Diagnostic Manual: Low Flow or No Delivery on Centrifugal Pumps

1. Problem description & scope of application

This diagnostic manual addresses the critical problem of low flow or complete loss of delivery in centrifugal pumps. These conditions can result in significant production loss, damage to the pump and associated system components, and increased operating costs. The symptoms and diagnostic procedures described here are applicable to a wide range of centrifugal pump types used in the automotive, aerospace, food, chemical and energy industries.

Symptom classification:

• Critical: No funding. Immediate action required as this indicates a complete system failure or significant damage to the pump.
• Major: Low flow (>20% below nominal). Requires prompt diagnosis to avoid loss of efficiency and progressive damage.
• Minor: Slightly reduced flow (<20% below nominal). May indicate incipient problems or slight abnormalities in the system; Monitoring and preventative diagnosis recommended.

2. Safety instructions

Before any diagnostic or maintenance work is carried out on centrifugal pumps, strict safety measures must be observed to prevent personal injury and damage to the system.

• POWER DISCONNECT (LOTO): The entire system must be safely isolated from the power supply and locked/marked (lockout/tagout) in accordance with DIN EN ISO 14118 (Safety of Machines - Avoidance of Unforeseen Start-ups). Make sure there is no residual energy.
• PERSONAL PROTECTIVE EQUIPMENT (PPE): Always wear appropriate PPE, including safety glasses (DIN EN 166), protective gloves (DIN EN 388), safety shoes (DIN EN ISO 20345) and, if necessary, hearing protection (DIN EN 352).
• CHEMICALS & TEMPERATURE: For pumps that pump dangerous liquids (pay attention to ATEX areas!) or media with extreme temperatures, additional protective measures and compliance with specific hazardous substance regulations (e.g. TRGS) are required.
• PRESSURE RELIEF: Completely relieve all system pressures in the suction and pressure areas of the pump before opening any lines or components. Be careful of residual pressure and liquid splashes.
• HOT SURFACES: Pump housing and motor may remain at high temperatures even after switching off. Allow equipment to cool before handling or wear heat-resistant gloves.
• ROTATION: The pump can rotate by external forces. Secure the shaft before performing any work that could cause accidental rotation.

3. Required diagnostic tools

4. Checklist for initial assessment

Before beginning detailed diagnosis, a systematic initial assessment of the system and environment is essential. This minimizes unnecessary dismantling work and provides crucial information.

5. Systematic diagnostic flowchart

This flow plan enables structured diagnosis to systematically isolate the most likely cause of low flow or loss of production.

1. Start: Low flow or no delivery detected.
2. Check for air in the system or air bubble in the pump:
   1. Stop the pump and use LOTO to be on the safe side.
   2. Bleed the pump. If there is a vent valve on the pump housing, open it carefully until liquid flows continuously and no more air escapes.
   3. Result:
      • If large amounts of air escape: Probable cause: Air bubble / air inclusion. Continue to 6.1 (air inclusion).
      • If no air escapes or only very little: The problem is not primarily due to air inclusions. Go to step 3.
3. Checking the suction line and suction conditions:
   1. Checking the suction container fill level: Is it below the minimum?
   2. Check suction filter/strainer: Visual inspection for blockages, dirt.
   3. Observe the pressure gauge on the suction side during operation: Does it show a strong vacuum (e.g. below -0.5 bar) or does it fluctuate strongly?
   4. Check suction-side pipes for leaks (acoustically with ultrasonic leak detector, visually, soapy water).
   5. Result:
      • If level too low or suction filter clogged: Probable cause: Insufficient suction / NPSH problem. Continue to 6.2 (suction problems / NPSH).
      • If strong vacuum or strongly fluctuating display on suction side: Probable cause: Cavitation (induced by suction problems) or air ingress. Continue to 6.3 (cavitation).
      • If leaks are detected in the suction line: Probable cause: Air inlet. Continue to 6.1 (air inclusion).
      • If the suction side is normal: Continue to step 4.
4. Check for impeller wear or blockage:
   1. Stop the pump and apply LOTO.
   2. Open the pump (if structurally possible) and visually check the impeller for wear, erosion, corrosion or blockages. (E.g. DIN EN 12163 for copper alloys, DIN EN 10088 for stainless steels).
   3. Move shafts axially and radially: is there any play?
   4. Result:
      • If impeller is visibly worn (blade edges thin, rough surfaces, loss of material) or blocked: Probable cause: Impeller wear or blocking. Go to 6.4 (Impeller wear) or 6.5 (Foreign bodies/blockage).
      • If the impeller is intact and can rotate freely: Continue to step 5.
5. Checking the system resistance and the system curve:
   1. Observe the pressure-side pressure gauge during operation: Is the pressure higher than expected with reduced flow?
   2. Check pressure lines for partially closed valves, check valves (stuck), or blockages.
   3. Use thermal imaging cameras on pipes and valves to identify temperature gradients that indicate blockages or restriction.
   4. Calculation of the system resistance and comparison with the pump characteristic curve.
   5. Result:
      • If pressure side is excessively high and/or temperature gradient visible: Probable cause: Increased system resistance / system curve shift. Continue to 6.6 (system resistance).
      • If all previous points are excluded: Further in-depth analysis of the entire hydraulic system and pump design is required.

