Fluid Dynamics and a Water Tank

I would think that there is a difference between pressure in the "top" of your sight glass and the head pressure in the vessel. Sight glasses/tubes/whatever need to be ported correctly.

The bottom of the sight tube, fitted with a valve, is ported to the bottom of the vessel. The top of the sight tube should be ported to the headspace of the vessel. When in use, the sight tube valve should be fully open.

Hope this helps,

Mike

Are you saying that if I connect the top of the sight tube back to the top of the tank I can cancel the effect?

Yes, it needs to be connected to both the top and bottom of the tank via valves, to allow the glass plastic to be replaced if needed.

To work, both valves must be open.

Look here:-

Where the sight tube connects to the bottom does it need to be in its own, isolated port?

GA, AG (seems like Deja Vu),

I was trying to draw a similar graphic, but my graphics pgm is on the fritz. That is exactly what I wanted to draw - however, I don't think the top valve is necessary.

Best Regards,

Mike

It is if you ever break the glass tube, or it gets dirty and needs cleaning.......look at any steam locomotive or ships boiler, they use the same principles.....they usually look something like this:-

I avoided connecting directly to the tank as it would require me to drain the tank, and, here in California, as summer is here, water is gold. Initially I had the valve to the sight tube wide open, but after 'Bernoulli's principle' sucked down the float and I stopped the water flow the float would shoot to the top of the tube and get stuck. But, ultimately does it matter as long as there is active fluid exchange? Is the effect amplified with a larger discrepancy between the volumes in the tank versus the sight tube?

The driving factor here is the velocity across the bottom of the sight tube.

And it only matter to us if it matters to you (her?). If she is trying to use the tube to measure outflow (I'd like to dispense 750 gallons) it matters.

If she just uses it so she knows when to order more water, probably not.

But if it matters, and she doesn't mind someone in her water, you could grab an assistant and relocate the bottom of the gauge tube. Shouldn't lose more than a few gallons. If it is drinking water I can see some reluctance.

It is drinking water. Either way I don't think I'd want to take a dive into her tank... being ten feet tall with a 24 in(or so) access lid. Something I failed to mention: just after the main line exits the tank it tees. One way is the main supply line, the other is for a fire hose (or fire truck) attachment. If I connected the sight tube to this side of the tee would it minimize the venturi effect because of minimal liquid flow? Or does this side of the tee get its fair share of sucking?

Something that you might try is running a upside down U tube from the bottom inside the tank out through the inlet hole to the bottom of the tank outside.(like a syphon hose). It doesn't have to be of any large diameter as the tank level should not vary all that quickly. Connect this to you sight tube and as long as the u tube remains full of water your float will follow the tank level without the venturi effect or drilling holes in your tank. The only down side of this is if it springs a leak it will drain your tank.

Good Answer, like this: hose and sight tube shown much thicker than needed for clarity:-

You would still see the same effect if you hook it up to the side tee. The pressure you are seeing at the tap into the tank under flow conditions is dynamic head. To see static head (actual level in the tank) you need a new tap at the bottom of the tank. And that new tap should be at least 30 degrees around (as seen from a top-down view) from the suction line. As someone already mentioned, she can either get used to mentally compensating for the difference or drain the tank and install a new tap. Unfortunately there is no way around it. On a side note, I'm suprised she is using the same port to fill the tank as is used to drain it. Typically a tank is filled from the top.

As JohnDG says...also called the Venturi effect, put in poor laymans terms to help you visualise it, the main flow through the pipe is trying to drag the liquid out of the sight pipe as it flows past the Tee.
Del

Hello lightasmass,

How are you?

Let me just suggest this............As long as the float level indicator cannot get past the bottom bends and "T", and I presume they are never likely to use all the water in the tank in one go/at one time, why does the float moving under the venturi effect matter? The float goes back as soon as the drawn water is turned off right?

The reason I say this is because although it would be possible to ask a local 'diver' to help in the 'tight' confined of the tank, to do whatever you want to do in there, you say you are reluctant to enter the tank. I can understand that.

Bearing that in mind, Is it possible to either pump the water into a tanker while you fit the 'Stub and vertical clear plastic pipe' at a different place on the tank?

Or, perhaps hire or borrow a tank from the same people you got the other tank from and pump the water into it, then fit the 'stub and vertical clear plastic pipe'?

