Oil Mist Concentration On-line Measurement by Optical Scintillation Method
In order to evaluate the efficiency and performance of the natural gas compressor outlet oil separator,a concentration detection method of micro-nano-scale oil mist droplets based on the light scintillation theory was proposed for the low-concentration lubricating oil mist downstream of the compressor outlet oil mist separator under the condition of 0～50 mg·m-3 low concentration oil mist concentration.Based on the particle phylolayer model,Lambert-Beer law and Poisson distribution,the theoretical model uses the total scattering theory to derive the particle concentration calculation model under the long-optical path parallel uniform beam by segmenting the measuring volume in the beam field and taking the transmittance of each monolayer measuring volume as the statistical random variable of Poisson distribution,and explains the causes and influencing factors of the optical scintillation method.For compressor lubricants in the natural gas industry,ISO viscosity grade 460 lubricants were selected,and spectral scanning and Principal Component Analysis(PCA)determined that the 400 nm wavelength beam had the best detection sensitivity and positive correlation in the visible light band.At the same time,the infrared spectrum and three-dimensional fluorescence spectrum of lubricating oil show that the infrared band has a strong absorption effect in hydrocarbon gas medium,and the fluorescence effect of lubricating oil in the ultraviolet band will aggravate the stray light in the optical path and affect the test accuracy.For the flicker frequency in the optical scintillation method,the selection of the flicker frequency is determined by the spectral change measurement,and the selection of the flicker frequency affects the detection accuracy by affecting the fitting variance of the scintillation signal,and the flicker frequency is determined according to the size of the measuring body,particle diameter and particle velocity.The results show that in the range of 0～50 mg·m-3 oil mist concentration,there is an obvious linear relationship between oil mist concentration and flicker signal variance under the appropriate flickering frequency by optical scintillation method,and the linear correlation coefficient R2=0.989.In the calibration test results,the linear correlation coefficient R2=0.989,and the test error is less than 10%.However,the error of the long-path light transmission method detection calibration results gradually decreases with the concentration,and the linear correlation coefficient R2=0.956.Compared with the long-path optical transmission method,the error of the optical flicker method is lower than that of the optical transmission method at very low concentration,and with the increase of the calibration concentration,the test error of the optical transmission method gradually decreases and is comparable to that of the optical flicker method.The above analysis shows that the optical flicker theory can realize the real-time online detection of oil mist concentration at low concentration,and the method is not affected by optical window pollution,and the optical structure is simple,which is expected to be used for real-time online monitoring of compressor outlet under high pressure in the future.
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