6. Error-cause matrix

This matrix summarizes the most common symptoms, their likely causes, recommended diagnostic tests, and expected results.

7. Root cause analysis for each error

7.1. Air inclusion in pump or suction line (Air Lock)

Detailed explanation: An air pocket or air bubble, often referred to as an “air lock”, occurs when air or gases become trapped in the pump housing or suction line and prevent the pump from sucking in or delivering the liquid. Centrifugal pumps are designed to pump liquids and not gases. Even small air pockets can significantly affect eligibility. Larger air bubbles can block the pump completely.

How to confirm: Typically, carefully opening a vent valve at the highest point of the pump housing results in the release of air, followed by an immediate or gradual build-up of pressure and flow. An ultrasonic leak detector can detect air ingress at flange connections or stuffing boxes in the suction line. An audible hissing sound indicates a leak.

Possible damage if not rectified: The pump can be operated dry, which leads to overheating of seals and bearings. This can result in premature failure of mechanical seals, shaft bearings and even the impeller.

7.2. Suction problems & falling below the NPSH_require (NPSH problems)

Detailed explanation: An inadequate net positive suction head (NPSH_previous) on the suction side is a critical cause of delivery problems. NPSH_previous describes the absolute pressure at the suction inlet of the pump, minus the vapor pressure of the pumped medium at the operating temperature, based on the pump centerline. If this value is lower than the net positive suction head (NPSH_erf required by the manufacturer), the pumped medium evaporates at the impeller inlet, which leads to cavitation.

Causes can be:

• The liquid level in the suction container is too low.
• Excessively long or thin suction line.
• Blockages in the suction line, e.g. due to dirty suction filters or sieves.
• Too many bends, valves or other resistance in the suction line.
• Media temperature too high, which increases the vapor pressure and thus reduces the NPSH_previous.
• Air leaks in the suction line that increase the vacuum on the suction side.

How to confirm: Measurement of the suction pressure directly at the pump inlet. A calculation of the NPSH_previous taking into account atmospheric pressure, filling level, media density, vapor pressure and friction losses in the suction line is required. A value below the NPSH_erf specified by the pump manufacturer confirms the problem. Typical values ​​for NPSH_ref are between 1-5 meters of liquid column.

Possible damage if not rectified: Leads directly to cavitation (see 7.3).

7.3. Cavitation

Detailed explanation: Cavitation is the formation and subsequent sudden collapse of vapor bubbles in a liquid due to local pressure drops below the vapor pressure of the liquid. This typically occurs at the impeller inlet, where the flow velocity is highest and the static pressure is lowest. When these bubbles reach areas of higher pressure, they implode with enormous force, creating microjets and shock waves that erode the surface of the impeller and casing.

How to confirm: Acoustically by a characteristic crackling or noise like “pebbles in the pump”. Visually caused by pitting or rough, eroded surfaces on the impeller and housing, often on the leading edges of the blades. Temperature measurements with a thermal imaging camera can show local cooling on the pump housing as evaporation removes thermal energy. Vibration analyzes often show broadband increases in vibration in the range from 200 Hz to 2 kHz.

Potential damage if not corrected: Severe erosion and loss of material on the impeller and pump housing, drastically reducing the efficiency of the pump. Damage to mechanical seals and bearings due to increased vibrations and dynamic loads, which can lead to premature failure of the entire pump. Sustained cavitation can reduce the lifespan of a pump by 50-70%.

