This article explains how the latest generation magnetic flowmeters, with built-in diagnostics and integrated meter verification, help users better manage production processes.
Magnetic flowmeters are widely used across a range of industries that include water and wastewater, food and beverage, chemical and pharmaceutical. They have 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.
Manufacturers of magnetic flowmeters are constantly striving to improve the design of their products by enhancing their performance and broadening their application scope. Powerful diagnostics and features such as meter self-verification, together with access to the latest smart wireless networks, make the products easier to install and use. These features are all designed to help manage production processes by increasing availability, reducing maintenance and operating costs and improving quality.
How they work
The operating principle of a magnetic flowmeter system is based on Faraday’s Law of electromagnetic induction, which states that a voltage will be induced in a conductor moving through a magnetic field. According to Faraday’s Law: E=kBDV, where the magnitude of the induced voltage E is directly proportional to the velocity of the conductor V, conductor width D and the strength of the magnetic field B. The law says that the magnitude of the induced voltage is directly proportional to the velocity of the conductor. The fluid acts as the conductor and coils in the meter generate the magnetic field. The sensor contains electrodes that detect the voltage generated, which the transmitter amplifies, performs a calculation based on the known cross sectional area of the sensor and outputs a flow signal.
The importance of ground wiring
To ensure a magnetic flowmeter performs accurately, it is important that it is correctly grounded. Improper grounding and wiring is the number one cause of magnetic flowmeter issues. This can occur in new installations where the magnetic flowmeter is not properly referenced to the process and in existing installations where corrosion, for example caused by an aggressive environment, results in deterioration of the electrical ground wire. Poor grounding allows electrical noise to be picked up by the sensor electrodes and consequently affects the signal to noise ratio and the stability of the transmitter output.
One way to resolve this issue is to use a flowmeter with a built in Ground/Wiring fault diagnostic as found in Emerson’s Rosemount E-Series magmeter. This works 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.
As an example of how grounding faults can be overlooked, a customer in the chemical industry was experiencing high measurement variability on a magnetic flowmeter installed in a water evaporator line. In order to fix the problem the flowtube sensor and transmitter was replaced. However, this did not resolve the issue and the flowmeter was still not functioning properly. It is not uncommon for this to happen with maintenance technicians often replacing a transmitter and/or flowtube sensor without first verifying the ground connection.
When the magmeter at the chemical plant was replaced with a unit having the Ground/Wiring diagnostic, this immediately indicated a fault. Upon further inspection, it was apparent that the old ground wires had been used with the new flowtube sensor which meant the bad ground connection remained undetected. The replacement of the original flow tube and transmitter interrupted the process and incurred considerable cost when in fact the actual source of the problem was a corroded ground wire.
Dealing with the effects of process noise
There are three basic types of process noise that can affect the performance of a magnetic flowmeter leading to inaccurate flow results.
1/f noise
This type of noise has higher amplitudes at lower frequencies, but generally degrades over increasing frequencies. Potential sources of 1/f noise include chemical mixing and the general background noise of the plant.
Spike noise
This type of noise generally results in a high amplitude signal at specific frequencies which can vary depending on the source of the noise. Common sources of spike noise include chemical injections directly upstream of the flowmeter, hydraulic pumps and slurry flows with low concentrations of particles in the stream. The particles bounce off of the electrode generating a ‘spike’ in the electrode signal. An example of this type of flow would be a recycle flow in a paper mill.
White noise
This type of noise results in a high amplitude signal that is relatively constant over the frequency range. Common sources of white noise include 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. An example of this type of flow stream would be a basis weight stream in a paper mill.
Process noise creates instability
For example, in applications like pulp stock slurries, the properties of the process fluid can lead to an unstable output from the flowmeter. If the flowmeter output is driving a valve, the unstable output will cause a lot of travel in the valve position as it tries to keep the flow rate steady at a set point.
The traditional signal filtering method for getting a stable output in noisy applications is to add damping. While this makes the output look stable, the flowmeter’s response time to actual process changes is compromised. Having a stable output from the flowmeter reduces the amount of valve travel reducing valve wear and the need for maintenance. However, the valve is slower to respond to actual process changes.
The end result of excessive damping can be a very inconsistent, out of control process. While the flowmeter output and valve travel may be indicating steady process flow conditions, they may just be hiding what is really happening which could be an increase in process variability.
