Hydrocarbon gas sampling offers a cost effective alternative to the installation of on-line chromatographs when a comprehensive analysis of the gas is required. Gas sampling systems are also often used to provide back-up measurement as well as continuous validation and calibration samples for chromatographs.
Gas sampling systems are bypass loops with a flow-through sample extractor pump mounted in the loop. The loop is extracted from the central third of the pipeline and must be installed in a location with minimum turbulence and pressure variations to avoid the formation of liquids in the gas stream. The bypass loop normally flows across a differential pressure in the main pipeline (such as an orifice plate). The loop extractor is designed to have sufficient strength to resist bending and vibrations and has a bevelled inlet to maximise representivity. Individual sample grabs are extracted from the bypass loop by a G6 gas sampler and discharged into a constant pressure sample receiver where they are kept at line pressure ready for laboratory analysis.
Gas sampling systems do not need the multi-stage pressure letdown systems used by chromatographs and can be designed for high pressure, arduous and sour service. They have minimal calibration requirements and avoid the need for complex field instrumentation to be installed in the field.