Troubleshooting Centrifugal Pumps: Low Flow or No Discharge

1. Description of the problem and scope

This diagnostic guide is designed for engineers and service technicians who encounter centrifugal pumps that exhibit a significant reduction in volumetric flow or a complete lack of discharge. Fluid supply problems are among the most common failures of centrifugal pumps in industrial settings, which can lead to production stoppages, equipment damage and significant financial losses. Effective and timely diagnostics are critical to minimizing downtime.

1.1. Definition of symptoms

• Low flow: The pump is running, but the flow rate is lower than nominal or expected.
• No priming: The pump is running, but no liquid is supplied at all, or the outlet pressure remains close to the suction pressure.
• Increased level of noise and vibration: Especially characteristic of cavitation.
• Pump or motor overheating: May indicate increased load or friction.
• Discharge pressure reduction: The pressure generated by the pump is insufficient to overcome the system resistance.

1.2. Field of application

This manual covers centrifugal pumps used in a wide range of industrial applications including:

• Water supply and drainage (EN 1092-1, EN ISO 9906)
• Chemical and petrochemical industry (API 610, ISO 13709)
• Food industry (compliance with hygienic standards)
• Energy
• Heating, ventilation and air conditioning (HVAC) systems

Classification of malfunctions:

• Critical: Complete lack of injection, which leads to an immediate stop of the production process.
• Basic: Low flow that significantly affects product performance or quality, potentially causing equipment damage.
• Insignificant: A small flow reduction that does not critically affect the process, but indicates the initial stage of the malfunction.

2. Safety measures

BEFORE ANY DIAGNOSTIC OR REPAIR WORK ON THE PUMPING EQUIPMENT, COMPLETE SAFETY OF PERSONNEL AND EQUIPMENT MUST BE ENSURED. FAILURE TO FOLLOW THESE PRECAUTIONS COULD RESULT IN SERIOUS INJURY OR DEATH.

• Lockout/Tagout (LOTO): Before any tampering with the pump or related systems, MUST perform a Lockout/Tagout procedure in accordance with internal company standards and DSTU requirements EN 1032. Disconnect all power sources (electrical, pneumatic, hydraulic) and install lockout devices and tags. Check for no voltage.
• Residual energy: Ensure that all residual energy (eg, pipeline pressure, storage potential energy, hot fluids) is discharged or safely diverted. Open the drain valves and depressurize the system.
• Personal protective equipment (PPE): Always use appropriate PPE: safety glasses, gloves, helmet, safety shoes, protective clothing. When working with aggressive or hot liquids - additional specialized PPE.
• Hot surfaces: Pumps and motors can reach high temperatures. Be careful to avoid burns.
• Chemical hazard: When working with chemically active liquids, follow all safety protocols associated with these substances (MSDS/SDS safety data sheets). Provide adequate ventilation.
• Confirmation of no movement: After turning off the power, make sure that all moving parts have stopped and cannot start accidentally.
• Works at height: When performing work at height, follow safety rules, use safety systems.

3. Necessary diagnostic tools

For effective diagnostics, the following set of tools that meet industry standards is required.

4. Initial assessment checklist

Before starting a detailed diagnosis, perform the following steps. This will allow you to collect important information that will help narrow down the potential causes of the malfunction.

5. Systematic diagnostic algorithm

Use this step-by-step algorithm to systematically identify the root cause of low flow or no boost.

