PSI needed at 136 ft. underwater to push water out. 1

[imjasper]

I know that at 136 ft underwater, there is approx 100psi on an object. If it were a barrel with an open bottom, and I were pumping air into the barrel to push the water out, would I need 100 psi to do this? Or would the fact that the water would be escaping out the bottom reduce this?
In other words, I know that there is 100 psi on the top of the barrel, but is there also 100 psi at the bottom of the barrel as well?
I realize this is not very analytically worded, but I need to know this for a project that I'm working on.

 

[DeepBlues]

The problem is a little vague, but I'll take a stab at it. First off, I get 60.5psi at a depth of 136ft salt water. Slightly less for fresh water(59psi).
Secondly, you also need to take into account the length of the barrel. If it's like an oil drum then there won't be much difference, but if you're trying to pump the inside of this tank dry, you need to overcome the pressure at the bottom of the tank. No, it does not matter that the tank is open bottomed. If you don't apply enough pressure then the air wont reach the bottom of the barrel. Also, keep in mind you need to account for the pressure drop along the length of hose you intend to use (which will need to be alot longer than 136'). This will further increase the required pressure.
And just a word of caution - please don't even think of using this as a 'diving bell' or any life support for humans. There are many regulations on that type of equipment, and I wouldn't even consider advising you how to do that based without knowing all of the factors involved.
Hope this all helps.
-DeepBlues

 

[imjasper]

I'm sorry, I mispoke myself, I meant approx 230 feet. The water would be held in a silo and the air would be being pumped from the bottom so that the air hose would actually be pumped through a hose only a few feet long. And the silo will be air tight with a pressure relief valve at the top.
I guess what I'm trying to find out is this:
If I put a 3.5 cubic foot barrel at the bottom of the silo of water, using a pump pumping 3.5 CFPM @ 100 psi, would this even out and pump exactly 3.5 CFPM since the water pressure at that depth is 100psi? And would the silo being air tight with a one way valve (allowing pressure to escape)
change the required pressure of the pump to move the 3.5 CF in less time?

 

[DeepBlues]

Sorry, running out of town. Don't have time to look at your question well enough to give an answer. Be back next week, or maybe someone else can help.

 

[DeepBlues]

I have re-read your question and I'm afraid it's a little too complicated for me to answer on a message board. My gut says no, but again this would be a lengthy analysis. Making the silo airtight makes it even more complicated.
The base equation you need to use is p1v1t1=p2v2t2. Temp will remain constant, or negligibly different (I think). You know p1 and p2. V1 is the kicker though... That may be the C.I.R. of your pump. Not sure.
Hope this helps.
-Deep Blues

 

[MrEngineer45]

As for the barrel: the interior will be nearly isobaric,
if it is a small barrel.
Figure 2.31ft/psig fresh water.