liquid level measurement

Thermo,

I've had a similar problem with level measurement. Our liquid generates foam and has suspended particles in it that can clog the diaphragms of pressure type level transmitters.

We tried using load cells on one tank and are now in the process of doing the same for the other tanks. They work fine and are quite reliable. Just make sure that you put grounding straps across them and give instructions to welders to put their grounding cables within one meter of their welding works. Welding current will easily damage the load cells and there are safety issues when you have to replace them. This would be especially true for your tank since it's quite large.

Go ahead and get your load cells.

Suggest a differential pressure transmitter connected at top and bottom of the vessel. Bottom connection may need to be diaphragm type if fluid is corrosive, and/or large bore if contents are loaded with suspended solids. We have used this system successfully for sewage surge vessels, and have also used load cells. DP system is preferred as using isolation valves allows the easy removal of unit for cleaning etc. Just select a DP transmitter to cover a range equal to the height of the vessel. If one with a readout attached is selected, you'll get local indication as a bonus.

I concurr with the very inexpensive DP transmitter. We have plenty that work well on storage tanks (i.e. not agitated). However, I've had tremendous luck with rather inexpensive (tho more than DP) radar devices for difficult applications, although you'll have to speak to a vendor regarding foam - I can't recall if the radar can be configured to see the top of the foam or if it penetrates.

check out this explosion proof level sensor. It may be of some help.

try calling sky sports they make 810 735 9433 they make all kinds of liquid measuring devices, i use them on airplane fuel tanks they are not mechanical and will custom make what you need.

Use the load cells under the tank or put in a stand pipe and measure it with redundant, high TDR, pressure transducers with the previously mentioned diaphrams. Your redundancy in the Stand Pipe could be a radar at the top. The stand pipe is less likely to be affected by the wave action. Keep in mind that all of them will measure the crust too.

I would suggest that you contact a manufacturer of guided wave radar units. Radar is very accurate and reliable. They should not be effected by the rotating elements you speak of. Because of the crusting and coating problems you speak of, you will probably want to stay away from a still pipe or an external level gauge. Depending on the dielectric of the liquid and foam, some radar units can detect both the surface level of the foam and the surface level of the liquid.

A second technology you might consider would be nuclear absorbtion level transmitters or back-scatter type transmitters. There are many reliable and informative vendors of these as well and because of the very small sources they contain they are extreamly safe to use. These are the absolute best for your application but may be an administrative issue because of licensing requirements and cost.

I think that the load cells are a good idea if there is not a lot of vibration and the specific gravity in the tank is constant. I suspect that the rotating elements in yoour tank are probably heavy because of the process conditions you speak of and this will definately impact the operation of the cells.

Directly connected differential pressure transmitters for level in this application will definately be a problem, even with seals, because of the dirty service, and potential for plugging. They should definately be avoided. However they could be used with a "bubbler" on each pressure leg to eliminate the problems. This is an often overlooked method but can be very effective in the most difficult of services.

You make no mention of temperature. You must consider the temperature with any method you choose.

I hope this helps.

Patrick

Your idea of using the Load cell is fine.

Have you thought about other methods such as :

1.Guided wave Radar based Level measurement. However one should need to know process conditions, specifically the temperature.You have not mentioned the temperature. If you are interested, suggest you to contact Magnetrol with all the process details.They will recommend you the better product.

2.Radiactive Level measurement. However this needs to take the permission to use the Radioactive material, lot of precautions, training.

Thermo: I have a lot of questions regarding your problem, but the technique used in the measurement of liquid hydrogen level, which I referred to in my 11-28 message, was to run two parallel conductors down the side of the dewar. The dielectric constant was slightly different between liquid and gas and the total capacitance between the conductors varied with the liquid level. That was many years ago and there may be better ways now. DickL

http://www.magnetrol.com/select_ctry.asp

http://www.endress.com/

Here's another guided wave radar that looks a little more robust:

http://www.mohr-engineering.com/efp-guided-radar-liquid-level.php

Is your tank made from insulating material? And are the properties of the contents reasonably consistent for conductivity and/or dielectric constant? If so, you could use a low-frequency capactitive sensor placed against the outside of the tank. This was tried for automotive use, but had a problem if the outside of the tank became contaminated with conductive material.

If your interest is purely in maintaining a maximum level, a relatively simple acoustic transducer (also placed outside the tank and at the desired level) would possibly meet your needs.

Obviously, either method would best use low (intrinsically safe) Voltages with low inductances and limited currents; the relevant levels should be in your manuals.

Whatever method you choose, there might well be periodic disturbances from the stirring mechanism. You would either need to include their effect in your controller, or filter them out, but both methods should be reasonably straightforward.

Hi all,

Few years ago I have a problem like that, and I have a simple stupid solution, but its worked very well.

Install pressure transmitter and air pipe to the bottom of the tank. Give air pressure to the bottom of the tank until the bubbles are coming, and it will give the different pressure for the different level. You can use the different pressure as the signal for your controller.

It's worked, I guarantee....

Hi all,

Few years ago I have a problem like that, and I have a simple stupid solution, but its worked very well.

Install pressure transmitter and air pipe to the bottom of the tank. Give air pressure to the bottom of the tank until the bubbles are coming, and it will give the different pressure for the different level. You can use the different pressure as the signal for your controller.

It's worked, I guarantee....

that's me.....LEE (I've given reply before registered....hehehe...)

How about a pressure transducer?

DOH! Never mind.

You can use standard pressure transmitter. It's installed after stop valve (at air pipe).

It's very simple...........

By the way, is there any job vacancy for instrument engineer for me ?????