ELECTROMAGNETIC FLOW METER

Introduction to Magnetic Flow Meters

         A magnetic flow meter (mag flow meter) is a volumetric flow meter which does not have any moving parts and is ideal for wastewater applications or any dirty liquid which is conductive or water based. Magnetic flow meters will generally not work with hydrocarbons, distilled water and many non-aqueous solutions). Magnetic flow meters are also ideal for applications where low pressure drop and low maintenance are required. 

                                           

Principle of Operation:

       

 The operation of a magnetic flow meter or mag meter is based upon Faraday's Law, which states that the voltage induced across any conductor as it moves at right angles through a magnetic field is proportional to the velocity of that conductor.

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Faraday's Formula:

E is proportional to V x B x D where:                       E = The voltage generated in a conductor             V = The velocity of the conductor             B = The magnetic field strength             D = The length of the conductor           To apply this principle to flow measurement with a magnetic flow meter, it is necessary first to state that the fluid being measured must be electrically conductive for the Faraday principle to apply. As applied to the design of magnetic flow meters, Faraday's Law indicates that signal voltage (E) is dependent on the average liquid velocity (V) the magnetic field strength (B) and the length of the conductor (D) (which in this instance is the distance between the electrodes).In the case of wafer-style magnetic flow meters, a magnetic field is established throughout the entire cross-section of the flow tube (Figure 1). If this magnetic field is considered as the measuring element of the magnetic flow meter, it can be seen that the measuring element is exposed to the hydraulic conditions throughout the entire cross-section of the flow meter. With insertion-style flow meters, the magnetic field radiates outward from the inserted probe .                      ELECTROMAGNETIC FLOW METER •         AC type magnetic flow meters apply line voltage to the magnetic coils. The signal generated is a low level AC signal in the high microvolt to low mV range. •        In the pulsed DC type Magnetic flowmeter, the magnet coils are periodically energized. DC excitation may be either on-off or plus-minus excitation. The principle is to take a measurement of the induced voltage when the coils are not energized and to take a second measurement when the coils are energized and the magnetic field has stabilized. •In all the pulsed DC approaches, the concept is to take a measurement when the coils are excited and store that information, then take a second measurement of the induced voltage when the coils are not excited. The voltage induced is a combination of both noise and signal when the coils are energized and it is only noise when the coils are de-energized. Subtracting one from the other will yield only signal.                                          AC/DC Technology Comparison                             DC                                    AC Accuracy                                 High                                   Low Auto Zeroing                           Yes                                    No Power consume.                      Low                                    High Effect of stray Noise                Low                                   High Process noise rejection             Not good                           Good •Design Temperature    Upto 120 °C with Teflon liners; Upto   180 °C with Ceramic                                                   liners. •Type of flow detected  Volumetric flow of conductive   liquids, including slurries of                                                  corrosive or abrasive materials. •Flow Ranges                 0.01 to 100000 GPM (0.04 to 378000   l/m) •Size Ranges                         From 0.1 to 96” in diameter •Accuracy                 ±1% of full scale for AC excitation.   ±1% of actual flow for 10:1 range   and ±0.5% of actual flow for 2:1 or   5:1 range.'

MAGMETER SELECTION:

The characteristics of the fluid to be metered, the liquid flow parameters, and the environment of the  eter  are the determining factors in the selection of a particular type of flowmeter.

Conductivity:

Electrical conductivity is simply a way of expressing the ability of a liquid to conduct electricity. Just as copper wire is a better conductor than tin, some liquids are better conductors than others. However  of even greater importance is the fact that some liquids have little or no electrical conductivity (such as hydrocarbons and many nonaqueous solutions, which lack sufficient conductivity for use with magmeters). Conversely, most aqueous solutions are well suited for use with a magmeter. Depending on the individual flowmeter, the liquid conductivity must be above the minimum requirements specified. The conductivity of the liquid can change throughout process operations without adversely affecting meter performance, as long as it is homogeneous and does not drop below the minimum conductivity threshold. Several factors should be taken into consideration concerning liquids to be metered using magnetic  flow meters. Some of these are:

1. All water does not have the same  conductivity. Water varies greatly in conductivity due to various  ons present. The conductivity of “tap water” in Maine might be very different from that of “tap water” in Chicago.

2. Chemical and pharmaceutical companies often use deionizer or distilled water, or other solutions which are not conductive enough for use with magnetic flow meters. 3. Electrical conductivity is a function of temperature. However, conductivity does not vary in any set pattern for all liquids as temperature  changes. Therefore, the temperature of the liquid being considered should always be known.

4. Electrical conductivity is a function of concentration. Therefore, the concentration of the solution should always be provided. However, avoid what normally is a logical assumption,  such as: That electrical conductivity increases as concentration increases. This is true up to a point in some solutions, but then reverses. For example, the electrical conductivity of aqueous solutions of acetic acid increases as concentration rises up to 20%, but then shows a decrease with increased concentration to the extent that, at some concentration above 99%, it falls below the minimum requirement.

