Containers and Storage Equipment: [Weekly Topic] The purposes of various processes in the manufacturing of spherical tanks and potential hazards (2011.05.16-22)
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This post was last edited by salesment on 2011-5-21 20:37. It covers all aspects of the manufacturing and installation process of spherical tanks, including heat treatment, flaw detection, pressure testing, etc. It also discusses the specific purposes of these processes in practice, as well as the potential safety hazards that can arise from operational errors How should practical problems be handled and resolved when they arise?
This post was last edited by SuperFree2316 on May 18, 2011, at 07:52. ] Welding of spherical tanks
The welding work for spherical tanks is extensive and quite challenging. The welds must be performed in various positions, including flat, vertical, overhead, and horizontal positions; thus, the technical requirements are extremely stringent. 3. Preheating before tank welding, post-weld heat treatment, and overall heat treatment (1) Preheating before welding. Preheating refers to heating the workpieces to be welded to a temperature higher than that of the surrounding environment before welding, and then carrying out the welding at this temperature. The material used in spherical tanks is mostly high-strength alloy steel; during welding, due to the cooling and contraction of this material after welding, cold cracks and brittle fractures can easily occur. The purpose of preheating is to prevent cracks from forming in the heat-affected zone of the welded metal, to reduce stress-induced deformation, to avoid a decrease in the plasticity and toughness of that heat-affected zone, and to remove surface moisture. According to the construction specifications, the preheating temperature of spherical tanks is determined based on the material and thickness of the welded components, the degree of restraint at the joints, the welding materials, and weather conditions. During preheating, it is necessary to heat the welding area evenly so that it reaches the temperature specified by the welding process. The preheating area should extend at least 3 times the thickness of the plate on each side of the welding joint’s center, with a minimum range of 100 mm. (2) Post-weld heat treatment. Post-weld heat treatment should be carried out immediately after the spherical tank is welded. The main purpose of post-weld heat treatment is, on one hand, to relieve residual stresses and improve the plasticity and toughness of the weld seam ； More importantly, it is to remove hydrogen ions from the weld and improve the mechanical properties of the welded area. The post-weld hydrogen removal treatment for spherical tanks shall be determined based on the results of welding procedure qualification. The required temperature for post-weld heat treatment should generally be the same as the preheating temperature (200–350°C), and the holding time should be 0.5–1 hour. Welds in the following situations shall undergo post-weld heat treatment to remove hydrogen immediately after welding. 1) High-strength steel with a thickness greater than 32 mm ； 2) Other low-alloy steels with a thickness greater than 38 mm ； 3) Butt welds between forged flanges and spherical shell plates. (3) Overall heat treatment 1) Purpose of overall heat treatment. The purpose of the overall heat treatment of spherical tanks is to eliminate stresses generated during their assembly and welding, stabilize their geometric dimensions, alter the weld metallographic structure, enhance the toughness and stress resistance of the metal, and prevent the occurrence of cracks. At the same time, due to the release of dissolved hydrogen, it prevents the formation of delayed cracks, avoids delayed failure, and enhances fatigue strength and creep strength. At present, in China, overall heat treatment is employed for spherical tanks made of various materials with a wall thickness greater than 34 mm. 2) Methods of overall heat treatment. There are two methods for the overall heat treatment of spherical tanks: the internal combustion method and the electrothermal method. 4. Inspection of spherical tanks
Welding quality inspection is an indispensable and important measure for ensuring the quality of spherical tanks. (1) Weld inspection. (2) Hydrostatic test. The hydrostatic test is conducted to check the strength of the spherical tank and to evaluate the quality of its assembly and welding, in order to ensure that the tank can withstand the design pressure without leaking. Subjecting to water pressure overload can improve the load-bearing capacity of spherical tanks. Although strict inspections are carried out on spherical tanks during manufacturing, assembly and welding, as well as after welding, undetected defects may appear during the hydrostatic test. Therefore, the hydrostatic test is also a relatively important inspection method. (3) Airtightness test. According to the regulations, after passing the hydrostatic test, the spherical tank must undergo magnetic particle testing or penetrant testing as well ； After excluding surface cracks and other defects, conduct the airtightness test. The airtightness test is conducted after all accessories of the spherical tank have been installed, and the pressure gauges, safety valves, and thermometers have been calibrated and found to be satisfactory. The gas used for the airtightness test should be dry, clean air or another inert gas, and the gas temperature must not be lower than 5°C. 3. Construction requirements and conditions 3.1 Conditions that should be met before construction 3.1.1 The construction site should have three connections and one level, that is, roads, water, and electricity, and the working site should be flat and compact. 3.1.2 The spherical tank foundation has been completed and has reached the acceptance standards. 3.1.3 Construction machinery and equipment have entered the site and been approved by the responsible engineer for tooling and equipment. 3.1.4 The main materials of the project have arrived and meet the standards. 3.1.5 The construction materials, tools, small machines, and calibrated measuring instruments have been fully prepared. 3.1.6 The welding process assessment report has been approved. 3.1.7 The design unit has made a design disclosure and conducted a joint review of the drawings. 3.1.8 The construction plan has been approved, and a detailed technical explanation has been made to all construction personnel. 3.1.9 The construction personnel have been organized, and all types of work are working with certificates. 3.1.10 The construction is started with the approval of the relevant units. 4. Spherical tank assembly and welding serial number 1 2 3 4 5 6 items Foundation center circle diameter D1 Foundation orientation adjacent pillar foundation center distance S Spacing between anchor bolt center and foundation center circle Elevation of the upper surface of the pillar foundation Elevation of the foundation elevation Elevation difference of adjacent pillar foundation Allowable deviation ±6.1mm 1° ±2mm ±2mm -6mm 3mm 2mm Flatness of the surface on a single pillar foundation 4.1 Check the dimensions of the spherical tank foundation, and the allowable deviations should comply with the following table: 4.2 The strength of the foundation concrete should meet the design requirements, and complete completion data should be available. The surface of the foundation should be free of exposed reinforcements, cracks, pores, pitted surfaces, mechanical damage and other defects. 4.3 Acceptance of spherical tank parts 4.3.1 Inspect the product quality certificate and other technical quality documents provided by the manufacturing unit according to the specification requirements. 4.3.2 The spherical shell plates must not be spliced, and defects such as cracks, scratches, bubbles, scars, inclusions, etc. are not allowed on the surface. Carry out penetrant or magnetic particle inspection on the surface of the gas-cut groove. The specific number of random inspections for each spherical tank is 3 pieces for the upper and lower pole plates, and 7 pieces for the equatorial plate. Grade I as specified in JB/T4730.5 or JB/T4730.4 is considered qualified. 4.3.3 Carry out ultrasonic testing and random inspection of the spherical shell plates. The specific number of random inspections for each spherical tank is 3 pieces each for the upper and lower pole strips, and 7 pieces for the equatorial strip. The level I specified in JB/T4730.3 is considered qualified. If excessive defects are found, the spot inspection should be doubled. If there are still excessive defects, 100% inspection should be carried out. 4.3.4 The thickness of the spherical shell plate should be randomly inspected. The number of random inspections is 2 pieces each for the upper and lower pole strips, and 4 equatorial strip plates. Each spherical shell plate should be inspected at 9 points, and the thickness measurement result should not be less than 44mm. If there is any unqualification in the spot check, the spot inspection should be doubled. If there are still unqualified, the spherical shell plate should be inspected one by one. 4.3.5 Inspection of the overall dimensions of the spherical shell plate. 4.3.5.1 Inspection of spherical shell plate curvature: Use a sample with a chord length greater than or equal to 2000mm for inspection. The gap between the middle measuring points (≥150mm from the peripheral groove) shall not be greater than 3mm, and the peripheral measuring points shall not be greater than 2mm. 4.3.5.2 The allowable deviation of the geometric dimensions of the spherical shell plate should comply with the provisions of the following table: Chord length in the length direction of the project L1, L2, L3 Chord length in any width direction B1, B2, B3 Diagonal chord length D An example of checking the distance between two diagonals is as follows: Allowable deviation (mm) ±2.5 ±2 ±3 5 4.3.5.3 The allowable deviation of the geometric dimensions of the groove of the spherical shell plate should meet the following requirements: (1) The allowable deviation of the groove angle (α) is ±2°30′. (2) The allowable deviation of the blunt edge of the groove (P) and the groove depth (h) is ±1.5mm. The inspection example is as follows: 4.3.6 Inspect the product test plates and welding process test plates provided by the manufacturing unit. Each spherical tank should not be less than 12 pieces. Its size should be 180mm × 650mm, and its material, steel grade, thickness, and groove type should be the same as the spherical shell plate. 4.3.7 The spherical tank nozzle, flange, manhole, tie rod, elastic joint, etc. should be free of defects such as cracks, mechanical damage, severe corrosion, inconsistent specifications, etc. 4.3.8 The allowable deviation of the full length of the spherical tank support is 3mm, and it should be kept vertical after welding with the bottom plate. The allowable deviation of the verticality is 2mm. The allowable deviation of the straightness of the full length of the pillar is 8mm. 4.4 Spherical tank assembly 4.4.1 Spherical tank assembly procedure: Laying the assembly platform → marking the spherical shell plate, welding the positioning block → upper and lower circumferential seam assembly welding of the pillar → equatorial belt assembly spot welding → lower plate group spot welding → upper plate group spot welding. 4.4.2 The platform of the spherical tank group is laid next to the spherical tank foundation. The length of the platform is not less than 14m, the width is not less than 4m, and the thickness of the platform plate is 20mm. The upper surface of the platform should be level, and the allowable deviation of the level is ±2mm. 4.4.3 Draw the center line of the accepted spherical shell plate, and then weld the positioning block. The welding process is the same as the formal welding. The welding angle height should not be less than 6mm. The welding angle height of the positioning block that also serves as a lifting lug should not be less than 8mm. 4.4.4 Release the center circle of the foundation on the foundation, correct the spacing between the foundations, determine the center position of each pillar, use the center position as the center of the circle, release the actual position circle of each pillar bottom plate, and make an obvious mark. 4.4.5 Lift the two adjacent equatorial strip plates with pillars onto the foundation, and adjust the center position of the pillars and the verticality of the pillars. The height of each set of pad irons should not be less than 25mm, and the inclined pad irons should be used in pairs and in close contact. Fix the bottom plate of the pillars with the foundation, and anchor the drag rope. 4.4.6 When installing the tie rods between the pillars, they should be tightened symmetrically and evenly. The deflection in the middle of the tie rod should not be greater than 2.68cm. 4.4.7 Insert the equatorial belt plate without the pillars in the middle, install the clamps, and then lift the equatorial plate and the insert plate with the pillars in sequence. After the equatorial belt is installed, adjust the geometric dimensions and perform spot welding. 