Level measurement of liquid in closed tank

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Thread Starter

Giri

Can any one explain how to calculate the quantity of the liquid (The fluid is liquid Nitrogen)in a closed tank.
The tank volume is 44,080 litres.Rosemount Dp transmitter (model 3051) is used for measuring the tank DP. The range set for the DP transmitter is 0 to 7400mm H20, which corresponds to 0 to 100% of the tank level.
I am not sure about the tank height. Based on the above transmitter range(0 to 7400 mm H20) can any one tell me 0 to 100% corresponds to how many litres. I mean what is the tank capacity at 0% and 100%.
 
First – for someone to help calculate they would need the tank position - is horizontal or vertical?

A horizontal tank level volume change is not proportional, as the tank gets larger near the middle. I made the calculation once – and built a proportional dial for our continuous transducer but have since forgotten. I saw a link once that demonstrated the volume but I can’t find it.

Bob Hogg
www.almegcontrols.com
 
T
We have a number of LN2 tanks around our facility. We measure liquid volume in them in two ways. For continuous measurment we mount the tank on load cells and weigh it. For point measurment of LN2 in cold traps and such we use type K thermocouples. Although somewhat less accurate than weighing, TCs works very well for cold trap level control

The DP will work OK but it will likely be non-linear. It will tell you if you are full, or if you are almost empty, and sometimes thats all you want to know, but with LN2 DP may not be very accurate in between, especailly if your tank is outside, as most are, and it also depends on if you are drawing liquid or gas off the tank and at what rate, and on the shape of your tank. LN2 is in a superheated state, meaning it is a cryogenic liquid that is above its boiling point, so disturbances can make big changes. If your fluid level is static, you're not drawing off gas or liquid, and your tank environment temperatue stays fairly constant, you can develop an aproximation of the DP to your tank volume though and probably be close enough, until there is a distrubance. The only way to know for sure is to use load cells. The pressures may change with temperatures and flows, but mass is constant, regardless of whether the N2 is liquid or gas. Weight methods are also independent of the changing shape of the liquid as it rises and falls in cylindrical or spherical tanks.
 
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David Bergeron

The volume at any level will depend on the shape and dimensions of the tank. If the tank is vertical and cylindrical and flat on the bottom, the volume will be pi*r^2*L, where r is the cross-sectional radius and L is the level of the fluid. There are appropriate equations for other tank shapes that can be looked up. Because of irregularities in the tank to really get an
accurate indication of the volume, the tank will have to be strapped. The result of tank strapping will be a table of levels and corresponding volumes.

David Bergeron, P.E.
Thompson Equipment Co.
125 Industrial Ave
New Orleans, LA 70121
[email protected]
http://www.thompson-equipment.com
Phone: 504-833-6381
Fax: 504-831-4664
 
Dear Giri,

I just did a similar job for 36-ton tank of carbon dioxide liquid a few months ago by using 5th order equation in DP transmitter. The local quantity display and output was assumed in a fixed condition 23 barg at -17oC. The 6-equation factors depend on tank geometry, density, installation, etc. The factor calculation was very long. You need HART communicator to enter these factors. Its accuracy is about +/-1.5%. I was regret to charge too little to our customer for such complicated calculation! Hope this help.

Hung Lam
http://www.thongphat.com
 
Ralph,

I have a fuel tank on our farm that is 36" in dia. and 60 long, and it is mounted horizontally. I can measure the level in the tank so I would like to be able to convert that to gallons. Does your spreadsheet do this?

Sam Cunningham
Farm Mgr, Boys' Home Inc.
Covington, VA
 
I know a company that makes level switches for closed tanks without any tank opening. if interested, email me at eng_h_elfalaky [at] yahoo.com
 
u told u have the capacity of the tank. if the transmitter is calibrated without dead level, then u can get it simply by the calculating the % of the total capacity in liters thats all..and also

P=$gh
$=density if Liquid nitrogen
g=acceleration due to gravity
h=height
ur URV is at 7400mmWC
u can get the height as 7400/$g = h
that's the max height of the tank. 1.25g/l and g=9.8m/s^2
 
its easy you just need this formula,
volume of cylinder = 22/7*r*r*h

you already know the range of transmitter, 0-7400mmhg, pressure = density*height*acceleration due to gravity. just use the density of nitrogen to solve this equation to find tank height for 100 percent, after that calculate the tank height for specific percentage level and put it in volume formula. easy.
 
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