BS ISO 23978:2020 pdf download – Natural gas — Upstream area — Determination of composition by Laser Raman spectroscopy
5.7 Sulfur compound absorber
Beakerflask, flask etc. filled with basic solution can be used as sulfur compound absorber in order to eliminate sulfur compounds discharged to the atmosphere. Sodium hydroxide solution (200 g/l), prepared by dissolving sodium hydroxide (chemically pure) in water is suggested as the basic solution.
6 Reagents and materials
6.1 Zero gas Pure argon or other monatomic gas is used to set the zero level of the Raman analyser. The purity should not be less than 99,999 %.
6.2 Base span calibration gases Base span calibration gas used to set software parameters shall be working standard pure gases or binary mixtures in accordance with ISO 6142-1, ISO 6144 or with ISO 6145. Binary gas mixtures consisting of blending zero gas and a single gas component present in natural gas are preferred when their typical concentrations in natural gas are below 75 %. For typical natural gas upstream area samples, 1 % H 2 S/Ar, 5 % CO 2 /Ar, 3 % N 2 /Ar, 3 % C 3 H 8 /Ar, 2 % C 2 H 6 /Ar, are suggested. As base span calibration gases for all but CH 4 which can be pure (100 %) or Ar diluted.
6.3 Working span calibration gases Regular calibration shall perform using working standard gas mixtures in accordance with ISO 6142-1, ISO 6144 or with ISO 6145. The working standard gas mixtures shall contain appropriate concentrations and cover the analytic range of the analyser. All components in the reference standard shall be homogenous in the vapour state at the time of use. The concentration of a component in the reference standard gas shall be close to the actual sample gas concentration, shall be less than relatively 20 % of the corresponding component in the test gas, the minimum concentration shall be greater than 0,02 %.
7 Measurement procedures
7.1 preparation Make sure the analyser placed on a flat, horizontal surface, in a clean location that maintained at a relatively constant environmental conditions:
— temperature: 15 °C to 35 °C;
— relative humidity: 10 % to 75 %. Set up the laser Raman gas analyser according to the manufacturer’s instructions, connect zero gas, base span calibration gas, measurement calibration gas, and sample in sequence to the instrument as shown in Figure 2.
7.2.1 Calibration frequency Calibration should be carried out when experimental conditions were changed. And it should be recalibrated according to the needs of the user.
7.2.2 Calibration procedure Connect the calibration gas with the analyser according to Figure 2. Adjust the input pressure and temperature to a stable condition. Sweep the analyser with the gas flow for at least 2 min. After the sulfur-containing calibration gas is injected, it should go through the absorption solution before it is vented.
7.2.3 Zero calibration According to 7.2.2, connect zero gas with the analyser, start calibration, update the background when the signal values are stable.
7.2.4 Base span calibration Before sample measurement, base span calibration should be done to overcome the instrument drift caused by temperature, pressure and general electronic fluctuations, and the cross-interference caused by the multiple components in the gas could be maximum eliminated by calibrating the cross- interference factors. According to 7.2.2, connect base span calibration gas to the analyser, start calibration, observe the signal values, adjust the cross-interference factors till the interference signal to a reasonable value (usually in a range of -1 to 3).
7.2.5 Working span calibration Using ISO 11095, the user shall choose the calibration method depending on the analytical accuracy that is required. The following are candidate methods and their requirements:
— bracketing calibration: the difference between calibration gas and analytical result should be less than relatively 20 %;
— response formula simulation: no less than three calibration gas should be used to cover the analytical range;
— one point calibration: the difference between calibration gas and analytical result should be less than relatively 20 %. If the method is used to monitor fixed natural gas source, one point calibration or bracketing calibration should be applied. If the method is used to analyse a non-constant natural gas source, response formula simulation should be applied.
7.3 Sampling and sample analysis Connect gas sample, gas pressure regulator, filters, and analyser in sequence adjust pressure and flow to the experimental conditions, start sample analysis, the sample will be automatically measured. Sampling shall be in accordance with ISO 10715. Sulfur-containing sample should go through the absorption solution before it is vented.BS ISO 23978 pdf download.