APPENDIX G – Pumps and Turbines
Surge Analysis and the Wave Plan Method
Supplementary Material: Example Problems and Solutions
Appendix G – Problem 21
G.21 A pump manufacturer has fabricated a new pump impeller and performed a dry-run to determine its moment of inertia. A motor was used to bring the impeller to a steady state speed of 1800 rpm during the dry-run (with no liquid in the pump casing) and the power supply to the motor was cutoff at time t. The torque measured on the pump shaft at time t was 2500 N-m. A tachometer showed a speed of 1635 rpm at time t+2 seconds. If the moment of inertia of the pump shaft and motor armature coil (or the rotor connected to the pump impeller) are 26 N-m2 and 853 N-m2 respectively, what is the moment of inertia of the impeller.
Solution:
Given the unbalanced torque of τ = 1200 N-m at time t, the change in speed of ∆N = 1800-1635 = 165 rpm in time ∆t = 2s, the combined inertia I of the pump-shaft-motor assembly based on Eq. G.6 is 138.9 kg-m2 or 1362.59 N-m2. The inertia of the pump impeller alone is 1362.59 – 853 – 26 = 483.59 N-m2.
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