Diagnostic Guide: Low or No Discharge in Industrial Centrifugal Pumps

1. Description of the Problem and Scope

This guide addresses the symptoms and diagnostic methodologies for low or no fluid discharge in industrial centrifugal pumping systems. This critical failure can manifest itself as a significant reduction in flow rate, insufficient discharge pressure or complete absence of flow, affecting operational efficiency, product quality and, in extreme cases, causing unplanned production shutdowns.

The affected equipment includes all centrifugal pumps used in sectors such as automotive, aerospace, food, chemical, energy and water treatment. The severity of this problem is classified as:

• Critical: Total production stoppage, risk of major damage to equipment or process.
• Major: Substantial reduction in system performance, non-compliance with process parameters, significant increase in energy consumption.
• Minor: Slight performance deviations, marginal increase in energy consumption, but potential for escalation if not addressed.

Root causes to investigate include cavitation, impeller wear or damage, air lock in the system, suction line problems (clogs, leaks, insufficient NPSHa), and mismatch with the system curve.

2. Safety Precautions

CRITICAL SAFETY WARNING: Before beginning any diagnostic or maintenance procedures on a centrifugal pump, it is essential to implement a strict Lockout/Tagout (LOTO) protocol in accordance with current workplace safety regulations (e.g., OSHA guidelines or EN 1037). Residual electrical, hydraulic or pneumatic energy must be dissipated. Make sure that there is no residual pressure in the pipes and that the engine cannot be started accidentally.

STORED ENERGY HAZARD: Pumps and pipes may contain fluids at high pressure and/or temperature. Uncontrolled release of this energy can cause severe burns, impact injuries, or exposure to hazardous chemicals. Always safely vent and drain the system before handling.

PERSONAL PROTECTIVE EQUIPMENT (PPE): Always use appropriate PPE, including at least: safety glasses (UNE-EN 166), chemical or mechanical protective gloves (UNE-EN 388/374, depending on the fluid), safety footwear (UNE-EN ISO 20345) and hearing protection (UNE-EN 352) in noisy environments. Assess additional hazards such as gases, vapors, or slips.

3. Required Diagnostic Tools

Correct selection and calibration of diagnostic tools is essential for accurate identification of the root cause.

4. Initial Evaluation Checklist

Before starting any deep diagnostics, it is crucial to collect contextual information and observe the behavior of the system under operating conditions. This checklist helps guide the diagnosis.

5. Systematic Diagnostic Flowchart

This flowchart describes a step-by-step approach to isolating the root cause of low or no discharge.

1. Initial Symptom: Low or No Centrifugal Pump Discharge
   1. Does the pump start?
      1. NO:
         • Check motor power supply:
           1. Check main switch, fuses, circuit breakers.
           2. Measure phase voltage at motor terminals with a multimeter. (Expected value: Nominal voltage +/- 5%).
           3. If the voltage is correct, check the motor windings (resistance, insulation).
         • Check mechanical lock:
           1. With LOTO pump, try to rotate the pump shaft manually.
           2. If it is blocked, there may be foreign objects in the impeller or serious bearing failures.
         • Probable Cause: Motor electrical failure, overload, severe mechanical blockage.
      2. YES: Continue to the next step.
   2. Is the pump primed (full of liquid)?
      1. NO:
         • Proceed to prime the pump according to the manufacturer's manual.
         • Check for air leaks in the suction line (check vacuum gauge).
         • Probable Cause: Air lock.
      2. YES: Continue to the next step.
   3. Is there flow in the discharge pipe (even if it is low)?
      1. NO (No Discharge):
         • Check for a completely closed discharge valve.
         • Check if there is a total obstruction in the suction or discharge pipe.
           1. Visual inspection if possible, or use of an endoscope.
         • Verify that the direction of rotation of the motor/pump is correct. (Use strobe tachometer or observe rotation).
         • Probable Cause: Valve closed, severe obstruction, reverse direction of rotation.
      2. YES (Low Discharge): Continue to next step.
   4. Evaluate the Suction Line:
      1. Measure the pressure/vacuum at the suction mouth with a calibrated vacuum gauge.
         • If the vacuum is excessive (> -0.5 bar or -380 mmHg), or if it fluctuates:
         • Inspect suction filter/grid: Clogged?
         • Check liquid level in the suction tank: Too low (insufficient NPSHa)?
         • Inspect suction pipe: Correct diameter? Too long or with too many elbows? Visible/audible air leaks?
         • Check fluid temperature: Too high (reduces NPSHa)?
      2. Probable Cause: NPSHa problems, suction obstruction, air leaks.
   5. Assess Impeller and Pump:
      1. Are there any gravel or knocking noises coming from the pump? Are there excessive vibrations?
         • (Use vibration analyzer. Alarm levels ISO 10816: RMS speed > 4.5 mm/s for large pumps on flexible foundations, > 2.8 mm/s for small rigid pumps.)
         • Inspect impeller internally (if possible with endoscope or after disassembly): Worn, corroded, damaged, or partially blocked by solids?
      2. Probable Cause: Cavitation, worn/damaged impeller, partial internal blockage.
   6. Evaluate the Discharge System and System Curve:
      1. Measure the discharge pressure with a calibrated manometer.
      2. Measure the discharge flow with an ultrasonic flow meter.
      3. Compare the operating point (flow vs. head) with the pump characteristic curve. Is the pump operating too far to the left (low flow) or to the right (high flow) of its BEP (Best Efficiency Point)?
      4. Inspect discharge line: Valves partially closed? Partial obstructions? Incorrect dimensioning (too small, high pressure losses)?
      5. Probable Cause: Operation outside of BEP, restrictive discharge valves/piping, incorrect system curve design.

