1. Problem description & scope of application

This diagnostic guide is aimed at maintenance technicians, maintenance managers and reliability engineers in the manufacturing industry in the DACH region. It systematically deals with the most common causes of reduced flow or the complete lack of delivery in centrifugal pumps. Affected system types include process pumps in chemical, food, automotive and energy systems as well as cooling water systems.

Symptoms:

Significantly reduced flow rate
No flow despite the pump running
Increased noise levels and vibrations
Fluctuating pressure on the suction or pressure side
Overheating of the pump or motor
Changed motor power consumption

Failure to correct these problems can result in critical equipment failures, increased energy consumption, premature wear and tear, and significant production losses. The severity classification is usually classified as critical if production is impaired, or major if there is impending damage to the system.

2. Safety precautions

WARNING: Before beginning any diagnostic or maintenance work on centrifugal pumps, it is mandatory to take the following safety measures to prevent serious injury or fatal accidents. Failure to comply results in the highest risk for people and systems.

System shutdown and activation: According to DIN EN ISO 14118, the system must be completely shut down, the pump must be electrically de-energized (switching off, locking, marking in accordance with VDE 0105-100) and secured against being switched on again (lockout/tagout procedure).

Residual energy: Always be aware of the possibility of stored energy in pressure lines, capacitors (in frequency converters) and rotating masses. Before opening lines, the pressure must be completely reduced.

Medium handling: Appropriate protective measures must be taken when handling aggressive, hot, toxic or explosive media (e.g. emptying, rinsing, ventilating). Personal protective equipment (PPE) in accordance with VDI 2242 must be worn (protective goggles, gloves, safety shoes, respiratory protection if necessary).

Hot surfaces: Pump housings and motors can reach high temperatures. Be careful when touching.

Rotating parts: Make sure all protective covers are in place before operating the pump.

3. Required diagnostic tools

Tool	Specification/Model	Measuring range	Purpose
multimeter	CAT III 1000 V, True RMS (TRMS)	Voltage: up to 1000 V AC/DC, current: up to 10 A AC/DC, resistance: up to 40 MΩ	Measurement of motor voltage and current, winding resistance, continuity test
Infrared thermometer / thermal imaging camera	Measuring range -20 °C to 500 °C, emissivity adjustable	Accuracy ±2 °C or ±2%	Detection of overheating (bearings, motor, housing), hotspots in the electrical system
Vibration meter	Acceleration: 0.1-200 m/s², Speed: 0.1-2000 mm/s	Frequency range: 10Hz - 1kHz	Detection of imbalance, bearing damage, misalignment, cavitation (as high-frequency vibration)
Pressure gauge	Differential pressure gauge, analog/digital	-1 to 25 bar (suction side), 0 to 60 bar (pressure side)	Checking suction and pressure pressure, differential pressure via filter
Flowmeter	Ultrasonic clamp-on, electromagnetic	Depending on the pipe diameter and medium, typically 0.1 - 10 m/s	Verification of the actual flow rate
Stethoscope / acoustic measuring device	Mechanical stethoscope or electronic listening device	Listening range 20 Hz - 20 kHz	Localization of unusual noises (cavitation, bearing noises)
Laser alignment device	Measuring accuracy 0.01 mm	Measuring range up to 3 m	Check and correct shaft misalignment between pump and motor
Digital stopwatch	Accuracy 0.01 s		Measuring emptying times for volume flow estimation

4. Checklist for initial assessment

Before you begin the detailed diagnosis, carefully document the following points.

Observation / measurement	Documentation/Checkpoint	Setpoint / acceptance range
Pump data	Manufacturer, type, nominal flow rate (m³/h), nominal pressure (bar), nominal speed (rpm), power consumption (kW)	Nameplate data
Operating conditions	Current flow rate (m³/h), suction pressure (bar), pressure pressure (bar), medium temperature (°C), medium viscosity (cSt), suction container filling level	Process specifications
Motor current consumption	Current (A) per phase, voltage (V)	Rated current ±5%, rated voltage ±10%
Noises and vibrations	Type of noise (rattling, hissing, crackling), noticeable vibrations on the pump housing/bearing	Quiet, even; Vibration speed < 4.5 mm/s (effective, according to DIN ISO 10816)
Leaks	Visual inspection of shaft seal, flange connections, housing	No leaks
Alarm history	Plant documentation, control system logs from the last 24-48 hours	No relevant pre-alarm messages
Maintenance history	Final maintenance work, repairs, replacement of components	Relevant for finding the cause
Media visual inspection	Clouding, foaming, discoloration of the pumped medium	Clear, homogeneous, without inclusions
Valve positions	Suction and pressure valves fully open? By-pass valves closed?	According to operating instructions

5. Systematic diagnostic flowchart

This decision tree guides you through diagnosing flow problems.

