Fluid dynamics

[Brian D]

OK, for all you fluid engineers out there...

Assume there is some internal pressure in the tanks. My assumption is that the weight of the fluid has a effect on the way the fluid is pushed out of the container

1. Say one has tandem container. Say that both containers are full. If both containers are connected to a consumption source, would it be safe to assume that the fluid form both containers are drawn equally?

2. Say one has tandem containers, but one container is 50% and the other 100%. Would fluid dynamics draw from the full full container until both containers are of equal pressure or will they both be drawn equally until one container is empty?

3. Would the full container assist by feeding into the partially filled container to equal the internal pressure?

This should be a good discussion.

 

[Stu Jackson]

Are both at the same level? Are both open and vented? Are you talking about two water tanks to a common header and shutoff valves for each?

 

[Scott T-Bird]

Brian, there are too many holes in your question but here goes:

When the 2 tanks are connected atmospherically above the liquid surface (vented) and also connected below the liquid surface by a pipe network that is connected to both tanks then the liquids will always seek the same surface level because the only internal pressure inside the tank is atmospheric pressure at the liquid surface and the liquid pressure + atmospheric pressure below the surface (the same at all levels). If this is the case, then the liquid in the tanks can't rest at differing surface levels unless there are shut off valves that disconnect the connectivity.

If the liquid is being drawn out of just one tank, and the tanks are connected by pipe below the surface, the liquid surface should always be the same until one tank empties, if the bottom is higher than the other. Then you will be sucking air out of the empty tank and not affecting the liquid level of the lower tank, unless the pipe is connected to the lower tank. If the bottom of the upper tank is higher than the top of the lower tank and you are drawing from the lower tank, then the lower tank will be filled until you've emptied the upper tank.

This assumes that the tanks are vented and you are not dealing with the vacuum that is created if you draw liquid out of a sealed tank. At some point, then, you will have problems.

 

[kpgraci]

Pressure does increase with liquid depth, if that helps.

 

[Brian D]

Stu, both containers are vented. Both containers are connected to a common feed for the consumption. Both containers are on equal plane and are of equal size. So basically both containers are exactly the same.

Thanks, Scott. The assumption is that the pressure is not an artificial vacuum.

One day I will take to gallon bottles of water and experiment. LOL

 

[Ross]

The level of the liquid in the containers will be the same. Take two buckets with holes in the bottoms and put them in the lake. They will fill equally

 

[kpgraci]

Stu, both containers are vented. Both containers are connected to a common feed for the consumption. Both containers are on equal plane and are of equal size. So basically both containers are exactly the same.

No doubt, the same. It will not be possible to fill one tank more than the other and both will draw at the exact same rate.

I build a 5 barrel rain barrel project, barrels connect at the base by a common pipe, only barrell #1 fills. I was concerned about barrel #5 becoming 'stale' but realized that as water enters the top of barrel #1 it will force water from the bottom of #1 into the common pipe where it distribute evenly to the other 4. I draw the water using a powerful pump, and you can see barrel #1 drop first, causing an inbalance that is quickly picked up by the remaining barrels - this effect cascades down the line, only noticable because I'm drawing it off faster then the pipe diameter allows over a gravity flow rate.

 

[Scott T-Bird]

Cool ...

I build a 5 barrel rain barrel project, barrels connect at the base by a common pipe, only barrell #1 fills. I was concerned about barrel #5 becoming 'stale' but realized that as water enters the top of barrel #1 it will force water from the bottom of #1 into the common pipe where it distribute evenly to the other 4. I draw the water using a powerful pump, and you can see barrel #1 drop first, causing an inbalance that is quickly picked up by the remaining barrels - this effect cascades down the line, only noticable because I'm drawing it off faster then the pipe diameter allows over a gravity flow rate.

Neat project, exactly right. My fluid dynamics prof was a woman from Montana State University ... I always liked that school because of her.

 

[Scott T-Bird]

Yes, for water ...

Pressure does increase with liquid depth, if that helps.

it is 62.4 pounds per square foot for every foot of depth. With a snorkel, you can only breath at a few inches depth. If you tried at a couple of feet down, you couldn't overcome the pressure on your lungs and you wouldn't breath.

 

[Brian D]

So as I understand it, if container A is 50% and container B is 100%, container B will be the main source until container A and B are at equal pressure. Then both containers will provide at about equal amounts until both are depleted.

Wonderful! (I think)

 

[Scott T-Bird]

Not really ... if they are connected by the same pipe network and there are no valves shutting off the water flow between tanks, the fluid level will equalize between Tank A and Tank B while everything is at rest. Then, when you are drawing the fluid out, it draws from both tanks equally. Neither (or both) will be the main source. The 2 tanks will be an equal source at all times that they are connected. The tanks can't rest at uneven levels of fill unless they are disconnected from each other with a valve that is shut off.

