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:
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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.
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Chemical and pharmaceutical companies often use deionizer or distilled water, or other solutions which are not conductive enough for use with magnetic flow meters.
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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.
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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.