1. Description of the problem and scope of application
This manual is intended for diagnosing centrifugal pumps that exhibit reduced performance (delivery below design) or a complete lack of discharge flow. The problem is classified as critical because prolonged operation in such conditions leads to cavitation damage to the impeller, overheating of bearing assemblies and premature failure of mechanical seals.
2. Safety measures
ATTENTION: Before starting any work with the pumping equipment, it is necessary to perform the blocking/marking procedure (LOTO - Lockout/Tagout). Ensure that the power source to the motor is de-energized and the system is free of stored energy (pressure is relieved through the drain valves). Use appropriate PPE (protective gloves, safety glasses, special shoes). Work according to safety techniques when working with industrial equipment.
3. Necessary diagnostic tools
| Tool | Specification/Model | Range | Purpose |
|---|
| Manometer (digital) | Accuracy class 0.5 | -1 to 16 bar | Measurement of suction and discharge pressure |
| Current clamps | True RMS | 0-200 A | Engine load check |
| Thermal imager | Resolution >= 160x120 | -20°C to +350°C | Diagnosis of local overheating (bearings) |
| Vibroanalyzer | Portable (ISO 10816) | 10 Hz - 1 kHz | Detection of cavitation and imbalance |
4. Initial assessment checklist
| What to check | action | norm |
|---|
| Working condition | Adjustment with operating point (head/flow) | Limits on the pump curve |
| Recent changes | Viewing the maintenance log (replacing valves, filters) | No recent interventions |
| History of accidents | Checking automation signals (PLC) | No active accidents |
| Suction line condition | Check for air leaks (flanges) | Complete tightness |
5. Systematic diagram of diagnostics
- Check for flow:
- If no flow:
- Check that all discharge and suction valves are open.
- Check whether the pump is filled with working fluid (elimination of air plug).
- Check the direction of rotation of the motor.
- If the flow is low:
- Check the condition of the suction filters (clogging).
- Measure the inlet pressure (NPSHa). If the pressure is below the calculated one, cavitation is likely.
- Check the condition of the impeller (wear/erosion).
6. Matrix "Symptom-Cause"
| Symptom | Probable Cause (Rank) | Diagnostic test | Expected result |
|---|
| Lack of flow | Air plug (1) | Open the exhaust valve | Air/water outlet |
| Lack of flow | Reverse rotation (2) | Checking the motor phases | The direction of the arrow on the pump |
| Low flow | Cavitation (1) | Suction manometer | Pressure < saturation pressure |
| Low flow | Clogging of the filter (2) | Pressure drop across the filter | High Drop (dP) |
| Low flow | Wheel wear (3) | Performance curve analysis | Deviation > 10% from the calculation |
7. Root Cause Analysis
Cavitation
It occurs when the pressure of the liquid at the inlet drops below its saturation (evaporation) pressure. Steam bubbles are formed, which explode when they hit the high-pressure zone on the wheel, causing metal erosion and vibration. Confirmation: Characteristic noise similar to gravel pumping. Consequences: Destruction of wheel blades, failure of seals.
Air traffic jam
Accumulation of air in the upper part of the pump housing due to leaks or incomplete filling. The pump cannot create pressure. Confirmation: The pump makes noise, the pressure does not increase. Consequences: Running dry, overheating.
8. Step-by-step elimination procedures
- Removing the air jam:
- Stop the pump (LOTO).
- Close the discharge valve.
- Open the air release valve (vent) on the pump housing.
- Pour in the liquid until a continuous stream without bubbles comes out of the vent.
- Close the vent.
- Inspection/cleaning of suction line:
- Check all flange connections for leaks (soap solution under vacuum).
- Dismantle and clean the strainer if the pressure drop dP > 0.3 bar.
9. Preventive measures
| The root cause | Strategy | Monitoring method | Interval |
|---|
| Cavitation | Maintaining NPSHa > NPSHr | Pressure sensors | Constantly |
| Clogging | Regular cleaning of filters | Manometers (dP) | Every month |
| Depreciation | Vibration analysis | Vibroanalyzer | Every quarter |
10. Spare parts and components
| Description of the part | Specification | When to replace | UNITEC-D category |
|---|
| Working wheel | According to the article number of the pump | With erosion > 2 mm | Pumps-Spare parts |
| Mechanical sealing | Material: SiC/SiC | When leakage occurs | Pumps-Seals |
| A set of bearings | C3 gap | Every 20,000 hours | Bearings-Industrial |
The complete spare parts catalog is available on our website: https://www.unitecd.com/e-catalog/
11. Links
- ISO 9906:2012 - Centrifugal pumps. Classes of tests of reception characteristics.
- ISO 10816-3 — Assessment of machine vibration.
- Technical documentation of the pump equipment manufacturer UNITEC-D.
