Ultrasonic Flow Meter Technology

Best Technology Guide to Ultrasonic Flow Meters.

What is an Ultrasonic Flow Meter? An ultrasonic flow meter is a volumetric flow meter that measures the flow rate of a medium based on the effect of the flowing medium on the ultrasonic velocity or ultrasonic pulse. There are two types of ultrasonic flow meter technologies: Doppler and transit-time.

We will mainly introduce and analyze ultrasonic flow meters for measuring liquids. There are non-intrusive type, insertion type, pipeline type, etc.

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What is an Ultrasonic Flow Meter?

Ultrasonic flowmeter refers to a flowmeter developed based on the principle that the propagation speed of ultrasonic waves in a flowing medium is equal to the vector sum of the average flow velocity of the measured medium and the speed of the sound wave in a stationary medium. It is mainly composed of a transducer and a converter. There are different types such as Doppler method, time difference method, beam offset method, noise method and correlation method.

For liquid media, Doppler ultrasonic flowmeters and transit-time ultrasonic flowmeters are commonly used in industrial processes.

Know more about Ultrasonic flow meter – Wikipedia

Features of Ultrasonic Flow Meters

Ultrasonic flowmeter is a non-contact instrument that has been applied with the rapid development of integrated circuit technology in the past ten years. It is suitable for measuring fluids that are not easy to contact and observe, as well as large pipe flow.

• Simple structure, easy to install and use.
• Energy saving. No additional resistance is generated and no pressure loss.
• Suitable for difficult to measure media and large pipelines.
• Simple structure, convenient installation, use and maintenance.
• Easy to maintain. The inspection parts are easy to repair and replace, and do not need to be cut off (except for the built-in type).
• The unique signal digital processing technology makes the instrument’s measurement signal more stable, strong anti-interference ability and more accurate measurement.
• Non-mechanical transmission parts are not easily damaged, maintenance-free, and long life.
• The circuit is more optimized and highly integrated; low power consumption and high reliability.
• Intelligent standard signal output. Friendly man-machine interface, multiple secondary signal outputs for your choice.
• Pipe section type small diameter measurement is economical and convenient. And the measurement accuracy is high.

Advantages and Disadvantages of Ultrasonic Flow Meter

Advantages

• This technology can be very accurate and is used for custody transfer (meaning accounting accurately for an expensive fluid) of natural gas and petroleum liquids.
• There is no pressure loss. A type that can perform detection from the outside of piping is available.
• Ultrasonic flowmeters are available in sizes to 72 inches and larger.
• High turndown (can read low as a percentage of the full scale or top reading), handles high pressures, is repeatable (consistent), handles extreme temperatures, can be used clamped to the outside of a pipe without penetration, is low maintenance, highly reliable and self–diagnosing.
• Ultrasonic flowmeters do not obstruct flow so they can be applied to sanitary, corrosive and abrasive liquids.
• Some ultrasonic flowmeters use clamp-on transducers that can be mounted externally to the pipe and do not have any wetted parts.
• Temporary flow measurements can be made using portable ultrasonic flowmeters with clamp-on transducers.
• Clamp-on transducers are especially useful when piping cannot be disturbed, such as in power and nuclear industry applications.
• In addition, clamp-on transducers can be used to measure flow without regard to materials of construction, corrosion, and abrasion issues.

Disadvantages

• A long section of straight pipe is required.
• Liquids that have a large solid content will cause malfunctions.
• Measurement is not possible when there are many air bubbles.
• High cost.
• Sensitivity to stray process vibrations, problems with pipe diameter change due to buildup and clamp-on units have lower accuracy.
• The use of clamp-on transducers introduces additional ultrasonic interfaces that can affect the reliability and performance of these flowmeters.
• In particular, if not properly applied and maintained, attenuation of the ultrasonic signal can occur at the interfaces between the clamp-on transducers and the outside pipe walls, and between the inside pipe walls and the fluid.

Types of Ultrasonic Flow Meters

In terms of measurement principle, ultrasonic flowmeters have Doppler and transit-time. In terms of structure, ultrasonic flowmeters are generally composed of sensors and transmitters (host). We can also classify them according to different forms of sensors and transmitters.

