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Introduction
This article takes an in depth look at Ultrasonic Flow Meters.
You will learn more about topics such as:
What is an ultrasonic flow meter?
How an ultrasonic flow meter works
Types of ultrasonic flow meters
Uses for ultrasonic flow meters
And much more…
Chapter One – What is an Ultrasonic Flow Meter?
An ultrasonic flow meter measures the flow of a liquid or gas by sending ultrasonic waves across a pipe, containing the flow in the direction of the flow and in the opposite direction of the flow. The ultrasonic waves and the velocity of the flow of the liquid or gas can be combined to determine the flow rate. An ultrasonic flow meter has two transmitters, and two receivers, with one of each mounted on either side of the pipe at a calculated distance to provide accurate readings.
Ultrasonic flow meters are highly accurate with a rating of ±1% and are essential for custody transfers. The clamp-on version is non-intrusive and can be easily installed by being clamped to the exterior of a pipe. Unlike other forms of flow meters, ultrasonic flow meters do not have moving parts and are versatile and bidirectional.
Chapter Two – How Do Ultrasonic Flow Meters Work?
The first ultrasonic flow meter was introduced by a Japanese inventor in 1959. It used doppler technology to measure blood flow. In the early 1960s, flow meters found use as industrial instruments for measuring the flow of gasses and liquids. From their early beginnings, they have become the main product for many instrument manufacturing companies.
A flow meter aims to measure the volume of flow of liquids and gasses to provide accurate readings for flow control. Many industrial applications, such as chemical companies, require exact readings for production processes.
How an Ultrasonic Flow Meter Works
An ultrasonic flow meter uses acoustic sounds to measure flow. There are two types of ultrasonic flow measuring methods, which are transit time travel and Doppler shift. The difference between the methods is how directional measurement is acquired. With the time travel method, sound waves are sent along a diagonal to the flow, and measurements are taken in both directions. With the Doppler method, sound waves are projected along the flow path, and the frequency of the return signal is measured.
The theory behind an ultrasonic flow meter’s operation rests on the concept that there is a change in velocity of ultrasonic wave pulses when there is a change in the flow rate of a fluid.
Construction Of An Ultrasonic Flow Meter
The transducer, the device that calculates the flow rate, can be mounted parallel or at an angle to the side of the pipe. Ultrasonic flow meters have two transmitters that send the sound and two receivers that receive the sound. The transmitters send out short bursts of ultrasonic signals or pulses through the flow of the fluid.
One transmitter, the preferred transmitter, sends sound waves along the direction of the flow. It is preferred because it happens in the direction of the flow. The other transmitter sends sound waves in the opposite direction of the flow. The signal from the first transmitter increases as the speed of the fluid increases. The signal from the second transmitter decreases since it is sent in the opposite direction of the flow.
Doppler Principle
The Doppler type of ultrasonic flow meter measures the Doppler frequency shift of the sound waves scattered by particulate matter in the fluid. The frequency shift provides information on the velocity of the fluid. The Doppler form of ultrasonic flow meter can be used in open channels to measure the flow of waste matter, water, and other open channel liquids.
What Transit Time Measures
Transit time ultrasonic flow meters measure flow speed by calculating the time it takes for ultrasonic pulses to travel through the fluid. The received rate of the ultrasonic pulses is then converted into flow rate. The unique nature of transit time ultrasonic flow meters is their ability to measure the flow rate regardless of the direction of the flow.
What Is Topology?
Topology is a study of spaces that do not change under deformation. It is called rubber geometry because the objects studied can be shaped, changed, contracted, and transformed without being broken. In ultrasonic transit time flow meters, it describes how the ultrasonic pulse passes through the flow. An alphabetical letter in the shape of the ultrasonic wave is used as the descriptor.
Z Transit Time
The ultrasonic waves travel in the shape of a Z, which can be seen in the diagram below.
V Transit Time
With the V-shaped transit time ultrasonic flow meter, the receiver, and transmitter are installed on the same side of the pipe. As shown in the diagram, the ultrasonic waves travel down to the pipe and bounce back to the receiver.
W Transit Time
The W transit time ultrasonic flow meter has the ultrasonic wave bounce off the bottom of the pipe, back to the top of the pipe, and down to the bottom of the pipe a second time before going to the receiver.
Open Channel Ultrasonic Flow Meters
Open channel ultrasonic flow meters measure the level, flow rate, and volume of water flowing through weirs, flumes, and channels. Open channel flow meters must be non-contact, which is why ultrasonic open channel flow meters are so widely used for this application.
