Principle of Pressure Opening of PE Pipeline

       In China's city PE gas pipeline opening and plugging technology is still in the initial and development stage, the application of this technology and the use of this opening and plugging equipment of the city and enterprises are still very few, but with the PE pipe in China's city gas network in the universal application, PE pipe opening and plugging technology will accompany, has its broad prospects for development.

       1. Opening and plugging of PE pipeline under pressure

       With the extensive use of PE pipelines in the city gas pipeline network, the maintenance and repair of PE gas pipelines has become an important task in the daily operation and management of gas transmission and distribution companies. There are three general situations:

       (1) For small holes and cracks in the local part of the pipeline, the electric fusion saddle pipe fitting can be used to block the damaged part, and then the saddle pipe fitting and the pipe wall can be fused together by the electric fusion welding machine, so as to achieve the purpose of repairing the pipeline.

       (2) When the pipeline below D200 is greatly damaged, the flexibility of the pipeline itself can be used to flatten the pipeline at both ends of the damaged part with a special gas pipe press (for D63 ~ D200 pipelines, the air source cannot be completely cut off with a flattener), then cut off the air source, and then repair the damaged part.

       (3) For large-diameter pipeline damage, drill holes (special PE pipe drilling tools) at both ends of the damage, and then plug the isolation ball plug (cup or bladder type) into the hole to seal the pipeline. Finally, cut and replace the damaged part in the middle. For emergency repairs that do not allow gas stoppage, two more holes can be drilled outside the isolation ball (cup) to connect the bypass pipe for temporary gas supply.

       1.1 drilling and plugging equipment under pressure

       The principle of opening and plugging under pressure in PE pipelines is the same as that of opening and plugging in metal pipelines. The equipment includes a pressure opening machine, a frame and a plugging downloader, which is suitable for pipe diameter D160 ~ D300mm, temperature -10~40 ℃, and medium pressure ≤ 0.4MPa.

       Welding between 1.2 saddle fittings and pipes to be perforated

       A key problem in the opening and plugging of PE pipes under pressure is how to ensure the welding quality of saddle fittings and pipes to be opened. Construction units in the PE gas pipeline maintenance, often encounter to be perforated pipe and pipe fittings do not match the situation. Since the welding parameters (such as voltage and heating time) of electrofusion connection machines or electrofusion fittings produced by different manufacturers may be different, the voltage and heating time of electric heating during electrofusion connection shall be carried out according to the parameters provided by the manufacturer of electrofusion connection machines or electrofusion fittings. Before welding, the construction unit should evaluate the welding process of the pipe (piece) to determine whether it can be welded. In fact, for the "special" construction of PE pressure opening and plugging, it is unrealistic for the construction unit to evaluate the welding process of PE pipes and fittings from different manufacturers. To sum up, due to the existence of the above problems, the popularization and application of the PE pipe pressure opening sealing technology is hindered.

       2. Electric fusion connection of PE pipes and fittings

       2.1 special cases requiring electrofusion connection

       The Technical Code for Polyethylene Gas Pipeline Engineering (CJJ63-2008) stipulates that when connecting pipes, fittings and pipeline accessories made of polyethylene raw materials with different grades and melt mass flow rate (MFR) difference (conditions are 190 ℃, 5kg) not less than 0.5 g/min, and polyethylene gas pipelines with different standard dimension ratios (SDR) of welded ends, electrofusion connection must be adopted. PE pipes with nominal diameter less than 90mm should be connected by electrofusion (because the pipe with smaller diameter has smaller welding section size when using hot melt butt welding, which cannot guarantee the welding strength, so only electrofusion socket welding can be used).

       Factors Affecting the Quality of 2.2 Electrofusion Connection

       "Research on Air Tightness Resistance of PE Gas Electrofusion Fittings" conducted welding tests on injection molded PE100 electrofusion fittings, and concluded that with the extension of power-on time, the air tightness resistance of PE gas electrofusion fittings increases continuously. When the air tightness resistance of PE gas electrofusion fittings and pipe electrofusion connections reaches the highest, the air tightness resistance of PE gas electrofusion fittings and pipe electrofusion connections begins to decrease instead with the extension of power-on time. This is mainly because: when the power-on time is too short, the pipes and fittings are not fully melted, and there is a gap between the pipes and fittings; If the power-on time is too long, the pipes and fittings will melt too much and holes will be generated after cooling. Only when the power-on time is moderate, there will be no gaps and holes between pipes and fittings, and the air tightness of PE gas pipe fittings and pipe fusion connection reaches the highest. With the increase of the resistance of the copper resistance wire, the air tightness of the PE gas pipe fittings increases continuously. When the resistance of the copper resistance wire increases to the maximum value, the air tightness of the electric fusion connection of the PE gas pipe fittings and the pipe tends to be stable. This is mainly because: when the resistance of the copper resistance wire is too low, the heat of the resistance wire is not enough, and the plastic resin of the pipe and pipe fittings cannot be fully melted, so that there is a gap between the pipe and the pipe fittings. The closer the MFR of the pipe and pipe fittings, the better the compatibility, and the better the air tightness after the electric fusion connection of the pipe fittings. The matching size between pipes and fittings increases, and the air tightness is reduced after electrofusion connection. Only when the matching size of pipes and fittings is between 0.05 and 0.20mm, gaps and voids will not occur after electrofusion, and the air tightness is good after electrofusion connection.

       Quality Inspection of 2.3 Electrofusion Saddle Connection

       For hot-melt joints, check the symmetry of the flanging of the joint, the alignment of the joint, and carry out the flanging cutting inspection. Generally, material problems such as mismatching of melt mass flow rate and moisture, and operation problems such as improper setting of pressure, temperature and time parameters can be found. For the electrofusion connection joint, the fusion of the joint is wrapped in the electrofusion pipe fitting. Under the condition that the molten material does not overflow a large amount, the quality inspection can only be judged from the observation hole on the electrofusion pipe fitting, and the quality problems caused by the inconsistency of the material itself and the axis are often not found.

       PE pipe pressure opening and plugging technology has certain practical application value and development prospect. With the increasing proportion of PE pipe in urban gas pipeline network, the further improvement and implementation of PE material quality standards and engineering construction standards, and the further standardization of PE material market, this technology will be popularized and applied like metal pipe pressure opening plugging technology, and will play its role in gas pipe network maintenance and repair.