If this is not drinking water but water for the garden etc, can you find an empty pool nearby and run a pipe to it and pump the water out as mention above?

Keep in touch please?

bb

Hi Babybear

I am fine thanks... on this day last year Northern California was waking up to plumes of smoke. A freak lightning storm swept across the region starting 2000 fires. There were a dozen fires within 5 miles of my house, two of which eventually burned to my property lines. With a local area-wide burn of 200,000 acres.

The person for whom I did the work lives about 5 miles from me and was in a similar predicament. She, reasonably, wants to make sure her water supply is maxed out all the time. But, as you say, it doesn't matter that much to do error correction whilst water is flowing out of the tank and get accurate readings when the pressure is static.

My interest in starting this thread is 1: Why, 2: Is there a reasonable work around. I have the why. As for the work around, it is reasonable when the rains come again and I can drain the tank. Although, with regards to a work around, I am now just curious as to a different solution. There must be a theoretical threshold with regards to the volume of the main line versus the volume of water moving through it and its effects on the sight tube. Or the point at which the venturi effect is negligible on the accuracy of the sight tube... but this is just mental exercise.

Her supply is a seasonal spring and a low output well. Out here we don't have much in the way of neighbors and bringing in a water tender to temporarily store the contents of the tank is, I think, prohibitively expensive for the nature of the project.

Thanks for your time and comments.

P.S.

"Ironside Fire"

This pict was taken on 6/27/08 looking North from my front yard. This is the result of a USFS contract crew, out-of-control, back burn "burnout"..... they almost lost it.

"Cedar Fire"

This pict was taken on 7/10/08 looking Southeast from my front yard. When the fire hit this hillside a nearly 1000 foot wall of flame rolled from top to bottom. The sound it produced was akin to a jet engine.

The problem with the original design is that if a pump is connected with a high output, it will probably start sucking air down this tube at some point, this is why the sight tube should a) not be connected to a pipe, it should ONLY be connected to the tank, twice as i ashowed in my diagram......

You/she are taking unfair risks otherwise.......

I do understand that emptying the tank is not an option till much later in the year, so I guess she will have to live with it till then...

Tell her to turn off the valve when she has finished he level check and turn it on again at the next check, that is the only safe wy to go....that way the pump cannot suck air.

I don't think I was clear on the supply-side setup. The tank gravity feeds the water system. A seasonal spring feeds the tank via gravity and when the spring can't keep up a well pump on the bottom of the property pumps up to the tank. There is no danger of the pump sucking air. Her main interest is being able to check the tank at a distance without climbing the hill to it and be sure volume is adequate. I'm confident my setup is safe... it's just not as accurate as I'd like during dynamic flows.

I meant when fighting a fire using the water from the tank......sorry if I was not clear enough on that point.....

Hi 1.Is the fishing bobber free to move in the sight tube?. 2. you obviously are creating a partial vacuum during the quick drain. look for the problem over the fishing bobber. Hope it helps Wangito.

The float moves freely in the sight tube. I'm sure there is no vacuum.

Best post up to now....GA from me.

Thank you for your comprehensive answer and for correcting my misnomer with regards to "sucking". The current setup will have to suffice for summer. I'll make the recommended changes when the rains come. Thanks again.

Hi lighthasmass,

Just wondering what your avatar is. What is it, if you don't mind my asking?

Mike

Happy new year,

So, how did the corrections go?

Hi Lightasmass,

Looks like some lateral thinking is called for!! The draw-off reduces the static head at the tee, therefore indicated level falls ;QED. I wonder just how far away the level is visible with the current arrangement?

Solution:You have access to the free water level via the top manhole, or a separate hole drilled thru' the top ; therefore use a simple float operated level indicator with two pulleys to transfer the motion of the float to the front face of the tank where you have a weighted pointer (bob) set against a graduated scale calibrated in gallons X000s. The only thing to remember with this set up is that the pointer is at the top of its travel when the tank is empty; but visibility will be greatly improved!

Benefits:No heroic diving tricks, it is fool proof,and pretty well obeys the KISS principle (Keep it Simple , Stupid!), you might even win some brownie points with the owner..........have fun!

If you look at the picture in one of my earlier posts, there is an even easier optical method that needs very little work, look here at the background on this picture:-