4.4.8 Lift the lower pole plate in order from the extreme side plate to the extreme middle plate. The upper ring seam and the equatorial plate are clamped with clamps, the geometric dimensions are adjusted, and the longitudinal and circumferential seams are spot welded in sequence. 4.4.9 Lift the upper plate into place in sequence from the extreme side plate to the middle plate. The lower ring seam and the equatorial plate are clamped with clamps. The drag rope is anchored, then the geometric dimensions are adjusted, and the longitudinal and circumferential seams are spot welded in sequence. 4.4.10 Mechanical methods are not allowed to be assembled forcefully during the entire assembly process. 4.4.11 Assembly technical requirements 4.4.11.1 After the pillar is installed and aligned, the verticality of the pillar should be checked in both the radial and circumferential directions of the spherical tank. The allowable deviation shall not be greater than 12mm. 4.4.11.2 The gap between the spherical shell plate sets should be 2±2mm; the misalignment of the pairing should not be greater than 2mm. Its value should not be greater than 7mm. 4.4.11.3 Use a sample with a chord length of not less than 1000mm to check the edges and corners of the assembled spherical shell plates. 4.4.11.4 The inspection of assembly clearance, misalignment and edges should be measured every 500mm along the butt joint. 4.4.11.5 After the equatorial belt is assembled, the horizontal error of the equator line of each spherical shell plate should not be greater than 2mm; the horizontal error of the equator line of two adjacent spherical shell plates should not be greater than 3mm; the horizontal error of the equator line of any two spherical shell plates should not be greater than 6mm. 4.4.11.6 After the spherical tank is assembled, the difference between its maximum diameter and minimum diameter should not be greater than 36mm. 4.5 Welding of spherical tank 4.5.1 Welding method 4.5.1.1 The spherical tank is welded using manual arc welding. 4.5.1.2 The welding seam is cleaned using carbon arc gouging and grooving, and the angle grinder is used for grinding and shaping. 4.5.2 Acceptance and storage of welding rods 4.5.2.1 PP.J607RH electric welding rods are used for welding between the spherical shell plates, between the spherical shell plate and the pre-welded plate, and the electrode specifications are φ 3.2 and φ 4.0. φ 5.0 welding rods are not allowed. Between the upper and lower sections of the pillars, between the positioning block and the spherical shell plate, and between the pre-welded plate and the structure, E5015 (J507) electric welding rods are used. 4.5.2.2 The welding rod should have a quality certificate. The appearance of the welding rod should be inspected and there should be no cracks, bubbles, impurities, rust, peeling, mildew and other defects in the welding core. The welding rod clamping end should be clear. 4.5.2.3 Special personnel should be assigned to store, dry and distribute welding materials. The welding materials warehouse should be kept dry, and the relative humidity should not be greater than 60%. The drying temperature of E5015 (J507) and PP.J607RH type welding electrodes is 350~400℃, and the drying time is 1 hour. The dried welding electrodes are stored in a constant temperature oven at 100~150℃, and the coating should not fall off or have obvious cracks. 4.5.2.4 Welders should carry an insulation cylinder that meets product standards. The storage time of the welding rod in the insulation cylinder should not exceed 4 hours. When it exceeds the original drying temperature, it should be re-dried at the original drying temperature. The number of times the electrode is re-dried should not exceed two times. 4.5.3 Material and Structural Features This spherical tank is made of 07MnCrMoVR alloy steel. Due to the large thickness of the spherical shell and many welds, especially when welded after the whole point is fixed, the structural restraint stress is large. In order to avoid cracks, Relevant technical requirements must be strictly followed when welding. 4.5.4 The welder performing welding must hold a valid certificate of qualification issued by the Technical Supervision Bureau. The type of steel welded, the welding method and the welding position are all consistent with the items that I have passed the examination. 4.5.5 Welding process requirements 4.5.5.1 The welding process is carried out strictly in accordance with the parameters and necessary conditions determined by the process assessment. 4.5.5.2 Welding process parameters: Welding position electrode diameter flat welding φ3.2 φ4.0 φ3.2 vertical welding φ4.0 φ3.2 horizontal welding φ4.0 φ3.2 overhead welding φ4.0 (mm) 100~ welding current (A) 140~ 180 13~18 26~30 90~ 110 5~11 24~28 11 130~ 160 8~17 26~30 100~ 120 7~11 22~26 11 140~ 180 11~18 26~30 90~ 110 5~10 22~26 130~ 160 8~15 24~28 120 Welding speed 8~12 (cm/min) Welding voltage (V) Welding level welding line energy 25~29 7(4) 12~25kJ/cm 7(4) 12~35kJ/cm 12~35kJ/cm 12~30kJ/cm 4.5.5.3 Preheating must be carried out before welding. For formal welds, use liquefied gas flame to preheat, spot welds, tool welds, lifting lug welds, weld repairs, and spherical surface repairs. You can use liquefied gas or oxygen-acetylene flames for preheating. The preheating range is no less than 150mm on both sides of the center of the weld. The preheating temperature should be measured symmetrically at 50mm from the center line of the weld. Each weld must be measured at no less than 3 points. Yes. The preheating temperature should be 75~100℃. The interlayer temperature should not be lower than 100℃ but should not be higher than 180℃. 4.5.5.4 When a weld is not completed, or when a weld is completely completed, post-heat treatment should be carried out immediately with liquefied gas. The post-heat temperature is 200~250℃ and the time is 0.5~1h. The post-heat temperature is the same as the measurement. 4.5.4.3 Measurement of strip preheating temperature. 4.5.5.5 Before welding, oil, moisture and other harmful impurities within 50mm on both sides of the groove surface must be removed. 4.5.5.6 The backward arc starting method should be used at the beginning of the weld bead, and the arc crater should be filled at the closing arc end. When changing the welding rod and re-starting the arc, the arc should be re-started as soon as possible before the arc crater of the previous welding rod has cooled down. The joints between layers should be staggered by more than 50mm. 4.5.5.7 The weld groove dimensions are as follows:: 4.5.5.8 Carbon arc gouging After all the external welds of the spherical tank are welded, gouge and root clean the inside. Remove all the deposited metal of the tack welding and the first layer of weld bead. The shape and width of the groove after cleaning should be consistent. Then use a grinder to remove the carburized layer and slag in the gouge, polish until the metallic luster is exposed, and then use the penetration method to inspect. JB/T4730.5 specifies Grade I as qualified. 4.5.5.9 It is strictly prohibited to use welding rods with peeling coating and rusty welding core. 4.5.5.10 When any of the following conditions occurs in the welding environment and there are no effective protective measures, welding is prohibited. Wind speed is greater than 8m/s; relative humidity is greater than 85%; ambient temperature is below -5℃; rainy or snowy days. Note: The temperature and relative humidity of the welding environment should be measured at a distance of 500~1000mm from the surface of the spherical tank. 4.5.6 Precautions for welding operations 4.5.6.1 The DC welding machine used to weld spherical tanks must have good performance and be equipped with DC current and voltmeters to check the welding parameters and the working condition of the welding machine. The current and voltmeters should be certified by the measurement department. 4.5.6.2 Before welding the spherical tank, a canvas should be used to surround it to prevent wind and rain. 4.5.6.3 The welding sequence is as follows: Longitudinal seam of the equatorial belt (outside)—Longitudinal seam of the upper and lower polar plates and flat seam of the middle plate (outer side)—Square annular seam between the polar side plate and the middle plate (outer side)—Large circular seam between the equatorial belt and the upper and lower polar plates (outer side)—Gouging of the inner weld Root cleaning, weld bead grinding - longitudinal seam of the equatorial belt (inside) - longitudinal seam of the upper and lower pole plates and flat seam of the pole middle plate (inside) - square annular seam between the pole plate and pole mid plate (inside) - large annular seam between the equatorial belt and the upper and lower pole plates (inside) 4.5.6.4 Eight welders are selected for each tank welding, and the welding is performed at symmetrical intervals. The welding speed of each welder should be as consistent as possible, and the welding should be performed simultaneously. The segmented de-welding method should be used for welding to reduce welding stress. 4.5.6.5 The weld meat of the longitudinal weld bead should be welded to the center of the circumferential weld bead and ground off before circumferential seam welding. 4.5.6.6 In order to prevent cracks in the spot-fixed weld bead, special attention should be paid when spot-fixing. The spot welding height is 8~10mm, and the weld bead length is not less than 100mm. All T-shaped joints should be continuously welded, the welding length is not less than 150mm, and the spacing should be 250~300mm. 4.5.6.7 For formal welding, positioning welding starts and extinguishes the arc in the groove. The welding arc starting point and arc extinguishing point of the work clamp should be on the weld bead of the work clamp. It is strictly forbidden to arbitrarily start the arc in non-welding positions to avoid the base metal being scratched by the arc. If arc scars or pits are accidentally caused, they must be removed by grinding and penetration inspection. 4.5.6.8 The welding process of tack welding and work clamps and the requirements for welders are the same as those of spherical shell welding. 4.5.6.9 When removing the tool clamp, do not damage the spherical shell plate, and polish the weld bead of the tool clamp smoothly. 4.5.6.10 During welding, the process parameters of each welder must be randomly checked and recorded at any time. The welding line energy should not exceed the upper limit of line energy that has passed the process assessment. Those who do not meet the requirements should be corrected immediately. 4.5.6.11 Product test plate welding requirements: a. The material of the test plate must be qualified and have the same grade, heat batch number and the same heat treatment status as the container; b. The test plate welding operation should be completed by a welder participating in the welding of the same spherical tank; c. One spherical tank should complete three groups of test plates (vertical welding, horizontal welding, flat + overhead welding); d. Use the same welding process as the spherical tank; e. The appearance and non-destructive testing of the weld of the product test plate are the same as the butt weld of the spherical tank; f The product test plate should be heat treated together with the spherical tank; g The welding of the product test plate should be carried out at the same time, at the same location and with the same environmental requirements as the welding of the main body of the spherical tank. 4.5.7 Post-weld geometric dimension inspection: 4.5.7.1 After welding, use a sample with a chord length of not less than 1000mm to check the edges and corners of the spherical shell weld, and the value should not be greater than 7mm. 4.5.7.2 The difference between the inner diameter between the two poles, the maximum inner diameter and the minimum inner diameter of the equatorial section shall not be greater than 80mm. 4.5.7.3 The difference between the inner diameter between the two poles, the maximum inner diameter and the minimum inner diameter of the equatorial section and the design inner diameter shall not be greater than 80mm. 4.5.7.4 The verticality of the pillars is measured in both radial and circumferential directions, and its value shall not be greater than 12mm. 4.6 Weld inspection and repair: 4.6.1 Appearance inspection of welds: 4.6.1.1 Before the appearance inspection of the weld, the slag skin, spatter, etc. should be cleaned up. 4.6.1.2 The surface of the weld and heat-affected zone shall be free of defects such as cracks, pores, undercuts, slag inclusions, pits, and incomplete welding. 