Question

2. A pump delivers 30o C water from a supply reservoir to an elevated storage tank at a discharge rate of 120 litres/sec. At this discharge the manufacturer’s data indicates the net positive suction head (NPSH) for the pump is 6.0 m. The water surface elevation difference between the reservoir and the tank is 45.0 m. The total length of pipe between the two is 150.0 m, 10.0 m of which is located on the suction side of the pump. Minor loss coefficients in the 10.0 m suction pipe total to 3.7. The pipe material is ductile iron and has an inside diameter of 35.0 cm. If the pump is located 1.0 m to 3.0 m above the water surface in the supply reservoir (the height of the pump above the water surface varies because the water surface in the reservoir can vary), will the pump be susceptible to cavitation?

Answer 1

AVAILABLE NET POSITIVE SUCTION HEAD SHOULD ALWAYS EQUAL OR EXCEED MANUFACTURERS NET POSITIVE SUCTION HHEAD

since manufacturers data says net positive head for pump is 6m.

we will check for availaable net positive suction head NPSH_a = p_atm / γ - h_e - h_l - p_v / γ    

p_atm is atmospheric pressurre, Y is vapour specific wieght, h_e is elevation of pump from surface of water tank.

h_l is head loss and p_v is vapour pressure.

so NPSH_a = p_atm / γ - h_e - h_l - p_v / γ    

The absolute vapor pressure(p_v) of water at temperature 30 ^oC is 4.2kN/m².

HEAD LOSS IS GIVEN 3.7, H_E IS GIVEN AS 1 TO 3 SAY 2ms, y is equal to 9.80kn/m³  If the pump is positioned above the tank, the elevation - h_e - is positive and the NPSH_a (net positive suction head) decreases when the elevation of the pump increases (lifting the pump).At some level the NPSH_a will be reduced to zero and the fluid will start to evaporate causing cavitation.

so, NSPH = (101.33 kN/m²) / (9.80 kN/m³) - 2-3.7-(4.2 kN/m²) / (9.80 kN/m³) = 4.21M

SINCE 4.2 IS LES THEN 6M HENCE WATER WILL BOIL AND CAUSE CAVITATION.