Centrifugal Pump Troubleshooting: Low Flow or No Head

1. Description of the Problem and Scope of Application

This manual is intended for diagnosing and eliminating malfunctions associated with reduced supply or complete absence of pressure in centrifugal pumps operated in industrial conditions of Ukraine. Typical symptoms include a significant drop in performance, the pump's inability to pump fluid to the required height, and characteristic noises and vibrations. The manual covers diagnostics on equipment used in the food, chemical, energy and automotive industries.

Classification of Severity:

• Critical: Complete lack of supply, which leads to a stoppage of the technological process. Requires immediate intervention.
• Significant: Feed significantly below nominal (>20%), affecting product quality or process efficiency. Needs urgent diagnosis.
• Minor: A slight decrease in feed (<20%), which may indicate the initial stages of the malfunction. Needs a scheduled examination.

2. Precautions

WARNING! Before starting any diagnostic or repair work on the pump unit, all necessary safety measures must be taken to prevent personal injury and equipment damage.

• LOCKOUT AND TAGOUT (LOTO): Always perform a lockout and tagout procedure for power sources (electrical, hydraulic, pneumatic) to prevent accidental starting of the pump. Make sure all valves on the suction and discharge lines are closed and locked.
• PERSONAL PROTECTIVE EQUIPMENT (PPE): Be sure to use appropriate PPE: safety glasses/face shields, gloves (chemically resistant when working with aggressive liquids), protective shoes, protective clothing. Heat-resistant gloves when working with hot liquids.
• CONSERVED ENERGY: Remember that residual pressure or vacuum may remain in the system. Carefully open the drain and vent valves. Hot liquids and surfaces can cause burns.
• HAZARDOUS SUBSTANCES: Make sure that the liquid in the pump and pipelines is safe for contact or take special measures to drain and dispose of it in accordance with current regulations (DSTU 8798:2018, EN 1530:2004).
• ROTATING PARTS: Never remove protective covers while the pump is running. Make sure all rotating parts have come to a complete stop before approaching them.

3. Necessary Diagnostic Tools

For effective diagnostics, it is recommended to use the following tools that meet the standards of DSTU EN ISO 10012:2005:

4. Initial Evaluation Checklist

Before starting a detailed diagnosis, it is necessary to collect initial data. This information will help narrow down the potential causes of the malfunction.

5. Systematic Flow of Diagnostics

Follow the following flow chart for systematic troubleshooting:

1. SYMPTOM: Low Delivery or No Head
   1. Step 1: Check Pump Operation
      • Diagnosis: Are the pump and motor running? Does the pump shaft rotate?
      • If NO:
        1. Possible Reason: No power supply, motor malfunction, clutch disconnection, pump jamming.
        2. Diagnostic Test: Check the motor power supply with a multimeter (voltage, current). Visually inspect the coupling. Try turning the pump shaft by hand (after LOTO).
        3. Go to: Motor/Coupling Electrical/Mechanical Troubleshooting.
      • If YES: Continue diagnostics.
   2. Step 2: Suction Check
      • Diagnosis: Measure the suction pressure with a vacuum gauge.
      • If Suction Pressure is Low (higher vacuum) or Extremely Low:
        1. Possible Cause: Suction problems (air plug, clogged filter, insufficient fluid level, leak in suction line, excessive hydraulic suction resistance).
        2. Diagnostic Test:
           • Visual inspection of the filter and suction line.
           • Checking the liquid level in the tank.
           • Listening for suction noise – a hissing sound may indicate air suction.
           • Use a soapy solution or spray to detect leaks on suction flanges/connections.
        3. Go to: Section 'Suction Problems'.
      • If the Suction Pressure is Normal: Continue diagnostics.
   3. Step 3: Checking the Injection
      • Diagnosis: Measure the injection pressure with a manometer.
      • If the Discharge Pressure is Low (and the Suction Pressure is Normal):
        1. Possible Cause: Impeller wear, cavitation, air jam in the pump housing, wrong direction of rotation, partially open shut-off devices.
        2. Diagnostic Test:
           • Listening to the pump (characteristic crunch during cavitation, hissing during air).
           • Visual inspection of the direction of rotation.
           • Checking the position of the shut-off valves.
           • Measurement of vibration with a vibration analyzer.
           • Using a thermal imager to detect local overheating.
        3. Go to: Section 'Impeller Wear', 'Cavitation', 'Air Locked Pump'.
      • If the Discharge Pressure is High (and the supply is low/absent):
        1. Possible Reason: Partially or completely closed shut-off valve on the discharge, clogged pipeline or heat exchanger, wrong selection of the pump (incompatibility with the system curve).
        2. Diagnostic Test:
           • Checking the position of all shut-off devices on the injection.
           • Measurement of hydraulic resistance of pipeline sections.
           • Comparison of the passport characteristics of the pump with the calculated system curve.
        3. Go to: Section 'System curve analysis'.

