Although there are numerous types of magnetic flowmeters available for measuring liquid flow rates, all of them function according to the fundamental principles of Faraday’s law, which dictates the relationship between a moving conductor and the voltage it creates within a magnetic field. But despite their similarities, some magnetic flowmeters are better suited to specific tasks than others, making flowmeter selection and proper application important concerns in a fluid measurement project. Issues of cost, accuracy, reliability, and ease of use are some of the significant factors involved in choosing a flowmeter.
In terms of application, the metallurgical properties of the fluid conduit and the measuring apparatus can greatly influence measurement effectiveness. In addition, a fluid’s velocity profile and the flowmeter system’s capacity for handling flow disturbances or other interference often determine the type of device that would be most appropriate for a given application. In order to ensure reliable performance over an expected period of operation and volume of moving fluid, flowmeter scale—both in size and velocity limits—should be taken into careful account.
For more information on Faraday’s law, please visit HyperPhysics.
Size and Capacity Factors
The physical characteristics of an electromagnetic flowmeter, particularly its size, affect its processing abilities and the velocity range that it can effectively handle. The majority of magnetic flowmeters have a meter size of between approximately 0.6 to 7.9 inches (15 to 200 millimeters) and a velocity range between .98 and 32.8 feet per second (0.3 to 10 meters per second). The larger a flowmeter’s size and the higher its velocity capacity, the greater the flow rate it can measure. Some magnetic flowmeters can provide measurements of velocities below the standard range, but the accuracy decreases to some degree. Likewise, having flowmeters handle velocities higher than roughly 16 feet per second (5 meters per second) increases the risk of pipe deterioration and damage.
Magnetic flowmeters can be highly effective for applications involving corrosive conditions and for measuring the flow rate of corrosive materials, such as abrasives or slurries. They are also commonly employed in measuring paper stock or pulp, as well as low flow rates and pipe networks with relatively short inside diameters. Many magnetic flowmeters improve their performance by switching between two different range capacities, allowing them to reset their characteristics according to changes in the operating conditions.
Piping System Design
Circular configurations are fairly common in piping systems because they offer a comparatively simple construction design. Magnetic flowmeters are more easily applied to a circular arrangement than to rectangular pipe networks, and usually offer more effective measurements. While fluid velocity is usually unaffected by the pipe symmetry in this sort of network, the circular shape can sometimes distort the magnetic field, creating a need for recalibration. In straight piping situations, the upstream and downstream requirements are often different for each type of flowmeter. In most cases, the straight section of a pipe needs to be a specific distance from the electrodes and the flowmeter face in both upstream and downstream flow. The magnetic coils can also be arranged in a flowmeter to reduce piping effects through magnetic field distribution.
Magnetic flow meters can be used to measure flow rates for combustible or explosive liquids, often under hazardous conditions. Explosion-resistant flowmeter housings are vital for these projects, and the design specifications and safety parameters for the housings are usually regulated by presiding authorities. Remote electronic controls may be used to increase flowmeter reliability in hazardous circumstances, and integrated controls are also available. Some magnetic flowmeters are equipped with a dividing housing that separates field wiring from electronic circuitry. A backlit LCD interface can help improve operator use.
Advantages and Disadvantages
There are numerous benefits to using electromagnetic flowmeters to perform fluid flow measurements. They are generally non-invasive and have no moving parts, reducing the risk of breakdowns and the frequency of repairs. A decrease in flowmeter pressure is also usually no greater than that of an equivalent pipe length, reducing the piping costs. Some of the other major advantages provided by magnetic flowmeters include:
• Power usage is relatively low, with electrical power requirements as low as 15 watts for some models.
• They are mechanically obstructionless and can be equipped with abrasion-resistant liners, making them effective for measuring slurries and other erosive fluids.
• They are capable of dealing with most kinds of acids and bases, as well as water and water-based solutions, due to lining materials that are both insulators and have corrosion resistance.
• Relatively small amounts of electrode metals are needed for magnetic flowmeters.
• They can measure both very low flows and very high volume flow rates, with a minimum diameter of roughly 0.125 inches and a maximum volume of up to 10 cubic feet.
• They can usually measure multidirectional flow, either upstream or downstream.
Despite these advantages, magnetic flowmeters also present certain difficulties for flow rate measurement. These flowmeters are only effective on conductive fluids, and materials such as unmixed hydrocarbons and gases cannot be measured. However, magnetic materials themselves may also present problems, as hydrodynamic effects can alter the normal flow pattern and disturb the velocity rate enough to interfere with operations. Depending on their size and capacity, magnetic flowmeters can be relatively heavy, and those with higher corrosion and abrasion resistance can be expensive.
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