6. Matrix of Failures and Causes

This table provides a correlation between observed symptoms, probable causes, diagnostic tests, and expected results.

7. Root Cause Analysis for Each Major Failure

7.1. Cavitation

Explanation: Cavitation occurs when the pressure at some point inside the pump falls below the vapor pressure of the liquid at that temperature. This causes the formation of vapor bubbles that violently implode as they move to areas of higher pressure, generating microjets and shock waves. There are two main types: suction cavitation (the most common, due to insufficient NPSHa) and discharge cavitation (due to operating the pump with very low flow or excessive recirculation).

How to Confirm: It is manifested by a characteristic “gravel” or “rattle” noise inside the pump, elevated vibrations (confirmed with a vibration analyzer, with energy spikes at high frequencies), fluctuations in the suction and discharge pressure gauge, and a drastic reduction in performance. An internal inspection will reveal pitting, erosion, and a rough appearance on the impeller and volute surfaces, especially on the leading edges of the blades.

Damage if Not Resolved: Persistent cavitation causes severe erosion and fatigue in the impeller and casing material, dramatically reducing component life. It leads to premature failure of bearings and seals due to vibration and mechanical stress. Pump efficiency degrades and energy consumption increases, potentially leading to catastrophic structural failure.

7.2. Impeller Wear or Damage

Explanation: The impeller, as a key component that transfers energy to the fluid, may suffer wear due to abrasion (solid particles in the fluid), corrosion (chemical attack), erosion (due to cavitation), or mechanical damage due to impact of foreign objects. Wear reduces the critical dimensions of the impeller, altering its hydrodynamic geometry.

How to Confirm: An internal visual inspection (with an endoscope or after disassembling the casing) is the direct method. You will look for thinned, punctured, bent, broken blades, or excessive material buildup. In initial phases, pump performance decreases (lower flow and pressure) and there may be a gradual increase in vibration (detectable with vibration analyzer, values ​​above the limits of ISO 10816 for mechanical wear, for example, RMS speed > 2.8 mm/s for small pumps).

Damage if not Resolved: The efficiency of the pump will progressively drop, increasing energy consumption for the same task. The imbalance generated by uneven wear will increase vibration, accelerating failure of the bearings, mechanical seal and, eventually, the motor. In extreme cases, the impeller can disintegrate, causing serious damage to the volute and casing.

7.3. Air Lock (Depriming)

Explanation: Centrifugal pumps cannot pump air. If the impeller case or suction line fills with air or gases, the pump "deprimes." The impeller rotates freely in air, without creating sufficient vacuum in the suction or pressure in the discharge. Common causes include inadequate initial priming, air leaks in the suction line (gaskets, gaskets, mechanical seals), too low a liquid level in the suction tank exposing the inlet, or the release of gases dissolved in the fluid.

How to Confirm: The pump starts and the motor rotates, but there is no fluid discharge (discharge pressure gauge at 0 bar) or the flow rate is minimal. The suction gauge may indicate a very high vacuum or unstable fluctuations. A "bubbling" or "air churning" noise is often heard inside the pump. The pump may overheat if operated unprimed for a long time.