6. Malfunction-Cause matrix

This chart provides a quick overview of common symptoms, likely causes, diagnostic tests, and expected results. The reasons are ranked by probability.

7. Root Cause Analysis for Each Malfunction

7.1. Cavitation

Explanation: Cavitation is the phenomenon of formation and subsequent rapid collapse of liquid vapor bubbles in a pumped flow. Occurs when the static pressure of the liquid at any point in the flow parts of the pump (usually at the entrance to the impeller) falls below the pressure of saturated vapors of this liquid at a given temperature. This occurs when the available cavitation margin (NPSH_A) becomes less than the required cavitation margin (NPSH_R) of the pump. (DSTU ISO 17769-1:2010).

Reasons:

• Low NPSH_A (e.g. due to too high fluid temperature, high hydraulic suction resistance, too high suction height).
• Incorrect selection of the pump (NPSH_R of the pump is too high for the given conditions).
• Underfilling of liquid in the suction tank.
• The suction valve is partially closed.

Confirmation: Characteristic noise similar to "gravel crunch", increased vibration (especially at frequencies from 20 to 200 Hz), rapid destruction of the surface of the impeller blades and the pump housing (cavitation erosion), overheating of the pump housing.

Damage (if not eliminated): Cavitation leads to progressive destruction of the metal of the flow parts, reduced pump efficiency, increased wear of bearings and end seals due to vibration, and ultimately to pump failure.

7.2. Air Locked Pump (Air Plug)

Explanation: Centrifugal pumps are not intended for pumping gases. If a significant amount of air or gas accumulates in the suction line or in the pump housing, the impeller cannot create enough pressure to move the liquid because the density of the air is much less than the density of the liquid. This creates an "air jam" that prevents the pump from working.

Reasons:

• Air leakage in the suction line due to leaky flanges, connections, shaft seals, oil seals.
• Insufficient liquid level in the suction tank, resulting in air suction through the vortex.
• Incomplete filling of the pump with liquid before starting (insufficient filling).
• Accumulation of gases released from the liquid.

Confirmation: The pump is working, but the discharge pressure is almost zero or much lower than normal. A characteristic noise of "grinding" air is often heard, the pump operates at "idle speed" with a reduced motor current. You can visually see air bubbles in the transparent sections of the pipeline.

Damage (if not eliminated): "Dry running" of the pump can lead to overheating of end seals and bearings, their rapid failure. It is also possible to damage the impeller due to friction.

7.3. Wear of Impeller and Internal Clearances

Explanation: The impeller is the key element of a centrifugal pump that transfers energy to a liquid. Over time, due to abrasive wear (from solid particles in the fluid), corrosion (from aggressive fluids) or cavitation erosion, the impeller vanes and the surfaces of the inner sealing rings (slotted seals) wear. This leads to an increase in internal liquid flows from the discharge side to the suction side inside the pump, which significantly reduces the efficiency and performance characteristics of the pump.

Reasons:

• Long-term operation with abrasive liquids.
• Pumping corrosive liquids without proper pump material.
• Work in conditions of constant cavitation.
• Normal operational wear and tear.

