Question

A pump move warm water at 120F from a reservoir open to atmosphere. The pipe is 6" diameter cast and has a standard 90 degree elbow and a fully open gate valve. The pump draws water from a pipe submerged below the water surface. The pump is 10 feet above the reservoir surface, and there are 20 feet of pipe in the inlet section. If the flow rate is 1,650 gpm, what is the NPSHA at the pump inlet?

Answer 1

Given data: Cast iron pipe size, D = 6" = 0.5 ft. Pipe length, L = 20 ft. Flow rate, Q = 1650 gpm Solution: The equation of NPSHA is NPSHA = h, +/-h-ht+hy - hvp ......Eq. (1) Where: h, is the atmospheric or absolute pressure. h, is the vertical distance from the surface of the water to the pump centerline. h, is the friction formed in the suction piping. h, is the velocity head at the pump's suction. h, is the vapor pressure of the water at its ambient temperature. Here, h. = 14.7 psi = 33.9 ft of water h = - 10 feet Calculate friction loss in pipe. LV2 hfi= f* * .... Eq. (2) D 29 Let us calculate Ronald's no., R. R = (DV)/v Here, D = 0.5 ft.

D2 220.57 20.52 V = 1123 ft/min V = 18.72 ft/s v = kinematic viscosity = 6.06 * 10 ft/s R = (0.5*18.72)/6.06*10* = 1544765 It is turbulent flow. Here, f = Darcy friction factor 0.0791 f=- RO 0.0791 15447651 f = 0.0022 Substitute values off, L, D and Q in Eq. (2) 20 18.722 hfi = 0.0022* * 0.5 * 2 * 32.2 * hfi = 0.478 ft Now, Calculate h.

ho=29 18.622 hv = 2 * 32 hy = 5.4 ft Head loss due to 90° bend is hf2Fkoh,=0.7*5.4 = 3.78 Calculate vapor pressure head, he hup = pg Here, P. = vacuum pressure of water at 120-F = 1.6951 psia =1.6951*144 psf P = 1.94 slugs/ft g=32 ft/s 1.6951 * 144 hup = 1.94 * 32 hyp = 3.93 ft Substituting the respective values in eq. (10 NPSHA = h. +/-h-h; +h.-H = 33.9 - 10 -(0.478+3.78) +5.4-3.93 NPSHA = 21.11 ft.