4.6.1.3 The width of the weld should be increased by 1~2 mm on each side of the groove. The entire weld surface after welding must be ground with a grinder to eliminate weld waves. The weld reinforcement on the outer surface of the spherical shell should be 0~1mm. The weld reinforcement on the inner surface of the spherical shell should be 0~0.5mm. 4.6.2 Non-destructive testing of welds 4.6.2.1 Personnel engaged in non-destructive testing of spherical tanks must hold a technical grade appraisal certificate for non-destructive testing personnel of boilers and pressure vessels issued by the Bureau of Technical Supervision. Only those who have obtained a certificate of level II or above can fill in and issue an inspection report. 4.6.2.2 Non-destructive testing of the weld can only be carried out 36 hours after the welding is completed. 4.6.2.3 The butt welds of spherical tanks should undergo 100% radiographic inspection. The inspection is carried out in accordance with JB/T4730.2. Level II is qualified. The re-inspection parts should include all circumferential seams, longitudinal seams within 700mm from the circumferential seams (including 20% ultrasonic re-inspection, including all T-shaped and Y-shaped welds) and parts with defects and doubts in radiographic inspection. The re-inspection shall be carried out in accordance with JB/T4730.3. Level I is qualified. 4.6.2.4 The internal and external surfaces of the butt weld of the spherical tank, the workpiece weld marks and their heat-affected zones, the repair welds, the defective grinding areas, and the fillet welds on the pillars and lower sections should be subjected to 100% magnetic particle or penetrant inspection. The inspection is carried out in accordance with JB/T4730.4 or JB/T4730.5, and grade I is qualified. After the overall heat treatment and the hydrostatic test, 100% magnetic particle or penetrant re-inspection should be carried out on the above-mentioned surface flaw detection parts. 4.6.2.5 The butt welds of spherical tanks should be subjected to 100% penetrant inspection after root cleaning. Level I specified in JB/T4730.5 is considered qualified. 4.6.2.6 If there are any impermissible defects in the welds that have been subjected to non-destructive testing, welding should be repaired after the defects are removed, and the part should be re-inspected according to the original flaw detection method until it is qualified. 4.6.3 Repair of spherical tanks 4.6.3.1 The surface defects of the spherical shell and the weld marks of the tool fixtures are ground with a grinder. The slope within the grinding range is at least 3:1. The maximum depth after grinding shall not be greater than 2mm. When it exceeds, welding repairs should be carried out. When the welding repair depth on the surface of the spherical shell plate exceeds 3mm, ultrasonic testing should be carried out. 4.6.3.2 When repairing welding surface defects, the length of the repair welding should be greater than 50mm. After repairing welding, the excess metal should be ground off to make a smooth transition with the main weld. 4.6.3.3 When welding and repairing surface defects of the spherical shell plate, the repair area of each place should be within 50cm2. When there are two or more repairs, the edge distance between any two places should be greater than 50mm. 4.6.3.4 The undercut and weld toe cracks on both sides of the weld must be removed with a grinding wheel and ground into a slope with a slope of 1:3 or less. The removal depth shall not exceed 0.5mm. If it exceeds, repair welding shall be performed. 4.6.3.5 The internal defects of the welds shall be carried out in accordance with the requirements of the company's QB/HEJ.GZ0102-03-1999 "Welded Joint Rework Management System". When the defects are repaired, the depth of the defects shall not exceed 2/3 of the thickness of the spherical shell plate. If defects remain at 2/3 of the thickness of the spherical shell plate, repair welding shall be carried out in this state, and then the defects shall be removed again on the back side for repair welding. The length of repair welding shall not be less than 50mm. 4.6.3.6 The number of welding repairs on the same part of the weld should not exceed two times. If the number of welding repairs exceeds two times, reliable technical measures should be proposed, and repairs can only be made after approval by the chief technical person in charge of the construction unit. 4.6.3.7 The repair process is the same as the formal welding process. The preheating temperature is 100°C, and post-heat treatment must be carried out immediately after welding. 5. Schematic diagram of the spherical tank weld layout (see attachment) 6. Preparation and testing of product welding test plates. The samples are prepared and tested according to GB150-1998 and JB4744-2000. The test results should meet the requirements specified in the standard. 7. The general layout of the construction site includes a tool room and a welding machine shed of 35t. Crane semi-finished product stacking area Oxygen-101 Semi-finished product stacking area Oxygen-102 Semi-finished product stacking area Oxygen-103 Grouping platform Semi-finished product stacking area 8. Labor organization team Electricity engineer 1 person 2 people 15 Engineers Lifter 1 person 4 hostages Security guards Welder 1 person 14 riveters 8 people 2 fitters 9. Materials used for machine tools and means 9.1 Main means used Tarpaulin scaffolding 2000M2 20t wooden frame board 30 pieces 600 pieces inverted chain steel plate 5t δ=20 3 sets, 3t 64 M2 2 sets 9.2 Main machine tools and equipment name model specification 18 sets Main characteristic parameters Rated current 500A ~ 450A, input current 108A, input capacity 41KVA Maximum temperature 500℃, holding time 0~20h Can hold 100Kg welding rod, rated power 6.4KW, maximum temperature 500℃, holding time 0~20h, can hold 60Kg welding rod, rated power 4.2KW, dehumidification capacity 3Kg/h, power 1.2KW, refrigerant R22, charging capacity 1.1kg AC and DC dual-purpose arc welding machine automatic far-infrared welding rod drying box automatic far-infrared electric welding rod drying box dehumidifier axial flow fan low voltage transformer grinding wheel cutting machine pipe table pump drill ZXEI-500 ZYHC-100 ZYH-160 CFZ3D DN500 36V J3GB-400 4〃 ZT4016-W 2 units 1 unit power 2.2KW Grinding wheel diameter 400mm, cutting line speed 60m/s, clamp rotation angle 0~40, maximum drilling diameter per unit 16mm 10. Construction quality control 10.1 Strictly follow the project management documents of Sinochem Second Construction Group. Specific requirements are as follows: 10.1.1 Establish a corresponding quality inspection agency: The project department establishes a quality inspection agency and works in correspondence with the quality inspection department of the Tiantie Company Engineering Construction Headquarters. Each team leader is a part-time quality inspector and is responsible for the quality inspection work of the team. Quality inspectors at all levels must be stationed at the construction site before the start of their profession. 10.1.2 Technical personnel should provide detailed technical explanations to the construction workers before construction. 10.1.3 All types of materials used in construction must have quality certificates. All types of materials entering the warehouse should be classified and properly kept. 10.1.4 Measurement and inspection measuring tools used in the project must be approved by China * * It must be calibrated by a recognized metrology department and must be accurate and have a calibration certificate before it can be used. 10.1.5 Concealed projects during construction must be inspected and confirmed by the quality inspection department of the construction unit before being concealed, and the notification should be delivered 24 hours in advance. 10.1.6 The construction progress should be synchronized with construction records and other information. Construction data should be complete, and visa procedures at all levels must be complete. 10.1.7 Process control points implement three-level quality management A, B, and C. A Level B is jointly inspected by the construction unit, the supervision unit and the construction unit, level B is jointly inspected by the supervision unit and the construction unit, and level C is controlled by the construction unit. 10.1.8 Construction must be carried out strictly in accordance with the design requirements. If there is no design change, no one is allowed to change the design. 10.2 Strictly implement the rules and regulations in the company's pressure vessel quality assurance manual and pressure vessel management system. 10.3 Conscientiously implement ISO9001, international standard quality system program documents, so that everything has its roots and every step is witnessed. 11. Technical measures for construction safety and environmental management 11.1 Strictly implement the "Site Safety Management Regulations" in the project management document of Sinochem Second Construction Group Company. The specific requirements are as follows: 11.1.1 Adhere to weekly safety meetings and daily on-site inspections. 11.1.2 All construction site workers should wear safety helmets and safety shoes, and be equipped with personal protection facilities and equipment according to the requirements of the type of work. 11.1.3 It is strictly prohibited to start a fire in warehouses, oil depots, flammable areas and other major fire prevention areas, and be equipped with appropriate fire prevention equipment. 11.1.4 Smoking is only allowed in designated areas under controlled conditions, and smoking is strictly prohibited in vehicles on site. 11.1.5 Fire extinguishers should be placed in places where the use of fire extinguishers is allowed, and all personnel should be trained to use fire extinguishers. 11.1.6 All personnel working in the air above 2 meters must wear safety belts. During unprotected construction on the outer frame, a safety net must be set up under high-altitude working points. 11.1.7 All on-site electricity installation and repair work must be performed by personnel with professional electrician certificates. All electrical equipment should be grounded and leakage protectors should be used. 11.2 Strictly implement the company's "Occupational Health and Safety Management Manual" and "Occupational Health and Safety Management System Procedure Documents". The specific requirements are as follows: 11.2.1 Carefully analyze the actual situation and operation process on site, and identify the hazard sources clearly and be aware of them. 11.2.2 Before lifting heavy objects, carefully check the rigging, clamps, etc., and make sure before lifting. When lifting, it is strictly forbidden to stand under the crane arm. 11.2.3 During preheating and post-heat treatment of welds, care should be taken to prevent burns, and the sphere should be kept fully grounded. 11.2.4 The lighting voltage inside the tank should not be greater than 36V, handheld power tools should be well insulated, and the sockets should be placed in the box. 11.2.5 When welding in the tank, use an axial flow fan to remove the smoke, and assign a dedicated person to conduct safety supervision to ensure good ventilation in the tank. 11.2.6 Outdoor electrical facilities should have rainproof measures to prevent electrical appliances from getting damp and causing electric shock accidents. 11.2.7 Provide sufficient fire-fighting equipment on site to prevent flammable materials from catching fire. 11.3 Conscientiously implement ISO14001 "Environmental Management System Document" and understand the connotation of the management policy of "obeying regulations, civilized construction, cherishing resources, and protecting the environment". 11.3.1 All materials, equipment and machinery should be neatly stored in designated areas. 11.3.2 All scraps, waste pens, waste wires, tarpaulins, waste welding rod heads, etc. should be collected and stored once a day. A clean-up of the entire site involving the participation of all personnel should be organized once a week.