1. Checking the basic operating conditions of the pump
   1. IF the pump does not start or the motor does not rotate:
      • Check the motor power supply (voltage, frequency).
      • Check the functionality of the starter, relay, switches.
      • Probable cause: Electrical fault or mechanical jam.
   2. IF pump starts but no flow or pressure:
      1. Go to step 2.
2. Evaluate the presence of liquid and air in the system
   1. IF no liquid in the pump housing (not filled):
      • Check the liquid level in the suction tank.
      • Check the suction valve (fully open?).
      • Refill the pump and suction line.
      • Probable cause: Insufficient filling (priming) or lack of liquid.
   2. IF there are signs of an air plug or air suction:
      • Observe transparent sections, listen for hissing sounds.
      • Check all connections on the suction line for leaks.
      • Check the pump packing seals.
      • Try to remove the air through the vent valves.
      • Probable cause: Air plug or air suction.
   3. IF pump is primed and there is no sign of air:
      1. Go to step 3.
3. Measurement of suction and discharge pressure
   1. Using pressure gauges and vacuum gauges (Chapter 3):
      • Measure the pressure at the inlet (P_vsm) and outlet (P_nagn) of the pump.
   2. IF P_vsm is very low (close to vacuum or well below nominal NPSHa) and there is cavitation noise/vibration:
      • Probable cause: Cavitation due to suction problems (high hydraulic resistance, low fluid level, high fluid temperature).
      • Check filters, suction pipe diameter, suction height.
      • Go to Root Cause Analysis: Cavitation.
   3. IF P_nagn is very low, but P_vsm is normal, and there are no signs of cavitation:
      • Probable cause: Problems with the impeller (wear, damage, clogging) or wrong direction of rotation.
      • Check the direction of rotation of the motor (see marking).
      • Shut down the pump, perform the LOTO procedure, perform a visual inspection of the impeller (using an endoscope or disassembly).
      • Go to Root Cause Analysis: Impeller Wear.
   4. IF P_nagn and P_vsm appear normal for closed injection valve, but no flow with open valve:
      • Probable cause: Excessive resistance in the injection system (clogged pipelines, closed valves, incorrectly calculated system characteristic).
      • Check all injection line valves, filters, heat exchangers.
      • Go to Root Cause Analysis: System Characteristic Analysis.
4. Vibration and Noise Analysis
   1. Using the Vibration Analyzer (Chapter 3):
      • Measure the vibration level on the pump housing and bearings.
      • Compare the indicators with the permissible values ​​according to ISO 10816 (for the equipment class).
      • Norm: Vibration speed ≤ 4.5 mm/s (RMS) for industrial pumps.
      • Alarm: Vibration velocity > 7.1 mm/s (RMS).
   2. IF high level of vibration with characteristic "crack" or "gravel" noises:
      • Probable cause: Cavitation.
      • Check the NPSHa (available suction head).
      • Go to Root Cause Analysis: Cavitation.
   3. IF vibration is high, but without characteristic cavitation noise:
      • Probable cause: Impeller imbalance (after wear) or misalignment of shaft centers.
      • Check the condition of the impeller and clutch.
      • Go to Root Cause Analysis: Impeller Wear.

6. Malfunction-cause matrix

This matrix provides a summary of symptoms, most likely causes, diagnostic tests needed, and expected results.

7. Analysis of the root causes of each malfunction

7.1. Cavitation

The essence of the problem: Cavitation occurs when the pressure of the fluid at the inlet to the impeller falls below the pressure of the saturated vapors of the fluid at a given temperature. This leads to the formation and subsequent collapse of vapor bubbles, which causes intense shock waves. These high-energy shock waves (up to 1500 MPa) damage the surface of the impeller and housing, causing pitting.

Reasons:

• Insufficient available suction head (NPSHa): NPSHa < NPSHr (pump requirements). This can be caused by:
  • Dynamic suction height is too high (the pump is located too high above the liquid level).
  • High resistance of the suction line (long pipes, many bends, clogged filters, too small pipe diameter).
  • High liquid temperature, which increases the pressure of saturated vapors.
  • Low atmospheric pressure (for open systems).
  • Low liquid level in the suction tank.
• Excessive flow: The pump is operating far to the right of the optimum performance point (BEP), causing the impeller inlet pressure to drop.

Confirmation: A characteristic noise similar to the movement of gravel in the pump, vibration (peaks in the range of 0.5-2 kHz), a decrease in performance and pressure, and over time - clear signs of pitting on the impeller blades.

Consequences: Rapid wear of the impeller and other internal parts, increased noise and vibration levels, reduction in pump efficiency and productivity, possible destruction of seals and bearings.

7.2. Impeller or casing wear

The essence of the problem: Wear of the impeller (impeller) or the internal surfaces of the housing (for example, gap seals) leads to an increase in the internal flow of liquid from the discharge side to the suction side. This reduces the efficiency of the pump, reducing the generated pressure and, as a result, the volume flow.

Reasons:

• Abrasive particles in the liquid: Pumping liquids with suspended solid particles (sand, sludge) causes erosive wear.
• Chemical corrosion: Aggressive liquids corrode the material of the impeller and housing.
• Cavitation: As described above, cavitation is one of the main causes of mechanical damage.
• Aging of the material: Natural wear and tear of the material over time.
• Incorrect material: Using a material that is not suitable for the pumped liquid or operating conditions.

Confirmation: Visual inspection of the impeller (irregularities, reduction in the thickness of the blades, potholes, blunt edges), increased gaps in the gap seals (measured with a feeler gauge). Reduction of injection pressure at nominal revolutions.

Consequences: Significant reduction in efficiency, increased power consumption to perform the same work, increased vibration and noise, need for more frequent replacement of components.