Start: flow insufficient or missing?
1. Pump is running, but no or little flow?
  1. Pump produces abnormal noises (rattling, crackling) and vibrations?
    - YES:
      1. Check the suction pressure. Is it below the critical NPSH_R value or is it very low/fluctuating?
        
        YES: Probable cause: cavitation. Go to step 7a (cavitation).
        
        NO: Could also be advanced impeller wear or air bag, as noise and vibration can also occur here. Continue checking.
    - NO: Noises inconspicuous, but no flow?
      1. Conveying medium with visible air pockets or foam?
        
        YES: Probable cause: Air entrapment / suction problems (leaks). Go to step 7c (air in pump housing) or 7d (suction side problems).
        
        NO:
      2. Suction and pressure sides of the pump hot?
        
        YES: Probable cause: air bag / overheating. Try to vent the pump. Go to step 7c.
        
        NO:
  2. Pump runs smoothly, but no or little flow?
    1. Check the manometric values (suction and discharge pressure).
      1. Suction pressure normal, pressure pressure low or 0 bar: Probable cause: Internal short circuit / impeller wear / check valve. Go to step 7b (impeller wear) or check check valve.
      2. Suction pressure very low / Vacuum too high, pressure pressure low: Probable cause: Suction side problems (clogging, suction leak, NPSH problem). Go to step 7d (Suction side problems).
      3. Suction pressure and pressure pressure are both normal, but flow is missing: Probable cause: Blocked impeller / pipe blockage (very rare). Switch off the pump and check.
    2. Check motor current consumption.
      1. Motor current consumption significantly below the rated current: Probable cause: No medium / air bag / cavitation / impeller loose. Go to step 7a, 7c or 7d.
      2. Motor current consumption normal or slightly increased: Probable cause: Wrong system characteristic / impeller wear. Go to step 7b or 7e.
2. Pump doesn't start or switches off?
  1. Check electrical supply (fuse, switch, motor protection switch).
    1. Electrical problem: Probable cause: Motor defective, frequency converter defective, wiring. Contact a qualified electrician.
    2. Mechanical blockage (motor hums but doesn't turn): Probable cause: impeller blocked / bearing stuck. Unlock the pump and try to turn the shaft manually.

6. Error-cause matrix

Symptom	Probable causes (ranking)	Diagnostic test	Expected result if cause is confirmed
Reduced flow, loud crackling / rattling noises, vibrations	1. Cavitation 2. Impeller wear 3. Blockage on the suction side	Measurement of the suction pressure at the pump inlet, visual inspection of the impeller after dismantling, examination of the suction strainer	Suction pressure too low (< NPSH_required), bubbles forming in the sight glass, pitting on the impeller, visible blockage
No flow, pump runs empty, low motor current consumption	1. Air bag in the pump housing 2. Pump not bled 3. Loss of absorbency (suction line empty)	Bleeding the pump via venting screw, checking the fill level in the suction tank, checking the tightness of the suction line	Air escapes when venting, no medium in the sight glass, falling fill level in the suction container
Reduced flow rate, constant suction pressure, low pressure pressure, increased motor current (constant if necessary)	1. Impeller wear (advanced) 2. Foreign bodies in the impeller 3. Incorrect system characteristic (too high back pressure)	Dismantling and visual inspection of the impeller, measuring the pressure after the pump and before feeding into the system, checking the system resistance	Significant wear on the impeller (rounded edges, thinner blades), blockage due to foreign bodies, high pressure after pump with low flow rate
Reduced flow, suction pressure very low / high vacuum, increased noise	1. Blockage on the suction side (strainer, filter) 2. Intake leakage 3. Too long/narrow suction line 4. Geodetic suction height difference too high	Testing the suction filter, leak test of the suction line (soap water, vacuum test), checking the pipe dimensioning and height difference	Suction filter clogged, hissing/bubbling in the event of a leak, pipeline data deviating from the norm
Flow rate does not match the pump characteristic curve, despite normal pressures and speeds	1. System characteristic has changed (e.g. new valves, pipe deposits, process change) 2. Incorrectly sized pump for the current operating point	Recording of an operating characteristic curve of the pump, comparison with the manufacturer characteristic curve and system characteristic curve	Operating point is outside the optimal range on the pump characteristic curve, deviation of the measured system characteristic curve from the original

7. Root cause analysis for each error

7a. Cavitation

Detailed explanation: Cavitation occurs when the pressure at the suction port of the pump falls below the vapor pressure of the fluid being pumped. This leads to the formation of vapor bubbles that implode when they move to areas of higher pressure. These implosions cause high pressure peaks and lead to material removal from the impeller and pump housing. According to VDI 2441, a sufficient NPSH (Net Positive Suction Head) is mandatory.
Confirmation: Typical “crackling” or “rattling” noises, reminiscent of pebbles in the impeller. Significant punctiform pitting and material removal on the impeller blades, especially on the inlet side. Measuring the suction pressure: If this is close to or below the vapor pressure of the medium, there is a risk of cavitation.
Damage if not repaired: Rapid destruction of the impeller, shaft seal and bearings, leading to leaks, increased maintenance and complete pump failure.