Damage if not Resolved: Dry or air-locked operation can cause rapid overheating of the pump due to lack of cooling fluid. This can severely damage mechanical seals (dry run failure), bearings and casing, causing premature component failure and, in severe cases, complete pump failure.

7.4. Suction Line Problems (Insufficient NPSHa, Clogs, Leaks)

Explanation: The suction of a centrifugal pump is the most critical side. The NPSHa (Net Positive Suction Head Available) must always be greater than the NPSHr (NPSH Required) of the pump. Insufficient NPSHa may be due to:

• Low liquid level in the suction tank.
• Excessively long suction line or with many elbows/fittings, increasing friction losses.
• Suction pipe diameter too small.
• Filters, grates or foot valves clogged by solids.
• Air leaks in the suction line, flanges or seals, which introduce air into the pump and reduce suction capacity (similar effect to depriming).
• Excessively high fluid temperature, which increases vapor pressure and reduces NPSHa.

How to Confirm: The suction vacuum gauge will indicate a high (e.g. < -0.5 bar) or unstable vacuum. The discharge flow meter will show a low flow rate. Visual inspection of the suction filter or use of an endoscope may reveal blockages. Air leaks can be detected visually (bubbles if the pipe is clear), audibly (hissing), or by applying a soapy water solution to the joints. Calculating the NPSHa and comparing it to the manufacturer's NPSHr is essential.

Damage if Not Resolved: Persistent suction problems are the primary cause of cavitation, with all the associated destructive consequences (impeller erosion, bearing and seal failure). Additionally, overall system performance is compromised, increasing operating costs and the likelihood of downtime.

7.5. Misalignment with the System Curve (Incorrect Operating Point)

Explanation: The characteristic curve of a pump shows the relationship between the flow rate and the head (pressure) that it can generate. The system curve represents the static and dynamic pressure losses (friction) of the system of pipes, valves and accessories. The ideal operating point of the pump is where its curve intersects the system curve, preferably near the BEP (Best Efficiency Point).

A mismatch occurs if the system resistance is greater or less than expected. If the resistance is too high (e.g., partially closed valves, excessively long or small diameter piping, too many elbows), the pump will operate to the left of its BEP (low flow, high pressure), which can cause internal recirculation, overheating, and discharge cavitation. If the resistance is too low, the pump will operate to the right of the BEP (high flow, low pressure), which may also generate cavitation and motor overload.

How to Confirm: Measure the flow rate and discharge and suction pressure with the appropriate instruments. Plot this point on the characteristic curve of the pump supplied by the manufacturer. If the operating point deviates significantly from the BEP or the recommended operating zone, there is a mismatch. Valve inspection, piping design and a detailed calculation of head losses in the system are necessary. An ultrasonic flowmeter (clamp-on) and pressure gauges are essential here.

Damage if not Resolved: Continued operation outside of BEP results in low energy efficiency (higher consumption for the same flow rate), increased vibrations, excessive heating, and reduced pump life due to cavitation, high radial forces on the impeller, and premature wear of seals and bearings. It can also cause motor failure due to overload.

8. Step-by-Step Resolution Procedures

8.1. Cavitation Resolution

1. SAFETY: Apply LOTO. Drain and vent the system.
2. Increase NPSHa:
   • Raise the liquid level in the suction tank.
   • Reduce fluid temperature (if feasible).
   • Reduce friction losses in the suction line: clean filters, widen the diameter of the pipe, reduce the length or number of elbows.
   • Check and ensure that the suction valves are fully open.
3. Reduce pump NPSHr:
   • If the cavitation problem persists and the NPSHa is optimized, consider replacing the pump with one with a lower NPSHr, or adjust the operating speed (if using VFD) to operate at a more favorable point on the system curve.
4. Verification: Monitor suction pressure, flow rate and vibrations after the intervention. Cavitation noise should cease and vibration levels should return to acceptable values ​​(e.g. < 2.8 mm/s RMS for small pumps).

