The Drying Technology Study Of The Long Distance Gas Pipeline

In order to eliminate the potential trouble and defects of the newly built long gas distance pipeline, hydrotesting must be performed before commissioning, using water as the media of hydrotesting. There still exists a small amount of water inside the pipeline after hydrotesting and pigging. If drying is not carried out before commissioning, oxygen corrosion to the inside wall of the pipeline is caused easily, and ice block will take place, which will reduce the gas transmission capacity and affect the gas quality. Therefore, drying must be carried out on the long distance gas pipeline before commissioning.No matter which drying method is selected for a long distance gas pipeline, it is necessary to know clearly its working principle, drying techniques, application scope and its advantages & disadvantages of the method. Only when the method selected in this way will the drying efficiency be the highest, the most economical, the safest and the most environmental. Therefore, it is necessary to study and compare the above mentioned factors of each drying method.Based on the drying project performed home and abroad, and by comparing the different drying methods, this essay puts forward the drying method for long distance gas pipelines under different conditions. This method is the most efficient, the most economical, the safest and the most environmental.Research shows that after hydrotesting and dewatering, if drying is not performed in time, oxygen corrosion will take place due to free water on the inside wall of the pipeline without internal coating and the air inside pipeline. The longer the time lasts, the greater the degree of corrosion will be. It will also affect the operational life and reduce the safety factors of the piepline.While for the pipeline with internal coating, corrosion will take place only when the internal coating is damaged. If dry air is adopted to dry the pipeline with internal coating, the water in free state on the inside wall is easily absorbed by the foam pig, and there is hardly wear abrasion on the pig, which will enable the water in free state to distribute evenly on the inside wall and to ensure the dry air to be absorbed fully.Therefore, the effect of using dry air to dry pipelines with internal coating is better. When a pipeline without internal coating is dried with dry air, due to the small undulations inside the pipeline the foam pig is unable to absorb enough water during running, and also due to the greater friction factors inside the pipeline the foam pig is running slowly. In order to reduce the dew point inside the pipeline, the quantity of the pigs will have to be increased. If vaccum air is used to dry the pipeline without internal coating, the effect will be ideal.Before vaccum air drying, firstly using air to run the mechanical pig to verify that there is no open water at the end of the pipeline, then connecting it to the drying tank, using dry air to run faom pig until the increament of the foam pig at the end is smaller than 1.5kg. Then using dew point instrument to measure, the dew point shall be close to that of the atmosphere environment.When reaching this state, block both ends of the pipeline and connet it to the vacuum equipment. The vacuum equipment at this state can founction to its maximum extent, the drying speed be increased and the ideal drying effect be achieved. During the station drying process, the pipeline diameters are different and the connection is complicated, only dry air can be used for pipeline purging because assistant pigging is not available. Some dead water cannot be purged thoroughly from the pipeline. If dry air is used for drying at all times, it not only takes longer time, but also the drying effect is not ideal.But vacuum air drying does not have such problem. Before station vacuum drying is carried out, firstly using dry air or air to purge the pipeline until no open water comes out from the piepline, then seal the entire station for vacuum drying. No matter how complicated the pipeline is, the moisture remaining inside the piepline can be evaporated completely.The purpose of saving costs and satisfying the demand of project can be achieved by studying this subject. This has laid a solid foundation for China Petroleum Pipeline Bureau to open up the drying construction markets domestically and abroad and create a high economic efficiency.The study of this subject will have noticeable effects in the aspects of technology feasibility, practicability and economic efficiency.