General Process Guidelines for Cold Working of Carbon Steel Equipment
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General processing rules for cold working of carbon steel equipment. 1. General provisions. These processing regulations apply to pressure vessels made of carbon steel. The manufacturing of equipment and products must comply with the requirements specified in the product drawings, technical conditions, and processing documents. 2. Materials 2.1 The quality and specifications of the carbon steel materials used to manufacture pressure vessels (such as sheets, forgings, steel pipes, etc.) must meet the requirements of national standards, industry standards, and relevant technical specifications; the content of the quality certificate must be completed in full. When using foreign materials, they shall comply with the provisions in the \"Safety Technical Inspection Regulations for Fixed Pressure Vessels\". 2.2 Materials with a carbon content greater than 0.25% shall not be used for welding pressure vessels. 2.3 The selection of materials for manufacturing pressure vessels shall comply simultaneously with the \"Regulations on Safety Technical Inspection of Fixed Pressure Vessels\", GB150.2 standards related to materials, and the provisions of the drawings. 2.4 For the material of the main components used in the manufacture of pressure vessels, those that fall under any of the following conditions shall be retested in accordance with the requirements of the raw material standards or the design drawings. 2.4.1 Forge materials of Grade IV for manufacturing Category III containers. 2.4.2 Materials with special requirements specified in the design drawings. 2.4.3 Materials whose material quality certificate lacks required items. 2.4.4 Materials whose properties and chemical composition are questionable. 2.4.5 Items requested by users to be added to the technical agreement. 2.4.6 Overseas materials and their grades. 3. Layout and marking3.1 In accordance with the construction drawings, technical requirements, and material specifications, a cutting layout drawing shall be drawn. 3.2 Marking should be done on a clean and flat steel plate or a smooth surface. 3.3 The characteristics of marking lines and the tools used for marking shall be as specified in the table below:
Characteristics of marking lines | Marking tools
Lines for cutting or drilling positioning | Chalk lines, steel pins, compasses, and punches for positioning
Auxiliary lines and positioning lines | Chalk lines, chalk sticks, straightedges
3.4 For mass-produced items, marking must be done according to a template (the template is made of thin iron sheet; it must pass inspection and be stamped with a “passed inspection” mark, after which it is kept by the workshop team). For single pieces produced without mass production, marking may be done without using a template; however, such pieces must undergo inspection and re-inspection before material cutting can proceed. 3.5 When the drawings and manufacturing procedures do not specify the arrangement of welds, the marking should be done in such a way as to minimize the length of the welds, while paying attention to the following points: 3.5.1 Weld arrangement: (1) For vertical vessels, the welds should be arranged symmetrically on the left and right sides. (2) Horizontal containers should be arranged at an angle of more than 140° from the bottom of the container. (3) Try to avoid making holes around welds. If holes must be made, then all welds within a circle centered at the hole’s center and having a radius equal to one times the hole diameter must pass 100% radiographic (or ultrasonic) testing. (4) Welds must not be covered by supports, etc.; otherwise, they must be covered only after passing 100% radiographic testing and being ground smooth. (5) The length of the outer arc between Class A joints of adjacent tube sections shall be greater than 3 times the thickness δs of the steel material and not less than 100 mm. (6) The outer arc lengths as specified in 3.5.2 among the Class A butt joints of the head, the Class A joints of the embedded nozzles on the head, and the Class A joints of the shell section adjacent to the head must all be greater than three times the steel thickness δs, and shall not be less than 100 mm. Note: The outer arc length refers to the distance along the outer surface of the shell between the centerlines of the joint welds. 3.5.3 In the assembled cylinder, the length of any individual section shall not be less than 300 mm. 3.5.4 Cross welds should not be used. 3.6 The developed length of the cylinder section is calculated based on the mean diameter or is equal to the perimeter of the head. 4. Cutting 4.1 Cutting is generally carried out by shearing, semi-automatic gas cutting, grinding wheel cutting, and manual gas cutting. 