The small voltage developed at the electrodes is proportional and linear with respect to flow through the pipe. Flow measurement with a magnetic flowmeter is independent of temperature, pressure, density or viscosity of the liquid. Accuracy over a wide range (typically 10 to 1) has evolved from 1% of full scale reading to 1% of rate as standard.
Applications of magnetic flowmeters vary from corrosive chemicals to sanitary requirements such as milk and other food products.
The operation of a magnetic flowmeter is based on Faraday’s Law which states that a voltage developed in a conductor is proportional to the strength of the magnetic field (B), the length of the conductor (L) and the velocity (V) of the conductor as it moves through the magnetic field. This relationship can be shown in the following formula.
In the conventional magnetic flowmeter (figure 2) the magnetic field is created by a set of coils mounted outside of the non-magnetic, non-conductive flow tube. These coils are excited by either an AC voltage or a DC pulse depending upon the type of meter used (more detail provided later in this unit). In the magnetic flow meter the actual fluid becomes the moving conductor (must be conductive) therefore the path length (L) is set by the distance between the pick-up electrodes. These probes are mounted on opposite sides of the pipe and protrude through the pipe and liner making the distance between them almost equal to the flow tube diameter. The velocity of the conductive fluid through the pipe becomes the ‘V’ term in Faraday’s equation.
Since the magnetic field (B) and flow tube diameter (L) are known and the induced voltage (E) can be measured by electronic circuitry, the velocity (V) of the fluid can be calculated. Magnetic flowmeters are consequently a type of velocity measuring flow meter. Using the continuity equation and knowing the cross-sectional area of the flow tube, this velocity can then be used to calculate the flow rate through the magnetic flowmeter.
Continuity Flow Equation: Q = V x A
Q – volumetric flow rate
V – average velocity of liquid
A – cross-sectional area of the pipe
It should be noted that most manufacturers have established the maximum flow velocity of 10 m/sec (32.8 ft/sec) and minimum velocity of 1 m/sec (3.28 ft/sec) for optimum meter operation. Abrasive slurries velocities are limited to 2 m/sec (6.5 ft/sec) to effectively increase service life of the liner and electrodes. Sludge and grease bearing liquids should be measured at velocities greater than 2 m/sec (6.5 ft/sec) in order to reduce coating of the liner and electrodes.
Parts of a Magnetic Flowmeter
The major components of a magnetic flowmeter are the:
• Flow tube and liner
• Magnetic coils
• Electronic (transmitter) housing
Although the process fluid may not be considered an actual part of the magnetic flow meter it is still the first parameter to consider when designing a flow system using a magmeter.
The process fluid has to be conductive as has been shown from Faraday’s Law – just how conductive depends upon the requirements specified by the manufacturer. The standard minimum is typically 1 micromhos/cm (mhos/cm) to 5 mhos/cm.
Note that the standard (imperial) unit of conductivity is the ‘mho’ which is just the word ohm written backwards.
Conductivity is thus defined as the ability to conduct current.
The SI unit of conductivity is microsiemens/cm (S/cm), which is numerically equal to mho/cm.
Typical conductivities are:
• Sodium hydroxide 80000 S/cm
• Sulphuric acid 107000 S/cm
• Water 10 S/cm
• Ethyl alcohol 0.44 S/cm
• Fuel oil 0.000001 S/cm
As conductivity levels drop below 5 S/cm electrical noise becomes a problem. As velocity increases the amount of noise tends to increase, thus low conductivity liquid applications should have magmeters sized to a maximum flow of about 3 ft/sec (0.914 m/sec).
Magnetic flowmeters require the process fluid to have a minimum conductivity with no maximum. Since gases have a low conductivity and thus are not measured, magmeters are generally referred to as liquid flowmeters.
The magmeter flow tube has a liner to electrically isolate the electrodes from the meter body and pipe. The liner can be made from a variety of materials, the most common are Teflon, polyurethane, and rubber. Since the liner is in contract with the process fluid (a wetted part) it must be selected to not deteriorate in the given application. Polyurethane and rubber are mainly used in abrasive services whereas Teflon is used in sanitary, highly corrosive, and high temperature applications.
The coils of the magmeter create the magnetic field. The two general types of coils are the AC excited and the DC excited which will be discussed later in this unit.
The electrodes are physically mounted in the flow tube such that they are at right angles to both the magnetic field and the axis of the pipe. Like the liner, the electrodes are also a wetted part and therefore must be made of a material that is compatible with the process. Some common materials are stainless steel, Hastelloy, Monel.
