6 replies to this topic

[Reeda]

Hey can anybody tell me how to calculate the pressure of air in a closed tank which is half filled with water.

Basically the tank has a valve at the top which is opened when tank is started to be filled with water from the bottom. When the tank is half filled with water, the top valve closes. So now the tank is half water and half tank right? Will the pressure of tank be pressure of water (by row x g x h) plus the atmospheric pressure or will the air pressure in the tank be more then the atmospheric pressure as it is trapped in a restricted volume?

If anybody can tell i'l b grateful..thank you

[fallah]

Reeda,

 

Per the situation you described, just at the moment the tank is half filled and top valve closes, the pressure at the space above the tank is to be balanced with the source pressure by which the tank is filled with water from bottom.

[Reeda]

The tank is filled from a tanker through a flexible hose...i dont get it..can u elaborate a little please

[fallah]

Reeda,

 

This is a fluid static matter...

 

Gauge pressure at the space above the tank when its top valve is closed, would be equal to the pressure due to liquid column at tanker side above the liquid level inside the tank...provided that the tanker is operated at atmospheric pressure...

[Reeda]

Ok so the pressure in the tank is basically the sum of pressure exerted by the two water columns; one in the tanker and one in the tank itself...? But wouldnt the pressure at the bottom of the tank exceed the pressure of the liquid in the container if the pressure above the liquid in the tank is equal to pressure of liquid in the tanker above the level of liquid in the tank? Wouldnt that create backflow of water in hose pipe?  When two vessels with different water levels are connected to each other, the liquid from the the vessel with higher level starts to flow in the second vessel UNTIL the level becomes the same and hence the pressure becomes the same. 

Will the case remain the same if a pump starts pumping water out from the tank such that the level of water in the tank is kept constant (by continuously supplying more water in the tank from the tanker)? I think in such a case the pressure in the tank will start decreasing as the level of water in the tanker will decrease every instant right?

[fallah]

Reeda,

 

Please note that if the tanker is atmospheric and you will close the top valve of the tank after half filled , the trapped air inside the tank can be pressurized to compensate its lower liquid level than that in the tanker; then the pressure would be ballanced...

[Reeda]

Ok ok now i get it..as long as we are not pumping water out of the tank, the pressure in the tank (that of air plus the water) must be equal to the pressure in the tanker due to which the water level in the tank stops rising more then it already has (upto half of the tank). As soon as we start the pumping the pressure in the tank starts to decrease and hence more water starts coming in the tank from the tanker. All right.

The size of the tank and the level up to which the water is to be filled is designed in such a way that when water reaches upto that level then the pressure becomes same.

Thank you so much sir for bearing with me and explaining so fully