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The Analysis of Typical Accidents of Different Gas Pipes

Posted: 06/28/2021 10:47:42 Hits: 10

At present, the mainstream gas pipelines are cast iron pipes, PE pipes and steel pipes. The materials used for steel pipes are divided into several kinds. Take 20 steel, Q345D and L290M as examples for research in this article. Some defects of the pipeline made from the above several materials are the same, some different. The characteristics of each material determine the different kinds of defects, and various defects will eventually affect the safety and service life of the actual use of the pipe. Therefore, the material of the pipeline should also be considered when the causes of the pipeline's leakage are analyzed. The main advantage of steel pipes is that their material strength is greater than that of cast iron pipes and PE pipes, and their corrosion resistance is also better than that of cast iron pipes and PE pipes. In the operation of the pipeline, it is necessary to protect the pipe from corrosion. Therefore, the common defects of steel pipes mainly are corrosion of pipelines and poor welding quality.

(1) Corrosion of pipelines
An inspector of a gas company discovered that a gas concentration alarm went off in the low-pressure pipeline area in front of a residential building in Beijing during the inspection. It was found that the leak was caused by corrosion of pipelines at the low pressure DN100 in front of the residential building, and the corrosion hole is about 30 mm. The pipe is the 20 steel spiral welded steel pipe, which has the advantages of low engineering costs and whose strength and toughness can meet the use of pipeline under normal environment for 3 to 5 years. The disadvantage of the steel pipe is that although it has the characteristics of steel pipes, its strength and toughness are higher than that of PE pipes and cast iron pipes. However, the steel pipe belongs to the welded pipe in terms of the manufacturing process, resulting in welding seams in the steel pipe itself and destroying the integrity of the pipeline. The welded pipe is easy to be corroded during the later service of the pipeline. Meanwhile, toughness and plasticity will be reduced at the formed welding seams. Therefore, for some places where the soil is corrosive and the pipeline is subject to external stress for a long time, leakages of pipelines easily occur.

Figure 1 A photo of corrosion perforation of a steel pipe

In the process of other rush repairs, the corrosion of pipelines is often encountered due to the operation of the buried pipeline for a long time; holes appear, resulting in leakages of gas finally.

The main reason for the leakage of steel pipelines is corrosion. Soil corrosion and electrochemical corrosion of the galvanic cell of the pipe are the main reasons for the leakage of buried steel pipes. A major disadvantage of steel pipes is poor corrosion resistance. It is necessary to emphasize the anti-corrosion measures in the construction process to increase the service life of steel pipelines. Generally, the welded port adopts reinforced anti-corrosion, and cathodic protection measures are also adopted on the pipeline to reduce the influence of corrosion on the steel pipe.

(2) Welding quality
Metal pipes require welders to use melt welding rods and applied them to the welding part through the high temperature welding gun. In the welding process, there will be some chemical reactions. Choose the right welding rode. The right temperature and operational proficiency will affect the quality of welding. Therefore, the weldability of the pipe's base material has become a decisive indicator of the welding quality.

At the weld joint of the metal, the chemical composition and organization of the fusion zone are not uniform, which belongs to the weak area of the pipeline. The fusion zone has good hardness, but its ductility and toughness are poor, easily forming stress concentration and causing the cracking of the welding seam. Therefore, the poor operating skill of the welder can cause defects such as cracks, slag inclusion, lack of fusion, incomplete penetration, gas holes, undercuts, and other defects in the welded joint.

Figure 2 Cracks

Figure 3 gas holes

Figure 4 Incomplete penetration

Figure 5 Undercuts

PE pipes are heated and pressured on both sides of the pipes that need to be connected through hot-melting or electric smelting equipment. The end faces on both sides of the pipe are melt to reconnect under the pressure. After the pipe connection is stable, the temperature should be lowered for the pipe to complete the welding of the PE pipe. In this process, the thermal stability of the PE pipe not only determines the strength of the joint part, but also has a certain effect on the operation time, heating temperature, and cooling temperature. Therefore, the thermal stability of PE pipe is an important indicator.

(3) Surface subsidence
In the process of surface subsidence, the shear stress generated will act on the pipeline for a long time. The pipeline is subjected to a downward force when the degree of subsidence exceeds the plastic deformation of the pipeline; the pipeline cannot meet the displacement of subsidence, causing the weak welding part of the pipeline to crack and leak.

Figure 6 The cracking of welding part of the pipe caused by surface subsidence

(4) The socket part of cast iron pipes

The cast iron pipeline is weak at the connecting part, and the connection method of the cast iron pipeline is socket connection, which is a flexible connection. The socket connection is divided into the N type and S type. The working surface of the socket is a single inclined surface. The socket working surface is a single inclined surface, and the interface gap fracture surface is wedge-shaped. It is consistent with the rubber ring, make the rubber ring deform to fill the gap of the working surface to achieve the purpose of the sealing effect by the pre-pressure during installation.

Figure 7 N type of socket interface

This socket connection does not require welding, so the construction speed is fast and the construction technology is low. Workers easily operate. However, this kind of connection has high requirements for the quality of the rubber ring at the connecting part. Therefore, the sealing and aging degree of the rubber ring has become the main factors affecting the sealing of the cast iron pipeline.

(5) Damage caused by the external force

Figure 8 Damage caused by the external force

The damage due to the external force caused by the third party construction is also one of the reasons for the leakage of the operating pipeline. The aging of the pipeline after a long time leads to a decrease in the strength of the pipe itself, and the pressure resistance will also decrease. It is necessary to choose pipes with good strength, good plastic deformation, and corrosion resistance, which can reduce the degree of leakage to a certain extent.

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