Nitrogen Fed between Two Srapers to Replace Natural Gas in Gas Pipeline Commissioning

During the commissioning of a gas pipeline, the most critical link is the replacement of the natural gas in the pipeline. A method of feeding nitrogen to replace gas between two scrapers is adopted in Beijing Gas Loop Line originating from Huabei Oil Fields which is the source of gas supply for Beijing City. This consideration provided possibility of safe commissioning. The gas pipeline is 64.16 km in length, 529 in diameter (DN), passing through many a railway, highway, river, villege and highly populated area.Introduced in the paper is how this method was realized and its procedure including the selection of scrapers, launching and receiving process, and that of replacement of natural gas. Electronic tracing scrapers of DQZ series are recommended. The operation procedure is like this: first, push the scraper into the pipeline by means of a mobile compressor, then feed 49 cylinders of nitrogen, close the valve of the launcher; (The internal, nitrogen pressure is 0.022 MPa, the quantity of nitrogen being about 220 m3;) and finally launch the second scraper for replacement by means of the outgoing natural gas. Through mornitoring and calculation, the replacing speed is 1.61-7.04 m/s. The nitrogen content inside the pipeline is less than 0.2% after replacement, which can meet the requirements of gas transportation.