×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Are you an
Engineering professional?
Join Eng-Tips Forums!
• Talk With Other Members
• Be Notified Of Responses
• Keyword Search
Favorite Forums
• Automated Signatures
• Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Can fluid pressure be overcome

Can fluid pressure be overcome

(OP)
If I have a tank of water 230 feet deep, and I have an open bottom barrel at the bottom that I want to fill with air, and I am pumping the air directly from the outside of the tank at the bottom (direct source of air so that I don't have to have 230' of hose to get to the bottom), I know that I need to be pumping the air at 100 psi to overcome the water pressure.
Can the pressure needed to pump the air be reduced by sealing the tank???
Example:  A 2 liter bottle of water turned upside down; the water will come out unless the opening makes contact with another body of water.  Because the bottle is sealed at the top, the surface tension of the water is enough to stop the flow of the water.
Finally what laws and equations can I use to determine this???

Thanks
Replies continue below

RE: Can fluid pressure be overcome

(OP)
I forgot to mention the fact that there would be a one way valve at the top of the tank of water to stop the build up of pressure inside the tank.

RE: Can fluid pressure be overcome

Surface tension has nothing to do with it.  If you turn a bottle upset side, the water will try to run out but as it does, the pressure at the top would decrease and the flow stops.  You simply have a pressure balance between the surrounding atmosphere and the static head of the water in the jar and the pressure inside the bottle at the top (which will be atmospheric pressure minus the static head).

If you do the same experiment with a tube of mercury, about 33 inches will remain in the tube when you invert the tube.  In this case, the pressure above the mercury is a vacuum (except for the vapor pressure of the mercury).  The 33 inches of mercury = the pressure exerted by the atmosphere.

So, back to your question, you'll still need 100 psi to pump air into that barrel.

(OP)

RE: Can fluid pressure be overcome

This sounds an awful lot like a homework problem.....

Is this for some type of real-world application ??? If so, please explain......

You are not trying to take advantage of us ......are you ??

MJC

RE: Can fluid pressure be overcome

If you seal the tank as suggested it is no longer a tank/barrel/bottle but it becomes a pressure vessel subject to external pressure. In this case you could fill it with less (or zero) pressure, but the vessel have to be able to withstand the 100psi in order not to collapse, and this is probably a lot more expensive than getting air at full pressure.

regards
Mogens

RE: Can fluid pressure be overcome

(OP)
I'm experimenting with a few theories of mine.  No I'm not an engineer but I have this problem with ideas and numbersin that when I get an idea in my head, and being as good with numbers as I think I am, I can't sleep at all until I get the idea either solved or eliminated as being  plausible.  Since typically I don't even know the area of math that I need to know to solve the equations that pop into my head, I search the internet to try to find the answer to my questions.  I don't understand why I seem to know that there is a particular equation involved with my idea, but the equation will usually come to me and then I need to find a proper source to see if my equation is accurate.  I don't even know what books would help me to just learn the proper math involved or I wouldn't be bothering anybody else.  But I figure that it can't hurt to ask. ...........(deep breath).
Now as to your question to whether this is for a real world application, yes, absolutely.  Problem is, that the idea is so delicate that I can't reveal the whole idea.  I wish the world was perfect and I could divulge all, but I can't.  All I can do is carefully figure out how to word a question as to not give away too much and hope that I worded it properly as to be understood.

RE: Can fluid pressure be overcome

(OP)
The best way I can put this is as follows:
A ten cubic foot open bottom barrel full of water at the bottom of a 230' sillo full of water.  An air compressor at the base of the silo to fill the 3 cubic foot barrel.  The air comp. will pump 3 cubic ft of air per minute at 100 psi (per manufacturer spec. sheet).  I know that at 230', the water pressure is approx. 100psi.  So theoretically, it should take 1 minute to displace 3cf of water.
Now, if I seal the silo and put a one way valve at the top to allow excess air pressure to escape but not to enter, will this change the amount of pressure required by the air compressor to pump that same 3 cf of air?  (I think your answer above indicates that it would take less than 100 psi)

I hope that it's not a problem that I am not an engineer, nor am I doing this for some corporation.  But I do have a huge thirst for learning and knowing that which is unknown to me, or at the very least puzzles me.  I'm sure I'm not the only one like this.
Anyway, thank you for your input and help, it is greatly appreciated.

RE: Can fluid pressure be overcome

If you seal the top of the silo with a lid capable of withstanding vacuum and then pump the air out above the surface of the water, the pressure you need to push air into your barrel at the bottm of the silo WILL decrease by the amount of suction you generate under the lid (maximum improvement is not quite one atmosphere - eventually the water will start to 'boil' ).

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

• Talk To Other Members
• Notification Of Responses To Questions
• Favorite Forums One Click Access
• Keyword Search Of All Posts, And More...

Register now while it's still free!