Acid/Caustics:

The chemical composition of the liquid slurry to be metered will be a determining factor in selecting the flow meter with the proper design and construction. Operating experience is the best guide to selection of liner and  electrode materials, especially in industrial applications, because, in many cases, a process liquid or slurry will be called by a generic name, even though it may contain other substances which affect its corrosion characteristics. Commonly available corrosion guides may also prove helpful in selecting the proper materials of construction.

Velocity:

The maximum (full scale) liquid velocity must be within the specified flow range of the meter for proper operation. The velocity through the flowhead can be controlled by properly sizing the meter. It isn’t

necessary that the flowhead be the same line size, as long as such sizing does not conflict with other

system design parameters. Although the meter will increase hydraulic head loss when sized  smaller than the line size (because the meter is both obstructionless and of short lay length), the amount of increase in head loss is negligible in most applications. The amount of head loss increase can be further limited by using concentric reducers and expanders at the pipe size transitions. As a rule of thumb, meters should be sized no smaller than one-half of the line size. Because of the wide rangeability of magnetic  lowmeters, it is almost never necessary to oversize a meter to handle future flow requirements. When future flow requirements are known to be significantly higher than start-up flow rates, it is imperative that the initial flows be sufficiently high and that the pipeline remain full under normal flow conditions.

Abrasive Slurries:

Mildly abrasive slurries can be handled by magnetic flowmeters without problems, provided  consideration is given to the abrasiveness of the solids and the concentration of the solids in the slurry. The  abrasiveness of a slurry will affect the selection of the construction materials and the use of protective orifices. Abrasive slurries should be metered at 6 ft/sec or less in order to minimize flow meter abrasion damage. Velocities should not be allowed to fall much below 4 ft/sec, since any solids will tend to settle out. An ideal slurry installation would have the meter in a vertical position. This would assure uniform distribution of the solids and avoid having solids settle in the flow tube during no-flow periods. Consideration should also be given to use of a protective orifice on the upstream end of a wafer-style

magnetic flow meter to prevent excessive erosion of the liner. This is especially true since Tefzel liner

have excellent chemical resistance, but poor resistance to abrasion. In lined or non-conductive piping

systems, the upstream protective orifice can also serve as a grounding ring.

Sludges and Grease-Bearing Liquids:

Sludges and grease-bearing liquids should be operated at higher velocities, about 6 ft/sec minimum, in order to reduce the coating

Viscosity:

Viscosity does not directly affect the operation of magnetic flowmeters, but, in highly viscous fluids, the size should be kept as large as possible to avoid excessive pressure drop across the meter.

Temperature:

The liquid’s temperature is generally not a problem, providing it remains within the mechanism’s operating

limits. The only other temperature considerations would be in the case of liquids with low conductivities (below around 3 micromhos per centimeter) which are subject to wide temperature excursions. Since most liquids exhibit a positive temperature coefficient of conductivity, the liquid’s minimum conductivity must be determined at the lower temperature extreme.

Advantages of the DC Pulse Style:

From the principles of operation, it can be seen that a magnetic flowmeter relies on the voltage generated by the flow of a conductive liquid through its magnetic field for a direct indication of the velocity of the liquid or slurry being metered. The integrity of this low-level voltage signal (typically measured in  undress of  microvolt's) must be preserved so as to maintain the high accuracy specification of magnetic  flow meters in industrial environments. The superiority of the dc pulse over the traditional ac magnetic meters in preserving signal integrity can be demonstrated as follows:

Quadrature:

Some magnetic flowmeters employ alternating current to excite the magnetic field coils which generate the magnetic field of the flowmeter (ac magnetic flowmeters). As a result, the direction of the magnetic

field alternates at line frequency, i.e., 50 to 60 times per second. If a loop of conductive wire is located in a magnetic field, a voltage will be generated in that loop of wire. From physics, we can determine that this voltage is 90° out of phase with respect to the primary magnetic field.

 The magnitude of this error signal is a function of the number of turns in the loop, and the change in  magnetic flux per unit time. In a magnetic flowmeter, the electrode wires and the path through the conductive liquid between the electrodes represent a single turn loop. The flow-dependent voltage is in phase with the changing magnetic field; however, flow independent voltage is also generated, which is 90°out of phase with the changing magnetic field. The flow-independent voltage is therefore an error voltage which is 90° out of phase with the desired signal. This error voltage is often referred to as  uadrature. In order to minimize the amount of quadrature generated, the electrode wires must be  arranged so that they are parallel with the lines of flux of the magnetic field. 

In ac field magmeters, because the magnetic field alternates continuously at line frequency, quadrature is significant. It is necessary to employ phase sensitive circuitry to detect and reject quadrature. It is this circuitry which makes the ac magnetic meter highly sensitive to coating on the electrodes.

