Tank Level Transducer Calculations

[gr8scot]

I need to spec a level transducer for a vertical tank filled with vegetable oil. I would like to learn the steps involved for doing this. Kind of an "idiots guide", if you will.

I know the height of the liquid in the tank: 17 feet.
Tank inside diameter: 89.75 inches
Specific gravity: 0.9450

I have looked at transducer manufacturers web sites and they seem to use different methods for defining the ranges available: psi and inches of water. What other info do I need and what are the steps involved in determining the correct range of transducer to use?

Is there an existing site for this type of info?

Thanks in advance.

 

[cbarn24050]

You need just under 200 inches of water.

 

[gr8scot]

And you determined that how?

17 feet X 12in/foot = 204

 

[itsmoked]

Veg oil is lighter.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[HEC]

Static Pressure is rho x g x height. I use Metric system, so

rho is density , in your case, approximately 945kg/m3
g is acceleration due to gravity, on earth the best approximation is 9.81m/sec2
height is 5.18m, This is at full scale.

Range required is 48kPa or 193 inches of water.


Mark Hutton

 

[danw2]

Level can be determined by head pressure, the force of gravity on the mass of liquid, which produces pressure.

A typical unit of measurement is inches of water column, that is, the pressure exerted by the height of a column of water above the measurement point. Commercial head pressure level transmitters have to use some standard pressure engineering units, one of those standard
engineering units in the US is inches of water. Others are psi or bar.

But the head pressure of a liquid of a different density than water will vary from the pressure developed by the same column height of water in proportion to the density differences.

Specific gravity is expressed as a ratio of the density of a liquid to the density of water. Hence, the level of any liquid can be expressed by measuring the head pressure in units of inches of water and dividing by the specific gravity of the liquid to get the actual level.
Level reading (inches of water) / Specific gravity = actual level

In practice, measurement is typically made by the level insturment in standard engineering units, like inches of water, and then corrected with the specific gravity factor to get the proper actual level;the correction typcially done in a read-out device, although some of the smart level transmitters can probably make the correction directly, nowadays.

In your case, a full tank is 17 feet = 204 inches.
If the tank were filled to the 204 inch level with a liquid lighter than water, the head pressure
would be less than the head pressure of water.

A level measurement with a head pressure level gauge of a full tank of a liquid with a S.G. of 0.945 would give a reading of 192.8" inches of water (204 x 0.954).

The instrument reading of 192.8" divided by the specific gravity, 0.954, provides the actual column height of 204".

Level measurements by pressure work great if
- you have access at the bottom of the tank, and that access is not the fill or unloading pipe.

Head pressure can not "see" any pressure below its own elevation., so a level transmitter mounted a foot above the bottom of the tank cannot measure below that 1" level. Flow past a pressure access tap produces error.

- the specific gravity of the liquid is constant.

Other technologies that mount from the top: no-contact like radar, ultrasonic or IR; or contact technologies like capacitive or magetostrictive are not affected by S.G. changes.

 

[waross]

A bubble tube works well if a little air is not a problem with vegetable oil.
A vertical tube is installed in the tank. A very small volume of air is fed into the tube, and a slow stream of bubbles issues from the bottom of the tube. The pressure in the tube is directly proportional to the head of liquid above the bottom of the tube. The pressure transmitter may be scaled to the specific gravity of the fluid.
I have seen this system used for sewage, control of flood water pumps, and in many buildings to determine the amount of boiler fuel in the storage tanks.
respectfully

 

[itsmoked]

Something not mentioned is that if the tank is not open to the atmosphere pressure sensing of the level is out the window without a complex system.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[gr8scot]

Thank you all for the great info., just what I needed.

One last question, how does the calculation differ for a horizontal tank?

 

[danw2]

Assuming the tank has flat ends (not domed ends), Precision Digital claims only 0.3% full scale error using a 10 point look-up table. Go here:

http://www.predig.com/download/download.asp?File=/products/manuals/LIM690.pdf

and check page 25 for the table (PD690 manual).

I've used their PD690 panel meter to linearize the readings and retransmit the linearized signal.

Like ItSmoked warns, if the tank is pressurized (even from vapor pressure), then the gas pressure above the liquid has to be subtracted out, which is done with differential pressure transmitter, where the gas pressure from the top of the tank is plumbed to a second port on the transmitter.

The Siemens ultrasonic level units, the ones with the electronic box separate from the top-mounted transducer (MultiRanger, SPL), have various algorithms for different shaped tanks, I suspect horizontal tanks with domed ends is one of them.

 

[HEC]

Gr8scot,

In relation to the horizontal tank measurement, If you only want to know the level i.e. how deep the fluid in the tank is there is no diference to the calculations given. If however you want the tank contents, then the calculation does vary. There have been a number of posts recently in relation to horizontal tank volume measurement.
Danw2 has raised a very valid point, the measurement point is always referenced to the centre of the measureing element. Headspace pressure will affect the level (pressure) reading as will density of the fluid. If you expect the fluid density to vary greatly a second pressure transducer can be used at a known level above the bottom transmitter. The fluid density can then be inferred from the differential pressure.


Mark Hutton