Remote Energy Ultrasonic Flowmeter in Urban Sewage Treatment Plant

The ultrasonic flowmeter is an instrument for measuring flow by detecting the effect of fluid flow on the ultrasonic pulse. Like the electromagnetic flowmeter, it belongs to the barrier-free flowmeter because there are no obstacles in the instrument flow channel. It is a type of flowmeter suitable for solving the problem of difficult flow measurement, especially in measuring flow with a large diameter. It is also one of the flowmeters that have developed rapidly in recent years.

Principle of ultrasonic flowmeter

Composition

The non-contact ultrasonic measurement system consists of two parts: an ultrasonic signal transducer and an electronic transmitter and receiver installed at a long distance. The transducer continuously emits a series of ultrasonic waves and receives the reflected echo returned from the liquid or solid surface being tested. The microprocessor in the transmitter further processes the signal and displays the data as an LCD digital display.

(1) Sound wave is a kind of mechanical wave with a frequency of more than 2 × A 104 Hz sound wave is an ultrasonic frequency that cannot be heard by the human ear. The relationship between its wavelength L, frequency f, and speed c is

L=c/f

(2) Reflection and refraction of ultrasonic wave

When sound waves propagate to the interface between two media, part of them can return to the original medium, which is called a reflected wave, and the other part of the energy can continue to propagate through the medium surface to another medium surface, which is called a refracted wave.

Reflection law: Sina/Sina '＝＝ C1/C2
Where, incidence angle a, incidence wave velocity C1, reflection angle a ', reflection wave velocity C2.
Refraction law: SINa/SINb ＝＝ C1/C2

Where, incidence angle a, refraction angle b, C1 is the * medium wave velocity, and C2 is the second medium wave velocity.

Features of ultrasonic flowmeter

The prominent feature of ultrasonic waves is their high frequency, so the wavelength, diffraction phenomenon, and directivity are small. It can be called a ray and propagates directionally. In liquid and solid, the attenuation is small and the penetration ability is large, and there will be significant reflection when encountering impurities or interfaces. Ultrasonic sensors are widely used in many scientific fields, that is, industrial production, by taking advantage of these remarkable features of ultrasound.

Ultrasonic transducer

An ultrasonic transducer, also known as an ultrasonic probe, is a device to realize the mutual conversion of electric energy and acoustic energy. In ultrasonic testing technology, the instrument that transmits and receives ultrasonic waves and converts them back into electrical signals is an ultrasonic transducer, namely a probe. There are many kinds of ultrasonic transducers used for testing, such as piezoelectric, magnetostrictive, electromagnetic, vibrating plate, elastic surface wave, etc. In the detection technology, the piezoelectric type is mainly used.

Brief analysis of advantages and disadvantages

advantage:

◆ It can be used for non-contact measurement;
◆ No flow obstruction measurement and no pressure loss;
◆ It can measure non-conductive liquid, and it is a supplement to the electromagnetic flowmeter for unobstructed measurement.
◆ The biggest advantage of the ultrasonic flow level is that it can be used for unobstructed flow measurement without cutting off the pipeline. It is a flow instrument that does not really touch the medium.

Disadvantages:

◆ The transmission time can only be used for cleaning liquid and gas; the Doppler method can only be used to measure liquid containing quantitative suspended particles and bubbles;
◆ The measurement accuracy of the Doppler method is not high.

Applications

Ultrasonic measurement is a method to measure the storage capacity of containers by measuring the material level. The ultrasonic sensor sends an ultrasonic pulse to the material surface from the top to receive the echo reflected from the surface. It measures the propagation time of the ultrasonic wave and calculates the distance from the reflecting surface.

The propagation time is an intuitive dimension of the distance between the ultrasonic sensor and the material level. The propagation distance of a sound wave is the product of propagation time and sound speed. If the position of the ultrasonic sensor is known, the material level (and reserves) can be converted. The speed of sound in the air varies widely. Therefore, the acoustic sensor needs temperature compensation. If it is used for other gases other than air, it should be calibrated accordingly.

Measuring flow

Among the flow measuring instruments in the sewage treatment plant, there are only two types of flow meters according to the installation type: pipe type and open channel type.
The open-channel ultrasonic flowmeter is widely used in sewage treatment plants. Therefore, we will simply explain here that the liquid flow in the open channel has a definite relationship with the liquid level somewhere on the tank, namely:
Q＝Kbhm
Where Q is the flow, K is the flow coefficient, b is the weir width, h is the liquid level height, and m is the index.

For a water tank with a certain shape, the values of k, b, and m are certain, so the flow can be calculated as long as the liquid level h is measured. It can be seen that the ultrasonic flowmeter used for open channel flow measurement actually measures the liquid level.

Measuring liquid level

The ultrasonic liquid level meter is widely used in sewage treatment plants due to its non-contact measurement, high degree, and less maintenance.

Fig. 1 Working principle of the ultrasonic level gauge
As can be seen from Figure 1, the ultrasonic transmitter/receiver is placed above the liquid, the ultrasonic wave is transmitted downward, passes through the air medium, is reflected back when it meets the water surface and is received by the receiver. According to the principle of ultrasonic propagation in the medium, if the pressure, temperature, density, humidity, and other conditions of the medium are certain, the ultrasonic propagation speed in the medium is constant. Therefore, when it is measured that the time required for the ultrasonic wave to be transmitted to the liquid surface and reflected is received, the distance through which the ultrasonic wave passes can be changed, and the liquid level data can be obtained.