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Quality problems of pipe fitting construction

Attention:     Issuing time:2020-06-19 10:19
Common quality problems and solutions of pipe fittings in construction
Pipe fittings, especially tees, elbows, and large and small heads, are more and more commonly used in the pipeline construction process. This is mainly due to their good forming, strong pressure resistance, and simple welding forms to ensure the pressure capacity of the pipeline. Therefore, the rational use of pipe fittings and the quality of the pipe fittings directly play a decisive role in the process pipeline engineering, especially in high-temperature, high-pressure and flammable and explosive pipelines such as petrochemical industry.
 
Because of this, in the pipeline design and installation process, especially in the high-temperature, high-pressure and flammable and explosive pipeline projects in the petrochemical industry, no matter from design selection or processing and manufacturing, to every step of construction, installation and inspection, design and manufacturing and The personnel of the installation unit must strictly control the quality of their selection, materials, manufacturing, installation and inspection, otherwise it will cause unpredictable losses in the construction and production of the device. From the construction process, we found that the quality of the pipe fittings to the goods There are the following problems




 
1
Uneven wall thickness of pipe fittings
 
The uneven wall thickness of pipe fittings mainly occurs in the parts with the largest deformation. For example, the wall thickness of the back of the elbow is thinner than other parts; the wall thickness of the nozzle and the body of the pipe varies. According to relevant national standards. The maximum wall thickness reduction of the pipe fittings shall not exceed 12, 5 of its nominal thickness, but it has been found on the spot that some wall thickness reductions have reached 2o-30. For the inspection of such problems, it is often difficult to find with common calipers and other measuring tools. At this time, it can only be measured by using an ultrasonic thickness gauge.
 
2
Hardness exceeds standard
 
The problem of excessive hardness is mainly due to the heat treatment process after forming. The solution is to use the correct heat treatment process to perform another heat treatment. This problem can usually be solved.
 
 
3
Defects generated in materials and processing
Such problems are the most dangerous to the safety of the device. It's more troublesome to check. The factors that produce defects are more and more complicated. It has both the defects of the material itself and the defects caused by the manufacturing process problems and the incorrect heat treatment process. So why do they cause safety hazards, and how to avoid such problems? Now take the three links as an example to analyze this one by one.
 
 
3.1 Problems with raw materials
We know that for pipes that withstand high temperature, high pressure and flammable and explosive media. The quality of raw materials directly affects the quality of the pipe fittings used to manufacture such pipe fittings. For carbon steel pipes, most of them use GB5310-85 "Seamless Steel Pipe for High-pressure Boilers" and GB6479-86 "High-pressure Seamless for Chemical Fertilizer Equipment" Steel pipes "GB9448-88 "Seamless steel pipes for oil cracking" and GB8163-87 "Seamless steel pipes for transporting fluids" and other standards. For this type of steel pipe, the steel plant shall conduct a hydraulic test one by one. For large and medium-sized steels, eddy current or ultrasonic wave flaw detection (uT) is often used instead of hydrostatic inspection. If the inspection is carried out in strict accordance with national and related standards, the quality of the pipe can still be guaranteed. But sometimes it is inevitable that there will be missed inspections and false inspections. What's more, due to the mixed pipeline purchase channels, some raw materials that have not been inspected have entered the circulation field. Therefore, for the pipelines that arrive, especially the non-batch purchases directly from the steel pipe factory s material. In addition to the comprehensive re-inspection according to the standard, the rate of re-inspection should also be increased, or even checked one by one According to the surveys of many large-scale pipe fittings we have contacted with, many manufacturers have not strictly checked the re-inspection of the purchased pipes. They blindly believe in the factory quality certificates of the materials (it is worth noting that some factories leave the factory There is a problem with the authenticity of the quality certificate), and the quality of the pipes made of these unqualified pipes is difficult to be guaranteed only for the appearance inspection of the double-pier pipes. In addition, the inspection methods of many manufacturing plants are neither advanced nor perfect. Therefore, some unqualified pipe fittings can be easily shipped as qualified products. For example, such problems are encountered in the hydrorefining unit of the refinery of Tianjin Petrochemical Company. In the process of pressing after the installation of the pipeline, there is a tee of 14mm×7ram of the machine. When the pressure rises to llMPa, the solid neck cracks and leaks, and the leaked part.
 
The following analysis of this crack was made:   
a) Chemical composition   
Through chemical composition analysis and arbitration re-inspection, it was determined that the carbon content and manganese content were relatively high (c0.26; Mn: 1.27, the material used was carbon steel No. 2O);   
b) Mechanical properties  
The normal No. 2O steel pipe is 410~550MPa, and the actual measured value=570MPa; 12) Metallographic structure   
By observing the normal part of the three-way fracture and the straight pipe part, it is found that the proportion of pearlite is significantly higher than that of the normal No. 20 steel, which is related to its higher carbon content. Moreover, if the content of carbon and manganese is too high, it will inevitably affect the processing performance of the three links.
 
 
3.2 Processing technology issues   
At present, the processing and forming of domestic manufacturers generally adopt the following methods:
(1) Cold stamping;
(2) Hot stamping;
(a) Normal part of 4ram tube (rolling direction) F―P
(h)~1] Normal part of 4mm tube (lateral +P
(e) ~61mm tube crack fracture VI (transverse JF+P
(d) Magnification F+P of (c)
  
Metallographic structure of tee material
(3) Hydraulic forming
Common quality problems and solutions of pipe fittings in construction
(a) Inner wall surface F+P
(b) Outer wall surface F+P
  
 
The metallographic structure of the inner and outer walls of the tee. For tees with larger diameters and thicker walls, the latter two forming methods are mostly used, and the forming time of these two methods generally takes 1 to 2 minutes, and even some Up to 3 minutes. In these few minutes, how to keep the temperature relatively stable is a key issue for the quality of the pipe fittings. If the temperature difference is too large, it will cause the uneven flow of the steel fiber structure, especially for the parts with large deformation. Because of its large plastic deformation, if the temperature is too low, it will cause cold work hardening and brittle failure.

 

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