A Calibration System for Tunable Diode Laser Spectroscopy Water Content Analyzers of Natural Gas

Abstract

Excessive water in natural gas can lead to pipeline blockages or equipment corrosion, and thus accurate measurements of the water content of natural gas are essential. Water content analyzers based on tunable diode laser absorption spectroscopy (TDLAS) can accurately measure the water content of natural gas. In the present study, a calibration system comprising a humid methane generator, using a permeation tube, and a chilled mirror hygrometer was developed for TDLAS analyzers. The generator can produce humid gas mixture of methane and nitrogen with a frost point ranging from –65 ℃ to –29 ℃ at the pressure of 101.325 kPa and an arbitrarily adjustable volume fraction of methane. This calibration system was used to calibrate two TDLAS analyzers from different manufacturers. When the volume fraction of methane (on a dry basis) was 100% and the frost point of humid gas was between –65 ℃ and –29 ℃, the indication errors of the frost points were between –0.41 ℃ and –0.03 ℃ for one analyzer and between –2.24 ℃ and –0.75 ℃ for the other analyzer. The indication errors of the frost point were fitted using a quadratic function. In the frost point range of –65 to –29 ℃, the typical uncertainty of the frost point indication error was 0.38 ℃ (U, k = 2), corresponding to a water content uncertainty of 0.3 to 16.4 ppmV. This calibration system can be used for factory calibration, pre-installation calibration, and periodic calibration of TDLAS water content analyzers of natural gas, ensuring the accuracy, reliability, and equivalent consistency of water content results.