8.2. Resolution of Worn or Damaged Impeller

1. SAFETY: Apply LOTO. Drain and vent the system. Remove the pump from the line.
2. Disassembly and Inspection: Disassemble the pump casing. Visually inspect the impeller, volute and wear rings. Document damage (photographs, measurements).
3. Replacement: Replace the damaged impeller with a new one from the manufacturer (UNITEC reference). Make sure the material is suitable for the fluid and operating conditions (e.g., Stainless Steel EN 1.4401 for corrosive/abrasive fluids). Also replace the wear rings if necessary, ensuring clearance tolerances according to the manufacturer (typically between 0.2-0.5 mm).
4. Balance: If an impeller is replaced, it is advisable to perform dynamic balancing to ensure operation without excessive vibration.
5. Assembly: Assemble the pump following the manufacturer's torque specifications for the bolts and the correct installation of mechanical seals and gaskets.
6. Verification: Perform an initial boot. Monitor flow, pressure and, fundamentally, vibration levels (RMS speed < 2.8 mm/s for small pumps, < 4.5 mm/s for large pumps).

8.3. Air Block Resolution

1. SAFETY: Apply LOTO if disassembly is required.
2. Priming: Open the discharge valve only partially or keep it closed. Open the priming valve or bleed plug on top of the pump casing. Fill the pump and suction line with fluid until a steady stream comes out without bubbles. Close the purge valve and gradually open the discharge valve.
3. Identify and Eliminate Air Leaks:
   • Inspect all threaded connections, flanges, mechanical seals and gaskets in the suction line.
   • Apply soapy water solution to connections under operation (if safe) to check for bubbles, or perform a pressure test with the pump off and the line full.
   • Retighten connections, replace gaskets, gaskets or mechanical seal if leaks are detected.
4. Check: The pump must be properly primed and maintain uninterrupted discharge. The bubbling noise should disappear and the suction gauge should stabilize.

8.4. Suction Troubleshooting

1. SAFETY: Apply LOTO.
2. Suction Level Optimization: Ensure that the minimum fluid level in the suction tank does not fall below that specified to guarantee the required NPSHa.
3. Cleaning Filters and Grates: Close isolation valves, apply LOTO, drain and clean or replace clogged filters, foot valves and grates.
4. Suction Line Check:
   • Evaluate the diameter of the pipe. If it is too small, consider an increase.
   • Minimize pipe length and number of elbows.
   • Remove any unnecessary restrictions.
5. Fluid Temperature Control: If the problem is high fluid temperature, evaluate cooling systems or relocate the pump.
6. Verification: Measure the suction vacuum. It should be within the normal operating range for the pump and fluid.

8.5. Misfit Resolution with the System Curve

1. SECURITY: Apply LOTO for system modifications.
2. Detailed System Curve Analysis: Recalculate the static and dynamic head losses of the system with the current design of pipes, valves and accessories. Verify that calculations are carried out in accordance with engineering standards (e.g., Darcy-Weisbach tables).
3. Valve Setting: For pumps that operate too far to the left (low flow), gradually open the discharge valve. For pumps that operate too clockwise (high flow rate, rarely causes low discharge, but does cause cavitation and overload), close the discharge valve slightly.
4. Pipe Modification: If the problem is structural (wrong diameter pipes, too many elbows), consider modifications to optimize head losses and bring the operating point closer to the BEP.
5. Use of Variable Frequency Drives (VFD): If the pump has a VFD, adjust the motor speed to shift the pump curve and make it coincide with the desired operating point on the system curve, optimizing efficiency and avoiding cavitation.
6. Verification: Measure discharge flow and pressure and plot the new operating point on the pump curve. Verify the reduction of vibrations and energy consumption.

9. Preventive Measures

The implementation of preventive measures is key to avoiding the recurrence of these problems and prolonging the useful life of the pumps.

10. Spare parts and components

The availability of quality original spare parts is essential for fast and reliable resolution of faults.

To purchase original and high-quality spare parts, visit our e-catalog: www.unitecd.com/e-catalog/

11. References

• UNE-EN ISO 9906: Rotodynamic pumps. Hydraulic performance tests.
• UNE-EN ISO 5199: Technical specifications for centrifugal pumps with cantilever bearings. Category 1 Requirements.
• UNE-EN ISO 10816-3: Mechanical vibrations. Evaluation of machine vibration by measurements on non-rotating parts. Industrial machines with nominal power greater than 15 kW and nominal speeds between 120 r/min and 15,000 r/min.
• UNE-EN 166: Personal eye protection. Specs.
• UNE-EN 388: Protective gloves against mechanical risks.
• UNE-EN 374: Protective gloves against chemicals and microorganisms.
• UNE-EN ISO 20345: Personal protective equipment. Safety footwear.
• Operation and Maintenance Manuals for Pump Manufacturers (OEM).
• Related UNITEC-D Maintenance Guides (available at www.unitecd.com/maintenance-guides/).