4.2.1. The Influence of Extraction Pressure on Mean Value of Sound Pressure Level in Characteristic Frequency Bands for Water Blockage
Under a transverse blockage intensity of 0.05 and a longitudinal blockage intensity of 0.07, in-depth analysis is conducted on the water blockage sound wave signal data under four different pressures of 12 kPa, 24 kPa, 36 kPa, and 48 kPa. To ensure the accuracy of the experimental data, each operating condition is measured twice to obtain the changes in the average value of characteristic frequency domain from the pipeline water blockage sound signals under different pressures.
As presented in
Figure 13, it can be clearly observed that as the extraction negative pressure gradually increases, the average value of the characteristic frequency band also shows a significant growth trend. As a result, the energy value of sound signals is positively correlated with the degree of blockage. When the extraction pressure is 12 kPa, the average amplitude of characteristic frequency band 1 is 5.1 dB, and the average amplitude of characteristic frequency band 2 is 3.2 dB. When the extraction pressure rises to 24 kPa, the average amplitude of characteristic frequency band 1 rises to 6.2 dB, and the average amplitude of characteristic frequency band 2 rises to 4.3 dB. Compared to the 12 kPa, the average amplitude of characteristic frequency band 1 increased by 21.6%, and the average amplitude of characteristic frequency band 2 increased by 12.5%. As the pressure further strengthens, the average characteristic frequency band of the sound signals also continues rising. When the pressure reaches 36 kPa, the average amplitude of characteristic frequency band 1 reaches 7.8 dB, and the average amplitude of characteristic frequency band 2 rises to 5.6 dB. Compared with 12 kPa, the average amplitude of characteristic frequency band 1 increased by 52.9%, and the average amplitude of characteristic frequency band 2 increased by 37.5%. When the pressure reaches the maximum of 48 kPa, the average amplitude of characteristic frequency band 1 is 9.7 dB, and the average amplitude of characteristic frequency band 2 is 7.1 dB. Compared with 12 kPa, the average amplitude of feature frequency band 1 increased by a remarkable 90.2%, and the average amplitude of feature frequency band 2 increased by 65.6%. It can be concluded that there is a close relationship between the extraction negative pressure and average value of the characteristic frequency domain from pipeline water blockage sound signals.
4.2.2. The Influence of Transverse Blockage Intensity on Mean Value of Sound Pressure Level in Characteristic Frequency Bands for Water Blockage
Under the pressure of 12 kPa and longitudinal blockage intensity of 0.07, a detailed analysis is conducted on the acoustic signal data of water blockage at four different transverse blockage intensities of 0.05, 0.1, 0.15, and 0.2. To ensure the accuracy of the data, each operating condition is measured twice to obtain the variation law of the average value of characteristic frequency domain from the pipeline water blockage sound signals under different transverse blockage intensities.
As demonstrated in
Figure 14, as the transverse blockage intensity of water blockage increases, the average amplitude of the characteristic frequency bands exhibits a growing trend, albeit with a relatively minor increase in amplitude. When the transverse blockage intensity is 0.05, the average amplitude of characteristic frequency band 1 is 5.1 dB, and the average amplitude of characteristic frequency band 2 is 3.2 dB. As the transverse blockage intensity gradually increases to 0.1, the average amplitude of characteristic frequency band 1 slightly rises to 5.2 dB, while the average amplitude of characteristic frequency band 2 remains at 3.2 dB. Compared to the transverse blockage intensity of 0.05, the average amplitude of characteristic frequency band 1 increased by 1.96%, while characteristic frequency band 2 remains unchanged. As the transverse blockage intensity further rises, the growth trend remains mild. For example, when the transverse blockage intensity reaches 0.15, the average amplitude of characteristic frequency band 1 increases to 5.3 dB, and the average amplitude of characteristic frequency band 2 rises to 3.3 dB. Compared to the initial transverse blockage intensity of 0.05, the average amplitude of characteristic frequency band 1 increased by 3.92%, and that of the characteristic frequency band 2 increased by 3.13%. It is worth noting that when the extraction negative pressure changes to −48 kPa, the average amplitude of characteristic frequency band 1 reaches 5.4 dB, and the average amplitude of characteristic frequency band 2 rises to 3.4 dB. Although the growth rate is still relatively slight, compared to the data at a transverse blockage intensity of 0.05, the average amplitude of characteristic frequency band 1 increased by 8.82% and characteristic frequency band 2 increased by 6.25%.
