Lecture Notes on Centrifugal Pumps (Chapter 11)

Introduction to Centrifugal Pumps

• Common types of pumps used in various applications (e.g. swimming pools, spas).
• Students will be conducting tests on centrifugal pumps in the fluids lab (ME313).

Testing Procedure

• Setup involves a pump in a circuit with water from a reservoir.
• Key components: flow meters, pressure gauges, and tachometer.
• Suction Side: Where water enters the pump.
• Discharge Side: Where water exits the pump.

Energy Equation

• The energy equation is written from the suction side (Point 1) to the discharge side (Point 2).

• The equation accounts for pressure, velocity head, and elevation:

  [ \frac{P_2}{\gamma} + \frac{V_2^2}{2g} + Z_2 = \frac{P_1}{\gamma} + \frac{V_1^2}{2g} + Z_1 + HP ]

• Neglect velocity head and elevation differences in most tests.

• Continuity relation: Q1 = Q2 (flow rate is constant).

Performance Curves

• Performance curves are created by running the pump at constant speed with varying flow rates.
• Typical Head Curve: Starts off constant and drops off steeply at higher flow rates due to losses.

Additional Curves

• Power Input Curve (W_dot_in):
  • Shaft power = Torque × Pump Speed (ω).
  • Power can be expressed in kilowatts or horsepower.
• Pump Efficiency Curve:
  • Defined as [ \eta = \frac{W_{out}}{W_{in}} ]
  • Peaks at the best efficiency point, declining at lower and higher flow rates.

Manufacturer's Data

• When acquiring a pump, manufacturers provide performance curves.
• Important performance metrics include shutoff head (max head with zero flow) and efficiency curves.
• Different impeller diameters affect the head developed by the pump.

Net Positive Suction Head (NPSH)

• Defined as the height of fluid above the pump suction to avoid cavitation.
• Equation for NPSH:
  [ NPSH = \frac{P_{atm} - P_v}{\gamma} - H_L ]
  • Where:
    • P_atm = atmospheric pressure,
    • P_v = vapor pressure of the liquid,
    • H_L = head losses in the piping.

Pump Selection

• Pump Specific Speed (N_s):
  • Calculated using flow rate (Q) and head (H) at the best efficiency point.
  • Different ranges of N_s guide pump selection:
    • 500 to 4000: select a centrifugal pump.
    • 4000 to 10,000: mixed flow pump.

    • 10,000: axial flow pump.

Pumping in Pipe Networks

• Example problem involving a centrifugal pump transferring water between two reservoirs.
  • Calculating flow rate (Q), pump head (HP), and power input.
• System Head Curve vs. Pump Head Curve:
  • Where these curves intersect indicates the operating point of the pump.

Homework

• Homework will be conducted via McGraw-Hill Connect.
• Example problem involves calculating ΔZ for flow rates through multiple pipes using the Moody chart for friction factors.

Closing

• Review of key calculations before the next class.
• Reminder to start on the Connect homework.