4.2 Before being unloaded into the workshop, the materials shall be marked and sorted; they should be stored separately according to the product name, part name, and drawing number. For materials of pressure-bearing components, in addition to the material identification stamp, there must also be a stamp confirming inspection. 4.3 Cutting and grooving of steel plates. 4.3.1 For carbon steel Q345R grade plates, δ; 4.3.2 For alloy steel plates and plates with δ>6mm, leave a 5 mm machining allowance (around the edges) when cutting, and use a edge planer to create bevels. For materials prone to hardening (such as 13MnNiMoR, SA387, etc.), a machining allowance of 10 mm should be left. 4.3.3 The groove surface shall be free from defects such as cracks, delamination, and inclusions. 4.3.4 Burrs and slag remaining after gas cutting shall be removed completely and the surface shall be smoothed by grinding. 4.3.5 The difference in diagonals after cutting should be less than the values shown in the table below: Cutting size – Difference in diagonals: ＜300 ±0.8, ＞300 ±1, ≥1000 ±2. 5. Cylinder manufacturing 5.1 Leveling and correction must be carried out on a smooth surface, and there should be no deep dents left after leveling. 5.2 The R-forming at both ends of the rolled section is pre-bent to the desired radius of curvature using a 160T bending machine and molds (this limitation does not apply when operating on a four-roller rolling mill) ； Operation on large three-roller horizontal displacement roll formers is not subject to this limitation). 5.3 The curling of the cylinder shell on a rolling machine should be carried out slowly, and the number of curling operations should be minimized; the number of curling operations for each section of the same cylinder shell should be roughly the same. 5.3.1 For vessels subjected to internal pressure, the difference between the maximum and minimum inner diameters at the same cross-section of the assembled shell shall not exceed 1% of the inner diameter Di at that cross-section, and shall not be greater than 25 mm. 5.3.2 When the distance between the inspected cross-section and the center of the opening is less than the diameter of the opening, the difference between the maximum and minimum inner diameters at that cross-section shall not exceed the sum of 1% of the inner diameter Di at that cross-section and 2% of the opening diameter; this value shall also not exceed 25 mm. 5.3.3 After the assembly of containers subjected to external pressure and vacuum is completed, the maximum positive and negative deviations measured radially along the outer or inner diameter of the shell using a gauge shall not be greater than the maximum allowable deviation values specified in Figure 10 on page 327 of GB150. When the intersection of Do/δe and L/Do lies above the curve e=1.0δe in Figure 10 or below the curve e=0.2δe, the maximum positive and negative deviations e shall not exceed the values of δe and 0.2δe, respectively. 5.4 Straightness of the cylinder: Unless otherwise specified in the drawings, the allowable deviation △L shall comply with the following table: (or as per the drawings) Shell length H (m) Allowable straightness deviation △L (mm) ≤20 ≤2H/1000 and ≤20 20＜H≤30 ≤H/1000 30＜H≤50 ≤35 50＜H≤70 ≤45 70＜H≤90 ≤55 >90 ≤65 Note: During measurement, horizontal and vertical planes passing through the shell’s centerline are used; that is, thin strings are stretched along four points on the circumference at 0°, 90°, 180°, and 270°. The measured straightness value should be adjusted by subtracting the thickness difference. 5.5 The misalignment amount b of the welds at the joints of cylinders A and B shall comply with the provisions in GB150, as shown in the table below: Nominal thickness at the joint; Misalignment amount b (mm) according to the category of the welded joint. A B ≤12 ≤1/4 δs ≤1/4 δs 12＜δn≤20 ≤3 ≤1/4 δs 20＜δn≤40 ≤3 ≤5 40＜δn≤50 ≤3 ≤1/8 δs ＞50 ≤1/16δs, and ≤10 ≤1/8δs, and ≤20 Note: ① The requirements for misalignment amount of welds in Category A listed in the table above do not apply to the circumferential welds connecting spherical heads to cylinders, nor to the welds where embedded nozzles are joined to cylinders or heads. ② The requirements for the alignment tolerance of Class B welds in the table above apply to the circumferential welds connecting the spherical head to the cylinder. ③ The misalignment b at the joints of composite plates shall not exceed 50% of the thickness of the steel composite plate, and shall not be greater than 2 mm. 5.6 When the thicknesses of the steel plates on both sides are unequal, the thickness of the thinner plate shall not exceed 10 mm, and the difference in thickness between the two plates shall exceed 3 mm. When the thickness of the thin plate is greater than 10 mm, and the difference in thickness between the two plates is more than 30% of the thickness of the thin plate or exceeds 5 mm, the thickness edges shall be thinned on one or both sides as required in the figure below, or the edge of the thin plate shall be welded into an inclined shape using the surfacing method according to the same requirements. 