Example(Cavitation)

Greetings of the day!!!!

Hope everyone got familiar with the terminology from the previous blog(Cavitation) do checkout to avoid chaos 👍.

Let us together evaluate various scenarios to better understand what NPSH is .....Screenplay of the blog is as follows

-->Example-1 (positive experiment ::Where NPSH 💔💔 cavitation)

-->Example-2(Negative experiment::Where NPSH 💕💕cavitation)

-->How to avoid cavitation????

Example -1

Question/headache:

Consider tank filled with water till 6m and at a pressure of Pabsolute= 1atm containing water at 80c which is to be transported through a pipe of length 30m to the pump .Will the pump cavitate?

Answer & Relief:

• Assume frictional losses in the 30m long pipe to be 🔺P=0.15bar or hfs=1.5m(More on frictional losses in the upcoming blogs)

• Assume density=1000kg/m^3

• Ignore minor losses due to valves and flanges

1. Pabsolute( absolute Pressure)=1atm or Pabsolute/rho*g=10.33m.

2. P6(Static pressure) =0.6 bar or P6/rho*g=6m.

3. 🔺P=0.15bar or🔺P/rho*g=1.5m

4. Pv(vapor pressure of liquid at 80c)=333.3 torr or Pv/rho*g=4.766m.

Formula/Zandu balm:

NPSH=Net head at suction side(or at point 2 in the diagram) - Vapor pressure..........(1)

• Net head at suction side is nothing but summation of all gains and losses i.e Pabs/rho*g+P6/rho*g-🔺P/rho*g

From (1)

NPSH =10.33+6m-1.5 m - 4.766m

=10.04m ....................(Hurray its positive!!!)

The answer obtained is NPSH available which should be greater than required for effective functioning of pump.

And the answer for the question will it cavitate no it doesn't...

Example -2

Question/headache:

Consider a tank filled with water till 6m and containing water at 80c which is to be transported through a pipe of length 30m to the pump .Will the pump cavitate?

Answer & Relief:

• Assume frictional losses in the 30m long pipe to be 🔺P=15bar or hfs=150m(More on frictional losses in the upcoming blogs)

• Ignore minor losses due to valves and flanges.

1. P6(Static pressure) =0.6 bar or P6/rho*g=6m.

2. Pabsolute( absolute Pressure)=1atm or Pgauge/rho*g=10.33m

3. 🔺P=15 bar or🔺P/rho*g=150m

4. Pv(vapor pressure of liquid at 80c)=333.3 torr or Pv/rho*g=4.766m.

Formula/Zandu balm:

NPSH=Net head at suction side(or at point 2 in the diagram) - Vapor pressure..........(1)

• Net head at suction side is nothing but summation of all gains and losses i.e P6/rho*g+Pabs/rho*g-🔺P/rho*g

From (1)

NPSH =6m+10.33-150m - 4.766m

= -ve value ..........(Zandu balm isn't working😁😅 )

And the answer for the question.. Will it cavitate yes it does....

How to avoid cavitation ?????

Well , you should have known by now that cavitation can be avoided by maintaining net head on the suction side greater than the vapor pressure which can be done by increasing the pressure in the tank or by increasing the height of the tank( which is not economical in this case but there are cases where this is employed....)

Next blog is of your choice.....you people comment and let me know will be releasing it as soon as possible......

Kudos to my boss for allowing me to write this blog in the office 😉😉......just kidding 😅

Disclaimer: Calculations are not exact to decimals.....Will not effect anything though

Image credits: rodelta.com