Instead of increasing the damping, some transmitters allow the user to switch to a higher coil drive frequency. For example the Rosemount E-Series allows users to switch to 37 Hz. In many noisy applications, this is enough to provide a stable output without having to increase the damping. The result is a stable output and no decrease in response time.
So why does switching to the higher coil drive frequency provide a more stable output?
At 5 Hz or other low frequencies of operation, the flow signal is relatively weak when compared to the noise signal. When the Signal-to-Noise Ratio (SNR) is too low, the output from the flowmeter is unstable.
By switching to the 37 Hz coil drive frequency, the flow signal is moved to where the noise is weaker. With a weaker noise level and the same level of flow signal (still driving coils at the same current), the SNR increases. With an improved SNR, the output becomes more stable without adding any damping.
If very high levels of 1/f or white noise are present, then switching to the 37 Hz coil drive frequency may not be enough. In such cases, additional digital signal processing (DSP) which provides improved process noise performance and electrical noise immunity, or a High-Signal magnetic flowmeter may be required to provide a stable output.
An example of these unstable conditions occurred at a leading global producer of newsprint and magazine paper. Here the customer experienced instability in the output from a magnetic flowmeter installed in the basis weight flow line. As a result of the instability and variation in the signal, the operators were unable to run their basis weight control loop in automatic mode. Even with damping applied in both the meter and the control system, there was still too much noise to control the speed of the basis weight pump effectively.
A Rosemount E-Series magmeter with a High Process Noise diagnostic was installed and this indicated a low signal to noise ratio at 5 Hz. By changing the coil drive frequency to 37 Hz, the noise was significantly reduced and within an hour of operation, there was enough confidence in the stability of the signal to restore automatic control of the loop. By reducing the measurement variation, the customer was able to control the process and this resulted in reduced raw material usage and cull, contributing to a 1,5% increase in paper production.
Reducing meter calibration costs
Certain industries require regular calibration of flowmeters to meet fiscal or regulatory requirements and traditionally this means that the measuring device has to be removed from the line to be calibrated. During this time a spare meter is installed so that the process can continue to operate while the original meter is being verified. This process often incurs third party costs and results in lost production.
As an alternative, there are now magnetic flowmeters that have the ability to carry out a self verification test – alerting the operator when the calibration drifts outside set parameters. For example, Emerson’s Smart Meter Verification diagnostic provides calibration verification without removing the product from the line or requiring extra equipment. The diagnostic uses the magnetic field signature of the flowmeter, which was taken at the time of factory calibration. This signature is independent of temperature and flow-rate and will change if there is a mechanical shift of the coils over time due to vibration, thermal cycling, etc.
By eliminating the need to send the flowmeter out for verification, customers eliminate the cost of a spare meter, laboratory costs and maintenance and shipping costs.
Using wireless technology to access diagnostics
Manufacturers continue to develop and enhance their magmeter products. Accuracy and reliability have been improved, and there are now diagnostics that can be implemented to improve how users install, maintain and verify magnetic flowmeters.
But how do they access the data that is now available from these modern devices? One of the advantages of HART-based field devices is their embedded diagnostics capability. Unfortunately, this diagnostics information often goes unused because many legacy control systems don’t support HART communications. As a result, plants with hundreds or thousands of HART devices already installed can miss out on opportunities that diagnostics offer for reducing maintenance costs and improving equipment performance.
The availability of wireless technology using IEC 62591 (WirelessHART) communications enables access to assets that previously were out of physical or economic reach. Connection to the network is provided by a Smart Wireless Adapter (for example Emerson’s THUM) which can be retrofitted onto two or four-wire HART devices, to enable wireless transmission of measurement and diagnostic information.
Wireless communications simplify in-field configuration and troubleshooting for efficient, safe preventative maintenance, eliminating trips to hazardous areas and enabling new operational practices.
Summary
Process industries are being challenged to increase production without compromising quality. Intelligent field devices such as magnetic flowmeters can contribute to this need by providing advanced diagnostic capabilities. These help drive the transition from reactive maintenance to preventive and predictive maintenance strategies, reducing unplanned plant upsets and maximising process up time.
Powerful process diagnostics will help maintain product quality by identifying process issues early. These diagnostics need to be reliable, actionable and easy to use. The examples of the magnetic flowmeters described in this article demonstrate what is possible today.
For more information contact Danielle Aychouh, Emerson Process Management, +97 155 727 5910, danielle.aychouh@emerson.com, www.emersonprocess.com
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