4.2.3. The Influence of Longitudinal Blockage Intensity on Mean Value of Sound Pressure Level in Characteristic Frequency Bands for Water Blockage
Under an extraction pressure of 12 kPa and a transverse blockage intensity of 0.05, in-depth analysis is conducted on the water blockage sound wave signal data at six different longitudinal blockage intensities of 0.07, 0.2, 0.34, 0.5, 0.66, and 0.8. As shown in
Figure 15, it can be observed that with the increase in water blockage height, the average amplitude of the characteristic frequency band also presents a clear increasing trend. When the longitudinal blockage intensity is 0.07, the average amplitude of characteristic frequency band 1 is 5.1 dB, and the average amplitude of characteristic frequency band 2 is 3.2 dB. As the longitudinal blockage intensity gradually rises to 0.2, the average amplitude of characteristic frequency band 1 increases to 8.1 dB, and the average amplitude of characteristic frequency band 2 rises to 3.7 dB. Compared to the blockage intensity of 0.07, the average amplitude of characteristic frequency band 1 increased by 5.8%, and the average amplitude of characteristic frequency band 2 increased by 3.1%. With further increases in blockage intensity, the growth rate become more significant. For example, when the longitudinal blockage intensity reaches 0.34, the average amplitude of characteristic frequency band 1 jumps to 9.8 dB, and the average amplitude of characteristic frequency band 2 also reaches 4.3 dB. Compared to the blockage intensity of 0.07, it has increased by 7.8% and 9.4%, respectively. When the blockage intensity reaches 0.5, the average amplitudes of characteristic frequency band 1 and 2 rise to 12.6 dB and 5.1 dB, respectively, with further significant increases in amplitude. At higher blockage intensities, such as 0.66 and 0.8, the average amplitude of the characteristic frequency band maintains a significant growth. Especially at a longitudinal blockage intensity of 0.8, the average amplitude of characteristic frequency band 1 reaches 18.2 dB, and the average amplitude of characteristic frequency band 2 reaches 7.4 dB. Compared to the blockage intensity of 0.07, the average amplitudes of characteristic frequency band 1 and 2 have increased by approximately 13.7% and 12.5%, respectively.
It can be observed that as the longitudinal blockage intensity rises, the average amplitude in the characteristic frequency band also increases, and the increasing rate exhibits a noticeable enlargement. From the mechanism of sound waves, the accumulation of blockages inside the pipeline gives rise to the obstruction of fluid flow, which triggers irregular movement and pressure disturbance. When these disturbances propagate within the pipeline, the sound waves will occur. As the longitudinal blockage intensity rises, the fluid is more obstructed and the irregular movement and pressure disturbance become more intense, which stimulates higher amplitude sound waves. As the longitudinal blockage intensity increases, the propagation path of sound waves in the pipeline becomes more complex, and the reflection and refraction also increase. These effects will enhance the energy of sound waves, and accelerate the average amplitude of the characteristic frequency band of sound waves. It can be concluded that as the longitudinal and transverse blockage intensity varies, the average amplitude of the characteristic frequency band inside the pipeline gradually rises. Due to the occurrence of water blockage in the pipeline, which hinders fluid flow, reduces flow velocity, and increases pressure, thereby exacerbating the occurrence of water hammer. The intense sound wave is generated by the water hammer, which causes an increase in the amplitude of the acoustic signals. Additionally, during the process of pipeline water blockage, tiny bubbles may be generated by the turbulence and vortex. The expansion and collapse of these bubbles in the liquid can produce oscillating sound waves, namely bubble noise. As the longitudinal blockage intensity increases, the turbulence and vortex in the fluid intensify, which enhances the bubble noise. Furthermore, the enhancement of bubble noise also contributes to the overall rise in the amplitude of the acoustic signals.
An in-depth analysis of the characteristics of the sound signals within the pipeline is conducted under different longitudinal and transverse blockage intensities and extraction pressures; it can be observed that there is a significant functional relationship between the extraction pressure, longitudinal blockage intensity, and the average amplitude of the characteristic frequency band. When the extraction negative pressure or longitudinal blockage intensity varies, the average amplitude of the characteristic frequency band will also change accordingly. The transverse blockage intensity has a relatively minor impact on the average amplitude of the sound source signal in the characteristic frequency band. Therefore, in the subsequent analysis, the transverse blockage intensity will be temporarily ignored and mainly focus on the influence of the longitudinal blockage intensity and extraction pressure on the average sound pressure level of the characteristic frequency band. Nevertheless, when analyzing the longitudinal blockage intensity or extraction pressure separately, it can be found that their respective influences are not entirely independent. Thereby, in order to more accurately describe this relationship, a fitting analysis should to be conducted on the longitudinal blockage intensity and extraction negative pressure. The fitting results are presented in
Figure 16.
The fitting function relationship is expressed as follows:
The parameters are given by: z0 = 1.791, a1 = 0.033, a2 = 39.4, a3 = 0.0051, a4 = −0.42, a5 = −71.52, a6 = 0.649, a7 = 61.91, R2 = 0.984, indicating a better fit.
This model is based on six levels of longitudinal blockage intensity and four levels of extraction negative pressure. In practical applications, the levels of blockage and extraction pressure could be expanded to refine the fitting model.
Compared to traditional methods such as the pressure wave technique and the time-frequency analysis of acoustic waves for pipeline blockage detection, the acoustic wave detection method proposed in this study offers significant advantages in terms of detection speed and non-destructive testing. By analyzing the acoustic wave spectrum, this method can quickly identify pipeline blockages and is particularly suitable for complex, multi-branch pipelines used in coal mine gas extraction, unaffected by the shape of the pipeline. Moreover, the required sensors and analytical equipment are simple, with low maintenance requirements and relatively low cost.
More importantly, this acoustic detection method employs dual-channel acoustic signal acquisition technology, allowing for the simultaneous and precise collection of both the internal pipeline acoustic signals and external environmental noise. This approach makes the detection process more comprehensive and detailed, effectively filtering out environmental noise in various conditions, thereby significantly improving the accuracy and reliability of the detection.