Applications and Considerations for Magnetic Flowmeters

by | Jun 15, 2011 | Measurement Instrumentation, Technologies | 0 comments

Update: My colleagues in the U.K. let me know that the article no longer requires registration to the site to see. I’ve updated the link to the open article.

Flow measurement is one of the most common measurements in the process industries. In the measurement category of this blog, we have addressed many flow measurement technologies and a quick check of the Wikipedia flow measurement page shows the multitude of technologies available.

In a recent Control Engineering Europe article, Magnetic flowmeters can contribute more [p16 – site registration required], Emerson’s Klaus Brockman describes good application fits for magmeters. He also shares how diagnostics and meter verification can help with the overall operation and reliability of this flow measurement instrumentation.

Klaus opens with magmeter strengths and application fits with:

…no moving parts to replace or maintain and are accurate and reliable. They are suitable for a wide range of pipe sizes, and can be used to measure volume flow for almost every conductive liquid. The availability of different liners makes them suitable for sanitary and hygienic applications as well as the handling of aggressive or corrosive fluids. Thanks to their straight through design, they do not introduce a pressure drop.

For those unfamiliar with how exactly magmeters work, Klaus explains [hyperlink added]:

Magnetic Flowmeter Cutaway…based upon Faraday’s Law of electromagnetic induction, which states that a voltage will be induced in a conductor moving through a magnetic field. The law goes on to say that the magnitude of the induced voltage is directly proportional to the velocity of the conductor. The fluid acts as a conductor and the sensor contains electrodes that detect the voltage generated. The transmitter amplifies the voltage, performs a calculation based on the known cross sectional area of the sensor and outputs a flow signal.

Magnetic flowmeters require good electrical grounding to operate correctly. Improper grounding leads to spurious measurements transmitted from the magmeter back to the automation system. Klaus shares how diagnostics embedded in the Rosemount E-Series Magmeter help spot grounding issues:

…by specifically looking at the signal amplitude at frequencies of 50 Hz and 60 Hz which are the common AC cycle frequencies found throughout the world. If the amplitude of the signal at either of these frequencies exceeds 5 mV, that is an indication that there is a ground or wiring issue and that stray electrical signals are getting into the transmitter. The diagnostic alert will activate indicating that the ground and wiring of the installation should be carefully reviewed.

Unfortunately, most process manufacturing plants are inherently electrically noisy. Klaus gives examples of noise types including 1/f noise, spike noise, and white noise. 1/f noise has higher amplitudes at low frequencies and examples include general plant background noise and chemical mixing operations. Spike noise is high amplitude signals found at specific frequencies. Examples include:

…chemical injections directly upstream of the flowmeter, hydraulic pumps, and slurry flows with low concentrations of particles in the stream.

White noise is a high amplitude signal across the frequency range. Klaus cites sources such as:

…chemical reactions or mixing that occurs as the fluid passes through the flowmeter and high concentration slurry flows where the particulates are constantly passing over the electrode head.

To address these different sources of noise, signal filtering has traditionally been used. The delayed response time created by this filter can impact the performance of the control loop. Klaus describes how magmeters can be switched to a higher coil drive frequency (37Hz) to provide a stable output at this lower noise frequency without needing to increase signal damping. If this 37Hz frequency is noisy, additional digital signal processing or a High-Signal magmeter can be used.

You’ll want to read this article if magmeters are a fit for your operations. Klaus also shares ways to reduce meter calibration costs through smart meter verification and using IEC 62591 wireless technology to access the magmeter diagnostics.


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