7.4. Impeller wear (impeller wear)

Detailed explanation: Impeller wear is the loss of material on the blades and the impeller hub due to erosion (due to abrasive solids in the pumped medium), corrosion (due to chemically aggressive media) or cavitation. This wear leads to a change in the impeller geometry, an increase in the gap between the impeller and the housing and thus a reduction in the delivery head and efficiency.

How to confirm: After opening the pump, impeller wear is visually evident through thin, sharp or rounded blade edges, rough surfaces, material breakouts or a reduction in impeller diameter. Measurements with a caliper can reveal deviations from the original dimensions and gap dimensions (e.g. >10% deviation from the target value). A performance test shows a significantly reduced efficiency.

Potential damage if not rectified: Significant loss of efficiency, increased energy consumption for the same delivery rate, increased vibrations due to imbalance and ultimately complete failure of the pump. If excessive wear occurs, the impeller may break and damage other internal components.

7.5. Foreign body/blockage

Detailed explanation: A foreign object in the impeller or pump housing can block the flow, prevent the impeller from rotating, or cause serious imbalances. This can be caused by particles in the pumped medium, loose parts from the system (e.g. seal residues, abrasion) or by improper assembly (e.g. forgotten tools).

How to confirm: Dismantling the pump is usually necessary. The foreign body can then be visually recognized in the impeller or the flow channels. In the event of a partial blockage, irregular noises or increased current consumption of the motor may occur.

Possible damage if not rectified: Motor overload, shaft breakage, bearing damage, destruction of the impeller and housing. A stuck foreign body can completely block the pump and trigger the motor protection.

7.6. Increased system resistance & system curve analysis

Detailed explanation: The pump characteristic curve describes the delivery head and the flow of a pump. The system curve describes the resistance of the pipe system depending on the flow. The operating point of the pump is the intersection of these two curves. When system resistance increases (e.g. due to partially closed valves, blockages in the pipework, additional fittings, or a change in process conditions), the system curve shifts upward. This causes the intersection (operating point) to shift to a lower flow and higher head, resulting in reduced throughput.

How to confirm: Measurements of pressure at various points in the system (before and after valves, bottlenecks) may reveal excessive pressure losses. A thermal imaging camera can visualize temperature gradients at blockages or throttled valves. A detailed calculation of the system curve, taking into account all pipeline data, fittings, filling levels and media parameters, can visualize the changed operating point. Check all valve positions for correct opening.

Possible damage if not rectified: The pump works against excessive resistance, which can lead to overloading, reduced efficiency and increased power consumption. If the back pressure is too high, the pump can no longer deliver the intended delivery rate or, in the worst case, can even overheat.

8. Step-by-step fix procedure

8.1. Eliminate air entrapment

1. Safety: Stop the pump, use LOTO according to DIN EN ISO 14118.
2. Check system: Check suction tank level and shut-off valves on the suction side – fully open?
3. Venting: Carefully open the venting valve at the highest point of the pump housing (if present). Provide a collection tray for any leaking liquid. Bleed until a continuous jet of liquid without air bubbles emerges. Close valve.
4. Look for leaks: Check suction-side pipes, flanges, seals and stuffing boxes for leaks. Seal any leaks or replace seals.
5. Recommissioning: After completing the work, cancel the LOTO measures, put the pump into operation and check the delivery rate and noise.

8.2. Fix suction problems & NPSH problems

1. Safety: Stop pump, apply LOTO.
2. Check fill level: Make sure that the suction container is always sufficiently filled and that the liquid level does not fall below the minimum level recommended by the manufacturer.
3. Clean the suction filter/strainer: Dismantle the suction filter or sieve and clean it thoroughly or replace it. Set regular maintenance intervals for filters.
4. Optimize suction line: If possible, shorten suction line, increase diameter or reduce the number of elbows and fittings. Replace sharp 90° bends with large radii.
5. Check media temperature: If the media temperature is excessively high, check whether cooling measures can be implemented for the pumped medium.
6. Check vacuum break valve: Some systems have a vacuum break valve that lets in air if the vacuum in the suction line is too high to prevent piston action. Check function.
7. Recommissioning: Cancel LOTO, start pump, monitor suction and pressure pressures.

