1. Description and scope of the problem
This guide is intended for engineers and maintenance technicians working on centrifugal pumps in the aerospace and energy sectors. It specifically deals with failures manifested by an abnormal reduction in flow or a total absence of backflow. Gravity is classified as critical if the pump provides a cooling or lubricating function for rotating machines.
WARNING: Mandatory safety instructions
Any intervention on a centrifugal pump must comply with the lockout procedure (LOTO - Lockout/Tagout). - Isolate electrical, mechanical and hydraulic energy sources.
- Wear appropriate personal protective equipment (PPE) (chemical-resistant gloves, eye protection, hearing protection).
- Be careful with pressurized fluids: systematically purge the pump body before opening.
- Risk of burns: the pump bodies can reach high temperatures when operating at zero flow.
3. Necessary diagnostic tools
| Tool | Specification/Model | Measuring range | Usage |
|---|
| Vibration analyzer | Type FFT / ISO 10816 | 10 - 10000 Hz | Detect cavitation, misalignment |
| Digital multimeter | True RMS (CAT IV 600V) | 0 - 600V / 0 - 10A | Check motor power |
| Thermal camera | Min resolution 320x240 | -20°C to +500°C | Identify hot spots (bearings/sealing) |
| Ultrasonic flow meter | Non-intrusive clamp-on | 0 - 20 m/s | Measurement of actual output flow |
| Standard pressure gauge | Class 0.6 | -1 to 25 bar | Check suction/discharge pressure |
4. Initial Checklist
| Verification | Action | Registration |
|---|
| Valve position | Check total opening to suction and discharge | Open / Closed / Partial |
| Tank level | Check the net positive charge at suction (NPSHa) | Level in meters (m) |
| Motor current | Compare with nominal current (In) | Amps (A) |
| Temperature | Measure the pump body and bearings | Temperature (°C) |
5. Diagnostic decision tree
- Symptom: No flow
- Check the direction of rotation of the motor.
- Check if the pump body is primed (presence of air).
- Check the opening of the suction valve.
- Symptom: Low flow / Unstable pressure
- Check the suction pressure (NPSHa vs NPSHr).
- Look for characteristic cavitation noises (gravel noise).
- Check the condition of the impeller (wear, obstruction).
6. Defect-cause matrix
| Symptom | Probable cause | Diagnostic test | Expected result if confirmed |
|---|
| Cavitation noise | NPSHa too low | NPSH / Pressure Gauge Calculation | Suction pressure < limit value |
| Low flow | Intake air intake | Leak test | Bubble detection or pressure variation |
| Absence of repression | Direction of rotation reversed | Motor phase check | Abnormally low motor current |
| Loss of performance | Impeller/ring wear | Flow/pressure measurement | Operating point under curve |
7. Root Cause Analysis
Cavitation
Cavitation occurs when the local pressure of the liquid falls below its vapor pressure. Bubbles form and implode violently, eroding the impeller and causing destructive vibrations (greater than 4.5 mm/s RMS). Confirmed by a drop in suction pressure and a spectral analysis of vibrations.
Intake air intake
A leak on the suction seal introduces air. The air, being compressible, deprimes the pump. Confirmed by unstable discharge pressure and fluctuating flow measurement.
8. Step-by-step resolution procedures
- Correction of cavitation: Reduce the pressure loss on suction (filter cleaning, increase pipe diameter). Increase the fluid level in the reservoir.
- Repairing wear: Dismantle the pump body, inspect the impeller. Replace if wear exceeds 0.5 mm on radial clearance.
- Correction direction of rotation: Reverse two phases at the output of the motor terminal block (intervention carried out by an authorized electrician).
9. Preventive measures
| Cause | Strategy | Method | Interval |
|---|
| Cavitation | NPSH Monitoring | Suction pressure gauge | Monthly |
| Wear and tear | Vibration analysis | FFT analyzer | Quarterly |
| Air intake | Joint inspection | Visual/Ultrasound | Annual |
10. Spare parts and components
| Description | Specification | Replacement | Category |
|---|
| Impeller | 316L stainless steel / Cast iron | If clearance > 0.5mm | Hydraulics |
| Mechanical seal | Standard EN 12756 | Leak detected | Waterproofing |
| Bearings | Class C3 / high speed | Vibration > 6 mm/s | Mechanics |
To order certified parts, visit: https://www.unitecd.com/e-catalog/
11. References
- ISO 10816: Mechanical vibrations.
- NF EN 809: Pumps and pumping units for liquids.
- OEM UNITEC-D Preventive Maintenance Manuals.