One of the common problems associated with a magmeter is electrode coating since the electrodes are in contact with the process. An insulating type coating on the electrodes can create a significant shift in the magmeter zero and span. Several different methods have and are being used to prevent or reduce electrode coating. The electrodes can be manually brushed, however the pipeline has to be shutdown during this procedure. A momentary AC voltage can be applied to the electrodes to ‘burn-off’ the insulating coating however there is a loss of flow signal during this time. Electrodes can be rounded and designed to protrude slightly into the process to create a turbulence which scrubs the tip and thus are classified as a self-cleaning electrodes. Some electrodes can be removed from the magmeter for cleaning and some of these without stopping the flow. These electrodes must be carefully re-installed as the distance between the probes is one of the determining factors in determine the quantity of induced voltage. Ultrasonic generators can also be to clean the electrodes without interrupting the process. The more modern approached to electrode insulating coating is to ignore it through the use of a pulsed DC excitation systems which will be explained late.
The transmitter component of the magmeter converts the small voltage induced at the electrodes to a useable output signal. Traditionally the output of the transmitter is an analog 4 -20 mA signal although more modern magmeter transmitters are microprocessor based and have a digital, frequency output. Microprocessor based transmitters can automatically measure bi-directional flow; can be configured for totalizing or scaling, can display different units of flow, alarm points can be set, bi-directional communication established, and many more features.
Magnetic Field Excitation
There are two common methods used to create the magnetic field in the magmeter
Pulsed DC excitation
AC excitation was the first to be used however the advantages of the pulsed DC type have resulted in its more predominate use.
An AC voltage with its sinusoidal nature is always changing. When an AC voltage excites the coils of a magmeter two signals are induced at the electrodes – one created by the velocity of the fluid (Faraday’s Law), the other by the changing magnetic field itself. The second signal is called noise and must be reduced or eliminated for an accurate flow measurement.
The amount of noise can be determined by stopping the liquid flow. Once stopped, the only signal the transmitter would see is the noise due to the changing magnetic field. This ‘noise’ signal could then be zeroed-out in the transmitter.
The non-practicality of periodically stopping the flow to re-zero the meter, the large size of the AC coils compared to DC coils, and the electrode coating problems of AC magmeters have lead to the decline of the AC magmeter.
It should be noted however that an advantage of the AC excited meter is its fast speed of response as the coils are continuously energized although with advances in pulsed DC technology even this may not be an advantage for long.
A pulsed DC excited magmeter purposely has the coils de-energized for a period of time and energized for a period of time. During the ‘on-time’ the voltage supplied to the coils is constant and therefore the voltage signal induced at the electrodes and seen by the transmitter is due only to the fluid velocity. There is no noise due to changing excitation current.
It should be noted that it is also possible for noise to be generated from sources external to the magmeter. These may be due to other magnetic fields from inductive loads (motors) or voltage build-up at the electrodes dues to stray electrical current on the pipe or even electrochemical reactions within the process liquid itself. These could create a voltage noise at the electrodes that is not a result of the flow rate. These external noise signals can be automatically (electronically in the transmitter) zeroed-out in a pulsed DC magmeter once every cycle when the coils are de-energized or turned off (figure 3).
This is not possible in the AC excited magmeter because the coils are never really turned off.
The advantages of a pulsed DC magmeter include lower power consumption and automatic re-zeroing due to its ‘turn-off’ time. The coils of an DC meter are smaller and therefore have less weight than an comparable AC magmeter. A limitation of a pulsed DC meter is its speed of response again due to its off time. During this off-time there is no flow reading and for very fast changing flow conditions this may result in a less accurate average flow rate reading. The more modern pulsed DC magmeters have an adjustable on/off time that allows flexibility for a given flow situation and thereby reduces flow errors.
The importance of installation of a magmeter cannot be overemphasized. The direction of flow, the orientation of the meter for given process, bolting, and grounding has a significant impact on the operation of the flowmeter.
A magmeter can measure flow in either direction – the only difference is in the polarity of the voltage induced at the electrodes. Unless the transmitter is designed to detect this change, reverse flow will give the meter a ‘zero flow’ indication. For this reason magmeters have a flow direction stamped on the housing.
Magnetic flowmeters are designed to measure liquids in full pipes thus the preferred meter orientation is vertical with the flow upward through the meter. Sloping upward or horizontal piping is acceptable however piping must be designed to ensure the flow tube is always full (see figure 4)
The minimum straight run pipe lengths for a typical magmeter is significantly less than for orifice plates – one resource states 20D upstream and 5D downstream for an orifice plate and 10D upstream and 3D downstream for a magmeter. The manufacturer’s information should be consulted for actual values for a given application.
