This article was printed in CONTROL's April 2009 edition.
By Walt Boyes
Ever since the invention in the 1790s of the Woltman-style mechanical turbine flowmeter, automation professionals have been looking for the one flowmeter that will work in every application. Unfortunately, there are 12 flow measurement technologies in common use for a very good reason. No single flow technology works well, or even acceptably, in all applications.
Of the more broadly based flow technologies, the one that works in the most applications, across most industries and with higher accuracy than even differential pressure is the electromagnetic flowmeter, or magmeter. According to Jesse Yoder at Flow Research, the total global market for flowmeters is roughly $4.7 billion, and magnetic flowmeters account for a little less than 20% of that total. There are a lot of magmeters shipped every year. Magmeters are used in every process industry vertical: water, wastewater, mining and minerals, utilities, food and pharmaceuticals. Magnetic flowmeters are designed for handling almost all water-based chemicals and slurries and are furnished with corrosion- and abrasion-resistant linings and even clean-in-place (CIP) designs for sanitary applications.
Magnetic flowmeters also are made in the widest size range of any flowmeter technology because they can be scaled up almost infinitely. The first use of the technology was in the huge sluices that drained the Zuider Zee in the Netherlands in the 1950s, and typically vendors supply a size range from ½ in. (12 mm) to 36 in. (914 mm), with several vendors supplying extended sizes up to 120 ins. (3048 mm). Several vendors sell sizes below ½ in. as well. How it is possible to scale up and down this broadly is directly related to the technology.
How a Magmeter Works
In 1831, Michael Faraday formulated the law of electromagnetic induction that bears his name. As used in an electromagnetic flowmeter, coils are placed parallel to flow and at right angles to a set of electrodes in the sides of the pipe, generating a standing magnetic field. The pipe itself must be non-magnetic and lined with non-magnetic material, such as plastic, rubber or Teflon. When the fluid (which must be conductive and free of voids) passes through the coils, a small voltage is induced on the electrodes, which is proportional to the deflection of the magnetic field. By Faraday’s Law, this deflection is the sum of all of the velocity vectors impinging on the magnetic field.
Modern magmeters operate on a switched DC field principle to zero out noise that can be induced from RFI, EMI and electrical noise actually in the process fluid. They follow a regimen of turning the field off, measuring the voltage that is still induced on the electrodes, then turning the field back on and subtracting the off-state voltage from the on-state voltage. They do this several times a second, which reduces zero drift to almost nothing.
What this means for automation professionals is that the voltage induced on the electrodes is directly proportional to the average velocity in the pipe, and is therefore significantly more accurate than any other velocity-based measurement principle that only looks at a point or line velocity. In fact, the magnetic flowmeter is generally considered the most accurate wide-application flowmeter in current use.
Magmeter accuracy is remarkable, approaching the accuracy of positive displacement flowmeters. They’re often used for custody transfer when the flow is of relatively long duration. Typical accuracy of a magnetic flowmeter is 0.5% of measured value from 0.3 ft per sec to 33 ft. per sec (0.1 to 10 m per sec) velocity. Some vendors indicate even higher accuracies over portions of the flow range, up to 0.1% of indicated flow rate.
Where Magmeters Won’t Work
Magmeters have such a wide application that it’s easier to say where they will not work than to list all the applications in which they will.
They will not work when the pipe is not full (with the exception of several versions designed specifically for this application). If the pipe is not full, there will be significant error. One of the most common application failures of magnetic flowmeters is on a gravity-fed line discharging to atmosphere in a tank. Very often, at very low flows, the pipe is actually not full, and the flowmeter will read in error. If the pipe fill drops below the line of the electrodes, the meter will not read at all. Sometimes, applications like this are designed with a u-tube in the line, which is supposed to keep the pipe full at all times. And, sometimes this actually works.
They will not work when the pipe is full of entrained gas or air. This changes the computed volume of the pipe and changes the volumetric flow through the meter in an uncontrolled fashion that’s proportional to the amount of bubbles (or void fraction) in the pipe.
They will not work well where the flow starts and stops repeatedly because there is a lag between the time the flow starts and the correct velocity is read by the meter. This means that (again with the exception of some units that are specifically designed to be very fast) magnetic flowmeters don’t work well in short-duration batching operations.
They don’t read out in mass flow units, but when combined with an ancillary density measurement device (often, for larger diameter pipes a gamma nuclear densitometer), they can produce a high-precision mass flow measurement. This combination of devices is used regularly in any water-based fluid flow situation where the pipe size is larger than 12 in. (nominally 300 mm). This application is commonly found in the mining industry and in dredging applications in harbors and rivers around the world.