In the following figure the pump delivers 3 cfs flow of 80 d

In the following figure the pump delivers 3 cfs flow of 80 degree F water, and the intake pipe diameter is 8 in. The pump intake is located 6ft above the water surface level in the reservoir The pump operates at 2750 rpm. What are the net positive suction head for these conditions?

Solution

GIVEN DATA: Delivery Q=3 cfs=0.0849 m³/sec Temperature T= 80 deg F Diameter D=8 inch= 0.203 m. Suction head h_s=6 ffeet=1.828m. Speed N=2750rpm. TO FIND: NET POSITIVE SUCTION HEAD.(NPSH) SOLUTION: NPSH=P_s/Specific weight + V²_s/2g - P_v/Specific weight of water.....Specific weight of water = 9768 N/m³ at 80 deg F. g= 9.81 m/s² acceleration due to gravity....Vapour pressure P_v at 80 deg F= 3560 N/m². TO FIND P_s: P_s= (h_s-V²_s/2g) × specific weight of water......TO FIND V_s= Q/A=0.0849/(3.14/4 × 0.203²) = 2.628 m/sec.....P_s=(1.828- 2.628²/2×9.81)× 9768= 14681.3 N/m^2.. THEREFORE NPSH= 14681.3/9768 + 2.628²/2×9.81 - 3560/9768 = 1.495 metres = 4.9 feet.