The welding work for spherical tanks is extensive and quite challenging. The welds must be performed in various positions, including flat, vertical, overhead, and horizontal positions; thus, the technical requirements are extremely stringent. 3. Preheating before tank welding, post-weld heat treatment, and overall heat treatment (1) Preheating before welding. Preheating refers to heating the workpieces to be welded to a temperature higher than that of the surrounding environment before welding, and then carrying out the welding at this temperature. The material used in spherical tanks is mostly high-strength alloy steel; during welding, due to the cooling and contraction of this material after welding, cold cracks and brittle fractures can easily occur. The purpose of preheating is to prevent cracks from forming in the heat-affected zone of the welded metal, to reduce stress-induced deformation, to avoid a decrease in the plasticity and toughness of that heat-affected zone, and to remove surface moisture. According to the construction specifications, the preheating temperature of spherical tanks is determined based on the material and thickness of the welded components, the degree of restraint at the joints, the welding materials, and weather conditions. During preheating, it is necessary to heat the welding area evenly so that it reaches the temperature specified by the welding process. The preheating area should extend at least 3 times the thickness of the plate on each side of the welding joint’s center, with a minimum range of 100 mm. (2) Post-weld heat treatment. Post-weld heat treatment should be carried out immediately after the spherical tank is welded. The main purpose of post-weld heat treatment is, on one hand, to relieve residual stresses and improve the plasticity and toughness of the weld seam ； More importantly, it is to remove hydrogen ions from the weld and improve the mechanical properties of the welded area. The post-weld hydrogen removal treatment for spherical tanks shall be determined based on the results of welding procedure qualification. The required temperature for post-weld heat treatment should generally be the same as the preheating temperature (200–350°C), and the holding time should be 0.5–1 hour. Welds in the following situations shall undergo post-weld heat treatment to remove hydrogen immediately after welding. 1) High-strength steel with a thickness greater than 32 mm ； 2) Other low-alloy steels with a thickness greater than 38 mm ； 3) Butt welds between forged flanges and spherical shell plates. (3) Overall heat treatment 1) Purpose of overall heat treatment. The purpose of the overall heat treatment of spherical tanks is to eliminate stresses generated during their assembly and welding, stabilize their geometric dimensions, alter the weld metallographic structure, enhance the toughness and stress resistance of the metal, and prevent the occurrence of cracks. At the same time, due to the release of dissolved hydrogen, it prevents the formation of delayed cracks, avoids delayed failure, and enhances fatigue strength and creep strength. At present, in China, overall heat treatment is employed for spherical tanks made of various materials with a wall thickness greater than 34 mm. 2) Methods of overall heat treatment. There are two methods for the overall heat treatment of spherical tanks: the internal combustion method and the electrothermal method. 4. Inspection of spherical tanks
Welding quality inspection is an indispensable and important measure for ensuring the quality of spherical tanks. (1) Weld inspection. (2) Hydrostatic test. The hydrostatic test is conducted to check the strength of the spherical tank and to evaluate the quality of its assembly and welding, in order to ensure that the tank can withstand the design pressure without leaking. Subjecting to water pressure overload can improve the load-bearing capacity of spherical tanks. Although strict inspections are carried out on spherical tanks during manufacturing, assembly and welding, as well as after welding, undetected defects may appear during the hydrostatic test. Therefore, the hydrostatic test is also a relatively important inspection method. (3) Airtightness test. According to the regulations, after passing the hydrostatic test, the spherical tank must undergo magnetic particle testing or penetrant testing as well ； After excluding surface cracks and other defects, conduct the airtightness test. The airtightness test is conducted after all accessories of the spherical tank have been installed, and the pressure gauges, safety valves, and thermometers have been calibrated and found to be satisfactory. The gas used for the airtightness test should be dry, clean air or another inert gas, and the gas temperature must not be lower than 5°C. 3. Construction requirements and conditions 3.1 Conditions that should be met before construction 3.1.1 The construction site should have three connections and one level, that is, roads, water, and electricity, and the working site should be flat and compact. 3.1.2 The spherical tank foundation has been completed and has reached the acceptance standards. 3.1.3 Construction machinery and equipment have entered the site and been approved by the responsible engineer for tooling and equipment. 3.1.4 The main materials of the project have arrived and meet the standards. 3.1.5 The construction materials, tools, small machines, and calibrated measuring instruments have been fully prepared. 3.1.6 The welding process assessment report has been approved. 3.1.7 The design unit has made a design disclosure and conducted a joint review of the drawings. 3.1.8 The construction plan has been approved, and a detailed technical explanation has been made to all construction personnel. 3.1.9 The construction personnel have been organized, and all types of work are working with certificates. 3.1.10 The construction is started with the approval of the relevant units. 4. Spherical tank assembly and welding serial number 1 2 3 4 5 6 items Foundation center circle diameter D1 Foundation orientation adjacent pillar foundation center distance S Spacing between anchor bolt center and foundation center circle Elevation of the upper surface of the pillar foundation Elevation of the foundation elevation Elevation difference of adjacent pillar foundation Allowable deviation ±6.1mm 1° ±2mm ±2mm -6mm 3mm 2mm Flatness of the surface on a single pillar foundation 4.1 Check the dimensions of the spherical tank foundation, and the allowable deviations should comply with the following table: 4.2 The strength of the foundation concrete should meet the design requirements, and complete completion data should be available. The surface of the foundation should be free of exposed reinforcements, cracks, pores, pitted surfaces, mechanical damage and other defects. 4.3 Acceptance of spherical tank parts 4.3.1 Inspect the product quality certificate and other technical quality documents provided by the manufacturing unit according to the specification requirements. 4.3.2 The spherical shell plates must not be spliced, and defects such as cracks, scratches, bubbles, scars, inclusions, etc. are not allowed on the surface. Carry out penetrant or magnetic particle inspection on the surface of the gas-cut groove. The specific number of random inspections for each spherical tank is 3 pieces for the upper and lower pole plates, and 7 pieces for the equatorial plate. Grade I as specified in JB/T4730.5 or JB/T4730.4 is considered qualified. 4.3.3 Carry out ultrasonic testing and random inspection of the spherical shell plates. The specific number of random inspections for each spherical tank is 3 pieces each for the upper and lower pole strips, and 7 pieces for the equatorial strip. The level I specified in JB/T4730.3 is considered qualified. If excessive defects are found, the spot inspection should be doubled. If there are still excessive defects, 100% inspection should be carried out. 4.3.4 The thickness of the spherical shell plate should be randomly inspected. The number of random inspections is 2 pieces each for the upper and lower pole strips, and 4 equatorial strip plates. Each spherical shell plate should be inspected at 9 points, and the thickness measurement result should not be less than 44mm. If there is any unqualification in the spot check, the spot inspection should be doubled. If there are still unqualified, the spherical shell plate should be inspected one by one. 4.3.5 Inspection of the overall dimensions of the spherical shell plate. 4.3.5.1 Inspection of spherical shell plate curvature: Use a sample with a chord length greater than or equal to 2000mm for inspection. The gap between the middle measuring points (≥150mm from the peripheral groove) shall not be greater than 3mm, and the peripheral measuring points shall not be greater than 2mm. 4.3.5.2 The allowable deviation of the geometric dimensions of the spherical shell plate should comply with the provisions of the following table: Chord length in the length direction of the project L1, L2, L3 Chord length in any width direction B1, B2, B3 Diagonal chord length D An example of checking the distance between two diagonals is as follows: Allowable deviation (mm) ±2.5 ±2 ±3 5 4.3.5.3 The allowable deviation of the geometric dimensions of the groove of the spherical shell plate should meet the following requirements: (1) The allowable deviation of the groove angle (α) is ±2°30′. (2) The allowable deviation of the blunt edge of the groove (P) and the groove depth (h) is ±1.5mm. The inspection example is as follows: 4.3.6 Inspect the product test plates and welding process test plates provided by the manufacturing unit. Each spherical tank should not be less than 12 pieces. Its size should be 180mm × 650mm, and its material, steel grade, thickness, and groove type should be the same as the spherical shell plate. 4.3.7 The spherical tank nozzle, flange, manhole, tie rod, elastic joint, etc. should be free of defects such as cracks, mechanical damage, severe corrosion, inconsistent specifications, etc. 4.3.8 The allowable deviation of the full length of the spherical tank support is 3mm, and it should be kept vertical after welding with the bottom plate. The allowable deviation of the verticality is 2mm. The allowable deviation of the straightness of the full length of the pillar is 8mm. 4.4 Spherical tank assembly 4.4.1 Spherical tank assembly procedure: Laying the assembly platform → marking the spherical shell plate, welding the positioning block → upper and lower circumferential seam assembly welding of the pillar → equatorial belt assembly spot welding → lower plate group spot welding → upper plate group spot welding. 4.4.2 The platform of the spherical tank group is laid next to the spherical tank foundation. The length of the platform is not less than 14m, the width is not less than 4m, and the thickness of the platform plate is 20mm. The upper surface of the platform should be level, and the allowable deviation of the level is ±2mm. 4.4.3 Draw the center line of the accepted spherical shell plate, and then weld the positioning block. The welding process is the same as the formal welding. The welding angle height should not be less than 6mm. The welding angle height of the positioning block that also serves as a lifting lug should not be less than 8mm. 4.4.4 Release the center circle of the foundation on the foundation, correct the spacing between the foundations, determine the center position of each pillar, use the center position as the center of the circle, release the actual position circle of each pillar bottom plate, and make an obvious mark. 4.4.5 Lift the two adjacent equatorial strip plates with pillars onto the foundation, and adjust the center position of the pillars and the verticality of the pillars. The height of each set of pad irons should not be less than 25mm, and the inclined pad irons should be used in pairs and in close contact. Fix the bottom plate of the pillars with the foundation, and anchor the drag rope. 4.4.6 When installing the tie rods between the pillars, they should be tightened symmetrically and evenly. The deflection in the middle of the tie rod should not be greater than 2.68cm. 4.4.7 Insert the equatorial belt plate without the pillars in the middle, install the clamps, and then lift the equatorial plate and the insert plate with the pillars in sequence. After the equatorial belt is installed, adjust the geometric dimensions and perform spot welding. 4.4.8 Lift the lower pole plate in order from the extreme side plate to the extreme middle plate. The upper ring seam and the equatorial plate are clamped with clamps, the geometric dimensions are adjusted, and the longitudinal and circumferential seams are spot welded in sequence. 4.4.9 Lift the upper plate into place in sequence from the extreme side plate to the middle plate. The lower ring seam and the equatorial plate are clamped with clamps. The drag rope is anchored, then the geometric dimensions are adjusted, and the longitudinal and circumferential seams are spot welded in sequence. 