Introduction

Thermal power plants use recycled gypsum slurry for flue gas desulfurization treatment. The slurry flow is an important parameter to evaluate the efficiency of the desulfurization system, but it is difficult to detect the slurry flow. As the slurry is a corrosive liquid containing solid particles, it must be transported through a corrosion-resistant rubber-lined composite pipe. It is unrealistic to use a contact flowmeter without affecting production, The common non-contact ultrasonic pipe flowmeter can only be used for carbon steel pipes. The fundamental reason is that its matching sensor can not be used for composite pipes. This paper studies the sensor suitable for the flow detection of slurry in composite pipes and designs an ultrasonic sensor based on the Doppler flow measurement principle. 

The transmitter-receiver integrated packaging is adopted, which is easy to install. Through the selection of sensitive elements (materials) and parameter design, The structural design improves the applicability of the sensor to the composite pipeline. In the indoor test of the sensor, the ultrasonic signal containing flow information can be collected in the composite pipeline fluid, and the matching detection system can realize the flow detection of the composite pipeline and achieve good results. At the same time, the sensor is also suitable for flow measurement of fluid in ordinary carbon steel pipes.

Sensitive element

The sensitive element refers to the element that can directly feel or respond to the measured physical quantity in the sensor, and it is the core of the sensor. The sensing element of the ultrasonic sensors used for flow detection is generally piezoelectric ceramic. piezoelectric ceramic has the advantages of high sensitivity, good temperature characteristics, easy processing, and so on, which is widely used in various ultrasonic detection. The composite pipeline has more rubber lining than the ordinary carbon steel pipeline, so the selection of sensor-sensing elements will be more strict. This design uses PZT-5 in lead zirconate titanate piezoelectric ceramics (PZT) as the sensor sensing element. Compared with other piezoelectric ceramic materials, PZT-5 has a high piezoelectric constant, which can achieve high transmission and reception sensitivity. At the same time, the mechanical quality factor is the lowest, which can achieve high resolution and a small blind area, It is very suitable for composite pipes.

Parameter design

The slurry in the composite pipeline contains solid particles, which is suitable for Doppler measurement. Therefore, the working frequency and sensitivity of the sensor are very important.

(1) Operating frequency design

The working frequency design of the sensor shall meet the requirements of the Doppler measurement method, that is, the ultrasonic wave must have a scattering echo, otherwise the echo signal cannot be received. According to the attenuation theory of ultrasound, only scattering attenuation can form a scattering echo, and the formation of scattering is closely related to the wavelength of ultrasound and the size of solid particles. Schematic diagram of the scattered sound field, λ Is ultrasonic wave length, 2a is solid particle diameter.

When λ At 2a, ultrasonic diffraction moves forward without scattering echo. When ultrasonic wave length λ When the ratio with particle diameter 2a decreases to a certain extent, a scattering sound field will gradually form λ When ≈ 2a, a variety of different scattering sound fields will be generated λ< At 2a, the particles are mainly used as reflectors, and the ultrasonic wave will be reflected, but too much reflection will reduce the penetration ability of the wave, or the wave can only be transmitted to the shallow interface of the fluid medium, which cannot completely represent the flow velocity information in the pipe [5]. Therefore, the scattering intensity and penetration depth in the fluid should be considered simultaneously in the ultrasonic frequency design. To sum up, the working wavelength of the sensor should be λ> 2a, but not too large. According to theoretical calculation and test analysis, the working frequency of the sensor is designed as 640kHz.

(2) Sensitivity design

Sensitivity is an important index of receiving sensors, and voltage sensitivity is often used in practical applications. Voltage sensitivity refers to the ratio of the output voltage of the sensor to the free sound field voltage at this point before the sensor is introduced into the sound field, which is generally expressed in decibels, as shown in Formula (1).

Where is the basic sensitivity, is the free field voltage sensitivity, which mainly depends on the piezoelectric material itself. The piezoelectric voltage constant of PZT-5 is very high, and the receiving sensitivity of the sensor meets the application requirements. However, when the sensitivity is high, the external interference signal will also be detected, which will affect the reception of the measured signal. Generally, the signal-to-noise ratio of the ultrasonic sensor must be greater than 18dB design sensitivity.

Structural design

The transmitter and receiver integrated structure design is adopted for the sensor, which is mainly composed of sensing elements, an acoustic wedge, a damping layer, matching impedance, and a protective shell. The sensing element is PZT-5, with a diameter of 20mm and a thickness of 2mm. The acoustic wedge is made of organic glass, the damping layer is a mixture of silica gel and tungsten powder, and the protective shell is made of aluminum alloy, which is sealed as a whole. This structural design avoids the separation of the sensor due to the mounting position.

Inappropriate results in a decrease in measurement accuracy. The sound wedge is an isosceles trapezoidal structure, and the angle of bottom angle is 33.5 °. This design is obtained through theoretical calculation and experiment, which effectively suppresses the strong interaction echo generated by the ultrasonic wave when it travels in the pipeline and fluid. The two ramps of the sound wedge are smooth and flat. After the two-component epoxy resin is mixed at a ratio of 4:1, the two ceramic chips are closely attached to the ramps of the sound wedge. 

The epoxy resin as a matching layer can improve the projection coefficient of sound waves when they are incident from PZT ceramic chips to the sound wedge. The positive and negative electrodes of the two piezoelectric ceramic chips are on the same side, and the impedance elements are connected after welding the wires to match the detection circuit. 704 silicone rubber is mixed with a certain amount of tungsten powder, and then it is smeared as a damping layer on the entire sound wedge slope to seal and solidify the piezoelectric ceramic sheet, with a thickness of about 2mm. Silicone rubber is a sound-absorbing material with high impedance and attenuation, which can absorb ultrasonic interference radiated from the back of the piezoelectric ceramic sheet. 

Finally, put the sound wedge block sealed with piezoelectric ceramic chips into the aluminum alloy protective shell. After the sound wedge block is placed in the shell, its plane is consistent with the shell plane. The protective shell is reserved with cable holes. TNC connectors are used for sensor signal output. Double-shielded cables are used for cable wires to reduce noise interference to the received signal.

Performance test

The indoor flow test platform is used to test the performance of the sensor. Through the test, whether the sensor can match the measuring circuit to effectively transmit and receive ultrasonic signals on the composite pipe and the actual measurement at different flow velocities is detected. The test platform is mainly composed of a pipe pump, mass flowmeter, composite pipe (pipe inner diameter is 50mm), frequency converter, and liquid storage tank. The fluid medium is gypsum powder and water, and the simulated slurry is configured in a certain proportion. During the test, the two sensors are respectively installed on the composite pipe and the ordinary pipe. The pipe pump drives the simulated slurry to maintain a stable circulation in the pipe. The speed of the pipe pump can be adjusted by the frequency converter to change the slurry flow rate. The actual flow value of the fluid can be measured by the mass flowmeter (± 0.1%). During the test, the transmitted and received signals of the sensor can be monitored.

The test shows that the sensor and the matching detection system can realize the flow detection of the composite pipeline, and the overall error is less than 5%. With the increase in the flow, the detection accuracy is improved, and the actual flow is higher than the flow during the test. Therefore, the sensor can not only meet the actual requirements but also has wider applicability.

Conclusion

In this paper, the flow detection sensor technology of composite pipelines is studied. Based on the ultrasonic Doppler measurement method, the sensor's sensitive elements, key parameters, structure, and other aspects are explored and designed. Good experimental results are achieved, and the technical feasibility is verified, which has reference value for the application of ultrasonic technology in the flow detection of the composite pipeline.