Grounding and Ground Rings
Proper process grounding is one of the most important installation details, as it ensures that the flowtube and process fluid are at the same potential and only the induced flow signal is therefore measured.
This is necessary to provide a stable reference for the measuring electronics, which is usually an ungrounded differential amplifier. In addition, stray electrical currents can be induced in metal pipelines by capacitive or inductive coupling from other electrical sources. These electrical currents can create voltage zero shifts at the magmeter electrodes. Proper bonding of the magmeter to either side of the pipeline connection allows these stray currents to pass by the meter rather than go through it.
By connecting the magmeter flowtube, the fluid, and the reference for the amplifier to a stable, noise free reference point, the user is ensured of getting the best performance from their magnetic flowmeter
Where necessary magnetic flow meters are provided with 2 grounding rings of a conductive material recommended for the intended service. These grounding rings although designed primarily to provide proper grounding on installations with insulated piping, also serve to protect and shield the flowtube liner edge from process abrasion.
Most manufacturers include grounding ring kits if installing on insulated or plastic pipe and a detailed explanation of how to properly ground their magmeter (see figure 5). However inexperienced construction personnel have been known to “forget” to install these components, so be aware.
One of the most common faults of initial magmeter failure is improper tightening of the flange bolts. The liner of the magmeter can be easily damaged beyond repair by over-torquing the flange bolts. Manufacturer’s literature should be consulted for the proper torque requirements for their particular meter.
Magnetic flowmeters measure the volumetric flow rate of conductive liquids. Since they are velocity flowmeters their signal output is linear and directly proportional to the flow rate.
Meters range in size from small (0.1 inches, 2 mm) to large (96 inches, 243 cm) and with flow rates from 0.01 USGPM (37 mL/min) to 500, 000 USGPM (1.89 ML/min). Generally, magmeters have a relatively wide rangeability (10:1) and inaccuracies of 1% of full scale to 1% of rate.
Magmeters are virtually obstruction less and therefore have no pressure drop. They are widely used in the pulp and paper industry and in mining applications. They are not used in the oil industry as hydrocarbons are not conductive.
Advantages and Limitations
Although the features of the magmeter have been discussed throughout this document they are outlined here.
• No moving parts
• Obstruction less, no pressure drop
• Low power requirements, especially for pulsed DC excited meters
• Wide application for corrosive and slurry process because of liner
• Can measure low and high flow rates, good rangeability
• Can measure bi-directional flow
• Relatively high accuracy
• Can only measure conductive liquids, no hydrocarbons or gases
• Relatively heavy, especially AC excited meters
• Caution must be used in installing the meter (grounding, torquing, full pipe)
• Relatively expensive
• Electrodes can become coated
• Velocity measurement is limited to a maximum of 30 ft/sec (10m/sec) in practice.
A magnetic flowmeter system has two major components – the magmeter body that is installed in the pipeline and the transmitter, which may be mounted remotely from the body.
Each of these has its own calibration techniques and can be done independently however better accuracy is achieved if they are done together as a system.
Before a flowmeter leaves the factory it is calibrated with its own unique K Factor.
Passing a volume of liquid through the meter performs this true hydraulic calibration. This specific volume is measured using a weigh tank or a master meter such as a turbine meter. This accurate volume is then compared to the frequency count (number of pulses). From several tests the number of pulses per unit volume (K Factor) is determined. Each magnetic flowmeter is then stamped with its own unique K Factor. If these calibrations are performed in a certified facility then a calibration certificate traceable to NIST (National Institute of Standards and Technology) can be obtained.
As ‘real’ flows are difficult to attain in the field, only the transmitter part of the magmeter is field calibrated. A field calibrator (usually manufacturer specific) is used to simulate the input electrical output from the electrodes and to test and troubleshoot the electronic circuitry of the transmitter.
With the many advantages of a magnetic flowmeter, it is a widely used flow meter in many industries other than petroleum. This unit described the basic parts of a magmeter: the flow tube and liner, the electrodes, the coils, and the transmitter. The basic principle of operation of the magmeter shows an application of Faraday’s Law. Some of the operational considerations of a magmeter were mentioned such as the coating of the electrodes and ways to reduce it. The importance of proper installation practices was emphasized. The methods of generating the magnetic field through AC and pulsed DC exciting were described.
Data relating to possible applications of the magmeter along with its advantages and limitations were outlined. A brief explanation of the calibration involved with magmeters concluded the unit.