1.1 This test method covers online determination of hydrogen sulfide (H2S) concentration in natural gas using a tunable diode laser absorption spectroscopy (TDLAS) analyzer also known as a TDL analyzer. The particular wavelength for H2S measurement varies by manufacturer, typically between 1000 and 10 000 nm with an individual laser having a tunable range of less than 10 nm. 1.2 Process stream pressures can range from 700 mbar to 700 bar gage. TDLAS is performed at pressures near atmospheric (700 to 2000 mbar gage); therefore, pressure reduction is typically required. TDLAS can also be performed in vacuum conditions with good results, however, the sample-conditioning requirements will be different. Generally, the vent of a TDLAS analyzer is tolerant to small pressure changes on the order of 50 to 200 mbar, but it is important to observe the manufacturers published inlet pressure and vent pressure constraints. Large spikes or steps in backpressure may affect the analyzer readings. 1.3 The typical sample temperature range is -20 to 65C. While sample system design is not covered by this test method, it is common practice to heat the sample transport line to around 50C to avoid adsorption and desorption of H2S along the walls of the sample transport line. 1.4 The H2S concentration in natural gas depends on the location of the sample tap. For example, natural gas exiting a well may have very high concentrations, and after the gas is treated, the H2S concentration should be in single-digit parts per million. 1.4.1 Pipelines used for long-distance transportation and local distribution to end users typically have H2S concentrations less than 4 parts per million by volume (ppmv). Depending on the specifications defined by government regulations or agreed-upon specifications between buyers and sellers, in these applications, it is customary to use a measurement range between 0-5 to 0-100 ppmv. 1.4.2 Upstream pipelines carrying natural gas from the production well to the point where H2S is removed can vary greatly in H2S concentration. The H2S concentration can be anywhere from undetectable levels to tens of percent. In these situations, the measurement ranges can be anywhere from 0-5 ppmv to 0-90 %. 1.5 Units--The values stated in SI units are to be regarded as the standard. No other units of measurement are included in this standard. TDLAS analyzers typically output concentration in molar ratios such as ppmv. Weight-per-volume units such as milligrams or grains of H2S per cubic foot or cubic metre can be derived from ppmv at standard conditions or standard temperature and pressure. 1.6 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety,health and environmental practices and determine the applicability of regulatory limitations prior to use.
hydrogen sulfide; H2S; natural gas; TDL; TDLAS; tunable diode laser absorption spectroscopy
Pipeline on-line measurement of H2S using TDLAS is currently performed without a consensus standard test practice or method. Such a consensus test method is needed to ensure that data is collected in a consistent and comparable manner.
The title and scope are in draft form and are under development within this