Magnetic flowmeters can contribute more

19 July 2011

Klaus Brockman of Emerson Process Management explains how the latest generation magnetic flowmeters, with built in diagnostics and integrated meter verification, can help to better manage the production process.

Magnetic flowmeters are 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, generally, accurate and reliable. They are suitable for a range of pipe sizes, and can be used to measure volume flow for most conductive liquids. The availability of different liners makes them suitable for sanitary and hygienic applications as well as the handling of aggressive or corrosive fluids.
Manufacturers are constantly enhancing performance and broadening application scope of their products. 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.

The operating principle of the magnetic flowmeter system is 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.

The importance of ground wiring
To ensure a magnetic flowmeter performs accurately, it is important that it is correctly grounded. Improper grounding can occur in new installations, where the unit is not properly referenced to the process and also 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 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 diagnostics. Emerson’s Rosemount E-series Magmeter contains this feature, which works by looking at the signal amplitude at frequencies of 50Hz and 60Hz. If the amplitude of the signal at either of these frequencies exceeds 5mV, 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.

It is important that grounding faults are not overlooked, but this does often happen. One Emerson customer in the chemical industry was experiencing high measurement variability on a magnetic flowmeter installed in a water evaporator line. To fix the problem the flowtube sensor and transmitter was replaced. However, this did not resolve the issue. 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 was replaced with a unit that incorporated ground/wiring fault diagnostics, a fault was immediately indicated. On further inspection, it was apparent that the old ground wires had been used with the new flow tube sensor which meant the bad ground connection remained undetected. The replacement of the original flow tube and transmitter interrupted the process and incurred considerable costs when the actual source of the problem was simply a corroded ground wire.

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 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.

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.

Process noise creates instability. 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, this will cause 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 an inconsistent process. While the flowmeter output and valve travel may be indicating steady process flow conditions, they may 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. The Rosemount E-series, for example, allows users to switch to 37Hz. 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.

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, the SNR increases.

If high levels of 1/f or white noise are present, switching to the 37Hz coil drive frequency may not be enough. In such cases, additional Digital Signal Processing (DSP) 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. The customer experienced instability in the output from a magnetic flowmeter installed in the basis weight flow line. The operators were unable to reliably run their basis weight control loop in automatic mode. Even with damping applied in both the meter and the control system, there was too much noise to effectively control the speed of the basis weight pump.

An E-series magmeter with a high process noise diagnostic was installed and this indicated a low signal to noise ratio at 5Hz. By changing the coil drive frequency to 37Hz, 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 more accurately 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
Some industries require regular calibration of flowmeters 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.

However, there are now magnetic flowmeters with the ability to carry out a self verification test – alerting the operator when the calibration drifts outside set parameters. Emerson’s Smart Meter Verification Diagnostic, for example, 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.

Wireless diagnostics
Being able to access the data that is now available from these devices is one of the advantages of HART-based field devices, with their embedded diagnostics capability. Unfortunately, this information often goes unused because many legacy control systems do not support HART communications. As a result, plants with many 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 gives access to these assets. Connection to the network is provided by a Smart Wireless Adapter which can be retrofitted onto two- or four-wire HART devices, without special power requirements, to enable wireless transmission of measurement and diagnostic information.

Process industries are being challenged to increase production, with reduced manpower, at lower costs, 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.

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