Problem Description and Scope
This diagnosis is applied when centrifugal pumps have insufficient flow or total absence of discharge, affecting critical processes in industrial plants. Symptoms include: flow rate below 80% of nominal, complete loss of discharge, excessive cavitation type noise, abnormal vibration, and motor overheating. The severity is classified as critical when it affects essential processes, major when it reduces productive capacity, and minor when it allows operation with reduced performance.
Safety Precautions
WARNING: Before starting any diagnostics:
- Apply lockout/tagout (LOTO) procedure on power supply
- Use mandatory PPE: helmet, safety glasses, gloves, dielectric footwear
- Check residual pressure in system - risk of stored energy up to 25 bar
- Do not open drain valves without completely depressurizing
- Housing temperature may exceed 80°C - risk of burns
- Chemical fluids: consult SDS before handling connections
Required Diagnostic Tools
| Tool | Specification/Model | Measurement Range | Purpose |
|---|---|---|---|
| digital multimeter | Fluke 87V or equivalent | 0-1000V AC/DC, 0-10A | Check motor electrical parameters |
| Differential pressure gauge | Class 1.0, G1/4" thread | 0-16 bar | Measure suction/discharge pressure |
| vibration analyzer | SKF CMXA 75 or similar | 10-1000Hz, 0-50mm/s RMS | Detect cavitation and imbalance |
| Ultrasonic flowmeter | Portable, accuracy ±2% | According to pipe diameter | Measure real flow without interruption |
| infrared thermometer | Range -32°C to +380°C | Adjustable emissivity 0.1-1.0 | Detect hot spots |
| Vacuum gauge | Bourdon, class 1.6 | -1 to 0 bar | Check aspiration conditions |
Checklist - Initial Evaluation
| Parameter | Registered Value | Nominal Value | Status |
|---|---|---|---|
| Current flow rate (m³/h) | ______ | ______ | □ Normal □ Reduced □ Null |
| Suction pressure (bar) | ______ | ______ | □ Normal □ Low □ Negative |
| Discharge pressure (bar) | ______ | ______ | □ Normal □ Low □ High |
| Motor current (A) | ______ | ______ | □ Normal □ Low □ High |
| Bearing temperature (°C) | ______ | <70°C | □ Normal □ Elevated |
| Global vibration (mm/s RMS) | ______ | <4.5mm/s | □ Normal □ Elevated |
| audible noise | ______ | No irregularities | □ Normal □ Cavitation □ Friction |
Systematic Diagnostic Flowchart
- Is there discharge flow?
- NO → Go to step 2
- YES, but reduced → Go to step 5
- Motor running at nominal speed?
- NO → Check power supply, contactors, drive
- Measure voltage: it must be between 380-420V (±10%)
- Verify phase sequence with phasimeter
- YES → Go to step 3
- NO → Check power supply, contactors, drive
- Is the pump primed?
- Check vent valve at high point
- Check level in suction tank
- Pump unprimed → Priming procedure according to manufacturer
- Primed bomb → Go to step 4
- Dump valve open?
- Closed or partially closed → Fully open
- Open → Check impeller obstruction (step 8)
- Suction pressure within range?
- NPSH available < NPSH required → Suction problem (step 6)
- Normal pressure → Go to step 7
- Diagnosis of suction problems:
- Excessive suction height (>6 meters)
- Clogged suction filter (ΔP >0.5 bar)
- Undersized suction pipe (speed >1.5 m/s)
- Air inlet through flanges or gasket
- Characteristic cavitation noise?
- YES → Measure vibration at cavitation frequencies (0.5-2 x blade passage frequency)
- NO → Go to step 8
- Check internal status of the impeller:
- Measure radial vibration: alarm if >7.1 mm/s RMS
- Analyze spectrum: peaks in blade passage frequency indicate wear
- Motor current: if <80% nominal indicates loss of hydraulic head
Failure Matrix - Root Cause
| Main Symptom | Probable Causes (in order of frequency) | Diagnostic Test | Expected Result if Confirmed |
|---|---|---|---|
| Zero flow | 1. Pump unprimed 2. Valve closed 3. Completely clogged impeller | Visual inspection of transparent pipe Valve position verification Motor current measurement | Visible air in suction Closed position indicator Current <30% nominal |
| Reduced flow rate 50-80% | 1. Impeller wear 2. Incipient cavitation 3. Reduced engine speed | Vibration spectral analysis NPSH measurement available Tachometer on motor shaft | Peaks in freq. blade pitch NPSH disp < NPSH req + 0.5m RPM <95% nominal |
| Reduced flow <50% | 1. Partial impeller obstruction 2. Severe impeller wear 3. Suction problem | Motor current vs curve Endoscopic inspection Negative aspiration pressure | Current 40-60% nominal Visible blade wear Pressure < -0.7 bar |
| Noise + vibration | 1. Cavitation developed 2. Impeller-volute friction 3. Shaft misalignment | FFT vibration analysis Axial clearance measurement Laser alignment | Broadband 0.5-20 kHz Clearance <0.1 mm Deviation >0.1 mm |
Failure Root Cause Analysis
Cavitation
Cavitation occurs when the local pressure in the impeller drops below the vapor pressure of the liquid, forming bubbles that collapse violently. Main causes:
- Insufficient available NPSH (< required NPSH + 0.5m safety margin)
- Elevated liquid temperature, reducing available NPSH
- Viscosity greater than design
- Excessive suction lift combined with high pressure losses
Diagnostic confirmation: Vibration with broadband components in the range 500-20000 Hz, characteristic metallic noise, visible erosion at the inlet of the impeller blades.