4.4.10 Mechanical methods are not allowed to be assembled forcefully during the entire assembly process. 4.4.11 Assembly technical requirements 4.4.11.1 After the pillar is installed and aligned, the verticality of the pillar should be checked in both the radial and circumferential directions of the spherical tank. The allowable deviation shall not be greater than 12mm. 4.4.11.2 The gap between the spherical shell plate sets should be 2±2mm; the misalignment of the pairing should not be greater than 2mm. Its value should not be greater than 7mm. 4.4.11.3 Use a sample with a chord length of not less than 1000mm to check the edges and corners of the assembled spherical shell plates. 4.4.11.4 The inspection of assembly clearance, misalignment and edges should be measured every 500mm along the butt joint. 4.4.11.5 After the equatorial belt is assembled, the horizontal error of the equator line of each spherical shell plate should not be greater than 2mm; the horizontal error of the equator line of two adjacent spherical shell plates should not be greater than 3mm; the horizontal error of the equator line of any two spherical shell plates should not be greater than 6mm. 4.4.11.6 After the spherical tank is assembled, the difference between its maximum diameter and minimum diameter should not be greater than 36mm. 4.5 Welding of spherical tank 4.5.1 Welding method 4.5.1.1 The spherical tank is welded using manual arc welding. 4.5.1.2 The welding seam is cleaned using carbon arc gouging and grooving, and the angle grinder is used for grinding and shaping. 4.5.2 Acceptance and storage of welding rods 4.5.2.1 PP.J607RH electric welding rods are used for welding between the spherical shell plates, between the spherical shell plate and the pre-welded plate, and the electrode specifications are φ 3.2 and φ 4.0. φ 5.0 welding rods are not allowed. Between the upper and lower sections of the pillars, between the positioning block and the spherical shell plate, and between the pre-welded plate and the structure, E5015 (J507) electric welding rods are used. 4.5.2.2 The welding rod should have a quality certificate. The appearance of the welding rod should be inspected and there should be no cracks, bubbles, impurities, rust, peeling, mildew and other defects in the welding core. The welding rod clamping end should be clear. 4.5.2.3 Special personnel should be assigned to store, dry and distribute welding materials. The welding materials warehouse should be kept dry, and the relative humidity should not be greater than 60%. The drying temperature of E5015 (J507) and PP.J607RH type welding electrodes is 350~400℃, and the drying time is 1 hour. The dried welding electrodes are stored in a constant temperature oven at 100~150℃, and the coating should not fall off or have obvious cracks. 4.5.2.4 Welders should carry an insulation cylinder that meets product standards. The storage time of the welding rod in the insulation cylinder should not exceed 4 hours. When it exceeds the original drying temperature, it should be re-dried at the original drying temperature. The number of times the electrode is re-dried should not exceed two times. 4.5.3 Material and Structural Features This spherical tank is made of 07MnCrMoVR alloy steel. Due to the large thickness of the spherical shell and many welds, especially when welded after the whole point is fixed, the structural restraint stress is large. In order to avoid cracks, Relevant technical requirements must be strictly followed when welding. 4.5.4 The welder performing welding must hold a valid certificate of qualification issued by the Technical Supervision Bureau. The type of steel welded, the welding method and the welding position are all consistent with the items that I have passed the examination. 4.5.5 Welding process requirements 4.5.5.1 The welding process is carried out strictly in accordance with the parameters and necessary conditions determined by the process assessment. 4.5.5.2 Welding process parameters: Welding position electrode diameter flat welding φ3.2 φ4.0 φ3.2 vertical welding φ4.0 φ3.2 horizontal welding φ4.0 φ3.2 overhead welding φ4.0 (mm) 100~ welding current (A) 140~ 180 13~18 26~30 90~ 110 5~11 24~28 11 130~ 160 8~17 26~30 100~ 120 7~11 22~26 11 140~ 180 11~18 26~30 90~ 110 5~10 22~26 130~ 160 8~15 24~28 120 Welding speed 8~12 (cm/min) Welding voltage (V) Welding level welding line energy 25~29 7(4) 12~25kJ/cm 7(4) 12~35kJ/cm 12~35kJ/cm 12~30kJ/cm 4.5.5.3 Preheating must be carried out before welding. For formal welds, use liquefied gas flame to preheat, spot welds, tool welds, lifting lug welds, weld repairs, and spherical surface repairs. You can use liquefied gas or oxygen-acetylene flames for preheating. The preheating range is no less than 150mm on both sides of the center of the weld. The preheating temperature should be measured symmetrically at 50mm from the center line of the weld. Each weld must be measured at no less than 3 points. Yes. The preheating temperature should be 75~100℃. The interlayer temperature should not be lower than 100℃ but should not be higher than 180℃. 4.5.5.4 When a weld is not completed, or when a weld is completely completed, post-heat treatment should be carried out immediately with liquefied gas. The post-heat temperature is 200~250℃ and the time is 0.5~1h. The post-heat temperature is the same as the measurement. 4.5.4.3 Measurement of strip preheating temperature. 4.5.5.5 Before welding, oil, moisture and other harmful impurities within 50mm on both sides of the groove surface must be removed. 4.5.5.6 The backward arc starting method should be used at the beginning of the weld bead, and the arc crater should be filled at the closing arc end. When changing the welding rod and re-starting the arc, the arc should be re-started as soon as possible before the arc crater of the previous welding rod has cooled down. The joints between layers should be staggered by more than 50mm. 4.5.5.7 The weld groove dimensions are as follows:: 4.5.5.8 Carbon arc gouging After all the external welds of the spherical tank are welded, gouge and root clean the inside. Remove all the deposited metal of the tack welding and the first layer of weld bead. The shape and width of the groove after cleaning should be consistent. Then use a grinder to remove the carburized layer and slag in the gouge, polish until the metallic luster is exposed, and then use the penetration method to inspect. JB/T4730.5 specifies Grade I as qualified. 4.5.5.9 It is strictly prohibited to use welding rods with peeling coating and rusty welding core. 4.5.5.10 When any of the following conditions occurs in the welding environment and there are no effective protective measures, welding is prohibited. Wind speed is greater than 8m/s; relative humidity is greater than 85%; ambient temperature is below -5℃; rainy or snowy days. Note: The temperature and relative humidity of the welding environment should be measured at a distance of 500~1000mm from the surface of the spherical tank. 4.5.6 Precautions for welding operations 4.5.6.1 The DC welding machine used to weld spherical tanks must have good performance and be equipped with DC current and voltmeters to check the welding parameters and the working condition of the welding machine. The current and voltmeters should be certified by the measurement department. 4.5.6.2 Before welding the spherical tank, a canvas should be used to surround it to prevent wind and rain. 4.5.6.3 The welding sequence is as follows: Longitudinal seam of the equatorial belt (outside)—Longitudinal seam of the upper and lower polar plates and flat seam of the middle plate (outer side)—Square annular seam between the polar side plate and the middle plate (outer side)—Large circular seam between the equatorial belt and the upper and lower polar plates (outer side)—Gouging of the inner weld Root cleaning, weld bead grinding - longitudinal seam of the equatorial belt (inside) - longitudinal seam of the upper and lower pole plates and flat seam of the pole middle plate (inside) - square annular seam between the pole plate and pole mid plate (inside) - large annular seam between the equatorial belt and the upper and lower pole plates (inside) 4.5.6.4 Eight welders are selected for each tank welding, and the welding is performed at symmetrical intervals. The welding speed of each welder should be as consistent as possible, and the welding should be performed simultaneously. The segmented de-welding method should be used for welding to reduce welding stress. 4.5.6.5 The weld meat of the longitudinal weld bead should be welded to the center of the circumferential weld bead and ground off before circumferential seam welding. 4.5.6.6 In order to prevent cracks in the spot-fixed weld bead, special attention should be paid when spot-fixing. The spot welding height is 8~10mm, and the weld bead length is not less than 100mm. All T-shaped joints should be continuously welded, the welding length is not less than 150mm, and the spacing should be 250~300mm. 4.5.6.7 For formal welding, positioning welding starts and extinguishes the arc in the groove. The welding arc starting point and arc extinguishing point of the work clamp should be on the weld bead of the work clamp. It is strictly forbidden to arbitrarily start the arc in non-welding positions to avoid the base metal being scratched by the arc. If arc scars or pits are accidentally caused, they must be removed by grinding and penetration inspection. 4.5.6.8 The welding process of tack welding and work clamps and the requirements for welders are the same as those of spherical shell welding. 4.5.6.9 When removing the tool clamp, do not damage the spherical shell plate, and polish the weld bead of the tool clamp smoothly. 4.5.6.10 During welding, the process parameters of each welder must be randomly checked and recorded at any time. The welding line energy should not exceed the upper limit of line energy that has passed the process assessment. Those who do not meet the requirements should be corrected immediately. 4.5.6.11 Product test plate welding requirements: a. The material of the test plate must be qualified and have the same grade, heat batch number and the same heat treatment status as the container; b. The test plate welding operation should be completed by a welder participating in the welding of the same spherical tank; c. One spherical tank should complete three groups of test plates (vertical welding, horizontal welding, flat + overhead welding); d. Use the same welding process as the spherical tank; e. The appearance and non-destructive testing of the weld of the product test plate are the same as the butt weld of the spherical tank; f The product test plate should be heat treated together with the spherical tank; g The welding of the product test plate should be carried out at the same time, at the same location and with the same environmental requirements as the welding of the main body of the spherical tank. 4.5.7 Post-weld geometric dimension inspection: 4.5.7.1 After welding, use a sample with a chord length of not less than 1000mm to check the edges and corners of the spherical shell weld, and the value should not be greater than 7mm. 4.5.7.2 The difference between the inner diameter between the two poles, the maximum inner diameter and the minimum inner diameter of the equatorial section shall not be greater than 80mm. 4.5.7.3 The difference between the inner diameter between the two poles, the maximum inner diameter and the minimum inner diameter of the equatorial section and the design inner diameter shall not be greater than 80mm. 4.5.7.4 The verticality of the pillars is measured in both radial and circumferential directions, and its value shall not be greater than 12mm. 4.6 Weld inspection and repair: 4.6.1 Appearance inspection of welds: 4.6.1.1 Before the appearance inspection of the weld, the slag skin, spatter, etc. should be cleaned up. 4.6.1.2 The surface of the weld and heat-affected zone shall be free of defects such as cracks, pores, undercuts, slag inclusions, pits, and incomplete welding. 4.6.1.3 The width of the weld should be increased by 1~2 mm on each side of the groove. The entire weld surface after welding must be ground with a grinder to eliminate weld waves. The weld reinforcement on the outer surface of the spherical shell should be 0~1mm. The weld reinforcement on the inner surface of the spherical shell should be 0~0.5mm. 4.6.2 Non-destructive testing of welds 4.6.2.1 Personnel engaged in non-destructive testing of spherical tanks must hold a technical grade appraisal certificate for non-destructive testing personnel of boilers and pressure vessels issued by the Bureau of Technical Supervision. Only those who have obtained a certificate of level II or above can fill in and issue an inspection report. 4.6.2.2 Non-destructive testing of the weld can only be carried out 36 hours after the welding is completed. 