 

[Maine Sail]

Critical question:

Are your tappings for the tanks at the bottom or fed in from the top and picking up close to the bottom via dip tube...

If you bottom tap two tanks and connect them via a pipe, and both tanks are vented, then the fluids in both will self level.

If you drop a tapping in from the top, with a dip tube, like a fuel tank, then they won't self level...

 

[Sandy Stone]

If you drop a tapping in from the top, with a dip tube, like a fuel tank, then they won't self level...

If both dip tubes and all connected piping is liquid full, as they would have to be to draw liquid out of the tanks, then the higher tank will siphon into the lower one until they equalize. Any significant amount of air in the lines will prevent this.

 

[Brian D]

Top tapped like fuel tanks. There is also an assumption that there are no antisiphon devices in the lines.

So if I open a valve on an empty container, the full container should start to transfer some of it contents to the empty until they are level. Is that correct?

Fluid dynamics. Interesting stuff, almost as interesting as satellite engineering.

 

[Sandy Stone]

So if I open a valve on an empty container, the full container should start to transfer some of it contents to the empty until they are level. Is that correct?

If the tank is really empty, then the dip tube will also be empty (full of air) all the way to the shutoff valve. Opening the valve will allow the interconnecting piping to drain and let air into the piping, preventing any siphon action.
If the tank has a low level, but the bottom of the dip tube is still submerged and the dip tube itself still contains liquid and not air, then the tanks will equalize.

 

[Scott T-Bird]

It sounds like you are attempting to run with 2 fuel tanks and a common fuel feed to the engine. If you want to control the levels in individual tanks, you will need to use valves. Once you break the siphon between tanks, or begin to draw from a tank that was empty, I suppose that you would have to bleed the air.

 

[kpgraci]

Top tapped like fuel tanks.

If tank #1 is 50% full and tank #2 is 100% full there will be a pressure differential and tank #2 will draw faster (all other things being equal), but that rate will slow as the tanks get closer to the same level - and I do not think there is any guarantee that tank #1 will not run dry before tank 2 equalizes. In this case I would either connect the 2 tanks at the bottom or treat one tank as a main and the other as aux and manually switch over when one runs dry - this has the advantage of never running out of fuel unexpectedly. :dance:

 

[Marc14857]

A few folks have referred to it but just to make sure we are all on the same page, at rest or under static conditions, the tanks levels will equalize as long as there is a an equivalent connection between the two water columns in the tanks and the tanks are vented to the same atmospheric conditions. That may not be the case under dynamic conditions when you are actually using the tanks. The tank levels under dynamic conditions may vary primarily due to the configuration of the tank outlet piping. If one tank has a different length of outlet piping or perhaps more turns before the two outlets meet, it is very possible that this tank will empty slower as it has a higher friction coefficient as compared to the other tank. This will be exacerbated at higher flows. When you stop drawing from the tanks the levels will equalize.

 

[Maine Sail]

A few folks have referred to it but just to make sure we are all on the same page, at rest or under static conditions, the tanks levels will equalize as long as there is a an equivalent connection between the two water columns in the tanks and the tanks are vented to the same atmospheric conditions. That may not be the case under dynamic conditions when you are actually using the tanks. The tank levels under dynamic conditions may vary primarily due to the configuration of the tank outlet piping. If one tank has a different length of outlet piping or perhaps more turns before the two outlets meet, it is very possible that this tank will empty slower as it has a higher friction coefficient as compared to the other tank. This will be exacerbated at higher flows. When you stop drawing from the tanks the levels will equalize.

Guys,

This discussion just jogged my memory..

A month or so ago I working on a J boat where the water tanks use "dip tubes" eg: drop into the tanks from the top.. There are two tanks isolated by valves. Both valves were closed when I got there.. The pump, a VSD, would not shut off so I was trouble shooting that problem. Turned out to be a bad pump.

The port tank was 100% full & vented to atmosphere and the starboard tank was 50% full & vented to the atmosphere. The owner has a slight list so always uses the starboard tank and keeps the port tank pretty much full. During the process both valves were opened and left that way for about 4 hours while I was there. The hoses are 1/2" diameter. When I was finished I closed the port valve and tanks were still at the same exact levels even after four hours..

I remembered thinking that was a pretty cool feature because on our boat the tanks are bottom tapped and will equalize in a short period of time if the tank isolation valves are left open.... We actually have to isolate port & stbd tanks when heeling or one tank drains completely into the other if there is room. When this happens our water pump sucks air so we have to keep the tanks isolated when sailing.......:cussing:

 

[Brian D]

So then another question. If both tanks are connected at the same point with the same length of hose and both valves are then opened, pulling fluid through one side should in effect vacate air in the hose on the other side? Meaning, since the tanks are vented, air has a way to escape and the second tank should start to equalize. At least in my mind that would happen. Like I said, satellite stuff is easier than fluid dynamics. LOL