Manufacturing process and quality control measures of pressure vessel

Manufacturing process and quality control measures of pressure vessel

In order to improve the manufacturing quality of pressure vessels and meet their actual production and application needs, taking the actual manufacturing process of a pressure vessel as an example, this paper studies the general situation of the pressure vessel manufacturing project, the main manufacturing process of the pressure vessel and the quality control measures of the pressure vessel manufacturing, hoping to provide scientific reference for the manufacturing and quality control of the pressure vessel. 

In manufacturing pressure vessels, relevant units and technicians must clarify the basic needs according to the specific conditions and take reasonable measures to carry out material selection, main component production, assembly, and welding. In this process, relevant units and technicians should formulate quality control measures for specific problems to ensure the manufacturing quality of pressure vessels and fully meet their practical application needs.

1. Project Overview

This study mainly discusses the production and processing of a gas storage tank. The gas storage tank is a type II pressure vessel with a specification of φ1000mm × 2410mm × 10mm, a design temperature of 40 °C, and a design pressure of 1.78 MPa. This study analyzes its manufacturing process and quality control measures to ensure its specific manufacturing quality.

2. Main manufacturing process of pressure vessel

The manufacturing process of the pressure vessel used in this project mainly includes four steps: first, reasonable selection of main materials; second, take reasonable technical measures to make the cylinder; third, take a reasonable technical solution to make the head; fourth, the whole pressure vessel is assembled and welded.

2.1 Main material selection

In the production and manufacturing of pressure vessels, material selection is a basic and key link. Only when the material is appropriate can the subsequent production and processing quality be ensured. Based on this, this study combines the actual design and application requirements of pressure vessels and reasonably chooses its main materials. Considering various factors, it was finally decided to select the supporting support as the container support, and the Q235B low alloy 16MnR steel plate was used as the cylinder material and the cylinder head material.

2.2 Tube section production

Before making the cylinder, the steel plate should be corrected by mechanical correction method, and the oxide skin, rust, and other dirt on the surface of the steel plate should be removed by chemical and mechanical methods. The second is marking and cutting. This project mainly uses CNC-4A CNC cutting machine to cut the cylinder section through the oxyacetylene flame. Therefore, the cutting method is hot. In the blanking process, it is necessary to arrange the plate and blanking according to the actual plate size and complete the drawing of the plate layout in the following way.

• (1) Strictly control the length of the cylinder section as required.
• (2) When blanking, the number of longitudinal splicing welds should be reduced as much as possible for the same cylinder section, and the arc length between adjacent longitudinal seams should be reasonably controlled.
• (3) The welds of the shell section and the shell section, the shell section, and the head cannot be set to the cross form, and the weld must be staggered, and the staggered amount should be more than twice the design thickness of the shell section.
• (4) For the weld between the cylinder body and the cylinder, the cutting should not be set at the edge of the girth weld of the cylinder itself.
• (5) The pressure vessel belongs to the horizontal vessel. Therefore, the girth weld should be set outside the support as far as possible, and the longitudinal weld should be set within a certain range below the cylinder as far as possible.
• (6) The setting of longitudinal weld and circumferential weld on the cylinder should try to avoid the reinforcement ring and hole position.
• (7) Cylinder blanking must be in strict accordance with the actual plate layout, and check whether the product number, quality mark, cylinder section number, part number, weld number, geometric size, plate thickness, and groove direction of each plate are qualified.
• (8) The cylinder’s circumference is all expanded according to the middle diameter, and it needs to be determined according to the actual size of the outer circumference of the head.
• (9) In cutting the cylinder section, the inspection line, the cutting line, and the actual material line must be marked on the plate.
• (10) The cutting allowance, edge machining allowance, and expansion length of the cylinder section are strictly controlled according to the design. The control of the main parameters in the underlined blanking of the pressure vessel is shown in Table 1.

This project mainly uses the symmetrical three-roll plate bending machine to roll the cylinder. In the specific processing process, the following four technical measures are mainly adopted:

• (1) For the flat section of the steel plate that cannot be bent at both ends, it is necessary to bend it by the pre-bending method according to the required curvature in advance. The thickness of the pre-bending plate is set to 20 mm, the radius of curvature is set to 900 mm, and the pre-bending length is set to 240 mm.
• (2) To avoid the occurrence of steel plate torsion and axial misalignment during the rolling process, it is necessary to straighten the steel plate before rolling and keep it in alignment with the rolling machine. The alignment method adopted in this project is the groove line method on the roller. In the alignment process, the midline is first drawn on the steel plate, and then the groove line on the surface of the lower roller is aligned with it to achieve a good alignment effect.
• (3) After the alignment is completed, the roll processing can be carried out. The upper roller must be pressed down during processing to make the steel plate produce corresponding curvature bending. In cold rolling, the deformation rate cannot exceed the allowable limit value of the material. At the same time, the downward pressure of the upper roller should be controlled to prevent slippage, and the downward pressure cannot exceed the power rating of the plate rolling machine. When rolled to the specified curvature, it is also necessary to roll several times under the curvature condition to fully release the internal stress to ensure the uniformity of its deformation and reduce spring back.
• (4) After completing the rounding and assembling the weld, it is necessary to round the cylinder again to make its roundness meet the requirements. In the specific processing process, it should be gradually loaded to the maximum value of the correction curvature. Then the key correction should be carried out in the front weld area. After the measurement is qualified, it should be gradually unloaded until the end of the rolling.

Table.1 Control of main parameters in this pressure vessel underlined blanking

Serial Number	Project	Parameter	Serial Number	Project	Parameter
1	Barrel length/mm	2300	5	Setting range of longitudinal weld seam below the cylinder/(°)	0-170
2	Arc length distance between longitudinal joints of the same cylinder section/mm	2500	6	Barrel cutting allowance/mm	50
3	Misalignment between barrel and barrel welds/mm	2100	7	Edge processing allowance/mm	50
4	Misalignment between cylinder section and head weld seam/mm	2100	8	Expansion length of cylinder section/mm	5800

2.3 Head production 

Before cutting the head, the diameter of the blank should be calculated according to the following formula:

D_p =k(D_g + s) + 2h + 100 (1)

In the formula:

• D_p represents the diameter of the blank itself, mm;
• D_g represents the head diameter, mm;
• k represents the influence coefficient of the shape of the head itself, constant;
• s represents the thickness of the head plate, mm;
• h represents the straight edge of the head, mm.

Because of the large diameter of the blank, this project decided to make the blank by the plate method. In the production process, to prevent the occurrence of cross welds, the pressure vessel head is made into two symmetrical steel plates, and the distance between the butt weld and the head center is controlled within 1/4 of the diameter of the blank. The inner cylinder head is composed of two plates welded together. The size of the outer plate is 2850mm × 1750mm × 8mm, and the size of the inner plate is 2850mm × 1100mm × 8mm.
There are many process methods for head pressing, including explosive forming, stamping forming, and manual hammering forming. This time, the stamping forming process is selected. The pressing of the head belongs to the heating drawing process. The hollow semi-finished products or flat blanks are drawn by drawing die, and finally, the open parts are formed. Before heating and drawing, the head must first heat the plate to improve its toughness, reduce deformation pressure, and provide sufficient convenience for subsequent drawing forming. During the heating and drawing delay, the heating treatment is carried out by the chamber heating furnace, the initial stamping temperature is controlled at 900-1050 °C, and the final stamping temperature is controlled at 700 °C and above. If the temperature during the period is lower than 700 °C, tempering treatment must be carried out after stamping to ensure the heating drawing effect. According to the design drawings, the gas tank head is a standard oval structure. In the specific pressing process, it is necessary to reasonably set the tonnage of the press in strict accordance with the designed calendering force and reasonably select the press according to the designed stamping diameter. Considering various factors, this project decided to use a 500 t tonnage four-column hydraulic press for the calendering of the head. In the process of calendering, considering that there are many unstable factors in the forming process, the accurate shape and deformation of the head are difficult to control, so the trimming allowance of 100 mm width is specially reserved for subsequent processing and trimming. To improve the processing efficiency, the secondary cutting of steel plate and groove processing can be carried out simultaneously. In the machining process, the head is clamped by a rotatable rotary platform to open upward, and the cutting gun is clamped to the side bracket of the platform and aligned with the cutting line. The motor provides a driving force for the rotation of the rotary platform. To ensure the stability of the cutting, in the specific cutting process, the spring is pressed to the head wall by way of the guide roller connecting the cutting nozzle to carry out the cutting treatment.