8.3. Repair cavitation damage

1. Safety: Stop pump, apply LOTO.
2. Damage assessment: Dismantle the pump. Check impeller and housing for pitting and erosion.
3. Impeller/housing replacement: If there is severe damage, replace the impeller and/or housing.
4. NPSH optimization: Carry out measures according to 8.2 to increase the NPSH_previous.
5. Adjust operating point: Check whether the pump is permanently operated too far to the right of its characteristic curve (high flow, low delivery head). If yes, close the throttle valve on the pressure side slightly to shift the operating point.
6. Recommissioning: Cancel LOTO, start the pump, closely monitor operating parameters and noise.

8.4. Repair impeller wear

1. Safety: Stop pump, apply LOTO.
2. Pumpe demontieren: Gehäuse öffnen und Laufrad zugänglich machen.
3. Measuring & Inspection: Measure the gap dimensions between the impeller and the housing (vernier caliper). Check impeller for wear, erosion, corrosion.
4. Replace the impeller: If there is significant wear (>10% deviation from the original dimensions or significant material removal), replace the impeller with a new original spare part. For extremely abrasive media, the use of an impeller made of more wear-resistant material (e.g. hard cast iron, ceramic-coated) can be considered.
5. Adjust gap dimensions: If adjustable, adjust the axial and radial play of the impeller according to the manufacturer's specifications (e.g. 0.2-0.4 mm) to minimize internal leaks.
6. Causal research: Check the type of pumped medium and filtering to minimize future wear.
7. Recommissioning: Cancel LOTO, start pump, monitor performance parameters.

8.5. Remove foreign body/blockage

1. Safety: Stop pump, apply LOTO.
2. Disassembly: Open the pump to gain access to the impeller and flow channels.
3. Remove foreign bodies: Carefully remove foreign bodies. Make sure there are no fragments or damage left on the impeller or casing. If necessary, use compressed air to clean hard-to-reach areas.
4. Damage check: Check the impeller and housing for damage caused by the foreign body (cracks, deformations). Replace components if necessary.
5. Prevention: Check the suction side for possible sources of foreign bodies (e.g. loose sieves, missing filters, damage in the container).
6. Recommissioning: Cancel LOTO, start the pump, check function and background noise.

8.6. Eliminate increased system resistance (system curve optimization)

1. Safety: Stop pump, apply LOTO. (For valve maintenance)
2. Check valve positions: Check all shut-off and control valves in the suction and pressure lines for complete opening. Make sure check valves are not stuck.
3. Locate blockages: Check pipes for blockages (deposits, corrosion), especially in bends and on fittings. Using a thermal imaging camera can help here (temperature differences at bottlenecks). If necessary, dismantle and clean pipe sections.
4. Check pipe dimensioning: If necessary, check whether the pipe cross-sections are sufficient for the required flow or whether the pipes are too narrowed due to corrosion.
5. Document system changes: Document all changes to the piping system (new valves, longer lines, new heat exchangers) and reassess the impact on the system curve.
6. Recommissioning: Cancel LOTO, start pump, monitor pressure and flow values. The pump should now work closer to the optimal operating point.

9. Preventive measures

Prevention is crucial to maximizing the lifespan and efficiency of centrifugal pumps.

10. Spare Parts & Components

The availability of high-quality spare parts is crucial for quick and efficient repairs. UNITEC-D offers a wide range of components that meet the highest industry standards and ensure reliable performance.

Visit our e-catalog for a complete overview of available spare parts and specifications: www.unitecd.com/e-catalog/

11. References

• DIN EN ISO 14118: Safety of machines – avoiding unforeseen startups.
• VDI 3832: Measurement and assessment of vibrations on industrial pumps.
• ISO 10816: Mechanical vibrations – measuring and assessing the vibration intensity of machines on non-rotating parts.
• DIN EN 166: Personal eye protection.
• DIN EN 388: Protective gloves against mechanical risks.
• DIN EN ISO 20345: Personal protective equipment – ​​safety shoes.
• VDE 0100: Setting up low-voltage systems.
• TRGS (Technical Rules for Hazardous Substances).
• Pump manuals from the manufacturers (e.g. KSB, Grundfos, Wilo).
• UNITEC-D maintenance guide for rotary machines (internal document).