11.3 Pump and System Characteristics

3 min readLast Updated on July 19, 2024

Pump performance curves are essential tools for understanding how pumps interact with fluid systems. They help engineers determine optimal operating points, efficiency, and power requirements. By analyzing these curves, we can select the right pump for a given system and optimize its performance.

Designing and troubleshooting pump systems involves considering factors like surging, stalling, and cavitation. By carefully calculating head loss, selecting appropriate piping, and ensuring sufficient net positive suction head, engineers can create efficient and reliable pump systems that avoid common problems.

Pump Performance and System Interaction

Interpretation of pump performance curves

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  • Pump performance curves provide essential information for determining the operating point and performance of a pump in a given system
    • Head-capacity (H-Q) curve relates the pump's discharge head to its flow rate (gpm, m^3/hr)
    • Efficiency curve shows the pump's efficiency at different flow rates (50%, 75%, 90%)
    • Power curve illustrates the power required by the pump at various flow rates (kW, hp)
    • NPSHR curve represents the net positive suction head required to prevent cavitation (m, ft)
  • Operating point is the intersection of the pump's H-Q curve and the system's resistance curve
    • Determines the flow rate and head at which the pump will operate in the system (200 gpm at 50 ft head)

Pump and system interaction analysis

  • System resistance curve represents the relationship between head loss and flow rate in a system
    • Influenced by factors such as pipe size (4 in, 200 mm), length (100 ft, 50 m), material (PVC, steel), and fittings (elbows, valves)
  • Pump selection involves choosing a pump with an H-Q curve that closely matches the system's resistance curve
    • Ensures the pump can deliver the required flow rate and head at the desired efficiency (80% efficiency at 500 gpm)
  • Pump operation can be optimized by adjusting the system's resistance to move the operating point closer to the pump's best efficiency point (BEP)
    • Variable speed drives or throttling valves control the flow rate and optimize pump performance (1750 rpm, 50% valve opening)

Effects of system resistance changes

  • Increased system resistance causes the operating point to move left on the H-Q curve, reducing the flow rate
    • Increases the head developed by the pump (from 50 ft to 60 ft)
    • May cause the pump to operate at a lower efficiency (from 85% to 75%)
  • Decreased system resistance causes the operating point to move right on the H-Q curve, increasing the flow rate
    • Decreases the head developed by the pump (from 50 ft to 40 ft)
    • May cause the pump to operate at a higher efficiency (from 85% to 90%) but could also lead to overloading (motor current exceeds rated value)

Pump System Design and Troubleshooting

Causes of pump surging and stalling

  • Pump surging occurs when the pump operates alternately between high and low flow rates
    • Caused by a mismatch between the pump's H-Q curve and the system's resistance curve (unstable intersection point)
    • Leads to excessive vibration, noise, and damage to the pump and piping (fatigue, seal failure)
  • Pump stalling happens when the pump cannot generate enough head to overcome the system's resistance
    • Caused by a significant increase in the system's resistance (clogged filter) or a decrease in the pump's speed (power failure)
    • Results in a drastic reduction in flow rate and potential damage to the pump due to overheating (seized bearings)

Design of pump piping systems

  • Head loss occurs due to friction, fittings, and changes in pipe size or direction
    • Calculate head loss using the Darcy-Weisbach equation: hf=fLDv22gh_f = f \frac{L}{D} \frac{v^2}{2g}
    • Minimize head loss by selecting appropriate pipe sizes (larger diameter), materials (smooth surfaces), and layouts (fewer bends)
  • Cavitation is the formation and collapse of vapor bubbles in the fluid due to localized low-pressure regions
    • Causes damage to pump impellers, reduced performance, and increased noise and vibration (pitting, erosion)
    • Prevent cavitation by ensuring sufficient net positive suction head available (NPSHA)
  • Net positive suction head (NPSH) is a critical factor in pump system design
    1. NPSHA is the difference between the fluid's absolute pressure at the pump inlet and its vapor pressure (10 ft, 3 m)
    2. NPSHR is the minimum NPSH required by the pump to avoid cavitation (8 ft, 2.5 m)
    3. Ensure NPSHA > NPSHR by proper design of suction piping (larger diameter, shorter length) and maintaining adequate suction pressure (flooded suction, suction tank)
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© 2025 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.

© 2025 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.
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