Flow Meter Tutorial - Magmeter - Flow Control June 2001

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Getting the Best from a Magnetic Flowmeter Run New Search by: Walt Boyes Pages: 54; June, 2001

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Magnetic flowmeters are widely used flow measurement devices for many kinds of conductive liquids. There are simple reasons for this. First, magnetic flowmeters are inherently extremely accurate. They measure the average of all of the velocities traveling through the pipe, from wall to wall, directly as a function of Faraday’s Law. Second, magnetic flowmeters are obstructionless, and have about the same pressure loss as an equivalent length of pipe. Third, they are remarkably maintenance free, even in cases of high corrosion or abrasion. Fourth, they can measure a wide range of fluids, from clean water to very dense slurries.

•A •C •F If I were going to build a plant somewhere in the world where rapid delivery of •F parts and replacement units was going to be a problem, magnetic flowmeters •G would definitely be in my arsenal. If I had to pick two liquid flowmeter types, and only two, to use at that remote location, magnetic flowmeters would be one • L •M choice, and I would be very torn what the other type would be. •P •P Of course, I am referring to full-bore magnetic flowmeters, whether of the spool-piece design or the wafer design. Insertion magnetic flowmeters of the •P typical design, with a single point-sensor, have about the same accuracy and •R repeatability as an insertion paddlewheel meter. Of course, there are always exceptions to the rule. (There are a few manufacturers who produce profiling insertion magnetic flowmeters, and they can be quite accurate, even coming close to full-bore meters in performance in good hydraulic conditions.)

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General Guidelines Nevertheless, like any other flowmeter, full-bore magnetic flowmeters aren’t magical, and there are quite a few ways to install and operate one that will give you the worst results. Here are some don’ts for installing and operating magnetic flowmeters. These are general guidelines and one should consult with the manufacturer to discuss the particular operating parameters of a given meter. In other words, these aren’t written in stone -- talk to your vendor!

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1. Don’t use a magnetic flowmeter on a non-conductive fluid. The minimum conductivity is generally between five and 20 uS. You should not approach those minimum values for normal use. Give yourself a safety factor. If you are measuring deionized water at about six uS, you might want to look at using

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another type of meter, like a Transit-Time meter or a turbine meter. This is very important since the minimum conductivity requirement can increase with the distance between the primary (the flow tube) and the converter (the transmitter). So, if you are building a water treatment plant, and you want to use a magnetic flowmeter on a DI water line, don’t run the cable 300 feet to where it is convenient to install the transmitter. 2. Don’t use a magnetic flowmeter on fluids with a lot of entrained air. Air or other gases may collect in the meter, especially if it is installed at a high point in the line (such as an inverted U-tube). While you may think you have forced the meter to remain full, there is likely to be a sizeable void caused by air or other gases coming out of solution and staying in the meter body. This will seriously impair the accuracy of the meter. 3. Don’t use a magnetic flowmeter on a self-dewatering slurry. Some slurries, such as calcium carbonate, or lime, have the property that at relatively high densities (three to four g/cc slurry gravity) they will dewater, that is the solids and the liquid will separate. The velocities of the solids and the liquids can become quite different, and since the liquid is conductive and the solids are not, the meter will be in substantial error. 4. Don’t use a garden-variety magnetic flowmeter in very rapid batching operations. Because of the time necessary to make a reading, the average magnetic flowmeter may not be fast enough to handle the step-changes in flow of a batch and fill operation. There are some magnetic flowmeters on the market that are faster response than others, so check the specifications to see if one will work in your batching operation. 5. Don’t use a magnetic flowmeter that is very much oversized for the flow rate. Very often, because of the exigencies of bureaucracy, we are forced to design plants for expansion, and run them under current conditions. This is especially true in the water and wastewater industries. However, any flowmeter’s performance degrades as you approach the lower end of its measuring range, and magnetic flowmeters are no different. If you must use an oversized meter, be sure that the pipe will remain full, that air or other gases will not become entrapped and check your supplier’s specifications because there is considerable variation in performance at the low end between magnetic flowmeter models. 6. Don’t use a magnetic flowmeter on a spiraling flow. Passing the fluid through two 90o elbows at right angles to each other can easily create these flows. Since the magnetic flowmeter’s raw signal output is the sum of all of the velocity vectors in the magnetic field, and in a spiraling flow, some of those velocity vectors have a negative sign, the output will be in error by as much as 40 percent. Magnetic flowmeters actually need very little straight run. A common specification is three to five diameters upstream of the electrode plane. Since the electrode plane is near the horizontal center of the meter body, if the meter body is long enough, the required straight run may actually be inside the meter body. The reductions in the cost of a full-bore meter since 1990 have made it possible to use them competitively with mechanical meters in water distribution and in industrial water treatment, as well as in wastewater treatment, mining, dredging, and the chemical industry. They are even beginning to be used in agricultural irrigation. Walt Boyes is a senior member of ISA, and current vice president of ISA's Publications Department. He is a writer and consultant who has delivered numerous technical papers, and has more than 25 years in the practice of flow

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