Question 12 from the 2010 case, morning session
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12. A screw pump that transports a medium with a density of 1300 kg/m3, located at a vertical height of 3.5 m above the liquid level on the suction side of the pump; the maximum pressure at the pump outlet is 0.6 MPa, and the maximum flow rate is 30 t/h. Based on a pump efficiency of 75%, what is the shaft power (kW) of the pump? (A) 4.7 (B) 5.1 (C) 3.5 (D) 6.112 Solution: Q = 30000 / (3600 × 1300) = 0.00641 (m3/s). H = ΔP / (ρgN) = HgρQ / (1000η) = ΔPQ / (1000η) = 60000 × 0.00641 / (1000 × 0.75) = 5.1 (kW). Answer: B. Is the height of the liquid level on the suction side, which is 3.5 m, not taken into account? It seems that H should be H1+H2=△P/ρg+3.5m=47.05+3.5=50.55m. And N should be HWsg/1000η=50.55*30000*9.81/(3600*1000*0.75)=5.5Kw!
A. A screw pump is a positive-displacement pump, and therefore the shaft power calculation formula for positive-displacement pumps should be used: N=100(P_outlet – P_inlet)×Q/(360×efficiency)