• Insertion-type ultrasonic flowmeter: can be installed and maintained without stopping production. The ceramic sensor is used for non-stop installation using a special drilling device. Generally, it is mono measurement. To improve the measurement accuracy, three channels can be selected.
• Pipe segment type ultrasonic flowmeter: needs to cut the pipeline to install, but future maintenance can be stopped. Choice of mono or tri-channel sensors.
• External clamp-on type ultrasonic flowmeter: can perform fixed and mobile measurement. Use special coupling agent (silicone rubber cured at room temperature or high-temperature long-chain polymer grease) for installation, and the pipeline will not be damaged during installation.
• Portable ultrasonic flowmeter: portable use, built-in rechargeable lithium battery, suitable for mobile measurement, with magnetic sensor.
• Hand-held ultrasonic flowmeter: small size, light weight, built-in rechargeable lithium battery, handheld use, equipped with magnetic sensors.

How Ultrasonic Flowmeters Work

Ultrasonic flowmeters use sound waves to determine the velocity of a fluid flowing in a pipe.

Doppler ultrasonic flowmeter:
At no flow conditions, the frequencies of an ultrasonic wave transmitted into a pipe. And its reflections from the fluid are the same.

Under flowing conditions, the frequency of the reflected wave is different due to the Doppler effect. When the fluid moves faster, the frequency shift increases linearly. The transmitter processes signals from the transmitted wave and its reflections to determine the flow rate.

Transit time ultrasonic flowmeter：
Transit time ultrasonic flowmeters send and receive ultrasonic waves between transducers in both the upstream and downstream directions in the pipe.

At no flow conditions, it takes the same time to travel upstream and downstream between the transducers.

Under flowing conditions, the upstream wave will travel slower and take more time than the (faster) downstream wave.

When the fluid moves faster, the difference between the upstream and downstream times increases. The transmitter processes upstream and downstream times to determine the flow rate. They represent about 12% of all flowmeters sold.

Principles

When the ultrasonic beam propagates in a liquid, the flow of the liquid will cause a small change in the propagation time. And the change in the propagation time is proportional to the flow velocity of the liquid. The relationship is as follows:

Which:
θ is the angle between the sound beam and the direction of liquid flow.
M is the number of linear propagations of the sound beam in the liquid.
D is the inner diameter of the pipe.
Tup is the propagation time of the sound beam in the positive direction.
Tdown is the propagation time of the sound beam in the reverse direction.
ΔT = Tup –Tdown

Let the speed of sound in a stationary fluid be c. The velocity of the fluid flow be u. And the propagation distance be L.
When the sound wave is in the same direction as the fluid flow (that is, in the downstream direction), its propagation velocity is c + u.

Otherwise, the propagation velocity is cu .

Place two sets of ultrasonic generators and receivers (T1, R1) and (T2, R2) at two places separated by L.

When T1 transmits ultrasonic waves in the forward direction and T2 in the reverse direction.

The time required for the ultrasonic waves to reach the receivers R1 and R2 is t1 and t2, then:
t1 = L / (c + u); t2 = L / (c-u)

Because in industrial pipelines, the velocity of the fluid is much smaller than the speed of sound, that is, c >> u, the time difference between the two is ▽ t = t2-t1 = 2Lu / cc.

It can be seen that when the sound wave propagates in the fluid c When known, the flow velocity u can be obtained by measuring the time difference ▽ t, and the flow rate Q can be obtained.

The method of flow measurement using this principle is called the time difference method. In addition, phase difference method and frequency difference method can be used.

What is non-intrusive ultrasonic flow meter technology?

The non-intrusive detection method means that the detection medium will not cause wear to the detection instrument, so there will be no leakage.

In the flow detection of liquid media, the clamp-type ultrasonic flowmeter is made by the ultrasonic detection method that has been used for a long time. And has become a standard flow detection product. In the field of chemical product production, especially in the measurement of corrosive media or media harmful to human health. People are most willing to use this non-immersive ultrasonic testing method. However, these advantages are not limited to instruments for liquid detection.

In recent years, ultrasonic flow detection technology has been increasingly used for the detection of gaseous media. Especially in the process of natural gas transportation, transportation, and storage, non-immersive detection methods have also been widely used. In the process gas detection, this non-immersive ultrasonic clamping detection technology is very beneficial.