Confirmation: Discharge pressure reduction with normal suction pressure and unchanged other parameters. A decrease in the efficiency of the pump, which is manifested in an increase in the consumption of electricity per unit of pumped liquid. When disassembling the pump - visual traces of wear, erosion, changes in the geometry of the blades, increased radial and axial clearances.

Damage (if not eliminated): Permanent decrease in performance, increased energy consumption, eventually - complete failure of the pump to perform its function. Increased vibration due to hydraulic imbalance.

7.4. Problems with absorption

Explanation: The efficient operation of a centrifugal pump is critically dependent on proper suction conditions. Any obstruction or abnormality in the suction line can lead to a decrease in NPSH_A and, as a result, cavitation or a complete loss of supply.

Reasons:

• Clogged filter, strainer, or tank inlet.
• The suction height is too high (the pump is located well above the liquid level).
• Suction line too long or too small in diameter, creating excessive hydraulic resistance.
• Air leakage in the suction line (see "Air plug").
• Improper design of suction fittings (elbows, transitions).

Confirmation: Low reading of vacuum gauge on suction (higher vacuum), indicating high suction resistance. Noise in the suction line, possible vibrations. Low pump performance.

Damage (if not eliminated): Cavitation, damage to the internal parts of the pump, reduction of the service life of the equipment.

7.5. Analysis of the System Curve

Explanation: The system curve reflects the dependence of the hydraulic resistance of the pipeline system on the flow of liquid. The operating point of the pump is determined by the intersection of the pump performance curve with the system curve. If the system curve changes (e.g. due to clogging, changing pipe lengths, opening/closing of valves) or if the pump has been incorrectly selected for the system, this will cause the pump to operate outside the optimum operating point, causing reduced efficiency, increased energy consumption and low delivery.

Reasons:

• Incorrect pump selection for specific operating conditions.
• Changes in the pipeline system (new elbows, smaller diameter, increased length).
• Partial closure of shut-off valves or clogging of pipelines at the discharge.
• High static pressure, which was not taken into account during selection.

Confirmation: Low delivery at normal or high discharge pressure. Calculation of the system curve and its comparison with the pump characteristics shows that the operating point is far from the point of maximum efficiency of the pump or too far to the left on the performance curve.

Damage (if not eliminated): Increased energy consumption, increased pump wear due to non-optimal operation (especially when operating in a "closed valve"), unstable system operation.

8. Sequential Troubleshooting Procedures

8.1. Elimination of Cavitation

ATTENTION: Before any intervention, the LOTO procedure must be performed!

1. NPSH check_A:
   1. Reduce fluid temperature if too high.
   2. Increase the static level of liquid in the suction tank.
   3. Reduce hydraulic suction resistance: check and clean filter, fully open all valves.
   4. Check the diameter and length of the suction pipeline; if necessary, increase the diameter or shorten the length.
2. Pump Check:
   1. Make sure the pump is properly filled with fluid.
   2. Inspect the impeller for damage caused by cavitation using an endoscope or after disassembly.
   3. If cavitation is a chronic problem, consider replacing the pump with a model with a lower NPSH_R or installing a pump that operates with positive suction head.
3. Verification: Start the pump, measure the flow, suction and discharge pressure, check the noise and vibration level. The feed must correspond to the design, the noise and vibration level - within the permissible values ​​(vibration speed up to 4.5 mm/s RMS according to ISO 10816-3).

8.2. Air Jam Removal

ATTENTION: Before any intervention, the LOTO procedure must be performed!

1. Filling the pump:
   1. Close the discharge valve.
   2. Open the suction valve and the air outlet valve on the pump body (if present).
   3. Fill the pump with liquid until the air is completely out. Close the vent valve.
   4. If there are vacuum pumps for filling, use them.
2. Detecting Air Leaks:
   1. Check the tightness of all connections, flanges, shaft seals on the suction line. Use a soapy solution or special leak detectors.
   2. Tighten the flange fasteners according to the tightening torque specified in the documentation (for example, 80 Nm for a DN100 PN16 flange).
   3. Replace damaged gaskets or seals.
3. Verification: Start the pump. Supply and pressure should be normal. Check for the absence of "hissing" and bubbles.