2.4 Assembly welding

The key points of assembly and welding technology of the pressure vessel and its related parts in this project include the following three items:

• (1) The cylinder section is assembled on the welded roller frame by the horizontal method, and the two cylinder sections are placed on the welded roller frame. The welding gap is reserved strictly according to the design requirements, and positioning welding can be implemented. After the joint assembly is completed, the amount of misalignment should be controlled within 15 % of the thickness of the container wall, and the maximum amount of misalignment should be controlled at 5mm.
• (2) For the cylinder and head, the inner cylinder must be placed on the welding roller frame during the assembly process. The head is lifted to the assembly position by the crane and then welded by positioning welding. The welding process and the cylinder section welding process are the same.
• (3) Other accessories on the pressure vessel should be assembled strictly with the specific design, and then the positioning welding should be carried out by arc welding. To ensure welding efficiency and quality, the construction unit must control its environmental conditions well. In addition to the bad weather conditions such as rain and snow, the construction unit needs to control the welding environment quality according to the parameters in Table 2.

Table.2 Main environmental quality control parameters in pressure vessel assembly and welding

Serial Number	Project	Parameter
1	Relative humidity of welding site environment/%	≤90
2	On site wind speed during manual welding/(m.s-1)	≤10
3	On site wind speed during gas shielded welding/(m.s-1)	≤2
4	Environmental humidity of welding material storage location/%	≤60

3. Pressure vessel manufacturing quality control measures

To further ensure the manufacturing quality of pressure vessels in the specific manufacturing process, relevant units and technicians should implement quality control through reasonable measures, including design and welding quality control, heat treatment quality control, and inspection quality control.

3.1 Design and welding quality control

In processing and manufacturing pressure vessels, relevant units should first control their design and welding quality to achieve good control of their overall quality. The design unit should fully grasp the main characteristics of the pressure vessel and reasonably design the pressure vessel according to its actual application requirements and on-site production and processing conditions to improve its processing efficiency and quality and meet the actual application requirements of pressure vessel. The welding of pressure vessels must be operated by professional and technical personnel with relevant qualifications and sufficient experience. Before welding all materials and equipment need to be strictly inspected before welding, and welding can be carried out only if it meets the requirements. All welding operations must be carried out strictly with the regulations to prevent quality problems caused by poor design and improper welding operations.

3.2 Heat treatment quality control

Heat treatment is a key technical link for the manufacturing process of pressure vessels. Only by reasonably controlling the quality of heat treatment can the quality of pressure vessel components be guaranteed. Based on this, when the components in the pressure vessel are heat treated, the relevant units and technicians must pay enough attention to their quality control, always control the heating temperature within the specified range, and implement heat treatment according to the specific design and operation specifications. For some complex manufacturing parts of pressure vessel workpieces, relevant units can adopt more scientific and advanced modern measures to simplify the process, reduce the complexity of its heat treatment process, and improve its heat treatment effect. For example, the current advanced digital three-dimensional simulation technology can be used to simulate the heat treatment process to judge the feasibility of the simplified post-heat treatment process and lay a solid technical foundation for improving its heat treatment quality.

3.3 Detection quality control

In the manufacturing process of pressure vessels, quality inspection is a key quality assurance measure. Only by controlling the quality of inspection can we find out the quality problems existing in the manufacture of pressure vessels in time so that they can be treated in a targeted manner to ensure the production and manufacturing quality of pressure vessels. In the current pressure vessel manufacturing process, non-destructive testing technology is the most commonly used quality testing technology, including ultrasonic non-destructive testing technology, magnetic particle non-destructive testing technology, ray non-destructive testing technology, etc. In the specific detection process, the relevant units and technical personnel should select the appropriate non-destructive testing technology according to the actual detection requirements and the actual situation of the site, and implement the non-destructive testing in strict accordance with its detection principle and operation specifications, record the specific test results truthfully, and deal with the detection error scientifically, to ensure that the actual manufacturing quality of the pressure vessel is truly fed back, find out its quality problems in time, and make targeted treatment in time to further ensure the pressure.

4. Conclusion 

A pressure vessel is a key piece of equipment in the field of the petrochemical industry. Its manufacturing quality is related to the operation quality and economic benefits of related enterprises and the safety of related fields and engineering projects. It is also closely related to its surrounding environment. In recent years, with the continuous development of petrochemical and other fields, the production and manufacturing of pressure vessels have also begun to receive widespread attention. To meet the practical application requirements in petrochemical and other fields, in the actual production and manufacturing of pressure vessels, relevant units, and technicians must take their actual design and application requirements as the basis, reasonably formulate their processing technology plans, and carry out pressure vessel processing and manufacturing through scientific, reasonable, advanced and targeted technical measures. In this process, relevant units also need to take reasonable measures to implement their manufacturing quality control, rationally optimize the manufacturing process of pressure vessels, further improve their production and processing efficiency and quality, meet the practical application needs of pressure vessels, and promote the coordinated and sustainable development between related fields and social economy and environment. 
Author: Jin Zhiyu