A certain pump delivers 1559 L/hr of water. The top of the tank shown in the Fig

Question

A certain pump delivers 1559 L/hr of water. The top of the tank shown in the Figure is open to standard atmosphere, thus the temperature of water is the same as Today's high outdoor temperature. The circular piping of total length 8 m is made out of 3-nominal schedule 80 PVC pipe and the fittings are two 90 degree long radius flanged elbows. The net positive suction head required is 1 m. State assumptions and show references and work. Use appropriate significant figures. What is z_s in m? Determine the Reynolds number of the water flow before entering the pump. Recommend a realistic z_s between the open tank surface and the pump to prevent cavitation in m. What happens to the pump blades or impellers if cavitation occurs?

Explanation / Answer

For Nominal pipe size 3 inch:

Inner diameter, d = 7.366 cm = 0.07366 m

Volume flow rate, Q = 1559 L/hr = 1.559 m3/hr = 68.678 gal/ min

Velocity of flow in pipe, V = Q/(area based on inner diameter) = 0.10162 m/s

Now from available data in engineering domain for schedule 80 3" PVC pipe:

for volume flow rate 70 gal/min friction head is 1.5 m/ 100m

Therefore , we can say that practically we can assume 1.5m/ 100 m

i.e. 0.015m per unit metre of pipe we will have this loss

other losses like the one in bend are minor and hence ignored.

So Zs will be 1.015 m for 1 m NPSH required.

b)

if dynamic viscosity of water is 8.90 × 104 Pa. s

Reynolds number = (density x velocity x diameter)/ dynamic viscosity = 8410.488

c) Practically while viewing this problem as a realistic we can take take Zs to be 1.2 m to prevent cavitation.

d) Vanes of impeller will get damaged if cavitation occurs.

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