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A brief discussion on the common quality problems of pipe fitting in construction

A brief discussion on the common quality problems of pipe fitting in construction

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  • Time of issue:2020-07-31 09:40
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(Summary description)Pipe fittings, especially tee, elbow and size, are more and more commonly used in the construction of pipeline projects, which is mainly due to their good forming, strong pressure resistance, simple welding form and other characteristics to ensure the pressure capacity of pipelines.

A brief discussion on the common quality problems of pipe fitting in construction

(Summary description)Pipe fittings, especially tee, elbow and size, are more and more commonly used in the construction of pipeline projects, which is mainly due to their good forming, strong pressure resistance, simple welding form and other characteristics to ensure the pressure capacity of pipelines.

  • Categories:News
  • Author:
  • Origin:
  • Time of issue:2020-07-31 09:40
  • Views:
Information

Pipe fittings, especially tee, elbow and size, are more and more commonly used in the construction of pipeline projects, which is mainly due to their good forming, strong pressure resistance, simple welding form and other characteristics to ensure the pressure capacity of pipelines.

In the process pipeline engineering, especially in the petrochemical industry and other high temperature and high pressure and flammable and explosive pipeline, the reasonable use of pipe fittings and the quality of pipe fittings directly plays a decisive role.Because of this, in the process of pipeline design, installation, particularly in the petrochemical industry and flammable and explosive of high temperature and high pressure pipeline engineering, from the design selection to processing and manufacturing, until the construction installation and inspection of each link, each unit personnel should be strictly controlled, to ensure its selection, material, manufacture, installation and inspection of quality, or in the process of plant construction and production will cause incalculable loss.From the construction process, we found that the quality of pipe fitting may exist in the following aspects:

1. The wall thickness of the pipe fitting is uneven

The uneven thickness of pipe fitting wall mainly occurs in the part with the largest deformation.If the wall thickness of the back of the elbow is thinner than other parts;The thickness of the pipe mouth and the wall of the fitting body are different.It is stipulated in the relevant national standards that the maximum thickness reduction of the wall thickness of the pipe shall not exceed 12.5 of its nominal thickness. However, the field measurement shows that the thickness reduction of some wall thickness reaches 20 ~ 30.For the inspection of this kind of problem, with the general calipers and other measuring tools are often difficult to find, at this time only the use of ultrasonic thickness gauge can be detected.

2. Excessive hardness

The problem of excessive hardness is mainly caused by the heat treatment process after forming. The solution is to conduct another heat treatment with the correct heat treatment process, which can generally be solved after treatment.

3. Defects in materials and processing

This kind of problem has the biggest harm to the safety of the device, and it is troublesome to check.There are many and complicated factors that cause defects, which are not only the defects of the material itself but also the defects caused by the processing and manufacturing process problems and the incorrect heat treatment process.So why do they create security hazards, and how to avoid such problems?

Now take the three links as an example to analyze them one by one:

1. Raw materials

We know that the quality of raw materials directly affects the quality of pipe fittings for pipe fittings that are subjected to high temperature, high pressure and inflammable and explosive medium.For carbon steel pipe, most of the use of gb5310-85 "seamless steel pipe for high pressure boiler", gb6479-86 "seamless steel pipe for chemical fertilizer equipment" gb9448-88 "seamless steel pipe for petroleum cracking" and gb8163-87 "seamless steel pipe for fluid transport" and other standard materials.For this kind of steel pipe, the steel mill shall conduct hydraulic test one by one.

For large and medium-sized steel, eddy current or ultrasonic transit inspection (uT) is often used instead of hydraulic test, such as strictly in accordance with the national and related standards to test, the quality of the pipe can be guaranteed.But sometimes, it is unavoidable that there will be missed inspection, false inspection, and even worse, due to the mixed purchase channels of pipe, some raw materials that have not been tested have entered the circulation field. Therefore, for the pipe after the arrival of the goods, especially the materials directly purchased from the steel pipe factory in batches.In addition to carrying out comprehensive reinspection in accordance with the standard, the rate of reinspection should also be increased, even by inspection.

However, according to the survey of several large-scale pipe fitting manufacturers we have contacted, many of them are lax in re-inspection of purchased pipes and blindly believe in the delivery quality certificate of materials (it is worth noting that the authenticity of some delivery quality certificates is problematic).

In addition, the inspection methods of many manufacturers are neither advanced nor perfect, so some unqualified pipe fittings are easy to leave the factory as qualified products. For example, such problems are encountered in the hydrotreating unit of tianjin petrochemical company refinery.During the pressing process after the completion of pipeline installation, the tee fitting with 14mm×7ram cracked at the fixed neck and leaked when the pressure rose to llMPa.

We made the following analysis on the leakage crack:

A) chemical composition

Through chemical composition analysis and reinspection by arbitration, the high content of carbon element and manganese element was determined (c0.26;Mn: 1.27, choose 2O carbon steel);

B) mechanical properties

The normal 2O steel tube is 410 ~ 550MPa, while the actual measurement is 570MPa.By observing the normal part of the three-way fracture and straight pipe, it is found that the proportion of pearlite is significantly higher than that of the normal no. 20 steel, which is related to the high carbon content.And its carbon and manganese content is too high, is bound to have an impact on the processing performance of tee.

2. Processing technology

At present, domestic manufacturers mainly adopt the following processing methods:

(1) cold stamping;

(2) hot stamping;

(3) hydraulic forming.

For the tee with larger diameter and thicker wall, the latter two forming methods are usually adopted, and the forming time of these two methods generally needs 1-2 minutes, or even up to 3 minutes.During these few minutes, how to keep the temperature relatively stable is a key problem for the quality of the pipe fittings. For example, if the temperature difference is too large, the steel fiber tissue flow will be unbalanced, especially for the parts with large deformation.Because of its large plastic deformation, such as too low temperature will cause cold hardening phenomenon and brittle failure.

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