7.3. Air lock (Air Lock)

The essence of the problem: Centrifugal pumps are not capable of pumping gases. If a significant volume of air or gas accumulates in the pump housing or suction line, the impeller begins to rotate in the air/gas without creating sufficient vacuum to draw in the liquid. This results in zero or very low flow.

Reasons:

• Insufficient filling (priming) of the pump: Before starting, the pump and the suction line were not completely filled with liquid.
• Air suction: Leaks in the suction line (leaky connections, cracks, damaged shaft seals, loose flange connections).
• Low fluid level in the reservoir: The suction port is exposed and the pump starts to draw air.
• Liquid Outgassing: Highly volatile liquids or those that are heated may outgas in the suction line.

Confirmation: The pump is running, the motor draws low current, the suction pressure is close to atmospheric (or even positive), the discharge pressure is very low or zero. A characteristic noise of working in the air or gurgling is often heard.

Consequences: No supply, possible overheating of the pump due to lack of liquid cooling, damage to mechanical seals.

7.4. Suction problems (other than cavitation and airlock)

Gist of the problem: Even without cavitation or airlock, problems in the suction line can significantly reduce the performance of the pump by limiting the flow of fluid to the impeller.

Reasons:

• Clogged suction filter/mesh: Accumulation of foreign particles causes an excessive pressure drop at the pump inlet.
• Closed or partially closed suction valve: Restricts fluid flow.
• Improperly designed suction line: Pipe diameter too small, excessive bends, sudden changes in direction, too long line resulting in high friction losses.
• Suction height: Static suction height is too high.

Confirmation: Low suction pressure (vacuum gauge shows high vacuum) with normal fluid level in the tank, which improves when the filter is cleaned or the valve is opened. Absence of characteristic cavitation noises.

Consequences: Reduced performance, increased pump load, potential cavitation with further deterioration.

7.5. System Curve Analysis

The essence of the problem: The pump operates at the intersection of its characteristic curve and the system characteristic of the pipeline. If the system resistance (system characteristic) increases significantly, the operating point of the pump shifts to the left on the characteristic curve, resulting in a decrease in flow, even if the pump is in perfect working order.

Reasons:

• Increasing static pressure: Increasing the height to which the liquid must be supplied.
• Increasing frictional losses: Clogging of pipelines with deposits, corrosion or growths; use of pipelines with a smaller diameter than calculated; adding new fittings or equipment (heat exchangers, filters) to the injection line without recalculation.
• Partially closed or clogged discharge valve: Increases hydraulic resistance.
• Increasing pressure at the outlet: Changing the conditions on the receiving side of the system.

Confirmation: The pump produces a high discharge pressure (often higher than nominal) but low flow. The motor draws increased current (if the pump is running far to the left of BEP). Inspection of the entire injection line. Calculation of actual system characteristics based on measured pressure and flow parameters.

Consequences: Reduced system performance, increased energy consumption, possible engine overload, fluid overheating (if the pump is in recirculation mode).

8. Step-by-step troubleshooting procedures

After identifying the root cause using the previous sections, follow the steps below to troubleshoot.

8.1. Elimination of cavitation

CAUTION: Hydraulic system modification work may require shutdown and LOTO.

1. Increase available NPSHa:
   • Check fluid level: Increase fluid level in suction tank if low.
   • Cleaning filters/screens: Clean or replace clogged filters and screens on the suction line.
   • Reduction of friction losses: Check the internal condition of the suction line for deposits. Consider replacing pipes with a larger diameter or reducing the number of fittings and elbows.
   • Decreasing fluid temperature: If fluid is pumped at high temperature, consider cooling.
   • Pump speed reduction: If possible, reduce the pump speed with a frequency converter to shift the operating point on the NPSHr curve.
   • Check suction height: If possible, lower the pump closer to the liquid level or install a support tank.
2. Pressure check: After corrective actions, remeasure suction and discharge pressures. Make sure that NPSHa > NPSHr (with a margin of 0.5-1.0 m).
3. Vibration and noise control: Start the pump and check the vibration (vibration speed indicators should be within the norm ISO 10816, ≤ 4.5 mm/s) and the absence of characteristic cavitation noises.

8.2. Elimination of wear of the impeller or housing

CAUTION: Requires full LOTO and pump removal.