4.6.2.3 The butt welds of spherical tanks should undergo 100% radiographic inspection. The inspection is carried out in accordance with JB/T4730.2. Level II is qualified. The re-inspection parts should include all circumferential seams, longitudinal seams within 700mm from the circumferential seams (including 20% ultrasonic re-inspection, including all T-shaped and Y-shaped welds) and parts with defects and doubts in radiographic inspection. The re-inspection shall be carried out in accordance with JB/T4730.3. Level I is qualified. 4.6.2.4 The internal and external surfaces of the butt weld of the spherical tank, the workpiece weld marks and their heat-affected zones, the repair welds, the defective grinding areas, and the fillet welds on the pillars and lower sections should be subjected to 100% magnetic particle or penetrant inspection. The inspection is carried out in accordance with JB/T4730.4 or JB/T4730.5, and grade I is qualified. After the overall heat treatment and the hydrostatic test, 100% magnetic particle or penetrant re-inspection should be carried out on the above-mentioned surface flaw detection parts. 4.6.2.5 The butt welds of spherical tanks should be subjected to 100% penetrant inspection after root cleaning. Level I specified in JB/T4730.5 is considered qualified. 4.6.2.6 If there are any impermissible defects in the welds that have been subjected to non-destructive testing, welding should be repaired after the defects are removed, and the part should be re-inspected according to the original flaw detection method until it is qualified. 4.6.3 Repair of spherical tanks 4.6.3.1 The surface defects of the spherical shell and the weld marks of the tool fixtures are ground with a grinder. The slope within the grinding range is at least 3:1. The maximum depth after grinding shall not be greater than 2mm. When it exceeds, welding repairs should be carried out. When the welding repair depth on the surface of the spherical shell plate exceeds 3mm, ultrasonic testing should be carried out. 4.6.3.2 When repairing welding surface defects, the length of the repair welding should be greater than 50mm. After repairing welding, the excess metal should be ground off to make a smooth transition with the main weld. 4.6.3.3 When welding and repairing surface defects of the spherical shell plate, the repair area of each place should be within 50cm2. When there are two or more repairs, the edge distance between any two places should be greater than 50mm. 4.6.3.4 The undercut and weld toe cracks on both sides of the weld must be removed with a grinding wheel and ground into a slope with a slope of 1:3 or less. The removal depth shall not exceed 0.5mm. If it exceeds, repair welding shall be performed. 4.6.3.5 The internal defects of the welds shall be carried out in accordance with the requirements of the company's QB/HEJ.GZ0102-03-1999 "Welded Joint Rework Management System". When the defects are repaired, the depth of the defects shall not exceed 2/3 of the thickness of the spherical shell plate. If defects remain at 2/3 of the thickness of the spherical shell plate, repair welding shall be carried out in this state, and then the defects shall be removed again on the back side for repair welding. The length of repair welding shall not be less than 50mm. 4.6.3.6 The number of welding repairs on the same part of the weld should not exceed two times. If the number of welding repairs exceeds two times, reliable technical measures should be proposed, and repairs can only be made after approval by the chief technical person in charge of the construction unit. 4.6.3.7 The repair process is the same as the formal welding process. The preheating temperature is 100°C, and post-heat treatment must be carried out immediately after welding. 5. Schematic diagram of the spherical tank weld layout (see attachment) 6. Preparation and testing of product welding test plates. The samples are prepared and tested according to GB150-1998 and JB4744-2000. The test results should meet the requirements specified in the standard. 7. The general layout of the construction site includes a tool room and a welding machine shed of 35t. Crane semi-finished product stacking area Oxygen-101 Semi-finished product stacking area Oxygen-102 Semi-finished product stacking area Oxygen-103 Grouping platform Semi-finished product stacking area 8. Labor organization team Electricity engineer 1 person 2 people 15 Engineers Lifter 1 person 4 hostages Security guards Welder 1 person 14 riveters 8 people 2 fitters 9. Materials used for machine tools and means 9.1 Main means used Tarpaulin scaffolding 2000M2 20t wooden frame board 30 pieces 600 pieces inverted chain steel plate 5t δ=20 3 sets, 3t 64 M2 2 sets 9.2 Main machine tools and equipment name model specification 18 sets Main characteristic parameters Rated current 500A ~ 450A, input current 108A, input capacity 41KVA Maximum temperature 500℃, holding time 0~20h Can hold 100Kg welding rod, rated power 6.4KW, maximum temperature 500℃, holding time 0~20h, can hold 60Kg welding rod, rated power 4.2KW, dehumidification capacity 3Kg/h, power 1.2KW, refrigerant R22, charging capacity 1.1kg AC and DC dual-purpose arc welding machine automatic far-infrared welding rod drying box automatic far-infrared electric welding rod drying box dehumidifier axial flow fan low voltage transformer grinding wheel cutting machine pipe table pump drill ZXEI-500 ZYHC-100 ZYH-160 CFZ3D DN500 36V J3GB-400 4〃 ZT4016-W 2 units 1 unit power 2.2KW Grinding wheel diameter 400mm, cutting line speed 60m/s, clamp rotation angle 0~40, maximum drilling diameter per unit 16mm 10. Construction quality control 10.1 Strictly follow the project management documents of Sinochem Second Construction Group. Specific requirements are as follows: 10.1.1 Establish a corresponding quality inspection agency: The project department establishes a quality inspection agency and works in correspondence with the quality inspection department of the Tiantie Company Engineering Construction Headquarters. Each team leader is a part-time quality inspector and is responsible for the quality inspection work of the team. Quality inspectors at all levels must be stationed at the construction site before the start of their profession. 10.1.2 Technical personnel should provide detailed technical explanations to the construction workers before construction. 10.1.3 All types of materials used in construction must have quality certificates. All types of materials entering the warehouse should be classified and properly kept. 10.1.4 Measurement and inspection measuring tools used in the project must be approved by China * * It must be calibrated by a recognized metrology department and must be accurate and have a calibration certificate before it can be used. 10.1.5 Concealed projects during construction must be inspected and confirmed by the quality inspection department of the construction unit before being concealed, and the notification should be delivered 24 hours in advance. 10.1.6 The construction progress should be synchronized with construction records and other information. Construction data should be complete, and visa procedures at all levels must be complete. 10.1.7 Process control points implement three-level quality management A, B, and C. A Level B is jointly inspected by the construction unit, the supervision unit and the construction unit, level B is jointly inspected by the supervision unit and the construction unit, and level C is controlled by the construction unit. 10.1.8 Construction must be carried out strictly in accordance with the design requirements. If there is no design change, no one is allowed to change the design. 10.2 Strictly implement the rules and regulations in the company's pressure vessel quality assurance manual and pressure vessel management system. 10.3 Conscientiously implement ISO9001, international standard quality system program documents, so that everything has its roots and every step is witnessed. 11. Technical measures for construction safety and environmental management 11.1 Strictly implement the "Site Safety Management Regulations" in the project management document of Sinochem Second Construction Group Company. The specific requirements are as follows: 11.1.1 Adhere to weekly safety meetings and daily on-site inspections. 11.1.2 All construction site workers should wear safety helmets and safety shoes, and be equipped with personal protection facilities and equipment according to the requirements of the type of work. 11.1.3 It is strictly prohibited to start a fire in warehouses, oil depots, flammable areas and other major fire prevention areas, and be equipped with appropriate fire prevention equipment. 11.1.4 Smoking is only allowed in designated areas under controlled conditions, and smoking is strictly prohibited in vehicles on site. 11.1.5 Fire extinguishers should be placed in places where the use of fire extinguishers is allowed, and all personnel should be trained to use fire extinguishers. 11.1.6 All personnel working in the air above 2 meters must wear safety belts. During unprotected construction on the outer frame, a safety net must be set up under high-altitude working points. 11.1.7 All on-site electricity installation and repair work must be performed by personnel with professional electrician certificates. All electrical equipment should be grounded and leakage protectors should be used. 11.2 Strictly implement the company's "Occupational Health and Safety Management Manual" and "Occupational Health and Safety Management System Procedure Documents". The specific requirements are as follows: 11.2.1 Carefully analyze the actual situation and operation process on site, and identify the hazard sources clearly and be aware of them. 11.2.2 Before lifting heavy objects, carefully check the rigging, clamps, etc., and make sure before lifting. When lifting, it is strictly forbidden to stand under the crane arm. 11.2.3 During preheating and post-heat treatment of welds, care should be taken to prevent burns, and the sphere should be kept fully grounded. 11.2.4 The lighting voltage inside the tank should not be greater than 36V, handheld power tools should be well insulated, and the sockets should be placed in the box. 11.2.5 When welding in the tank, use an axial flow fan to remove the smoke, and assign a dedicated person to conduct safety supervision to ensure good ventilation in the tank. 11.2.6 Outdoor electrical facilities should have rainproof measures to prevent electrical appliances from getting damp and causing electric shock accidents. 11.2.7 Provide sufficient fire-fighting equipment on site to prevent flammable materials from catching fire. 11.3 Conscientiously implement ISO14001 "Environmental Management System Document" and understand the connotation of the management policy of "obeying regulations, civilized construction, cherishing resources, and protecting the environment". 11.3.1 All materials, equipment and machinery should be neatly stored in designated areas. 11.3.2 All scraps, waste pens, waste wires, tarpaulins, waste welding rod heads, etc. should be collected and stored once a day. A clean-up of the entire site involving the participation of all personnel should be organized once a week.
According to GB12337-1998, spherical tanks meeting any one of the following conditions shall undergo post-weld overall heat treatment prior to the pressure test: a) When the drawings specify that post-weld overall heat treatment is required； b) Carbon steel and 07MnCrMoVR steel spherical shells with a thickness greater than 32 mm (greater than 38 mm when preheated to over 100°C prior to welding) ； c) 16MnR steel spherical shells with a thickness greater than 30 mm (greater than 34 mm if preheated to over 100°C prior to welding) ； d) Spherical shells made of 15MnVR steel with a thickness greater than 28 mm (or greater than 32 mm when preheating to over 100°C prior to welding) ； e) Other low-alloy steel spherical shells of any thickness ； f) Tanks marked in the drawings as being susceptible to stress corrosion, such as those used for storing liquefied petroleum gas, liquid ammonia, etc ； g) The diagram indicates spheres used to contain materials with extremely or highly hazardous toxicity. GB12337-1998 specifies that personnel engaged in non-destructive testing of spherical tanks must hold a technical qualification certificate for non-destructive testing personnel of boilers and pressure vessels, issued by the labor department and valid for the relevant scope. Only personnel who have obtained a certificate at level I or above for the relevant item may fill out and issue the inspection report. After the shape, dimensions, and appearance of the weld surface have passed inspection, non-destructive testing is carried out. For spherical tanks manufactured from steel prone to delayed cracking, non-destructive testing of the welds shall be carried out at least 36 hours after welding is completed.