Unlike closed pipe flow meters, open channel ultrasonic flow meters require the input of the geometry of the channel, including its depth and width. The flow meter uses the height of the water to determine the flow rate. They perform best in channels where sediment does not collect since sediment collection severely influences the meters readings.
Using A Multi-Channel Ultrasonic Flow Meter
A common problem with ultrasonic flow meters is measuring the flow rate when the Reynolds number, which is used to predict flow patterns, changes sporadically. When one ultrasonic beam is used, the flow rate will constantly be changing as the Reynolds number changes. A way to overcome this problem is the use of a multi-channel ultrasonic flow meter.
The multi-channel ultrasonic flow meter uses four or five beams to measure more than one sound wave in the fluid flow. The meter compares the transit time, or chord, of each beam and develops an average flow rate. With the four-channel method, two chords measure the flow rate at the center of the pipe while the other two measure the flow rate at the top and bottom of the pipe closer to the walls.
If you look at the image below, there are two transmitters at the front of the pipe and two at the rear of the pipe, for this two channel ultrasonic flow meter.
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Chapter Three – Types of Ultrasonic Flow Meters
There is a flow meter for any type of application, from measuring the consistency of gasses to determining how much fluid a customer purchased. The different forms of flow meters can provide precise and accurate data on volume, flow rate, mass, and quantity, depending on the requirements of the application.
Ultrasonic flow meters are among the more popular flow meters due to the exceptionally accurate data they provide and being noninvasive, not requiring the placement of an instrument in the flow of the material. The variations in types of ultrasonic flow meters are determined by where the sensors are placed, how they attach to the pipe, and the type of material being monitored.
Ultrasonic Flow Meter Types
Dual and Single Clamp-On Ultrasonic Flow Meters
A clamp-on ultrasonic flow meter is the least invasive of the various types of flow meters. They are attached to the outside of the pipe and are available in single and dual versions. With the single-sensor version, the transmit and receive crystals are placed in the same sensor body and are clamped at a single point of the pipe surface.
The dual version of clamp-on ultrasonic flow meter has a separate body for the one sensor and another for the receiver sensor. Whether the clamp-on ultrasonic flow meter is a dual or single version, they can only work with certain types of pipe and will not work if the pipe is lined or insulated.
Hybrid Ultrasonic Flow Meters
Hybrid ultrasonic flow meters are the most versatile of the various types of flow meters. They can measure the flow of any type of fluid and are considered hybrid because they contain both Doppler and transit time methods. With Doppler and transit time systems available, the hybrid ultrasonic flow meter allows the user to select the method for flow measurement. If the fluid flow has particulate matter, the Doppler system is used while the transit time method is used with clean fluids.
In-Line Ultrasonic Flow Meters
In line flow meters are installed in the path of the flow to measure the flow rate. Ultrasonic in-line flow meters are widely used to measure flow rates since they do not require cutting the pipe, shutting down the operation, or causing delays in the process. With the ability to be immediately used, ultrasonic flow meters offer an inexpensive and convenient measuring device for in-line measurement applications.
For an in-line ultrasonic flow meter to be put to use, all that is required is programming the pipe parameters into the display, measuring the transducer spacing, and clamping the transducers onto the pipe, which can be done while there is fluid in the pipe. The
total installation time is measured in minutes.
Portable Ultrasonic Flow Meters
Portable flow meters come in various sizes, from ones that are similar to a calculator to ones like a laptop or notebook computer. They have a built-in rechargeable battery or AC current plug and can be connected to any outlet. Data is displayed as graphs or columns, which can easily transfer to other computers.
The sensors for a portable ultrasonic flow meter are connected to wires, which are clamped, fastened, screwed, or magnetically attached to the pipe. For optimum operation of the meter, the transducers are placed at a specific location apart from one another. As with all types of ultrasonic flow meters, the signals are sent through the wires into the pipe to measure the flow rate.
Radar Flow Meters
Technically, radar flow meters are not ultrasonic flow meters since they use microwave radar signals to measure flow, mass, and volume. If a substance has a dielectric constant that is greater than two, such as water or ammonia, it can deflect radar signals. The higher the dielectric constant, the greater the signal that is deflected. Though radar flow meters are ideal for dielectric materials greater than two, they cannot be used with substances that are below that constant, such as air, vapors, gasses, or foam.
