-10: Safety in Electrical Installations and Services.
   • NR-12: Workplace Safety in Machines and Equipment.
   • Operation and Maintenance Manuals (OMM) from pump manufacturers.
   • UNITEC Maintenance Guides related to hydraulic and rotating systems.