1. Introduction The purpose of the overall heat treatment of spherical tanks is to eliminate stresses generated during their assembly and welding, stabilize their geometric dimensions, alter the weld metallographic structure, enhance the toughness and stress resistance of the metal, and prevent the occurrence of cracks. Meanwhile, due to the evolution of dissolved hydrogen, it can prevent the occurrence of delayed cracking and subsequent failure, improve fatigue strength and creep strength; thus, it is of great significance for ensuring the safe operation of spherical tanks. The Storage and Transportation Department of Sinopec Anqing Branch has constructed a new 2,500 m3 propylene spherical tank, designed by Anqing Shihua Engineering Design Co., Ltd. The basic parameters are shown in the table below: Table 1: Performance parameters of the spherical tank. As can be seen from Table 1, the material used for this spherical tank is 07MnCrMoVR, a material that is widely used in large spherical tanks in China. According to GB12337-1998 \"Steel Spherical Tanks\", spherical tanks made of this material with a wall thickness of ≥32 mm must undergo post-weld heat treatment. This steel grade is a low-alloy quenched and tempered high-strength steel that possesses excellent strength-to-toughness ratios, weldability, as well as resistance to HIC and hydrogen sulfide corrosion; it meets the requirements regarding low sensitivity to welding cracks. Its Rm value is ≥610 MPa and its Rel value is ≥490 MPa, which is why it is referred to as the CF-62 series of steels. It was initially imported from abroad, but production of this steel has now become domestic; the product used in this project is manufactured by Wuyang Iron and Steel Company. 2. Procedures and measures for supervision of quality control. As we know, in order to ensure the quality of construction projects, supervision engineers must carry out comprehensive quality supervision, control, and inspection throughout the entire construction process. For the whole construction process, control can be implemented before, during, and after the work is carried out; for a specific task as well, quality control involves actions taken before, during, and after its execution. Below, we focus on the heat treatment process for spherical tanks and outline the various quality control measures that supervisors should implement during this process. 2.1 Familiarize with standards and design documents **Industry standards and design documents serve as important bases for supervision and quality control. Currently, the main standards and regulations related to the construction and acceptance of spherical tanks in the petrochemical industry are listed in Table 2: Standards and specifications related to spherical tank installation. Among these three standards and regulations, Article 8.8 of GB12337, Chapter 6 of GB50094, and Chapter 8 of SH/T3512 all contain provisions regarding the overall heat treatment of spherical tanks. Among them, GB12337 and GB50094 specify the scope of heat treatment, the processes involved, the requirements regarding heat retention, as well as the requirements related to the displacement of column feet during heat treatment. As a petrochemical industry standard, SH/T3512 focuses on providing guidance on the construction processes for contractors. In addition to the aforementioned provisions, detailed descriptions are provided on the methods for overall heat treatment and the temperature measurement techniques during the heat treatment process, so that construction companies can carry out these tasks in accordance with unified standards. Generally speaking, supervision personnel must be familiar with and proficient in the relevant provisions of standards and regulations, especially some mandatory clauses. At the same time, it is also necessary to be familiar with the specific requirements outlined in the design documents, with a focus on understanding the design’s requirements regarding the overall heat treatment of the spherical tanks in this project. If needed, communication with the designers should be established to grasp their intentions, especially regarding the specific requirements for the heat treatment process, such as the heating rate, cooling rate, constant temperature values, and the relevant range limits, all of which must be strictly controlled during the heat treatment process. In addition to the above three professional standards, there are also various industry standards and regulations related to the installation of spherical tanks, including those concerning pressure vessel safety supervision, on-site equipment installation, welding, and non-destructive testing. Supervisors must be familiar with and proficient in these as well; they will not be listed in detail here. 2.2 Qualification review of heat treatment units
According to the provisions of Document No. 194 [2003] issued by the General Administration of Quality Supervision, Inspection and Quarantine, “Conditions for Permits on the Manufacturing of Boilers and Pressure Vessels”, heat treatment of boilers and pressure vessels is generally entrusted to units possessing equivalent manufacturing qualifications. That is, installation units qualified for the fabrication and on-site welding of spherical storage tanks also possess the qualifications for overall heat treatment of such tanks. However, since 07MnCrMoVR belongs to the CF series of steels, per Document No. 41 [2006] issued by the Special Correspondence Office of Quality Inspection, “Opinions Concerning the Use of CF Series Steel Plates in the Manufacturing of Pressure Vessels”, the National Standardization Committee for Boilers and Pressure Vessels (hereinafter referred to as the Committee) has specific requirements regarding the qualifications of units undertaking overall heat treatment of spherical tanks made from this steel type. Only after obtaining a confirmation letter from the Committee can a unit be deemed qualified to perform overall heat treatment on spherical tanks fabricated from CF series steels. For this project, the contractor possesses the qualifications for on-site welding of large spherical tanks, but has not obtained a confirmation letter from the Rating Committee regarding the qualifications for overall heat treatment of spheres made of CF series steel. Therefore, when reviewing the qualifications of the contracting party, the supervisor, in close consultation with the owner, required the contracting party to submit an application to the **Rongbiao Committee for the qualification needed to carry out overall heat treatment on CF-series steel spherical tanks prior to the commencement of the heat treatment work. Experts from the Rongbiao Committee were invited to conduct on-site and site-based evaluations of the contracting party’s personnel qualifications, equipment, and other relevant conditions during the heat treatment process, and their approval was obtained. Through the implementation of the overall heat treatment for this project, the contracting unit ultimately received a confirmation letter from the Bid Evaluation Committee, thereby gaining the qualification to carry out overall heat treatment on CF series steel spherical tanks. 2.3 Review of heat treatment plans: As a technical document for construction companies, the construction plan serves as the basis for construction operations, as well as a reference for supervisors in controlling aspects such as construction safety, construction quality, and project investment. Reviewing the construction plan is an important measure for supervising pre-construction quality control. By examining the construction plan, it is possible to determine whether the contractor’s quality control measures are in place, whether the plan is appropriate and feasible, and whether the contractor’s internal review procedures comply with relevant regulations. Currently, there are two methods for the overall heat treatment of spherical tanks: the internal combustion method and the electric heating method. The internal combustion method is further divided into the fuel-based method and the gas-based method. For large spherical tanks, the internal combustion method is generally used, and the fuel internal combustion method was adopted for the spherical tank in this project. Since the construction site of this project is located within the operating tank farm of Anqing Petrochemical, which is a key area for fire and explosion prevention, the requirements for safety protection are extremely stringent when using the exothermic method for heat treatment. Ensuring that the heat treatment process proceeds continuously and safely also constitutes a requirement for quality control. Therefore, the supervisor attached great importance to the review of this plan. It organized multiple specialized meetings with relevant parties, including the owner, design unit, and contracting unit, to thoroughly discuss aspects such as the selection of heat treatment equipment, the placement of temperature measurement points, the insulation system, the heat treatment process, and safety protection measures. During the discussion of the plan, the contracting party pointed out that the constant temperature value specified in the design documents was 575±15°C, resulting in a temperature difference of only 30°C between the upper and lower limits; this was difficult to achieve in actual practice. Ultimately, under the coordination of the supervisors, the designers adjusted the constant temperature value to 578±18°C. This adjustment ensured the effectiveness of the heat treatment while also improving its operability. Additionally, it should be emphasized that for the reviewed plan, the technical supervisor of the contracting unit must provide a technical briefing to the workers, so as to ensure the effective implementation of the plan. Supervisors shall inspect the extent to which this briefing has been carried out. 2.4 Confirmation of heat treatment conditions Before heat treating the spherical tank, the following tasks must be confirmed: a) All welding work connecting to the pressure-bearing components of the spherical tank must be completed ； b) All non-destructive testing tasks prior to heat treatment have been completed and verified as satisfactory ； c) The overall geometric dimensions of the spherical tank after welding meet the requirements ； d) The strut tie rods and foundation bolts have been loosened, and it has been confirmed that the strut feet can move properly during the heat treatment process ； e) The product welding test plates have been placed on the outside of the high-temperature area during the heat treatment of the spherical tank ； f) f) The nozzles that are not related to heat treatment have been sealed with blind flanges ； g) Before heat treatment, the thermocouples and recording instruments must have passed calibration and be within their valid period ； h) h) The heating system has been tested and is in good working order ； i) Preventive measures such as protection against rain, wind, fire, and power outages have been taken. These inspection tasks are all essential; supervisors must conduct thorough inspections and ensure that each item is addressed. In accordance with the supervision quality control procedures, the inspection and verification of the above conditions shall be carried out after the contractor has completed its own inspections and confirmed that everything is in order; thereafter, the contractor shall submit a written request to the supervisors, who will then organize a joint on-site inspection together with representatives from the owner, inspection agencies, and other relevant parties. When checking item a to determine whether all welding work has been completed, the contracting unit should be requested to obtain signatures from various specialized teams. By referring to the construction drawings, it is necessary to confirm that all welding tasks related to the spherical tank itself have been finished – such as the welding of the gaskets for the spray pipes and ladder platforms, as well as those for the instrument level gauges – and that the positioning is accurate. During the inspection under item g, we found that the quality certification documents for the thermocouples purchased by the contractor were incomplete, and there was no valid calibration certificate issued by a authorized metrology authority. Therefore, we did not approve the use of this batch of thermocouples. The contractor then repurchased the thermocouples; only after having them appraised by an authorized metrology authority and obtaining approval from the supervisors were they permitted to be used. This effectively ensures the accuracy of temperature data collection during the heat treatment process. Regarding the inspection of item h, as the thermal treatment equipment for spherical tanks becomes more complete and specialized, its reliability also increases. DCS control systems are employed, and temperature control has become increasingly precise, allowing for deviations of ±20°C or even less. In this project, the contractor has adopted DCS-HY intelligent fuel heat treatment equipment, as well as EK8.550L-R light diesel burners from the German company Oeko. The use of these devices ensures a reliable achievement of the desired heat treatment results and the fulfillment of quality control objectives. When inspecting the inspection reports submitted for heat treatment equipment upon its arrival, supervisors should pay close attention to the equipment’s certificates of conformity and quality certification documents to verify that the equipment meets the requirements for use in the project. After the heat treatment equipment is installed, the contractor should be required to submit an installation and commissioning report for inspection. At the same time, before the official ignition, the contractor should be required to conduct a trial ignition of the heating system in order to check the reliability of the fuel system, control system, ignition device, and temperature measurement system. 2.5 Quality control during heat treatment: During the heat treatment process, supervisors should monitor the entire process, paying special attention to whether the personnel assigned by the contracting party are present on site, and addressing any abnormalities promptly. At the same time, special attention is paid to checking whether the rates and holding times during heating, maintaining a constant temperature, and cooling meet the requirements specified in the design and relevant standards. For this project, the specific requirements for the heat treatment process are as follows: a) Heating. Pay close attention to the heating rate. During the heating phase, the heating rate does not need to be controlled at 300°C and below ； Above 300°C, the heating rate should be controlled within the range of 50°C/h to 80°C/h. b) b) Constant temperature. Pay close attention to monitoring the constant temperature and time. The design requirements of the project call for controlling the temperature at 578±18°C for 2 hours. To achieve the predetermined heat treatment temperature through overall stress-relief heat treatment, a certain holding time is required to allow the stress-relaxation process to proceed sufficiently, thereby achieving an effective reduction in stress and an improvement in the properties of the joint. c) Cooling. Pay close attention to the cooling rate. During the cooling phase, the cooling rate should be kept within the range of 30°C/h to 50°C/h; below 300°C, natural cooling in air is sufficient. d) d) Temperature difference. The temperature difference during heat treatment is the difference between the highest and lowest temperatures at various measurement points during this process; it reflects the overall temperature uniformity of the spherical tank. The smaller this temperature difference, the more uniform the temperatures are throughout the tank, and the more thoroughly the residual stresses are eliminated. During the heating and cooling phases at temperatures above 300°C, the temperature difference between any two temperature measurement points on the surface of the spherical shell must not exceed 130°C. During heat treatment, the temperature of the outer surface of the insulation layer should not exceed 60°C. e) Movement of the column foot. During the heat treatment process, the displacement of the strut base plate should be monitored. Adjustment should be made every time the temperature changes by 100°C; the foot of the column must be moved promptly to maintain its verticality. The movement of the column foot should be done smoothly and slowly. Although it is advisable to avoid rainy days during heat treatment, weather conditions should still be closely monitored during the ignition process, and the contracting party should be required to take appropriate rainproof measures; protective actions must be taken promptly if it starts to rain. 2.6 Submission for inspection and verification after heat treatment: Upon completion of the heat treatment, the contracting party shall submit the process documentation related to the heat treatment to the supervisor for inspection. When reviewing the submitted documentation, the supervisor should focus on checking whether the deviation between the actual heat treatment record curves and the design requirements is within the allowable range. To evaluate the overall effect of heat treatment and its impact on the welds and heat-affected zones, surface non-destructive testing of the internal and external butt welds of the sphere shall also be carried out in accordance with the requirements of the design documents, in order to detect any surface defects such as cracks. The welding test plates of the products involved in heat treatment are sent to a qualified laboratory for mechanical property testing, and the supervisor shall witness this process. As a post-event quality control measure, these inspection and verification processes merely serve to check whether the quality control objectives have been met; should there be significant deviations from those objectives, only passive remedial actions can be taken, resulting in unnecessary losses. Therefore, for quality control measures, emphasis should be placed on pre-control and in-process control, with proactive control methods being adopted as a priority to focus on prevention, thereby ensuring the effectiveness of quality target control. In this project, after the overall heat treatment of the spherical tank was completed, 100% magnetic particle testing was carried out on the internal and external butt welds as well as the heat-affected zones in accordance with the design requirements; no defects were detected. The mechanical property tests of the three welding test pieces also met the design specifications, achieving the desired results. 3. Conclusion: As the direct implementers and responsible parties for construction quality, construction contractors must have a sound quality management system in place that functions effectively; supervision by inspectors serves to ensure this. Although the overall heat treatment of spherical tanks requires certain expertise, the methods for quality control by supervisors remain the same. During the implementation of this project, thanks to the supervision staff’s focused control over various factors that could affect quality control objectives, strict adherence to quality standards, and the adoption of effective quality control measures, all the data parameters related to the overall heat treatment of the spherical tank met the design specifications, resulting in satisfactory outcomes.