5.7 When the difference in thickness between the two plates is less than the aforementioned value, the values specified in the table under item 5.6 a shall be applied. The misalignment amount b is determined based on the thickness of the thinner plate; when measuring this misalignment amount b, the difference in thickness between the two plates shall not be taken into account. Image L1, L2 ≥ 3(δ1−δ2) 5.8 The corners E formed circumferentially due to welding should be inspected using an internal (or external) template whose chord length is equal to Di/6 and not less than 300 mm. The value of E must not exceed 5 mm. Due to the angular protrusion E formed axially as a result of welding, it is advisable to use a straightedge with a length of no less than 300 mm for inspection. The value of E must not exceed (δs/10 + 2) mm, and must also not be greater than 5 mm. Image 6. Head fabrication
6.1 The dimensions of the head blank are provided by a designated head forming factory, or calculated using the following formulas:
| Name | Illustration | Blank diameter Dp (including machining allowance) |
|------|--------------|---------------------------------------------------|
| Flat-bottomed head | Image | DP = 1.01Dg + r + 2s + 2h |
| Torispherical/elliptical heads | Image | DP = 1.21(Dg + s) + 2h |
| Spherical heads | Image | DP = 1.42(Dg + s) + 2h |
| Spherical heads without flanges | Image | DP = Image + C; where constant C ≥ 4δ and C > 100 mm |
6.2 For heads fabricated by first joining plates and then forming them, the inner surfaces of the joint welds as well as the outer surfaces of such welds that may affect the forming quality must be ground flush with the base material prior to forming. 6.3 The requirements for head forming are as follows: 6.3.1 When the head is assembled from lobes and a top disc, the weld directions should be radial and circumferential. The minimum distance between various non-intersecting weld joints on the head shall be not less than 3 times the nominal thickness δn, and not less than 100 mm. 6.3.2 The actual thickness of the head after stamping shall be not less than the minimum forming thickness specified in the design drawings. 6.3.3 After the head is formed, its shape and dimensional tolerances are checked in accordance with the relevant head standards: including the inclination of the straight edges for oval, disc, and folded-edge conical heads ； Perimeter deviation at the straight edge of the end cap ； Inner diameter deviation at the straight edge of the head ； Roundness tolerance at the straight edge of the head ； Total thickness deviation of the head after trimming ； Shape deviation of the inner surface after head forming ； Deviation in the height of the straight edges of the end cap. The shape and size deviations are within the standard tolerance range. After the head is formed, an appearance inspection must also be carried out. For disc-shaped and flanged conical heads, the radius of curvature at the transition zone shall not be less than the value specified in the drawings. 6.3.4 The dimensional tolerance requirements for the segments used in the stamping of spherical shells or spherical heads shall comply with the provisions of GB12337 \"Steel Spherical Storage Tanks\". 6.4 After the head is stamped into shape, the mark should be re-applied using a stamp, with the same content as that used during material cutting. 6.5 The allowance lines for the head should be based on the straight edges of the head and the friction marks resulting from pressing. If machining is required on the end faces, a margin of >5 mm should be left; for materials with a tendency to harden (such as 13MnNiMoR, SA387, etc.), a machining allowance of 10 mm should be provided. 7. Bending of pipes 7.1 Pipes should generally be bent cold using a pipe bender; before bending, the molds and auxiliary tools must be smooth, free of iron shavings and oil stains. 7.2 For bending thin-walled pipes and pipes with a small bending radius, high-pressure water is used inside the pipe to induce bending; however, it should be noted that the maximum pumping pressure inside the pipe must be less than the yield limit σs of the pipe material (or as specified by the pipe standards). 