Since coatings cause a phase shift in the voltage signal, phasesensitive circuitry leads to rejection of the true voltage flow signal, thus leading to error.

                  Since dc pulse magmeters are not sensitive to phase shift and require no phase sensitive circuitry, coatings on the electrodes have a very limited effect on flowmeter performance.

Wiring:

In ac magnetic flowmeters, the signal generated by flow through the meter is at line frequency. This  makes these meters susceptible to noise pickup from  any ac lines. Therefore, complicated wiring systems are  typically required  to isolate the ac flowmeter signal lines from both its own  and from any other nearby power lines, in order to preserve signal integrity. In comparison, dc pulse magmeters have a pulse frequency much lower (typically 5 to 10% of ac line frequency) than ac meters. This lower frequency eliminates noise pickup from nearby ac lines, allowing power and signal lines to be run in the same conduit and thus simplifying wiring. Wiring is further simplified by the use of integral signal conditioners to provide voltage and current outputs. No separate wiring to the signal conditioners is required.

Power:

By design, ac magnetic flowmeters typically have high power requirements, owing to the fact that the  magnetic field is constantly  being powered. Because of the pulsed nature of the dc pulse magmeter, power  is supplied intermittently to the magnetic field coil. This greatly reduces both power requirements and heating of the electronic circuitry, extending the life of the instrument.

Auto-Zero:

In traditional ac magnetic flowmeters, it is necessary after installation of the meter to “null” or “zero” the nit. This is accomplished by manual  adjustment which requires that the flowmeter be filled with process

liquid in a no-flow condition. Any signal present under full pipe, no-flow conditions is considered to be an error signal. The ac field magmeter is therefore “nulled” to eliminate the impact of these error signals.

Many Magmeters feature automatic zeroing circuitry to eliminate the need for manual zeroing. When the magnetic field strength is zero between pulses, the voltage output from the electrodes is measured. If any

voltage is measured during this period, it is considered extraneous noise in the system and is subtracted from the signal voltage  generated when the magnetic field is on. This feature insures high accuracy, even in electrically noisy industrial environments.

Magmeter Selection

                   The key questions which need to be answered before selecting a magnetic flowmeter are:

•	Is the fluid conductive or water based?
•	Is the fluid or slurry abrasive?
•	Do you require an integral display or remote display?
•	Do you require an analog output?
•	What is the minimum and maximum flow rate for the flow meter?
•	What is the minimum and maximum process pressure?
•	What is the minimum and maximum process temperature?
•	Is the fluid chemically compatible with the flow meter wetted parts?
•	What is the size of the pipe?
•	Is the pipe always full?

Installation Considerations:

          Select a location for the sensor where the flow profile is fully developed and not affected by any disturbances. A minimum of 10 pipe diameters of straight run upstream and 5 diameters downstream is recommended. Some situations may require 20 pipe diameters or more upstream to insure a fully developed turbulent flow profile. The insertion magmeter is sensitive to air bubbles at the electrodes. If there is any question that the pipe is absolutely full, mount the sensor at a 45 to 135 angle.

Grounding requirements: 

          Magnetic flow sensors are sensitive to electrical noise which is present in most piping systems. In plastic piping systems, the fluid carries significant levels of static electricity that must be grounded for best magmeter performance. Instructions are included with the installation manual on how to best ground the magnetic flow meter.


In Line Magmeters

          The in line type magnetic flow meters offer a higher accuracy. They can be as accurate as 0.5% of the flow rate. The insertion styles offer a 0.5 to 1% accuracy. Omega's FMG-600 series in line flange and wafer style meters offer higher flow rates of 1 to 10 m/sec. These in line meters are offered in pipe sizes up to 12".

Minimum conductivity:

           5 micro Siemens/cm

Installation Considerations: 

            In line flow meters do not require as much straight pipe as the insertion styles. A minimum of 5 to 10 pipe diameters of straight run upstream and 1 to 2 diameters downstream is recommended. In vertical pipe runs, flow should always run up and not down. These flowmeters are very sensitive to air bubbles. The magmeter cannot distinguish entrained air from the process fluid; therefore, air bubbles will cause the magmeter to read high.

Advantages :

1)  Obstruction less and has no moving parts. No pressure drop across the meter.

2)  Bi-directional

3)  Viscosity of liquid has no effect on metering.

4)  Can be used for slurry service.

5)  Can measure very low to very high volume flow rate with   minimum size less than 1/8” inside diameter to size as large as 10 feet.

6)  Suitable for most acids, bases, waters and aqueous solutions.

Limitations :

 1)  Can work only with conductive fluids. Pure substances,   hydrocarbons and gases cannot be measured.

 2)  Electrical installation care is essential.

3)  To periodically check zero on AC type Magnetic Flowmeters,   block valves are required in either side to bring the flow to zero   and keep meter full. Cycled DC units do not have this   requirement.