Efficiency Calculation
Pump hydraulic efficiency:
ρ = fluid density (kg/m³) — water = 1,000
g = 9.81 m/s²
Q = flow rate (m³/s)
H = total differential head (m)
Pshaft = shaft input power (kW)
Worked example
Pump delivers Q = 0.02 m³/s of water at H = 25 m. Motor input power = 9 kW, motor efficiency = 92%, so shaft power = 9 × 0.92 = 8.28 kW.
η = (1000 × 9.81 × 0.02 × 25) / (8280) × 100 = 4905 / 8280 × 100 = 59.2%
Best Efficiency Point (BEP)
Every centrifugal pump has a BEP — the single operating point of maximum efficiency. The pump manufacturer specifies the BEP flow and head on the pump curve. Operating at BEP minimises:
- Internal recirculation and hydraulic losses
- Radial and axial thrust loads on bearings
- Heat generation (fluid temperature rise is lowest at BEP)
- Vibration — hydraulic noise is minimum at BEP
Industry guideline: operate within 70–120% of BEP flow for reliable long-term operation. Below 70% flow, internal recirculation at the impeller eye causes vibration and erosion. Above 120% flow, cavitation risk increases and NPSH margin decreases.
Causes of Efficiency Loss
| Cause | Efficiency impact | Diagnosis |
|---|---|---|
| Worn wear rings (increased internal recirculation) | 2–10% reduction | Measure wear ring clearance; compare to new clearance spec |
| Impeller erosion / corrosion (rough surface) | 3–8% reduction | Visual inspection on disassembly; surface roughness increase |
| Operating far from BEP | 5–30% reduction | Flow measurement vs pump curve |
| Increased pipe system resistance | Unchanged pump efficiency but increased motor power for same flow | Pressure survey; compare to commissioning data |
| Air entrainment in suction | Significant — output becomes unstable | Visual inspection of suction; check NPSH margin |
Impeller Trimming to Match System
If the pump is oversized for the system (operating at throttled condition with control valve), trimming the impeller diameter reduces head and flow to match the system without throttling losses. The Affinity Laws predict the effect of trimming:
- Flow ∝ impeller diameter
- Head ∝ (impeller diameter)²
- Power ∝ (impeller diameter)³
Trimming from 250 mm to 220 mm (12% reduction) reduces flow by 12%, head by 22.6%, and power by 33%. This is a permanent, low-cost modification that can save significant energy compared to throttling. Note: trimming is irreversible — ensure the trimmed size is correctly calculated before machining.
Monitoring Efficiency Trends
Calculate and record pump efficiency at regular intervals. A declining efficiency trend indicates internal wear, increased clearances, or changing system conditions. A 5% efficiency drop below BEP efficiency is a trigger for detailed inspection on most critical pumps.
Use flow meters, differential pressure transmitters, and power meters installed on the pump to calculate real-time efficiency. Compare to the original efficiency curve from the manufacturer's test data to quantify degradation.