With the rapid expansion of oil and gas pipelines in China and the growing implementation of urban gas pipelines, pipeline leakages have garnered significant attention. These leakages directly affect the safety of human lives and property. Consequently, pipeline leakages have emerged as a prominent research area. Volatile gases released during oil and gas leaks consist not only of methane but also of characteristic gases such as propane and butane. Precisely measuring the volume fractions of these gases can help in addressing the limitations of isolated measurements of methane. This comprehensive approach has significant value in terms of safety and environmental protection.

Because the stretching vibration of C—H chemical bonds in alkane macromolecules can cause the superposition of absorption spectra in the near-infrared region, it is difficult to achieve accurate measurements of propane and butane using traditional tunable diode laser absorption spectroscopy technique. In this study, a traditional tunable diode laser absorption spectroscopy technique was combined with a stoichiometric algorithm. Direct absorption signals and second harmonic signals within the range of 1685.9?1686.8 nm were recorded using a tunable diode laser absorption spectroscopy technique platform. The quantification of the two gas components was then achieved through the application of a partial least squares regression algorithm, which effectively addresses the challenges posed by overlapping absorption spectra. The study shows that this approach significantly enhances the accuracy and sensitivity of the quantitative analysis model.

Initially, a regression relationship between the volume fractions of elementary propane and butane gas and the second-harmonic signal was established using partial least squares analysis. This relationship enabled the prediction of unknown gas volume fractions below 2000×10^-6. The experimental results demonstrate that the maximum prediction error for propane is 14×10^-6, whereas for butane, it is 41×10^-6. The correlation coefficients R² are 0.9999 and 0.9995 for propane and butane (Fig.6), respectively. These findings serve as preliminary evidence for the partial least squares algorithm and its ability to accurately demodulate wide-spectrum absorption gas lines. Next, based on the mixture of propane and butane, the study observed that the amplitude of the second-harmonic signal demodulated from propane and butane at the same volume fraction differs by two orders of magnitude (Fig.7). This difference does not pose an issue for the inversion of their respective volume fractions in the presence of elementary gases. However, in the case of mixed gas, the butane signal with a smaller amplitude is submerged within significant variations in the propane signal. Consequently, the amplitude of the second-harmonic signal at the absorption center of propane is significantly reduced. Thus, modeling the second harmonic signal alone results in unacceptable errors. To address this challenge, the characteristic absorption information represented by the second harmonic signal was combined with the spectral band absorption information represented by the direct absorption signal. Both signals were collected and used as independent variables to train the regression model for gas mixtures. This approach ensures a more comprehensive and accurate analysis of the volume fractions of mixed gases. The developed model is capable of detecting low volume fraction gas mixtures, including propane and butane at volume fractions of 0.8% and 0.9%, respectively, of the lower explosive limit. The maximum prediction errors in the low volume fraction group, ranging from 100×10^-6 to 800×10^-6, are found to be propane at 34×10^-6 and butane at 51×10^-6 (Fig.8). Similarly, the maximum prediction errors in the high volume fraction group, ranging from 2000×10^-6 to 10000×10^-6, are found to be propane at 64×10^-6 and butane at 148×10^-6 (Fig.9). Importantly, all the prediction errors remain below 3% of the lower explosion limit, which aligns with the safety requirements of the petroleum industry. This methodology caters to the specific safety requirements of the petroleum industry by enabling the precise and sensitive detection of low volume fraction gas mixtures and ensuring production safety and hazard prevention. To further validate the dynamic reliability of the model during continuous operation, two continuous tests were conducted at low (Fig.10) and high (Fig.11) volume fractions. These tests successfully confirmed the stability and reliability of the partial least squares regression model in predicting the volume fraction of each component in the propane and butane mixtures throughout the dynamic process.

This study relies on a tunable diode laser absorption spectroscopy technique and leverages multiple measurement signals that represent gas absorption within a narrower scanning range. This approach enables the quantitative analysis of the overlapping spectral lines of propane and butane. Consequently, it offers a practical solution for accurately measuring the volume fractions of various volatile oils and gases. This solution is particularly well suited for addressing the specific needs encountered at oil and gas storage sites. The approach exhibits tremendous potential for expanding its applications and will undergo further validation in the field of oil and gas pipeline leakages.