Can you suggest what type of level indicators are good for liquid tanks (cylindrical) with foam on the surface? Foam thickness is about 7". This foam is developed during stripping because of soaps present in our raw material.
Depending on the foam density and Dielectric properties and fluid properties you could consider two technologies.
For continuous - guided radar and hydrostatic level. Check our website, http://www.endress.com for Levelflex M - FMP40 (guided radar) and Deltapilot S FMB70 (hydrostatic).
I sure can. If this liquid is going to always be the same specific gravity, the cheapest solution would be a Klay/HiTECH pressure transducer mounted on the bottom of the tank. Here is a link to what I am referring to:
Foam is a difficult application for any technology. Non-contacting technologies (through air radar and ultrasonic) are preferred where applicable because they do not reqire maintenance and cleaning. The surface of very wet, dense foam provides a reflective target that is required by these technologies. In this instance the device will accurately read the top of the foam. Dryer foam may absorb the return signal causing unreliable performance. A floating target can be used with these technologies to read the liquid surface level. This solution utilizes a float on the liqid surface with a rod extended through the foam and a target plate above. An offset is entered to the level device and the true level is given regardless of the foam type.
TDR radar and continuous capacitance are contacting technologies that mount on the top of the vessel with cables or rods extending to the bottom. The dryer the foam the better the performance of these devices. If the foam characteristics are right some of these devices may be able to read both the foam level and the liqid level. Very wet, dense foams may cause problems with these technologies because they have similar characteristics as the liquid. If this case the device will measure the top of the foam and not be able to detect the interface level. Build-up on the rod or cable can also be a maintenance issue with these technologies. These devices will also have an issue with "loading" on the cable which requires extra support at the mounting location.
Pressure works well in many foam applications but you are measuring hydrostatic pressure and not true level. If the specific gravity on the material remains constant then the pressure indicator will provide reliable level readings. In dirty liquids these devices will require regular cleaning as the lay on the bottom of the vessel. Installation of pressure can be difficult if the tank is aggitatited.
As you can see each technology has its pros and cons. Foam applications are difficult and should not be approached with "one type fits all" solution. My advice is to find a solutions provider that you can trust, has expertise with all of these technologies, employs them, and guarantees results. That is the only way you can be certain you are getting the right solution for your needs.
I tried Ultrasonic and RADAR type transmitters but did not work. Another problem in this vertical tank is high agitation. High turbulence on the surface is also hindering accurate level measurements. For some grades of product we make, foam bubbles in the tank would be as big as 16" dia. Defoamer trial failed to remove the foam. Is magnetostrictive transmitters good for this purpose?
Yes, foam can be a real killer. I recently had problems with an E+H ultrasonic which couldn't deal with even a small amount of foam. They
recommended and we switched to E+H Radar sensors, which aren't much better. The system currently runs perfectly on float switches, at 1% of the price of the 'technology' and they don't need 1.3Gb of software to set up.
You have found one of the problem children of measurement.
The fact is, you are trying to make a measurement that does not exist.
You want to measure a variable we'll call "what-the-level-would-be-if-there-weren't-any-vortexing-or-turbulence-or-foa m."
Problem is, that measurement is exceedingly difficult to infer, and inferential measurement is all you have.
Most people, at this point, give up and try either a load cell, or if they really need some sort of "it goes up and it goes down, and sort of resembles a level measurement" they use a nuclear level gauge.
There are some measurements that are currently impossible with state-of-the-art technology.
This is obviously very close to being one.
If you had no agitation, you could make the measurement. Throwing agitation into the equation makes it near impossible.
If you have already tried radar, ultrasonic, and bubbler level (not likely to work because of the air entrainment that causes the foam), and you have an agitator in the tank so no mechanical in-tank measurement is possible, you are stuck with outside the tank measurements, of which there are only
two: weigh the tank and subtract the tare, or use a nuclear gauge and
calibrate out the foam and the agitator's passage.
Of the two, the nuclear gauge is most likely to give you the best results as far as accuracy or precision of measurement (and they are not the same thing) are concerned.
As long as the dirty liquid is plyable in the liquid, IOW, the dirt bends and flexes in the liquid, the pressure transducer will still work and not need regular cleaning.
Magnetostrictive transmitters are very accurate but need maintenance cleaning depending on what they are measuring. The HiTECH unit has an accuracy of 1mm.
I would also suggest a Klay/HiTECH pressure transducer mounted at the bottom for your application. If what you are measuring is plyable, it will always read pressure at the sensor. IOW, as long as it doesn't harden up on the tranducer, it will measure the pressure. HTH...
Yes, go with a float and magnetostrictive. The float carries a magnet which surrounds the magetostrictive wire. As long as the crud in the tank does not cause the float to hang up on its central guide, this will work fine.
Robert Scott
Real-Time Specialties
Embedded Systems Consulting
Thanks Mr. Robert. Concern with the float and magnetostrictive is that the rubber crumbs in the tank might stick to the float and create nuisance. Our agitator shaft will always have at least 2" thick rubber. We do hydroblasting once in 2 months to get rid of it.
In this vertical closed tank, we have rubber (density- 790kg/cu.m) and organic solvent. This is stripped with steam from the bottom to coagulate rubber. This process also washes off the soap from the rubber, which causes lot of foam. Agitation makes the rubber to stick on the walls and the shaft. My concern is that rubber might stick to the float also. Appreciate your comments/suggestions.
What do you think about point level radiometric devices for this type of applications? Initially I considered Magnetostrictive but, the rubber crumbs in the tank will stick to the float and might corrupt the device. Appreciate your comments.
If you mind me asking, what are you currently using for a level measuring device if this is an existing tank application or is this a newly additional tank for you.
Radiometric would be fine... assuming you do not want to measure the level of foam. Having worked with radiometric devices for many years, they would be a good choice. They are not inexpensive however.
Any manufacturer of these devices would need to know vessel diameter, materials of construction, and vessel wall thicknesses. Does the vessel have a jacket? If so, what is in the jacket?
We are currently using DP cells. This works well During start up, readings goes erratic. We will have an operator standing and monitoring the level through the sight glass (top of the tank) for four such tanks until the level is adjusted.
Thank you for your suggestion. Yeah these tanks are water jacketed. material is SS 304.
What type of level transmitters do you suggest for level measurement in water pits. My point here is inexpensive and efficient level transmitters. This cuboidal concrete pit contains storm water which is about 10' deep, 6feet length and 5' width. Appreciate your suggestions.
Depending on the solids and grease content in the stormwater, you can use anything from a float to an ultrasonic level transmitter. Flowline makes a good inexpensive ultrasonic, as do several other manufacturers, including Siemens (Milltronics), Endress+Hauser, Emerson (from their Mowbray line), and MANY others. As David Spitzer and I have found when we produce our "Consumers' Guide to Non-Contacting Level Transmitters" there are over 65 manufacturers of ultrasonic level transmitters in the world.
For this application, I'd use an inexpensive ultrasonic, but I admit to some bias.
This unit will solve several problems - is rugged and can give you either 4-20 or switch points and has a a visual to boot! If the glass breaks - nothing happens as the unit is magnetically coupled.
Click on the pdf flyer at the bottom of the page. Each unit is custom made to fit.
You could also have a SS tube with the Almeg continuous transducer inside the tube. The fluid intakes would be screened to keep stuff out. They could build a larger I.D. float so there is more space between the float and tube for product build-up.
Like this link but SS - longer and with less vents and a fitting on top.
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