Operating Point and Correct Pump Selection in Centrifugal Pumps
Operating Point, System Curves, and Engineering Selection Methods in Centrifugal Pumps
1. The Basis of the Working Point: The Intersection of Pump and System Curves
A centrifugal pump operates at the point where the pump performance curve (H = f(Q)) intersects the system curve (H_system) . The system curve is expressed by the following basic formula:
Friction loss ( Hf ) is calculated according to the Darcy–Weisbach equation:
Therefore, the system curve has a quadratic structure: as the flow rate increases, the losses increase rapidly.
2. Technical Analysis of Pump Curves
A pump performance diagram includes the following curves:
- QH Curve: Flow Rate – Head Relationship
- QP Curve: Power consumption relative to flow rate.
- Q-η Curve: Efficiency curve and BEP point
- NPSHr Curve: Cavitation Resistance
Proper pump selection requires the operating point to be as close as possible to the BEP (Best Efficiency Point) region. Operation far from the BEP region results in vibration, increased radial load, and higher energy consumption.
3. Technical Consequences of Incorrect Operating Point
- Low pressure head
- High speed → pipe erosion
- The motor is overloading
- Vibration – increased radial load.
- Bed temperature increase
- Uneven flow and hammering
- According to test data, bed lifespan can be reduced by up to 40%.
- Energy costs rise.
4. Example of a Hydraulic Calculation:
The following example is a real engineering calculation:
| Flow Rate (Q) | 30 m³/h |
| Pipe Length (L) | 120 m |
| Pipe Diameter (D) | 65 mm |
| Coefficient of Friction (f) | 0.024 |
Speed relative to flow rate:
Friction loss:
If the static load is 12 m:
Intersection point with the pump curve: Q = 30 m³/h – H ≈ 21 mss .
5. Correction Factors for Viscous Liquids
According to viscosity correction charts:
- Yield may decrease by 10–40%.
- The head pressure decreases by 10–30%.
- Flow rate decreases by 5–20%.
- Power consumption increases by 10–60%.
Correction factors must be applied to oils, glycols, and high-viscosity chemical fluids.
6. Operating Principles of Parallel and Series Pumps
Series Pump: Discharge heads are added together → where high H is required.
Parallel Pump: Flow rate is collected → where high Q is required.
7. ORFA's Engineering Approach
ORFA applies international engineering principles in the selection of centrifugal pumps:
- System curve calculation
- IEP control
- NPSH security analysis
- Viscosity correction
- Motor power control
- Parallel/serial operation optimization
These methods ensure the correct pump selection, reduce operating costs, and increase equipment lifespan.
Conclusion
Accurate determination of the operating point in centrifugal pumps is crucial for system performance. Performing hydraulic calculations according to engineering principles is essential for energy savings, longer equipment life, and safe operation.