Gene Vogel
Pump & Vibration Specialist
Electrical Apparatus Service Association
St. Louis, MO
In the paper "Understanding Pump Curves for Troubleshooting Pump Performance," presented at the EASA Convention 2010, Gene Vogel provides a comprehensive guide to interpreting pump curves and using them to diagnose and resolve pump performance issues. The paper emphasizes the importance of understanding key parameters such as capacity, head, efficiency, and horsepower, which are essential for troubleshooting pumps, much like current and voltage are for motors.
Pump curves depict the relationship between capacity and head, with capacity plotted on the horizontal axis and head on the vertical axis. The curve shows the pump's performance from shut off (zero flow and maximum head) to run out (maximum flow and minimum head). The ideal operating point is near the Best Efficiency Point (BEP), where the pump runs most smoothly and efficiently. Efficiency curves, plotted alongside pump curves, indicate how efficiency varies with flow, peaking at the BEP and decreasing towards shut off and run out conditions.
Horsepower requirements at any operating point can be calculated using capacity, head, and efficiency. For most centrifugal pumps, horsepower increases with flow. Viscosity, which represents fluid resistance to flow, affects efficiency and must be considered when evaluating pump performance. Multi-trim curves show how trimming the impeller to a smaller diameter can adjust pump performance to match system needs, though this reduces efficiency slightly. Variable-speed drives (VSDs) offer a more flexible and efficient alternative to trimming impellers.
System curves represent the resistance to flow presented by the entire hydraulic system, including pipes, valves, and other components. The total dynamic head (TDH) is the sum of static head, velocity head, and friction head. Static head is the height difference between the suction and discharge levels, velocity head is the energy required to accelerate the fluid, and friction head is the resistance due to fluid contact with pipe walls and other surfaces.
Cavitation, a common issue in pumps, occurs when pressure drops below the fluid's vapor pressure, causing bubbles to form and implode, damaging the impeller and causing vibration. Net Positive Suction Head (NPSH) is critical for preventing cavitation, with NPSH Available (NPSHA) needing to exceed NPSH Required (NPSHR).
Common errors in pump installations include over-sizing pumps, mismatched pumps in series or parallel, restrictions on the suction line, poor base and alignment, unsupported piping, and inadequate protection from erosion and corrosion. Troubleshooting involves verifying fluid properties, checking for cavitation and air entrainment, and ensuring the pump operates on its curve. If the pump operates on its curve but performance issues persist, the problem likely lies within the system.
Key Points Covered:
- Importance of understanding pump curves for troubleshooting
- Relationship between capacity, head, efficiency, and horsepower
- Effects of viscosity and impeller trimming on pump performance
- Role of system curves in determining operating points
- Causes and solutions for cavitation
- Common errors in pump installations
- Steps for troubleshooting pump performance
Key Takeaways:
- Understanding pump curves is essential for diagnosing and resolving pump performance issues.
- The Best Efficiency Point (BEP) is the ideal operating point for pumps.
- Viscosity and impeller trimming affect pump efficiency and performance.
- System curves help determine the maximum operating point of a pump.
- Preventing cavitation requires ensuring NPSHA exceeds NPSHR.
- Common installation errors can lead to inefficient and unreliable pump operation.
- Thorough troubleshooting involves checking fluid properties, cavitation, and system issues.
To read the full technical paper and view the slides, download the PDF below.
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