Flowrate of compressed air required

[Torres88]

How do I determine what flowrate of compressed air at fixed pressure is required to force a ballast tank full of water submerged at 70m with a volume of 60m^3. The outlet line size is 2inches for the water outlet and the air inlet is 1 inch. Please help if you can. The air will expand as the vessel starts floating up right which makes it even more complicated.

 

[quark]

Calculation is not that difficult but executing the work is. Why do you want to do it? What precautions you are going to take during the execution? At what rate you are planning to empty out the vessel? Air expands as the vessel empties out but not as it starts floating up. The reduction in static head can (or can't) compensate the expanding air depending upon the water out flowrate you require.

Just tell us your safety precautions and we will try to help.

 

[Torres88]

This is basically a submerged buoy which has to be retrieved by blowing air into the 6 compartments completely filled with water to a total volume of 360 cubic meters. We would like to raise the buoy in about an hour. So the total flowrate of air has to be 360 m^3/hr. Water will be evacuated at depending on how fast the air comes in I guess.

 

[Torres88]

I was wondering if we could pump in a fixed volume of compressed air and since switch of the compressor. The air will expand as the buoy rises to the surface. So for example if you are at 70 m depth and you rise to the surface the pressure changes from 8 bar to 1 bar right so you have an expansion factor of 8 which causes the volume of the air to increase 8 fold. As long as this volume is more than 60 m^3 all the liquid would be out and the buoy will rise to the surface right? Am i correct or is this totally wrong?

 

[abcmex]

Dear Torres,
Perhaps you should consult this with someone who has experience in this kind of manoeuvres, a supplier delivers not only manpower but also experience

Now to your question from a very theoretical point of view . To lift the element you need to equalize the gross weight of the element with the weight of the displaced water by air. Therefore you need to know the weight of the element plus the weight of the water inside of the element. Once you have displaced more weight than the remaining water and the net weight then the buoyancy will start lifting the element.

http://en.wikipedia.org/wiki/Buoyancy

Now comes the second part of your problem. I agree to your conclusion that the compressed air inside of the element will start to expand and displace more water out of the compartments as soon as the upward movement starts, increasing the buoyancy and the accordingly the speed. This could be dangerous, that is why I think you need some professional help.

Submarines perform sometimes emergency surfacing, the huge ships partially jump out of the water. This is a good example of uncontrolled buoyancy.

Only as example:
If you put 60 m3 (Pressurized at the operation conditions 60 m) and the buoy was designed to float with 360 m3 (pressurized at operation conditions 4 m3?) there will not be enough buoyancy.

rgs

 

[Torres88]

If I have a compressor delivering the compressed air at 8 bar. I am assuming this pressure would need to be regulated at some point. How do i do this? At 70 m depth 8 bar is required to overcome the static liquid head but as the buoy rises the pressure decreases and the pressure differential increases, not to mention the fact that the air is expanding. Is there a way to account for this? We don't want the buoy shooting out of the water.

 

[roydm]

The air will only expand if you let it, If the outlet is blocked when it starts to float the air will not be able to expand however the vessel would have to withstand the high pressure at the surface. Perhaps a pressure relief valve that will vent pressure as it rises.
Roy