The concept behind a radar flow meter is the same as the one used by legal authorities when determining the speed of a vehicle. When the beam is sent into the flow, reflected signals are sent back at a slightly different frequency. The shift in frequency is a measure of the velocity and direction of the flow.
Time of Flight Ultrasonic Flow Meter (ToF)
With a time of flight ultrasonic flow meter, sound is sent from both sides of the pipe. The signal that is traveling with the flow arrives faster than the signal being sent against the flow. The arrival time of each signal can be measured by the phase shift and is proportional to the speed of the fluid in the pipe.
Accurate placement of the transducers is necessary to ensure the accuracy of the flow meters readings. A key to the process is the refraction angle of the ultrasonic signal as it passes through the pipe. An ultrasonic couplant is placed between the transducer and the pipe since an air gap will distort the signal.
For a time of flight ultrasonic flow meter to work properly, the fluid flow has to be free of gas bubbles, laminar flow, and solids that could scatter or disperse the signal, which is unlike a doppler flow meter that requires bubbles or particles. Changes in fluid composition or density caused by temperature or pressure variations have little effect on time of flight flow meters.
Totalization Ultrasonic Flow Meter
A totalization ultrasonic flow meter provides a running total of the amount of gas, liquid, or stream that passes through a pipe. They are designed for use with pipes that contain at least 100 parts per million (ppm) of 100 microns (μm) or larger suspended particulate matter or bubbles. Since normal ultrasonic flow meters have their sound deflected by particles or bubbles, totalization ultrasonic flow meters are ideal for dealing with the problem.
Uses for totalization ultrasonic flow meters include flow measurement of wastewater, reactivated sludge, waste activated sludge, sludge, slurries, crude oil, lime slurries, phosphates, and pulp stock. The Doppler ultrasonic method is used with totalization flow meters due to the bubbles and particulate matter in the flow. There are a variety of styles of totalization ultrasonic flow meters, including clamp-on types.
Chapter Four – Uses for Ultrasonic Flow Meters
Ultrasonic flow meters are widely used as industrial fluid flow measurement devices because they use sound to measure flow rates and volume and are non-intrusive. The lack of moving parts and insertion components eliminates the need for repair or replacement of parts, which lowers the cost of using them.
Any industry that relies on data regarding the flow of fluids uses ultrasonic flow meters to get accurate and precise information. From residential use to ones for the oil fields, ultrasonic flow meters offer companies data on the supply and movement of fluids.
Ultrasonic Flow Meter Uses
Measuring Water Movement
The first flow meter, developed in Germany, was designed to measure the flow of water. Within a year after its introduction, the technology of the flow meters spread across the globe. Fifty years later, in 1959, the first ultrasonic flow meter was introduced that used the Doppler method for measuring blood flow. In 1963, ultrasonic flow meters were introduced for industrial use.
Ultrasonic flow meters have carried on the tradition of the first flow meters and are used for open channel water measurement and water flow through pipes. The flexibility of ultrasonic flow meters and the lack of moving parts make them ideal for water monitoring.
Measuring In The Oil Industry
The demanding conditions of the oil industry require instruments capable of enduring the challenging environment. The flexibility of clamp-on ultrasonic flow meters has made them an ideal choice for oil extraction and production. Since oil is pumped at high pressure and flow velocity, an invasive type of flow meter is not capable of meeting the conditions.
Ultrasonic flow meters do not have to be installed inside the pipe, making them easy to install and time-saving. Also, cutting into the pipe to place another form of flow reading device can be dangerous.
Custody Transfer of Liquids
An area where accuracy is essential in the custody transfer of a liquid from a seller to a buyer. The custody transfer process, or fiscal metering, happens when a gas or fluid is exchanged between agreeing parties. Payment is dependent on the quantity and amount of material that is transferred. Due to the economic nature of the exchange, the volume and amount of liquid or gas transferred must be precisely and accurately recorded. Minute and minor errors can add up quickly, leading to errors in the custody transfer transaction.
In 1998, the American Gas Association (AGA) approved ultrasonic flow meters for custody transfer transactions. The approval was based on the fact that ultrasonic flow meters can measure volume on large natural gas pipelines, which range in size between 20 in to 42 in or 51 cm to 107 cm.
The accuracy and flexibility of ultrasonic flow meters have made them useful as custody transfer devices from oil wells to the refinery and the distributor. As with other applications, the lack of moving parts and the simplicity of operation have made ultrasonic flow meters ideal for custody transfer applications.