Damage if not resolved: Progressive impeller erosion, premature bearing failure, complete loss of hydraulic capacity in 6-12 months.
Impeller Wear
Wear reduces the effective diameter of the impeller and internal clearances, decreasing the energy transferred to the fluid. Contributing factors:
- Abrasive particles in the pumped fluid
- Prolonged operation with cavitation
- Excessive peripheral speed due to overspeed
- Improper impeller material for fluid
Diagnostic confirmation: Reduced motor current (70-85% of nominal value), lower discharge pressure, hydraulic efficiency <70% of original value.
Vibration indicators: Increase in blade passing frequency (number of blades × RPM/60) and their harmonics.
Aspiration Problems
They include air entry, excessive suction lift, and suction line restrictions. Specific causes:
- Low level in suction tank creating vortices
- Loose flanges or damaged gasket on the suction side
- Clogged filters generating pressure loss >0.5 bar
- Insufficient pipe diameter (speed >1.5 m/s)
Diagnostic confirmation: Negative suction pressure (< -0.5 bar), fluctuations in flow and pressure, presence of air in the suction window.
Step-by-Step Resolution Procedures
Cavitation Resolution
- Check NPSH available:
- Calculate: NPSH available = (P atm + P gauge - P steam - h losses)/ρg
- Must exceed required NPSH + 0.5m minimum
- Reduce suction losses:
- Clean filters (ΔP <0.3 bar)
- Check pipe diameter (speed <1.2 m/s)
- Minimize accessories (elbows, reductions)
- Adjust operating conditions:
- Reduce liquid temperature if possible (<60°C for water)
- Increase suction tank level
- Install booster pump if height >6m
- Post-repair check:
- Vibration <4.5 mm/s RMS
- Absence of metallic noise
- Stable suction pressure
Replacing Worn Impeller
- Safe Disassembly:
- Apply LOTO, completely drain system
- Mark volute position with respect to casing
- Measure axial/radial clearances before disassembly
- Dimensional inspection:
- Outer diameter: tolerance -2% maximum from nominal
- Blade thickness: reduction <10% acceptable
- Radial clearances: 0.2-0.8 mm depending on pump size
- Installation of new impeller:
- Apply anti-seize grease on key
- Tighten impeller nut: torque according to manufacturer specification
- Check axial clearance: 0.1-0.3 mm typical
- Post-installation verification:
- Manual free rotation without friction
- Shaft alignment: deviation <0.05 mm
- Startup vibration <2.8 mm/s RMS
Elimination of Air in System
- Manual pump priming:
- Close discharge valve, open high point vent
- Fill casing through auxiliary connection until overflow
- Close vent, open discharge gradually
- Check suction tightness:
- Negative pressure test: maintain -0.5 bar for 10 minutes
- Inspect flanges, gaskets, threaded connections
- Retighten screws: uniform torque according to pipe DN
- Eliminate pipe air pockets:
- Install vent valves at high points
- Gradual start increasing flow
- Systematically bleed air from suction to discharge
Preventive Measures
| Root Cause | Prevention Strategy | Monitoring Method | Recommended Interval |
|---|---|---|---|
| Cavitation | Maintain NPSH available >1.2 × NPSH required | Continuous suction pressure, vibration analysis | Monthly measurement, continuous trending |
| Impeller wear | Proper fluid filtration, optimal speed | Quarterly characteristic curve, efficiency | Every 3 months |
| Obstruction | Filtering system, sand trap if required | Filter pressure drop, motor current | Weekly |
| Aspiration problems | Minimum tank level, filter maintenance | Negative pressure, air inlet | Visual diary, instrumental weekly |
| Misalignment | Post-maintenance laser alignment | Radial/axial vibration, bearing temperature | Monthly |
Spare Parts and Critical Components
| Description Spare part | Technical Specification | When to Replace | UNITEC-D Category |
|---|---|---|---|
| Centrifugal impeller | Bronze/Stainless steel, depending on pumped fluid | Wear >5% diameter or efficiency <75% | Pumps - Impellers |
| wear rings | Bronze, nominal clearance 0.2-0.8mm | Clearance >2× nominal, high vibration | Bombs - Rings |
| Mechanical packing | Carbon-ceramic-NBR, depending on pressure/temp | Leakage >50 drops/minute, every 8760h operation | Seals - Mechanical |
| Roller bearings | SKF 6308 or equivalent, ABEC-3 grade | Vibration >7.1 mm/s, temperature >80°C | Bearings - Pumps |
| Elastic coupling | NBR elastomer, nominal torque +20% | Visible cracks, residual imbalance | Transmission - Couplings |
| Glycerin pressure gauges | Class 1.6, G1/2" thread, range 0-16 bar | Error >4% full scale, every 24 months | Instrumentation - Pressure |
To check availability and detailed technical specifications of these components, visit our digital catalog: https://www.unitecd.com/e-catalog/
Technical References
- UNE-EN ISO 9906:2013 - Rotodynamic pumps. Hydraulic reception tests
- UNE-EN ISO 5199:2003 - Axial suction centrifugal pumps. Technical specifications
- API 610 - Centrifugal Pumps for Petroleum, Petrochemical and Natural Gas Industries
- ANSI/HI 9.6.3-2012 - Rotodynamic Pumps for Vibration Measurement and Allowable Values
- Related guides UNITEC-D: "Vibration Analysis in Rotating Equipment", "Axis Alignment with Laser"
