Draining a Closed Tank

[robinhill510]

Supposing you had a tank mounted to the outside of a submersible vehicle.
The tank lid is left open, and the vessel that taken down to 200m sea water depth, to top of tank.

The tank lid is now closed at this depth; it is assumed that the tank is completely full, with no air gap/bubble on top.

The interior atmosphere of submersible vehicle is at normal atmospheric pressure (1 bar). If the valve on the bottom of the tank is opened, how do you calculate the quantity of water that you can drain from the tank into the vessel (ignore pressure rise in the vessel due to the ingress of water) before the low pressure generated in the tank prevents further sea water egress?

http://files.engineering.com/getfile.aspx?folder=ff256411-4753-47d8-90ef-ca10430b41ce&file=1.jpg

 

[BigInch]

No water will drain out, unless you let in some air from the submersible interior. One atmosphere inside the submersible will be more than enough to keep all but a couple of drops from coming out of the water tank outlet. You need an outlet large enough that can let in air at the same time it lets water out, or actually it's best to have an air inlet line connected from the inside of the submersible to the top of the water tank. Essentially it will work the same as it does in your kitchen. Get a couple of rigid milk bottles, connect them together with a 1/4" plastic tube and test it there. Put water in the bottom bottle and invert. Then connect them mouth to mouth, without the 1/4" tubing and try it again.

Independent events are seldomly independent.

 

[racookpe1978]

Your question has the "tank lid" closed while at depth? Then the tank is drained into a submersible without venting?

Sounds like a homework problem to me/ What is the diameter of the drain piping into the submersible? Show your schematic/P&ID.

 

[btrueblood]

The "couple of drops" that BigInch mentions will depend on the volume of the tank. The water in the tank is intially compressed at 200m depth, and then is decompressed to 1 atm pressure when the valve is opened. The compressibility of water Link will cause the volume of water in the tank to expand, this causes the few drips of water to come through before the pressure equalizes.

 

[BigInch]

I wasn't really considering pressure or temperature at all. I was only thinking about gravity effects.
He'll get a lot more drops if he considers a possible temperature change, rather than the 20 bars he'll have from the 200 m depth. I suggest that 20 BarA to 1 BarA pressure expansion of 1m3 of water will only be about 1/2 of a teardrop. I always say everything, including water, is compressible, but the truth is that water isn't very compressible at all, least as far as the usual compressible things go. Maybe another 2 teardrops will be squeezed from the tank as it compresses from the exterior pressure, but we don't know the size, shape, or what the tank's made of.

Independent events are seldomly independent.

 

[77JQX]

Another concern with gravity only effects is the height of the tank. If it's over ~ 33 feet (depending on temperature) then it will go vapor at the top, and only the bottom 33 feet or so will remain in the tank (ignoring, as the OP said, the pressure rise in the reveiving vessel).

 

[berkshire]

I think one thing that is not being considered here is the contraction of the outside tank when the valve is opened to the larger vessel at 1atm.
The outside water pressure will be somewhere around 6atm, for a fraction of a second ,there will be an 80psi spurt of water until the pressures equalize.
Unless the tank is made from a material so thick ,it cannot expand or contract.
B.E.

You are judged not by what you know, but by what you can do.

 

[chicopee]

An analogy to the OP is the watering can used in chicken coops. As the chickens drink from the trough, the water level in the trough will drop below the openings of the water can thereby breaking the water seal and thereby empty some of the water from the watering unti the water is restored than the water will no longer drop, until the water is broken again and again. So if the drain water become water sealed in the submersible than there is no more leakage from the tank. The physics behind this scenario is deals with partial pressure. Yes folks when I was in high school and college, I worked in farms and ranches.

 

[berkshire]

I just realized I got my units mixed up ,I calculated that at 200 feet not 200 metres.
B.E.

You are judged not by what you know, but by what you can do.

 

[BigInch]

Nothing like practical experience hey Chicopee.

Independent events are seldomly independent.

 

[chicopee]

Yep! BI

 

[BigInch]

After taking a couple of structural analysis classes, I really learned how to apply it at my part time job designing Gangnail roof trusses.

Independent events are seldomly independent.