7.3 For bends that cannot be processed on large-diameter pipe bending machines, the bend can be achieved by filling it with clean and dry sand and then heating it thoroughly; however, the following points should be noted: 7.3.1 The mold should conform to the bending angle specified in the drawings. 7.3.2 The heating length should be estimated correctly; the length = outer diameter of the bend R + 50 as a margin. 7.3.3 The heating temperature should be >850°C, with an orange-yellow color and uniformity. 7.3.4 Use the highest speed during bending to prevent cooling. 7.4 After bending, the pipes shall meet the following requirements: 7.4.1 Deviation of the bending angle: The deviation of the bending angle within the same plane is ±1°; for the angular separation between two bends that are not in the same plane, it is ±1° when the angle is 90°, and ±1.5° otherwise. 7.4.2 The flatness of pipe elbows shall be selected as specified in the following table: Length ≤500: 3; 500-1000: 4; 1000-1500: 5; >1500: 6. The waviness S of the inner surface at the bent portion of the pipe shall not exceed the values given in the table below, and the wavelength t shall be greater than 4S. Outer diameter of tube D1: ＜76, ＞76–108, 133, ≥159–219, ≥273–325, ＞377. Wave height S: ≤2, ≤4, ≤5, ≤6, ≤7, ≤9. When the tube is bent by more than 60 mm, an ellipticity check should be carried out in accordance with the following table: Bending radius R: ＜2.5D1, ≥2.5D1. Ellipticity: 12% (15% for U-shaped tubes), 10%. Calculation formula for Dmax – Dmin: a =————————————— ×100% Dg. When the tube is bent with R＜60 mm, a ball-passing test should be conducted; the diameter of the ball used for this test is specified in the following table: Bending radius R: R≥3.5D1, 2.5D1≤R＜3.5D1, 1.8D1≤R＜2.5D1, R＜1.8D1. Diameter of the ball for testing: D ≥0.9Dg, ＞0.85Dg, ≥0.8Dg, ≥0.75Dg. The reduction in the nominal wall thickness at the bends of the tube should be calculated as follows, with the value not exceeding those specified in the table below: So (wall thickness) – Smin (thinnest part of the wall thickness): b =—————————————————— ×100% So. Bending radius R: ≤2D1, 2D＜R＜3.5D1, R≥3.5D1. Reduction amount b: 20%, 15%, 10%. Note: Dg is the inner diameter of the tube, and D1 is the outer diameter of the tube. 8. Assembly and welding: 8.1 Cross welds should not be used during container assembly. The longitudinal seams between adjacent tube sections, as well as the welds joining the end caps to these sections, should be offset from each other. The distance between the centers of these welds should be greater than three times the nominal thickness of the tube body, and at least 100 mm. 8.2 The assembly gap and misalignment of the butt welds in assembly welding shall not exceed the values specified in the table below. 8.3 After the reinforcement rings are welded together, a gas-tightness test at a pressure of 0.4–0.5 Mpa must be conducted; only if this test is successful can the process proceed to the next stage. 8.4 Welding of welds on pressure vessels shall be carried out by welders who have passed the relevant examinations and possess the appropriate qualifications. 8.5 When welding temporary lifting lugs and tie rods onto pressure vessels, materials identical to those used for the vessel’s shell, or materials with similar mechanical and welding properties, should be employed. Appropriate welding materials and welding procedures must be used as well. The weld scars remaining after the backing plates of the temporary lifting lugs and tie rods are removed must be smoothed out; the thickness of these smoothed areas should not be less than the designed thickness for that area. Surface non-destructive testing should be performed if necessary. 8.6 Forced assembly is not allowed. 8.7 Weld repair shall be carried out in accordance with the \"Safety Technical Inspection Regulations for Fixed Pressure Vessels\" and the relevant repair procedures. 8.8 Pressure testing shall be carried out in accordance with the general guidelines for pressure testing. Assembly gap, groove, and misalignment gauge – Plate thickness, groove type, gap: straight seam gap, curved seam gap; Misalignment: longitudinal seam misalignment, circumferential seam misalignment; Groove edge sharpness; Welding method: 1 – Image; 0+0.3, 0+1 – Manual welding; 2 – 0+0.3, 0+1, 0.2, 0.3 – Manual welding; 3 – 0+0.5, 0+1.5, 0.3, 0.5 – Manual welding; 4 – 0+1, 0+1.5, 0.4, 0.8 – Manual welding; 5–6 – 0+1, 0+2, 0.5, 1 – 70°, 1.5+0.5 – Manual welding; 8–10 – 0+1.5, 0+2, 1, 1.5 – 60°–70°, 1.5+0.5 – Manual welding; 12–16 – 0+2, 0+2.5, 1.5, 2.5 – 60°, 1.5+0.5 – Manual welding; 1 – Image; 0+0.5 – Automatic welding; 2 – 0+0.5 – Automatic welding; 3 – 0+0.5 – Automatic welding; 4 – 0+0.5, 0+0.8, 0.4 – Automatic welding; 5–6 – 0+0.5, 0+1, 0.5 – Automatic welding; 8–10 – 0+0.8, 0+1.2, 1–1.2 – Automatic welding; 12–16 – 0+1, 0+1.5, 1.2–1.5 – Automatic welding