1. Disassembly and visual inspection:
   • Perform LOTO procedure, drain fluid, remove pump.
   • Inspect the impeller, housing, slot seals for wear, erosion, corrosion, cracks, foreign objects.
   • Measure gap seals. Allowable clearances must comply with the manufacturer's technical documentation. Usually, for medium-sized pumps, an increase in clearance by 0.2-0.3 mm already significantly reduces efficiency.
2. Component replacement:
   • Replace worn impellers, gap seals, housing or its liners. Use only original spare parts or certified analogues that meet the requirements of DSTU and ISO in terms of material and dimensions.
   • When replacing the impeller, ensure proper balancing (dynamic balancing according to ISO 1940-1, class G6.3 or G2.5) to avoid vibration.
3. Shaft Alignment: After replacement and assembly, BE SURE to accurately align the pump and motor shafts (use a laser alignment device). Permissible misalignment: radial runout up to 0.05 mm, angular runout up to 0.01 mm/100 mm.
4. Test run: Start the pump, check pressure, flow, vibration and temperature.

8.3. Elimination of air traffic jam

CAUTION: Make sure the system is depressurized before opening the bleed valves.

1. Filling the pump (priming):
   • Stop the pump, perform LOTO.
   • Open the vent valve (if present) at the top of the pump housing and slowly fill the pump with liquid until liquid without air bubbles starts to come out.
   • Close the vent valve.
   • Make sure the suction line is also completely filled.
2. Inspect for air leaks:
   • Inspect the entire suction line, including flanged connections, threaded connections, shaft packings.
   • Use a soap solution or other leak detector to locate leaks. Eliminate leaks - tighten fasteners, replace seals, gaskets or oil seals.
   • For stuffing boxes, make sure there is a small, constant drip of fluid (1-2 drops per minute) for lubrication and cooling, which also prevents air from being sucked in.
3. Topping up the liquid level: Ensure that the minimum liquid level in the suction tank is always above the suction port.
4. Test run: Start the pump, check flow and pressure.

8.4. Solving suction problems

CAUTION: Requires full LOTO of system and drainage of pipeline sections.

1. Cleaning the intake tract:
   • Perform LOTO, drain the system.
   • Clean or replace clogged suction filters, screens, mud traps.
   • Check the internal condition of the suction pipe for deposits or foreign objects.
2. Valve Check: Make sure the suction valve is fully open and working properly. Eliminate any mechanical obstructions or valve malfunctions.
3. Evaluation of pipeline configuration: If the problem is constant and not related to clogging, it may be necessary to revise the design of the suction line: reduce the number of bends, increase the diameter of the pipe, minimize the length. (Correspondence ISO 9905).
4. Test run: Start pump, check suction pressure (vacuum gauge should show less vacuum) and flow.

8.5. Adjustment of system characteristics

CAUTION: Changes in piping can affect the hydraulics of the entire system.

1. System resistance analysis:
   • Determine which components in the discharge line are creating excessive resistance (clogged filters, heat exchangers, partially closed valves).
   • Clean or replace clogged elements. Fully open the discharge shut-off valves.
2. Configuration change:
   • Consider increasing the diameter of the discharge pipes, reducing the number of fittings or bends to reduce friction losses.
   • If the static head has increased significantly (for example, a change in piping route or discharge point), it may be necessary to reconsider the pump selection or add an additional pump.
3. Using a Variable Frequency Drive (VFD): If the pump is running at a fixed speed, installing a VFD will allow the pump to adjust its rotation speed to operate at the optimum point for the changing system characteristic, providing the desired flow and energy savings.
4. Test run and verification: Start the pump, measure flow and pressure. Ensure that the pump duty point meets the process requirements and is close to the BEP zone.

9. Preventive measures

Implementation of effective preventive measures is key to preventing repeated failures and extending the service life of pumping equipment.

10. Spare parts and components

For quick and efficient repairs, it is important to have critical spare parts available.

To order spare parts and view the complete range of UNITEC-D GmbH, visit our E-catalog UNITEC.

11. Links

• DSTU EN ISO 9906: Centrifugal pumps. Hydraulic acceptance tests. Accuracy classes 1, 2 and 3.
• DSTU EN ISO 10816-3: Mechanical vibration. Evaluation of machine vibration on stationary parts by measurements. Part 3. Industrial machines with a rated power of more than 15 kW and a rated speed of 120 rpm to 15000 rpm under the conditions of on-site operation.
• ISO 1940-1: Vibration. Requirements for balancing rotors in a rigid state.
• API 610 / ISO 13709: Centrifugal pumps for the oil, petrochemical and gas industries.
• Operation and maintenance manuals from pump manufacturers (e.g. Grundfos, KSB, Wilo, Sulzer).
• UkrSEPRO: Product certification in Ukraine.