7b. Impeller wear

Detailed explanation: Impeller wear is the loss of material on the blades and casing due to erosion (due to solid particles), corrosion (due to chemical attack) or cavitation. This reduces the efficiency of the pump because the geometry of the impeller and the gap to the housing change. The choice of material in accordance with DIN EN ISO 5199 is crucial.
Confirmation: Disassembly of the pump and visual inspection of the impeller. Rounded edges, thinner blades, increased gaps between impeller and housing, often visible on the wear rings. The motor current can decrease as wear progresses as the pump does less work.
Damage if not repaired: Significant loss of efficiency, higher energy consumption, increased vibration and noise, which can lead to premature failure of bearings and seals.

7c. Air in the pump housing (air bag)

Detailed explanation: Centrifugal pumps are designed to pump liquids. Gas or air pockets in the pump housing can completely interrupt delivery because the pump is unable to compress and deliver gas. This leads to an “air pocket” and loss of absorbency.
Confirmation: The pump is running, but produces no flow or only a very low flow. The motor current consumption is often significantly lower than normal because the pump has hardly any resistance. Noises can sound “gurgling” or “running on empty”. Check the vent screw - air escapes instead of liquid.
Damage if not rectified: Overheating of the pump and the shaft seal (dry running), which leads to damage to seals, bearings and possibly the impeller.

7d. Suction side problems

Detailed explanation: Suction side problems include any factor that impedes fluid entry into the pump or reduces the required NPSH_A (Net Positive Suction Head available). This can range from clogged sieves to leaks in the suction line to an incorrectly sized suction line.
Confirmation: Extremely low or strongly fluctuating suction pressure, possibly audible hissing in the event of suction leaks. Visual inspection of the suction filter or strainer will reveal a blockage. Leak test of the suction line with soapy water or vacuum test. Verification of pipeline sizing (diameter, length, bends) and geodetic height difference according to hydraulic calculation.
Damage if not repaired: Frequent cavitation, reduced efficiency, increased wear on the pump and seals.

7e. Analysis of the system characteristic

Detailed explanation: Every pump installation has a specific system characteristic curve that represents the pressure loss over the flow of the pipeline and the system components (sum of friction losses and static head). The operating point of the pump results from the intersection of the pump characteristic curve and the system characteristic curve. A change in the system characteristic curve (e.g. due to valve changes, deposits, new process conditions) can shift the operating point and lead to a reduced flow rate.
Confirmation: Measurement of the actual delivery flow and the associated differential pressure of the pump. Compare these values with the original pump characteristic and the calculated system characteristic. If the pump has to generate a higher differential pressure at the nominal flow rate than at the design point, the system characteristic curve has shifted.
Damage if not corrected: Operating the pump outside the optimal efficiency range (BEP - Best Efficiency Point), which can lead to increased energy consumption, vibrations, cavitation and premature wear.

8. Step-by-step troubleshooting

The following measures must be carried out after diagnosis and identification of the root cause.

Fix cavitation:
1. Improve suction side: Increase suction pressure by raising the suction container fill level or lowering the pump. Reduction of pressure losses in the suction line (larger diameters, fewer bends, cleaned filters).
2. Reduce medium temperature: If practical, reduce the temperature of the pumped medium to reduce the vapor pressure.
3. Throttling of the pressure side: Only in an emergency and in the event of slight cavitation, slightly throttle the pressure side in order to reduce the flow and thus reduce the NPSH_required of the pump. Be careful, this shifts the operating point and reduces efficiency.
4. Checking the flow rate: Make sure that the pump is not operated far to the right of the optimal operating point (too high flow requirement).
Resolve impeller wear:
1. Pump revision: Disassemble the pump according to the manufacturer's instructions.
2. Replace impeller: Mount a new impeller according to OEM specification (consult UNITEC article number). The correct torques for fastening screws (according to DIN EN 1515-1) and the gaps for wear rings must be adhered to exactly.
3. Check wear rings: If necessary, replace wear rings to minimize gap losses.
4. Optimize material selection: If there is recurring wear, analyze the cause (abrasiveness, corrosion) and, if necessary, adjust the impeller material (e.g. harder alloy or coating).
Fix the air bag (bleed the pump):
1. Switch off and activate the pump.
2. Open the vent screw: Carefully open the vent screw at the highest point of the pump housing.
3. Refill/bleed medium: Wait until an even jet of liquid (without air bubbles) comes out. For self-priming pumps, the housing may need to be filled initially.
4. Close vent screw: Tighten firmly, observe torque according to manufacturer's instructions.
5. Check suction leaks: Check the entire suction line for leaks and seal any leaks.
Resolve suction-side problems:
1. Clean suction filter/strainer: Pump stop and activation required. Open the suction filter and clean it thoroughly or replace it.
2. Seal intake leaks: Check all flange connections, stuffing boxes and fittings for leaks and replace seals or components if necessary.
3. Optimize suction line: If the cause is a suction line that is too narrow or too long, a hydraulic recalculation and adjustment (larger pipe diameter, fewer bends, ball valves instead of slide valves) is required.
4. NPSH_A increase: Increase the fill level in the suction container or reduce the geodetic suction height.
Adapt system characteristic curve / optimize operating point:
1. System check: Check all components of the pipeline (valves, filters, heat exchangers) for blockages, misalignments or unexpected resistance.
2. Check valve positions: Make sure that all valves are correctly positioned and that no throttling reduces the flow unnecessarily.
3. Pipe cleaning: If deposits are suspected, carry out cleaning (mechanical or chemical).
4. Pump replacement/adjustment: If the system characteristic has shifted permanently and the current operating point is far away from the BEP of the pump, a pump replacement or an impeller adjustment (threshing diameter) must be considered in order to optimize the efficiency.

