Magnetic Level Transmitters

Calling all experts!
There seems to be a bit of trend these days to use:
1 - Magnetic level gauges instead of glass.
2 - Magnetostrictive level transmitters in conjunction with the magnetic
level gauges.
The mag gauges seem to be an excellent replacement for glass in all cases. Safety is the main issue but ease of installation is also important. Finally, improved visibility. It's hard to knock it when safety, price and ease of use all favour the same product.
The case for the associated transmitters is less obvious. Does anyone have any experience? In the past I had always mistrusted transmitters that were attached to other instruments as an after thought but this seems to be good deal.
Finally, have any of you used them in interface service such as oil on water? This has been the last holdout for displacers.
Walter.
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and
I assume you mean those metal tubes with the flappers on them hanging off the outside of the tanks.. (sorry! ;-)
Other Advantages: You can fully test the unit outside the tank (ie. not in the product) and you can visually see what the setpoints are.
Other Disadvantages: Some mug closing the isolation valves!

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I have used the KIP mag level switches on oily-water tanks and slops vessels, but not on anything major. As far as magnetostrictive transmitters go - I have seen them in use, once, but would never rely on them as the only source of data, for the abovementioned reasons. I seem to remember problems caused by surge or tank vaccuum disrupting the level in the tube...
> Finally, have any of you used them in interface service such as oil on

Yep. This is what they're good at.. but I can't see them replacing displacers on larger tanks for simple reliability reasons - it's too easy to stuff up the reading.
Cameron:-)
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and
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MDT are common for position feedback in hydraulic servo applications. The MDT is placed inside the hydraulic cylinder and extend up through the piston rod. A magnet is screwed on to the back of the piston. Obviously the MDT is immersed in oil and at high pressure. Some MDTs have field bus interfaces which should provide positions fast enough for process control. The most common brands I or my customers have used are Temposonic and Balluff. There are also intrinsically safe versions.
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Definitely the trend in New Zealand. One of the reasons is the ease with which a glass gauge can be stressed when re torqing after maintenance. Mag column is definitely safer. BUT Some operators swear by glass. 1) It is visual 2) Emulsions can be seen 3) Fluid interfaces can be seen 4) Rate of rise of level can quickly be assessed. I have no arguments with these. Problems with mag gauges: 1) One local plant has said "no flag ('flip flop') indicators", as they can stick. Instead they insist on the mag follower type. I am installing a 300# 600mm mag flag gauge at present (Gems) and I have to say that I am a little nervous, as sometimes the flags seem to stick.
2)Also there is only about a 1/4" gap between the float and the wall, and we are measuring very waxy crude (thus kept at 50 C / 122F). Not a big gap to bridge.
3) There is a concern often expressed about using one primary element (the float) for both a visual and electronic feedback. I have no argument to counter this.
4) Have to be careful that the float is chosen correctly for the density, and also for the pressure. Two years ago, a float collapsed on 70 bar nat gas service.
The reason we chose mag flag above, was that we needed hi and lo level trip switches, and this allows us to attach them to the side of the column, without needing to commit to fixed position threadolets in the side of the vessel, as traditional switches would require. Being installed as intrinsically safe, by the way. I haven't installed a transmitter yet, but it is very cost effective, being an additional cost of about US $1000. I would probably go for magneto restrictive to avoid the little step changes cause by the reed switch type.
Cheers ______________________________________________________________
Walter Driedger wrote:

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Alan Hooker
Electrical and Instrument Engineer
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Thanks for the input. The very visual nature of glass gauges means that also have greater potential to deceive. A single section of glass can only show two phases accurately. If the top is in oil and the bottom in water, emulsion can be trapped in the glass and be quite visible without there being any in the vessel at all. The reverse is even more likely. I wrote this up in "Controlling Vessels and Tanks", Figure 6-8 found at www.driedger.ca.
Walter.

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Walter, The applications where we have used magnetic level transmitters (or magneto-restrictive) have always been 'clean' i.e., no emulsions or coating of surfaces inside the tube. Also please ensure that the clearance between the float and the inner tube wall is adequate (otherwise the float tends to stick and the operator soon ignores the reading from the magneto-restrictive transmitter). Cheers, Raj

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Thanks for the comments on magnetic level gauges. Now I have another qeustion. In the past it was common practice to 'rod' displacers and float swtiches to make sure they were working. At the bottom of the chamber there was a drain with a gate valve. A thin rod was pushed through this to put pressure on the displacer so that it would read a higher level.
Is anything like that necessary or advisable with magnetic level gauges? It would be a bit more difficult to arrange. A displacer needs only a very small amount of movement to cover the entire span since it is a force balance device. A ten foot mag gauge would need a rod in excess of ten feed long to push the float from bottom to top. Does anyone have any experience with this?
Walter.
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I have used magnitostrictive level transmitters on turbine lube oil and electrohydraulic oil resevoirs with great success. They ignore foam, and require no holes below fluid level. We had a requirement to test the activation of alarms. The magnitostrictive transmitters allowed this without changing oil levels simply by raising or submerging the magnetic sensor.
On Wed, 08 Oct 2003 03:45:52 GMT, "Walter Driedger"

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Walter,
A couple of points to add to what has already been said:
(1) only on clean fluids - no entrained particulate to cause the float to hang. (2) repeatable SG (3) be cautious with the magnetostrictive electronics, they are relatively new to the market and have been known to be "flakey". I have had problems with at least two different client installations and am currently working at an upgrader that has several hundred installed. A number of the magnetostrictive electronics are being replaced by traditional dP cells with diaphragm seals. (4) it would be wise to consider the use of a secondary measurement in addition to the magnetostrictive transmitter. It's not that I don't trust them...but better to be safe than not. Consider opposing nozzles or a well designed bridle. (5) oil in water is a good application. With that said I would prefer to see a fairly stable interface, not something that is turbulent or fluctuating. (6) insulate and trace at the factory. It's a small price adder to pay compared to doing it in the field at $50 /hr as construction is winding down. (7) add them right away to the plant PM schedule for regular cleaning and inspection
Hope that this helps
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tHe PuNx

Scarecrow: I haven't got a brain...only straw.
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new to the

least two different

several hundred

by traditional

MDTs have been around for 25+ years. I wouldn't call that new.
Peter Nachtwey.
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Really? I knew Magnet Level gauges to be around for that long, but I thought "magnetostrictive" technology used with MLG's was relatively new.
I'm only familiar with one MLG manufacturer that uses "magnetostrictive" technology in their transmitters and that is ISI-Magtech.
Do you know of more? Please post them since I'm actively searching for an alternative to Magtech.
--

tHe PuNx



"Peter Nachtwey" < snipped-for-privacy@comcast.net> wrote in message
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What is "MDT"? My web search gets me everything from Mountain Daylight time to Maintenance Data Terminal to Microwave Device Technology. But nothing that looks releveant to this thread.
Walter
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time
OK, I was wrong. MTS claims over 3 decades. MDT is short for Magnetostrictive Displacement Transducer. MDT is a common acronym in hydraulic motion control. Since everyone interested in level sensing I posted a link.
http://www.mtslevelsensors.com /
Peter Nachtwey
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I have used MTS level transmitters in Ammonia Refrigeration Drum Level application and it works really well (provided you have specified the correct density for the float). Raj
Walter, In response to your query about testing the instrument with very long stand-pipes, we closed the isolation valves (which connects the stand-pipe to the drum), drained off the liquid ammonia and then used a known density liquid to calibrate and test the instrument. I am not sure how you could test low level trip / alarm functions if the stand-pipe length is >> 2.5 m. Raj

common
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