Polyethylene pipeline(PE Pipe) has many characteristics, such as economy, corrosion resistance, aging resistance, good flexibility and flexibility, and good ability to resist rapid cracks. It has been widely used in urban medium and low pressure gas pipeline network, and is also trying to be used in sub high pressure gas pipeline network. The construction quality of polyethylene pipe joint has always been the focus of our attention. At present, polyethylene gas pipeline with diameter greater than 90 mm is laid, and the interface form is mainly hot-melt butt joint connection. The welding quality control of hot-melt butt joint has been specified in the corresponding specifications. There is no mature and reliable inspection method for the internal quality of hot-melt butt joint, which can only be judged by the visual inspection and flanging removal inspection of the joint. In the actual construction process, we found that the quality of some hot-melt butt joints was qualified after flanging and cutting according to the method specified in the specification, but failed in the inspection test. Here we will discuss this problem and puts forward solutions.
A section of dn250mm polyethylene gas pipeline (sdr11 PE100) was laid before. The quality of a hot-melt butt joint was inspected according to the specifications. On the premise that the flanging symmetry inspection and hot head alignment inspection were qualified, the flanging removal inspection was carried out. The appearance of the cut flanging strip was qualified. When the flanging back bend (180 degree back bend every 50mm) test was carried out, There is no crack, crack or exposed fusion line, and the quality of the butt joint is determined to be qualified. However, occasionally, it is found that the qualified flanging can be pried off along the fusion surface at a local section with a slotted screwdriver, and the separated fusion surface is smooth and flat. We infer that the quality of the joint is unqualified.
Therefore, the flanging of all completed and qualified butt joints was strengthened, including slotted screwdriver pry, maximum distortion and stretching, etc. in this way, many flanging problems of the same kind were found. Therefore, it is doubtful whether the flanging removal inspection method specified in the specification can effectively prove the quality of hot-melt butt joint.
Executive Standards and Relevant Regulations
Special tools shall be used for flanging removal inspection to remove the external welding flanging without damaging the pipe and joint. Flanging and cutting inspection shall meet the following requirements:
- Flanging shall be solid and smooth with wide root;
- The lower side of flanging shall be free of impurities, holes, distortion and damage;
- 180 ° back bend test shall be carried out every 50 mm, there shall be no cracks and cracks, and the fusion line shall not be exposed at the joint.
In case of dispute over the welding quality inspection of hot-melt butt joint, the qualification inspection shall be carried out according to table 1.
*** Table 1 requirements for qualification inspection and test of hot melt butt welding process***
|No.||Inspection and Test Items||Inspection and Test Parameters||Inspection and Test Requirements|
|(1) Toughness, through|
|(2) Cytoplasmic, failed|
|2||Compressive (hydrostatic) Strength Test||(1) Sealed joint, type A||There is no damage or leakage at the welding position|
|(2) Direction, arbitrary;|
|(3) Adjustment time, 12 h;|
|(4) Test time, 165 h;|
|(5) Ring stress: ① PE80, 4.5 MPa; ②PE100,5.4 MPa|
|(6) Test temperature. 80 C|
According to relevant regulations, two joints in doubt shall be selected for each test. One joint shall be tested for compressive (hydrostatic) strength, and the other joint shall select the pipe section corresponding to the unqualified flanging section as the sample for tensile performance test . The test methods and parameters shall comply with the relevant provisions in Table 1. During the pressure (hydrostatic) strength test, the ring stress is increased because the pipe material is PE100 σ Take 5.4 MPa. The test pressure in the pipe is calculated according to the following formula:
Verification Contents and Results
For the two projects, a total of 3 batches of tests have been carried out, and the test conditions are shown in Table 2.
Table 2 test verification contents and results
|Project||Test Batch||Selection of Unqualified Joints||Test Content||Test Result|
|Item 1||Batch 1||Strengthen the inspection, the flanging is smooth and flat, and the length of the cracked surface is greater than 50% of the circumference.||Compressive (hydrostatic) Strength Test Group 1||Qualified|
|Tensile Property Test Group 1||Unqualified|
|Batch 2||Strengthen the inspection, the flanging is smooth and flat, and the length of the cracked surface is greater than 25% of the circumference||Compressive (hydrostatic) Strength Test Group 1||Qualified|
|Tensile Property Test Group 1||Unqualified|
|Item 2||Batch 1||Strengthen the inspection, the flanging is smooth and flat, and the length of the cracked surface is greater than 25% of the circumference||Compressive (hydrostatic) Strength Test Group 1||Qualified|
|Tensile Property Test Group 2||Qualified|
Analysis of Validation Results
Conduct quality inspection for a hot-melt butt joint according to the regulations. The flanging symmetry inspection and joint alignment inspection are qualified. During flanging removal inspection, the flanging appearance inspection is qualified, and the flanging back bending test is also qualified. However, if other methods are adopted, the flanging cracks at the fusion surface, and the cracking section is a smooth and flat brittle cracking surface, When the hot-melt butt joint is subjected to tensile test, the cracking type may be brittle cracking, and the quality of the hot-melt butt joint may be unqualified.
The inspection standard of flanging back bending test "180 degree back bending test shall be conducted every 50mm, there shall be no cracks and cracks, and the fusion line shall not be exposed at the joint" is not strict enough and needs to be further strengthened.
In engineering practice, through communication with technicians of material manufacturing and hot-melt connection equipment manufacturers, we found that the flanging with quality problems can be inspected by 360 ° face bending and stretching. This method will create a large shear force on the fusion surface during the final tension, which is more effective than 180 ° back bending.
Qualified judgment: the flanging is not cracked, or even if it is cracked, the cracking surface is a ductile cracking surface, which is determined to be qualified; If the flanging cracks and the cracking surface is a smooth and flat brittle cracking surface, it is determined as unqualified. The unqualified joints shall be cut off and re welded. If there are a large number of unqualified joints, but the flanging brittle crack length is less than 25%, the final determination of whether such welded joints are qualified shall be made by selecting typical specimens for tensile property test.
For the welding of hot-melt butt joint of large-diameter PE100 polyethylene pipeline, even if full-automatic equipment is used for welding, unqualified joints may still appear, and the causes of the problems are quite complex, and even some factors are difficult to overcome in the construction process. In particular, it is difficult to judge whether the material itself has an impact on the welding quality and how much the impact is through a certain test data. Therefore, the field inspection should be strengthened in engineering practice, especially in the early stage of the project, it is best to carry out 100% flanging and cutting inspection, and add the inspection method in this paper, which can further improve the welding quality control level of hot-melt butt joint. Our company has added this flanging inspection method in engineering practice and achieved good results