9. Preventive measures

Cause	Prevention strategy	Monitoring method	Recommended interval
Cavitation	Optimization of the suction line, ensuring sufficient NPSH_A, regular checking of the process parameters (fill level, temperature, pressure)	Vibration analysis (high-frequency components > 1 kHz), suction pressure monitoring, noise analysis (acoustic monitoring)	Continuously (online) or weekly
Impeller wear	Use of wear-resistant materials, filtration of the pumped medium, compliance with the optimal operating point	Regular efficiency measurement (flow rate vs. power consumption), vibration analysis (low frequencies), endoscopy/visual inspection during revision	Annually (efficiency), 6-monthly (vibration), upon revision (visual)
Air in the pump housing	Correct venting procedures after every shutdown/revision, leak test of the suction line	Visual control (sight glasses), noise analysis, motor current consumption (waste)	Before each start-up, monthly (tightness)
Suction side problems	Regular cleaning of suction filters/sieves, leak testing of the suction line, correct design of the suction line	Differential pressure monitoring via suction filter, suction pressure monitoring, visual inspection of the suction line	Monthly (filter), 6-monthly (tightness, visual)
Incorrect system characteristic	Regular hydraulic system analysis for process changes, compliance with design conditions	Recording and comparison of pump characteristics and system characteristics, flow and pressure measurements	When there are significant process changes or annually

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 (DIN, EN).

Part name	Specification	When to replace?	UNITEC category
impeller	Material: e.g. AISI 316, nodular cast iron EN-GJS-400-18LT, bronze; Diameter and blade geometry according to pump OEM	If there is significant wear (cavitation, erosion), imbalance, cracks or damage	Pump components
Wear rings	Material: e.g. bronze, gray cast iron GG25, PTFE; Fitting tolerances according to OEM	If the maximum permissible gap dimensions are exceeded or if there is visible wear	Pump components
Shaft seal (mechanical seal)	Material pairing: e.g. silicon carbide/silicon carbide, carbon/ceramic; Design according to OEM (e.g. single-acting, double-acting)	In the event of leaks, overheating, unusual noises or as a preventative replacement according to the maintenance schedule (e.g. every 8,000 operating hours)	Sealing technology
Pump bearing	Type: e.g. deep groove ball bearing 6308-2RS1, cylindrical roller bearing; Tolerance class P6 or better according to DIN 620	With increased running noise, increased storage temperature (> 70 °C), increased vibration (e.g. > 4.5 mm/s)	Rolling bearings
Suction filter/strainer	Mesh size: e.g. 200 µm, 500 µm; Material: stainless steel 1.4401	In the event of blockage, damage or as a preventive replacement according to the maintenance plan	Filter technology
Seals (flange, O-rings)	Material: e.g. EPDM, FKM, NBR, PTFE; Pressure class according to DIN EN 1514-1	In the event of dismantling, visible damage or leaks	Sealing technology

For a comprehensive selection of high-quality pump spare parts and components, visit our UNITEC-D e-catalog.

11. References

DIN EN ISO 14118: Safety of machines – avoiding unexpected start-ups
VDE 0105-100: Operation of electrical systems
DIN ISO 10816: Mechanical vibrations - evaluation of machine vibrations through measurements on non-rotating parts
VDI 2441: Cavitation in fluid machines
VDI 2242: Personal protective equipment against falls
DIN EN ISO 5199: Technical requirements for centrifugal pumps – Class II
DIN EN 1515-1: Flanges and their connections - screws and nuts - Part 1: Selection of screw connections
TÜV guidelines for pressure equipment and system safety
Manufacturer maintenance instructions for specific pump types