We specialise in measuring the flow of a liquid in a pipe by using clamp on /strap on sensors. These are high frequency sensors that penetrate through the pipe wall and the liquid to determine the fluid velocity, flow rate and total.  The most common types of clamp on technologies use a doppler or transit time propagation technique.

How to Use Ultrasonic Flowmeters

• Ultrasonic flowmeters are commonly applied to measure the velocity of liquids that allow ultrasonic waves to pass. Such as water, molten sulfur, cryogenic liquids, and chemicals. Transit time designs are also available to measure gas and vapor flow.
• Be careful because fluids do not pass ultrasonic energy. Such as many types of slurry, limit the penetration of ultrasonic waves into the fluid.
• In Doppler ultrasonic flowmeters, opaque fluids can limit ultrasonic wave penetration too near the pipe wall, which can degrade accuracy and/or cause the flowmeter to fail to measure.
• Transit-time ultrasonic flowmeters can fail to operate when an opaque fluid weakens the ultrasonic wave to such an extent that the wave does not reach the receiver.

Application Cautions for Ultrasonic Flowmeters

• The flowmeter has two modes diagonal and reflection. When the reflection mode cannot be measured, the diagonal mode may be able to measure, so we have always used the reflection mode.
• The flowmeter has high requirements on the pipeline, and the insulation layer must be scraped off, otherwise it cannot be measured.
• When we measure the air-conditioning water system, use a knife to cut off the insulation layer to prevent the sensor, and then stick the cut insulation layer after the measurement. The surface of the pipe is as bright as possible. If it is too rough, it must be polished with sandpaper.
• It is difficult to measure when the fluid in the pipeline is not full, so the measurement location should be as straight as possible, away from elbows, valves, and other places.
• The authenticity of the flowmeter reading depends on the signal strength. When the signal is too low, the result is basically unreliable, and generally, it is more than 60% or even more.
• Since the readings may vary greatly, the approach we take is to allow the flowmeter to take continuous readings, such as continuously reading for one minute and then taking the average.
• Measuring hot water lines is more difficult than cold water lines. Because the temperature of the hot-water pipe wall surface is high, the coupling is easily made at high temperatures. In addition to the product’s own couplant, we have all tried toothpaste.
• For transit time ultrasonic flowmeters, be sure that the fluid can adequately conduct ultrasonic waves. Because the flowmeter will not measure when the ultrasonic waves cannot penetrate the flow stream between the transducers.
• Ultrasonic waves must be able to penetrate the fluid for Doppler flowmeters to operate accurately.
• When the fluid is relatively opaque and does not penetrate the fluid, Doppler flowmeters tend to measure the velocity of the fluid at or near the pipe wall, which can cause significant measurement error and/or cause the flowmeter to fail.
• For Doppler ultrasonic flowmeters, be sure that the fluid adequately reflects ultrasonic waves. Because the flowmeter will not operate without a reflected ultrasonic signal.
• Depending upon design, reflections can occur due to small bubbles of gas in the flow stream or the presence of eddies in the flow stream. If not already present in the flowing stream, generating these sources of reflection can be difficult in practice. Fortunately, some combination of bubbles of gas and/or eddies are present in most applications.
• The velocity of the solid particles in slurry can be different than its liquid carrier fluid. Be careful applying ultrasonic technology when the solid particles can become concentrated in one part of the flowing stream. Such as in a horizontal pipe flowing at a relatively low velocity.
• Be careful when applying Doppler ultrasonic flowmeters in slurry applications. Because the solid particles can produce strong signals that can cause the Doppler flowmeter to measure the velocity of the solids and not the velocity of the liquid.
• Avoid fluids that can coat wetted transducers or coat the pipe wall in front of non-wetted transducers. Because the flowmeter will not measure when the ultrasonic waves cannot enter the flow stream.
• Be sure to maintain reliable clamp-on transducer connections to the pipe wall. Because the flowmeter will not measure when the ultrasonic waves are not able to reach the fluid.
• Be sure to understand the process and apply these flowmeters properly. For example, a periodic cleaning process upstream may cause the flowmeter to stop working because the dirt may not allow ultrasonic energy to pass through the fluid.
• Further, if the dirt coats wetted transducers, the flowmeter may fail to operate until it is cleaned.

How to install an ultrasonic flow meter?