8.3. Elimination of the Consequences of Impeller Wear

ATTENTION: Before disassembling the pump, it is mandatory to perform the LOTO procedure and drain the liquid!

1. Disassembly and Inspection:
   1. Disassemble the pump part according to the manufacturer's instructions.
   2. Visually inspect the impeller, housing, gap seals for wear, erosion, corrosion.
   3. Measure the internal clearances (radial and axial) and compare them with the permissible values ​​(usually within 0.2-0.5 mm, according to the manufacturer's instructions).
2. Component Replacement:
   1. Replace the worn impeller with a new one (UNI EN 13355:2006).
   2. Replace worn gap seals and sealing rings.
   3. When assembling, observe the specified tightening moments and clearances.
3. Verification: After assembling and filling the system with liquid, conduct a test run, measure operating parameters.

8.4. Solving Suction Problems

ATTENTION: Before any intervention in the suction line, the LOTO procedure must be performed!

1. Inspection and Cleaning:
   1. Check and clean the suction filter or mesh filter. Replacement as needed.
   2. Check the level of liquid in the tank, ensure that it corresponds to the minimum permissible level.
   3. Visually inspect the suction nozzle for blockages or foreign objects.
2. Optimization of the Suction Line:
   1. Check the diameter of the pipeline: the suction pipeline should not be smaller than the diameter of the suction pipe of the pump, and preferably one size larger.
   2. Reduce the length of the suction line to a minimum.
   3. Reduce the number of elbows and shut-off fittings on the suction.
   4. Eliminate any air pockets or high spots in the suction line.
3. Verification: Start the pump, monitor suction vacuum, delivery and discharge pressure.

8.5. Adjustment of the System Curve

1. System Analysis:
   1. Perform a detailed calculation of the system curve for the current operating conditions, taking into account all supports (pipelines, fittings, equipment).
   2. Compare the obtained system curve with the performance curve of the installed pump.
2. Actions:
   1. If the system resistance is too high:
      • Clean clogged pipelines and heat exchangers.
      • Fully open all shut-off devices on the discharge.
      • If necessary, increase the diameter of the injection pipeline or reduce its length.
      • Consider installing a pump with a higher pressure (for example, a two-stage pump).
   2. If the pump was selected incorrectly:
      • Consider replacing the impeller with another one with different characteristics (if allowed by the manufacturer).
      • Replace the pump with a model that better matches the calculated system curve.
3. Verification: After corrective measures, perform control measurements of supply and pressure.

9. Precautions

10. Spare Parts and Components

To ensure trouble-free operation of pumping equipment and quick troubleshooting, it is critical to have UNITEC-D GmbH spare parts that comply with EN and ISO standards.

Refer to UNITEC-D GmbH electronic catalog to order original spare parts.

11. Links

• DSTU EN ISO 10012:2005. Measurement control systems. Requirements for measuring processes and measuring equipment.
• DSTU EN ISO 10816-3:2016. Vibration is mechanical. Evaluation of machine vibration based on the results of measurements on non-rotating parts. Промислові машини з номінальною потужністю понад 15 кВт та номінальною швидкістю від 120 об/хв до 15 000 об/хв.
• DSTU ISO 17769-1:2010. Centrifugal, centrifugal and screw pumps. General technical requirements and terminology. Part 1. Definitions, designations, terms and indicators of hydraulic efficiency.
• DSTU EN 13355:2006. Iron and steel castings. Surface treatment and inspection of cast parts.
• DSTU EN 1514-1:2005. Flanges and their connections. Dimensions of gaskets for flanges of pressure class PN. Part 1. Non-metallic flat gaskets with or without fillers.
• DSTU 8798:2018. Quality management systems. Guidelines for the application of ISO 9001:2015 in the chemical industry.
• Operating and maintenance instructions from pump manufacturers (eg Grundfos, KSB, Wilo).
• UNITEC Maintenance Guides: Section "Maintenance of End Seals", "Diagnostics of Bearings".