Views:16 Author:Site Editor Publish Time: 2020-03-20 Origin:Site
Water and wastewater professionals rely on accurate flow measurement to achieve process operation and regulatory compliance. Choosing the best flow meter for each application is critical to obtaining accurate flow data.
The starting point is detailed knowledge of the application and the fluid being measured. It is also necessary to understand the characteristics of different meter types to determine the best match. Electromagnetic flowmeters and ultrasonic flowmeters are very popular in the water industry. Below is a comparison of these two meter types to help make decisions.
Ultrasonic flow meters use sound waves to measure the velocity of fluid flowing through pipes and convert this data into volume. The two basic types of ultrasonic meters are transit time and Doppler meters.
The transit time meter uses transducers clamped or inserted into the pipe at two locations to send ultrasonic signals into the stream. The time difference between the sound propagation upstream and downstream between the two sensors is proportional to the flow rate.
Doppler meters use sound waves reflected from substances in the fluid, such as bubbles or particles, to measure flow velocity.
Electromagnetic flow meters, called "mag" meters, work according to Faraday's law. The principle basically states that when a conductor (such as water) passes through a magnetic field, it generates an electrical signal. The full-caliber electromagnetic flowmeter uses an electromagnet installed on the outside of the pipeline, perpendicular to the flow direction. When a fluid passes through a magnetic field, charged ions will accumulate, with negative ions on one side and positive ions on the other. The resulting voltage change is proportional to the velocity of the fluid passing through the pipe and can be converted to volume.
The operation of the plug-in electromagnetic flowmeter is slightly different. For plug-in meters, a very small local magnetic field is inserted into the pipe. The electrodes cause the charge carriers to separate. Using a full profile plug-in meter, electrodes are placed along the entire flow cross section to measure the average velocity through the centerline of the pipe.
Both electromagnetic flowmeters and ultrasonic flowmeters can be used for a variety of water and wastewater applications, including the measurement of sludge, mud, and some chemicals.
Transit time ultrasonic meters can measure conductive and non-conductive fluids. These meters may have problems when measuring fluids containing suspended solids, debris, or bubbles, which can interrupt the path of the acoustic signal. Temperature compensation may be required to maintain accuracy. Corrosion, pitting or biofilm buildup on the pipe wall can also cause problems.
Doppler meters require a certain type of material-bubbles or solids-in the flow to measure flow.
The performance of the electromagnetic flowmeter is not affected by temperature, pressure or viscosity. These meters can handle rapid changes in flow. They can accurately measure clean fluids (such as drinking water) or fluids with heavy solids (such as sewage lifting stations and biosolids facilities). However, electromagnetic flowmeters cannot measure non-conductive fluids such as oil, steam or gas. The electromagnetic flowmeter can measure the flow of vertical or horizontal flow. Using today's high-resolution microprocessors, they can measure flow rates as low as 0.2 to 0.3 feet per second.
Both ultrasonic and electromagnetic flowmeters can be installed on various pipe materials and sizes from ½ inch to more than 100 inches in diameter.
For narrow sites or renovation projects, electromagnetic flowmeters need fewer straight pipes to avoid inaccuracy of flow jammers. Full-bore electromagnetic flowmeters only need one diameter length upstream and two downstreams, while ultrasonic flowmeters usually require five upstreams and 10 downstreams.
The plug-in electromagnetic flowmeter can be installed with a hot water tap without the need to shut down the process.
An important consideration for the installation of electromagnetic flowmeters is to ensure a quiet electrical environment, good grounding and shielded cables.
Transit time Ultrasonic instrument can be clamped or heat tapped into the pipeline. The coating on the outside of the pipe must be removed, and material must be added between the pipe and the sensor. In order to obtain the required accuracy, the exact thickness and material of the pipe must be known.
The cost comparison depends largely on the specific project rather than the cost of the instrument. Both instruments can provide cost-effective flow measurement. Will this project use a full-bore electromagnetic flowmeter or a plug-in type? Jet lag ultrasound or Doppler? The additional straight pipes required for ultrasonic instruments can add significant capital costs.
Electromagnetic flowmeters have higher accuracy, extremely low requirements for straight pipes, long service life, and extremely low life cycle costs. The New York City electromagnetic flowmeter installed in 1920 is still working and providing accurate flow data.
The electromagnetic flowmeter is very accurate, with an error of ±0.5% or less. In contrast, the time difference ultrasonic meter has an error of ±1% to 2%.
The electromagnetic flowmeter has no moving parts and requires no maintenance.
Ultrasonic meters also have no moving parts. The clip-on type requires the material between the sensor and the pipe to properly transmit sound. This material will degrade over time and needs to be replaced. In addition, the clamp must be tightened occasionally.
Both types of meters have lightning and power surge problems.
Both electromagnetic flowmeters and ultrasonic flowmeters provide high-precision flow readings for water and wastewater applications.
However, electromagnetic flow meters provide a higher level of accuracy and greater flexibility. Electromagnetic flowmeters can be used for full-bore installations, but can also be used in situations where openings are required to avoid process shutdown. The full-profile plug-in electromagnetic flowmeter can provide high-precision level full-hole installation without cutting into the existing pipeline.
For renovation projects or projects with limited space, only one straight pipe section with one diameter upstream and two straight pipe sections with downstream diameters are required to reduce capital costs and make life easier.
The electromagnetic flowmeter requires no maintenance, has an extremely long service life, and has a low life cycle cost, and can realize reliable and accurate flow measurement.
Water professionals have the responsibility to protect public health and the environment. Accurate flow measurement is an important part of process operation. Choosing the right flow meter can save time, money and trouble.
The manufacturer’s representatives have deep knowledge and experience to help make the right decision. Ask for their help when deciding which meter is best for your project.