The transit-time ultrasonic flowmeter is a competitive flow measurement method in the world today. And its measurement line accuracy is higher than 1.0%. Due to the diversity of the industrial site, especially the environment around the pipeline, how to install and debug the ultrasonic flowmeter according to the specific environment has become an important subject in the field of ultrasonic flow measurement. This regulation details the installation details of the ultrasonic flowmeter. It further fully reflects the advantages of accuracy, reliability and stability of the ultrasonic flowmeter, greatly reducing future maintenance work and even maintenance-free.

Detailed installation

The ultrasonic flowmeter should be aware of the situation before installation, including:

1. What is the distance from the host where the sensor is installed;
2. Pipe material, pipe wall thickness and pipe diameter;
3. Pipeline age
4. Fluid type, whether it contains impurities, bubbles, and whether it is full;
5. Fluid temperature
6. Whether there are interference sources at the installation site (such as frequency conversion, strong magnetic fields, etc.);
7. Four season temperature at the place where the host is placed;
8. Whether the power supply voltage used is stable;
9. Whether remote signals and types are needed;

According to the on-site conditions provided above, manufacturers can configure the on-site conditions, and can also make special models if necessary.

Installation location

The selection of the installation pipe section has a great impact on the test accuracy. The selected pipe section should avoid the two situations that have a large impact on the measurement accuracy.

Generally, the selection of the pipe section should meet the following conditions:

1. Avoid installing machines in pumps, high-power radio stations, frequency conversion, that is, where there is a strong magnetic field and vibration interference;
2. Select pipe sections that are uniform and dense and easy for ultrasonic transmission;
3. To have a long enough straight pipe section, the upstream straight pipe section of the installation point must be greater than 10D (Note: D = diameter), and the downstream must be greater than 5D;
4. the installation point upstream from the pump should be 30D distance;
5. The fluid should fill the pipeline;
6. There should be enough space around the pipeline for on-site personnel to operate. Underground pipelines need to be tested wells.

Installation method

Ultrasonic flowmeters generally have two probe installation methods, namely Z method and V method.
However, when D <200mm and the site conditions are one of the following conditions, the Z method can also be installed:

1. When the turbidity of the fluid to be measured is high, no signal is received or the signal is weak when measured by the V method;
2. When the inner wall of the pipeline is lined;
3. When the service life of the pipeline is too long and the inner wall has serious fouling;

For those with better pipeline conditions, even if D is slightly larger than 200mm, in order to improve the measurement accuracy, V method can be used for installation.

Probe position

1. Enter the parameters of the pipeline into the instrument and select the probe installation method to obtain the installation distance;
2. On horizontal pipelines, generally choose the middle of the pipeline, avoiding the top and bottom (the top may contain air bubbles and the bottom may have sediment);
3. V method installation: first determine a point and measure another point at a horizontal position according to the installation distance.
   Z method installation: first determine a point, measure another point at a horizontal position according to the installation distance, and then measure the symmetrical point of this point on the other side of the pipeline.

Pipeline processing

After determining the position of the probe, within the range of ± 100mm between the two installation points, use an angle grinder, file, sandpaper and other tools to grind the pipe to a bright and smooth pit.
Requirements: uniform gloss, no undulations, smooth and round feel. Special attention should be paid to the fact that the grinding point must have the same arc as the original pipe. Do not sand the mounting point to a flat surface, and wipe this area with alcohol or gasoline to facilitate probe bonding.

Fine-tune the probe

After connecting the wires, fill the inside of the probe with silicone. Leave it for half an hour. Then fix the probe to the polished pipe with silicone and clamps (note the direction of the probe, the lead end is outward). Then observe the signal strength Ratio to transmission time. If it is found to be bad, finely adjust the probe position until the signal of the instrument reaches the specified range

How to select a clamp on ultrasonic flow meter?

Before ordering, the following questions must be considered. With answers to the following questions we can select the appropriate flow device and ship it to your location.

• What are the pipe sizes of the liquids that are going to be measured?
• What is the minimum and maximum process temperature of the application?
• Do you require a portable battery-operated unit or a permanent fixed flow monitor powered by mains?
• What are the output requirements? E.g. analog, digital, pulse?
• What is the minimum and maximum flow rate for the flow meter?
• What is the minimum and maximum flow velocity?
• Is there a length of straight pipe away from bends and pipe disturbances?
• Is the pipe always filled with liquid?

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