What is ultrasonic flow meter?
Ultrasonic flow meter is a special transducer that can determine the level of stream flow of liquids or gases in a pipe section using ultrasonic signals.
- Design and principle of operation
- Types of Ultrasonic Flow Meters
- Installation manual of ultrasonic flow sensors
Ultrasonic flow sensors use a special signal - an ultrasonic pulse. Such ultrasound starts at frequencies above 20,000 Hz and can reach up to 1000 MHz.
The flow meter consists of three main parts:
- Signal source
In addition, a special counter can be additionally built into the main structure.
The principle of operation is based on a signal from the source that passes through stream and goes to receiver. Then it goes to the converter, which analyzes result.
For such work usually installed several pairs of sensors. They are located against each other and exchange signals. The most often they are located diagonally from the pipeline axis.
Such an arrangement makes it possible to exchange sound signals both in the direction of the substance flow and against it. When pipeline is empty, both signals pass this distance with the same speed.
When the substance moves through pipe, the sound along the flow accelerates. Such impulse directed against the flow moves more slowly. The greater flow velocity, the greater the time difference between these signals.
The ratio of these values is proportional to the flow rate. Hence, if you know the pipe section dimensions and the substance characteristics, you can determine the flow rate.
There are several types of ultrasonic flow meters depending on the measurement method:
- Transit time
- Open channel
Transit time ultrasonic flow meters record the phase of the signal on the receiving piezoelectric elements. In the receiver it is calculated the amplitude and speed ratio with which ultrasound passes in the flow direction. A separate schedule is drawn up for signals that move against the stream. These measurements are made at the same distance. Then device compares the results and determines a change in the measurements.
Dopler flowmeters allow determining the speed depending on the scattering of ultrasonic waves, in contact with small particles that are located in the stream. It can be solid particles of impurities or even gas bubbles. Sensors analyze the change in reflection frequency of the signal from such elements.
This method allows the measurement of consumption in substances with a large proportion of impurities and pollution. The disadvantage is lower accuracy since data is collected from a wide variety of particles.
Open channel ultrasonic flow meters analyze the time of signal passage from the source to the receiver in two directions (along and against the flow). That is, they analyze the demolition of a sound signal by a moving stream. Such a measurement of the time difference is proportional to the flow rate.
The duration of the signal move depends on the temperature, pressure and density of the substance. That is, if you know these parameters, you can calculate the ratio and determine consumption of a substance. Difference in the propagation time of ultrasonic signal in this case is proportional to the flow rate.
Unique feature of ultrasonic signal sensors is the wide range of installation possibilities.
There are several types of sensor depending on the installation:
- for open channels
Built-in sensors can be mounted through the pipe walls. The flow meter also can have its own pipe section in which the calculations take place. In such case, it is often attached with a flange.
The main advantages of this method are direct contact with the substance, which increases the calculation accuracy. The disadvantage is a mechanical intervention in the pipeline cross-section and the need to stop all flow movement in case of dismantling.
Overhead sensors help to avoid such problems. They can be installed with special clamps outside the pipe. The signal passes directly through the pipe wall and is reflected from them, direct into the receiver.
The advantages of the method are the possibility of the quick and painless sensors dismantling at any time. This method also allows to use flow meters with large diameter pipes.
The disadvantage of this scheme is less accuracy. Its installation requires additional fasteners in the form of clamps. Sensors of this type can be installed directly on the rail.
Sensors for open surfaces involve using in open channels and tanks. Additional devices for suppressing false signals are also installed in the design. The advantages of the method are the possibility of measuring the flow rate in open sources. The disadvantages include the influence on the accuracy of environmental factors.
Mounting of any sensor types requires admitting several conditions. It is important to observe minimum distance of the straight pipe section. This is approximately 10 pipe diameters before sensors and 5 after them. This avoids flow turbulence.
Sensors should not be mounted on the bottom and top wall. It is necessary because bubbles may appear at the top of the stream, and particles may settle at the bottom. It must be mounted on the side surfaces or in vertical sections.
Installation points must be cleared from insulation, paint and rust. To do this, you need to use a lubricant indicated by the manufacturer, for example, a conventional automobile lithol or grease.
In conclusion, it should be said that ultrasonic sensors can be a good solution in the work on flow measurement for harsh conditions. A wide range of installation methods and types of sensors allows the client to choose the most suitable one.