Biopharmaceutical Ultrasonic Flow Meters
Biopharmaceutical ultrasonic flow meters are used to measure flow rates in pharmaceutical labs. They are a clamp on type of flow meter that can be attached to flexible tubes and are media contact free, an essential requirement for laboratory testing.
A biopharmaceutical ultrasonic flow meter uses the transit time method for measuring flow rate. Every aspect of the flow process can be carefully monitored and checked by the calibration table on the device, which allows for customer customization of the controls. In the image below, the clamp on the transducer is the red and black device attached to the tubing.
Ultrasonic Blood Flow Meter
Blood flow is an essential physiological parameter but is difficult to measure. The flow velocity of blood varies widely and depends on the diameter of the blood vessels. The principle of an ultrasonic blood flow meter is based on the echo signal from erythrocytes in the blood. The ultrasound signal is scattered by the blood cells before being received by the receiver. The frequency shift of the scattered wave provides an image of the velocity of the scatterers.
The Doppler shift is used to measure the size and direction of the flow velocity. To ensure the accuracy of the Doppler shift readings, the ultrasound signal is pulsed. By range gating the return signal, the diameter, and velocity of the blood stream can be measured.
Chapter Five – Benefits of Using Ultrasonic Flow Meters
Since their introduction for industrial use in the 1960s, ultrasonic flow meters have been steadily growing as the ideal solution for fluid flow measurement and monitoring. The aspect that makes them the most popular is their lack of moving parts and the need to make adjustments in pipes to install them.
Unlike more complex and intrusive flow meters, ultrasonic flow meters can be instantly installed and available in less than a half-hour. This single feature of rapid and easy installation has made them the number one choice as a flow monitoring instrument.
Ultrasonic Flow Meter Benefits
Accurate Measurement
The measurements taken by an ultrasonic flow meter are not influenced by pressure, density, temperature, conductivity, or the fluid’s viscosity.
Motionless Parts
A benefit that is constantly emphasized regarding ultrasonic flow meters is the lack of moving parts. This factor removes the need for repairs or the replacement of parts. Once the flow meter is installed, no part of it is in motion, yet, readings are constant and consistent.
Long Service Life
With the removal of moving parts that can wear out or be damaged, ultrasonic flow meters are guaranteed to have a long service life. Since they are noninvasive, there isn’t any concern for wear from corrosive substances or abrasions.
Flexibility and Repositioning Availability
Whether the ultrasonic flow meter is in line or clamp-on, it can be easily repositioned or removed depending on the need of the application. While other flow meters are permanent and require a great deal of effort to repair or replace, ultrasonic flow meters can be temporary and installed when needed.
Pipe Size
Ultrasonic flow meters have been approved for custody transfer applications due to their ability to monitor and measure flow in large pipe sizes. They are used for transfers through six-inch or fifteen centimeters or bigger pipes, which are too big for other flow meters.
Materials That Can Be Measured
Ultrasonic flow meters can measure the flow of non-conductive liquids, gasses, and steam, which is a limitation of magnetic meters. For this reason, ultrasonic flow meters are used in oil and gas production, transportation, and refining since oil and gas are nonconductive.
Precision and Rangeability
Ultrasonic flow meters offer high rangeability and can measure any flow condition from low to high with exceptional long-term reliability. Their measurements are accurate and repeatable with the ability to perform in extreme temperature conditions.
Environment Shifts and Self Diagnosis
When an ultrasonic flow meter is installed, it is calibrated to the conditions and environment where it is placed. If there is a shift or change, the flow meter notices the change and shifts in the measurements.
Nonintrusive, Safe, and Sanitary Measurements
Regardless of how an ultrasonic flow meter is installed, it will not block the flow of fluids or materials. They never come in contact with the material, and therefore, do not influence the flow or the integrity of the material. This particular feature ensures safe and sanitary measurements.
Conclusion
An ultrasonic flow meter measures the flow of a liquid or gas by sending ultrasonic waves across the pipe, containing the flow in the direction of the flow and the opposite direction of the flow.
An ultrasonic flow meter uses the acoustic method to measure flow.
There are two types of ultrasonic flow measuring methods, which are transit time travel and Doppler shift.
Ultrasonic flow meters are among the more popular of flow meters due to the exceptionally accurate data they provide and being noninvasive, not requiring the placement of an instrument in the flow of the material.
Any industry that relies on data regarding the flow of fluids uses ultrasonic flow meters to get accurate and precise information.
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