According to the provisions of Document No. 194 [2003] issued by the General Administration of Quality Supervision, Inspection and Quarantine, “Conditions for Permits on the Manufacturing of Boilers and Pressure Vessels”, heat treatment of boilers and pressure vessels is generally entrusted to units possessing equivalent manufacturing qualifications. That is, installation units qualified for the fabrication and on-site welding of spherical storage tanks also possess the qualifications for overall heat treatment of such tanks. However, since 07MnCrMoVR belongs to the CF series of steels, per Document No. 41 [2006] issued by the Special Correspondence Office of Quality Inspection, “Opinions Concerning the Use of CF Series Steel Plates in the Manufacturing of Pressure Vessels”, the National Standardization Committee for Boilers and Pressure Vessels (hereinafter referred to as the Committee) has specific requirements regarding the qualifications of units undertaking overall heat treatment of spherical tanks made from this steel type. Only after obtaining a confirmation letter from the Committee can a unit be deemed qualified to perform overall heat treatment on spherical tanks fabricated from CF series steels. For this project, the contractor possesses the qualifications for on-site welding of large spherical tanks, but has not obtained a confirmation letter from the Rating Committee regarding the qualifications for overall heat treatment of spheres made of CF series steel. Therefore, when reviewing the qualifications of the contracting party, the supervisor, in close consultation with the owner, required the contracting party to submit an application to the **Rongbiao Committee for the qualification needed to carry out overall heat treatment on CF-series steel spherical tanks prior to the commencement of the heat treatment work. Experts from the Rongbiao Committee were invited to conduct on-site and site-based evaluations of the contracting party’s personnel qualifications, equipment, and other relevant conditions during the heat treatment process, and their approval was obtained. Through the implementation of the overall heat treatment for this project, the contracting unit ultimately received a confirmation letter from the Bid Evaluation Committee, thereby gaining the qualification to carry out overall heat treatment on CF series steel spherical tanks. 2.3 Review of heat treatment plans: As a technical document for construction companies, the construction plan serves as the basis for construction operations, as well as a reference for supervisors in controlling aspects such as construction safety, construction quality, and project investment. Reviewing the construction plan is an important measure for supervising pre-construction quality control. By examining the construction plan, it is possible to determine whether the contractor’s quality control measures are in place, whether the plan is appropriate and feasible, and whether the contractor’s internal review procedures comply with relevant regulations. Currently, there are two methods for the overall heat treatment of spherical tanks: the internal combustion method and the electric heating method. The internal combustion method is further divided into the fuel-based method and the gas-based method. For large spherical tanks, the internal combustion method is generally used, and the fuel internal combustion method was adopted for the spherical tank in this project. Since the construction site of this project is located within the operating tank farm of Anqing Petrochemical, which is a key area for fire and explosion prevention, the requirements for safety protection are extremely stringent when using the exothermic method for heat treatment. Ensuring that the heat treatment process proceeds continuously and safely also constitutes a requirement for quality control. Therefore, the supervisor attached great importance to the review of this plan. It organized multiple specialized meetings with relevant parties, including the owner, design unit, and contracting unit, to thoroughly discuss aspects such as the selection of heat treatment equipment, the placement of temperature measurement points, the insulation system, the heat treatment process, and safety protection measures. During the discussion of the plan, the contracting party pointed out that the constant temperature value specified in the design documents was 575±15°C, resulting in a temperature difference of only 30°C between the upper and lower limits; this was difficult to achieve in actual practice. Ultimately, under the coordination of the supervisors, the designers adjusted the constant temperature value to 578±18°C. This adjustment ensured the effectiveness of the heat treatment while also improving its operability. Additionally, it should be emphasized that for the reviewed plan, the technical supervisor of the contracting unit must provide a technical briefing to the workers, so as to ensure the effective implementation of the plan. Supervisors shall inspect the extent to which this briefing has been carried out. 2.4 Confirmation of heat treatment conditions Before heat treating the spherical tank, the following tasks must be confirmed: a) All welding work connecting to the pressure-bearing components of the spherical tank must be completed ； b) All non-destructive testing tasks prior to heat treatment have been completed and verified as satisfactory ； c) The overall geometric dimensions of the spherical tank after welding meet the requirements ； d) The strut tie rods and foundation bolts have been loosened, and it has been confirmed that the strut feet can move properly during the heat treatment process ； e) The product welding test plates have been placed on the outside of the high-temperature area during the heat treatment of the spherical tank ； f) f) The nozzles that are not related to heat treatment have been sealed with blind flanges ； g) Before heat treatment, the thermocouples and recording instruments must have passed calibration and be within their valid period ； h) h) The heating system has been tested and is in good working order ； i) Preventive measures such as protection against rain, wind, fire, and power outages have been taken. These inspection tasks are all essential; supervisors must conduct thorough inspections and ensure that each item is addressed. In accordance with the supervision quality control procedures, the inspection and verification of the above conditions shall be carried out after the contractor has completed its own inspections and confirmed that everything is in order; thereafter, the contractor shall submit a written request to the supervisors, who will then organize a joint on-site inspection together with representatives from the owner, inspection agencies, and other relevant parties. When checking item a to determine whether all welding work has been completed, the contracting unit should be requested to obtain signatures from various specialized teams. By referring to the construction drawings, it is necessary to confirm that all welding tasks related to the spherical tank itself have been finished – such as the welding of the gaskets for the spray pipes and ladder platforms, as well as those for the instrument level gauges – and that the positioning is accurate. During the inspection under item g, we found that the quality certification documents for the thermocouples purchased by the contractor were incomplete, and there was no valid calibration certificate issued by a authorized metrology authority. Therefore, we did not approve the use of this batch of thermocouples. The contractor then repurchased the thermocouples; only after having them appraised by an authorized metrology authority and obtaining approval from the supervisors were they permitted to be used. This effectively ensures the accuracy of temperature data collection during the heat treatment process. Regarding the inspection of item h, as the thermal treatment equipment for spherical tanks becomes more complete and specialized, its reliability also increases. DCS control systems are employed, and temperature control has become increasingly precise, allowing for deviations of ±20°C or even less. In this project, the contractor has adopted DCS-HY intelligent fuel heat treatment equipment, as well as EK8.550L-R light diesel burners from the German company Oeko. The use of these devices ensures a reliable achievement of the desired heat treatment results and the fulfillment of quality control objectives. When inspecting the inspection reports submitted for heat treatment equipment upon its arrival, supervisors should pay close attention to the equipment’s certificates of conformity and quality certification documents to verify that the equipment meets the requirements for use in the project. After the heat treatment equipment is installed, the contractor should be required to submit an installation and commissioning report for inspection. At the same time, before the official ignition, the contractor should be required to conduct a trial ignition of the heating system in order to check the reliability of the fuel system, control system, ignition device, and temperature measurement system. 2.5 Quality control during heat treatment: During the heat treatment process, supervisors should monitor the entire process, paying special attention to whether the personnel assigned by the contracting party are present on site, and addressing any abnormalities promptly. At the same time, special attention is paid to checking whether the rates and holding times during heating, maintaining a constant temperature, and cooling meet the requirements specified in the design and relevant standards. For this project, the specific requirements for the heat treatment process are as follows: a) Heating. Pay close attention to the heating rate. During the heating phase, the heating rate does not need to be controlled at 300°C and below ； Above 300°C, the heating rate should be controlled within the range of 50°C/h to 80°C/h. b) b) Constant temperature. Pay close attention to monitoring the constant temperature and time. The design requirements of the project call for controlling the temperature at 578±18°C for 2 hours. To achieve the predetermined heat treatment temperature through overall stress-relief heat treatment, a certain holding time is required to allow the stress-relaxation process to proceed sufficiently, thereby achieving an effective reduction in stress and an improvement in the properties of the joint. c) Cooling. Pay close attention to the cooling rate. During the cooling phase, the cooling rate should be kept within the range of 30°C/h to 50°C/h; below 300°C, natural cooling in air is sufficient. d) d) Temperature difference. The temperature difference during heat treatment is the difference between the highest and lowest temperatures at various measurement points during this process; it reflects the overall temperature uniformity of the spherical tank. The smaller this temperature difference, the more uniform the temperatures are throughout the tank, and the more thoroughly the residual stresses are eliminated. During the heating and cooling phases at temperatures above 300°C, the temperature difference between any two temperature measurement points on the surface of the spherical shell must not exceed 130°C. During heat treatment, the temperature of the outer surface of the insulation layer should not exceed 60°C. e) Movement of the column foot. During the heat treatment process, the displacement of the strut base plate should be monitored. Adjustment should be made every time the temperature changes by 100°C; the foot of the column must be moved promptly to maintain its verticality. The movement of the column foot should be done smoothly and slowly. Although it is advisable to avoid rainy days during heat treatment, weather conditions should still be closely monitored during the ignition process, and the contracting party should be required to take appropriate rainproof measures; protective actions must be taken promptly if it starts to rain. 2.6 Submission for inspection and verification after heat treatment: Upon completion of the heat treatment, the contracting party shall submit the process documentation related to the heat treatment to the supervisor for inspection. When reviewing the submitted documentation, the supervisor should focus on checking whether the deviation between the actual heat treatment record curves and the design requirements is within the allowable range. To evaluate the overall effect of heat treatment and its impact on the welds and heat-affected zones, surface non-destructive testing of the internal and external butt welds of the sphere shall also be carried out in accordance with the requirements of the design documents, in order to detect any surface defects such as cracks. The welding test plates of the products involved in heat treatment are sent to a qualified laboratory for mechanical property testing, and the supervisor shall witness this process. As a post-event quality control measure, these inspection and verification processes merely serve to check whether the quality control objectives have been met; should there be significant deviations from those objectives, only passive remedial actions can be taken, resulting in unnecessary losses. Therefore, for quality control measures, emphasis should be placed on pre-control and in-process control, with proactive control methods being adopted as a priority to focus on prevention, thereby ensuring the effectiveness of quality target control. In this project, after the overall heat treatment of the spherical tank was completed, 100% magnetic particle testing was carried out on the internal and external butt welds as well as the heat-affected zones in accordance with the design requirements; no defects were detected. The mechanical property tests of the three welding test pieces also met the design specifications, achieving the desired results. 3. Conclusion: As the direct implementers and responsible parties for construction quality, construction contractors must have a sound quality management system in place that functions effectively; supervision by inspectors serves to ensure this. Although the overall heat treatment of spherical tanks requires certain expertise, the methods for quality control by supervisors remain the same. During the implementation of this project, thanks to the supervision staff’s focused control over various factors that could affect quality control objectives, strict adherence to quality standards, and the adoption of effective quality control measures, all the data parameters related to the overall heat treatment of the spherical tank met the design specifications, resulting in satisfactory outcomes.
To eliminate the residual stresses resulting from the assembly and welding of spherical tanks, stabilize their geometric dimensions, improve the structure and properties of the welded joints and heat-affected zones, reduce hardness, and enhance plasticity and toughness, as well as to further release harmful gases from the welds and prevent hydrogen embrittlement and cracking in those welds, it is necessary to carry out heat treatment on the spherical tanks