Practice and Reflections on the Supervision of Chemical Equipment Manufacturing

Thread Content

Chemical processing equipment is usually manufactured using welding methods, which can easily conceal defects. Chemical processing equipment often has openings or connections on its body, and it is difficult to achieve precision even with stress analysis. When in use, chemical processing equipment not only is subjected to pressure; some of it also has to operate in high-temperature and corrosive environments. Errors in operation or abnormal reactions can lead to overheating and overpressure, ultimately resulting in damage to the equipment. To ensure that chemical equipment manufacturers maintain high standards of quality in their production, strictly control quality, and strive to eliminate common, frequent, and recurring quality issues, any potential defects in the products are addressed and controlled before they leave the manufacturer’s facility. For key projects, representatives are assigned to supervise the manufacturing process on-site, overseeing the technical procedures, manufacturing quality, as well as the manufacturer’s quality management system, while also ensuring that suppliers deliver goods in accordance with the contractual requirements. Based on the quality of chemical equipment and the supervision processes related to its production in a certain organization over the past few years, this article offers some preliminary insights into the supervision of chemical equipment, using pressure vessels as an example. It is hoped that these insights will be useful for future supervision activities related to chemical equipment in this industry. 1. It is necessary to grasp the main bases and standards for the supervision of chemical equipment manufacturing. The main bases and standards for such supervision should include design drawings, as well as **regulations and relevant standards applicable to that equipment (such as TSG 21 \"Regulations on Safety Supervision of Pressure Vessels\", GB/T150 \"Steel Pressure Vessels\", GB151 \"Shell and Tube Heat Exchangers\", JB4710 \"Steel Tower Vessels\", JB4730 \"Non-Destructive Testing of Pressure Vessels\"), along with standards for the materials and components used in various chemical equipment. Additionally, commercial contracts and technical appendices signed by both the supplier and the buyer, the supplier’s \"Quality Management Manual\", quality control procedure documents, and equipment manufacturing process documents are also important. 2. Supervision plan and control of quality critical points: Different chemical processing equipment has distinct requirements; therefore, a supervision plan should be developed in advance for each specific type of equipment. This plan should cover the frequency of inspections, the timing of equipment arrival at the site, as well as the technical documents and measuring instruments required for supervision. It is also necessary to identify the quality critical points and key aspects that need to be addressed during the supervision process. 3. The quality critical points in the supervision of chemical processing equipment can generally be divided into: a) Review points: It is sufficient for the supervision personnel to examine documents such as the supplier’s inspection records and reports for post-event verification in order to meet the requirements ； b) Inspection points: Supervisors should generally be present to conduct random inspections of processes such as electrode drying, groove preparation, the welding process, and post-weld visual inspection ； c) Stop points: Processes such as material inspection, assembly, and pressure testing are generally designated as stop points, and supervision personnel must be present to verify them. 4. Review of process documents: During supervision, the supplier’s technical personnel should be required to assess the completeness and rationality of the process documents prepared for our equipment. It is also necessary to examine whether the supplier’s manufacturing processes, quality assurance measures, testing methods, and timing of tests can ensure compliance with the technical requirements specified in the product design. For example, for the shell-and-tube heat exchangers manufactured by a certain company, the connection method required between the heat exchange tubes and the tube sheet is \"strength welding + expansion fitting\". When reviewing their process documents, the inspection personnel found that the specifications regarding expansion fitting were very simple – the process sheet merely mentioned \"expansion fitting\" in passing. There were no requirements regarding the hardness difference between the tubes and the tube sheet, no requirements for testing the expansion fitting process, nor any specific guidelines for carrying out the expansion fitting operation or criteria for determining its success; these specifications fell far short of what was required. A further review of the company’s general process guidelines also did not provide more detailed specifications for the expansion jointing process. Later, it was requested to check the hardness of the heat exchange tubes and the tube sheet; as a result, the hardness of the heat exchange tubes was even higher than that of the tube sheet. The expansion tool used was also inappropriate, as it couldn’t achieve proper expansion. When carrying out the expansion operation, the workers simply passed the expansion tool over the tubes once. Such process documents cannot effectively guide production, and therefore it is not possible to ensure that the equipment meets its design and operational requirements. The welding procedure cards for pressure vessels prepared by the process engineers at a chemical equipment factory do not specify any requirements regarding the qualifications of welders. It is up to the production supervisor to decide which level of skill is required for welding certain seams on the vessels, and this approach can lead to problems in practice. During actual inspections, it was found that the welders responsible for surfacing work on pipe plates and connections did not possess the necessary qualifications. 5. Material quality control: Special attention should be paid to the re-inspection requirements specified in the drawings or technical attachments regarding the chemical composition and mechanical properties of certain materials, to ensure that the manufacturing unit has indeed fulfilled these requirements. For example, the equipment technical annex signed with a certain factory stipulates that when retesting the equipment cylinder using the imported material SA387 Gr11 Cl2, it is necessary to analyze the contents of elements such as Sb (antimony), Sn (tin), and As (arsenic). By the time the supervision personnel arrived at the factory to oversee the production, the equipment casing had already been assembled, but upon reviewing the relevant documents, it was found that tests for these elements had not yet been conducted; therefore, they were forced to request that such tests be carried out. Furthermore, for any missing items or areas of concern identified in the material quality certificates of the pressure vessel’s stressed components, the manufacturer should be requested to carry out supplementary inspections or re-inspections. If necessary, magnetic particle testing or penetrant testing can be conducted on the surface quality of the materials, while ultrasonic testing can be used to assess their internal quality. For example: A batch of heat exchangers manufactured by a certain factory failed to pass the pressure tests repeatedly due to problems with the stainless steel heat exchange tubes. When accompanying the purchasing staff to the steel tube factory to investigate, it was found that this factory did not have any specialized equipment for eddy current testing and ultrasonic testing on each individual tube, as required by the standard GB13296 for stainless steel tubes used in boiler heat exchangers ； No chemical composition analysis, mechanical property tests, process performance tests such as flaring and crushing, nor grain size inspection were carried out in batches ； There are no temperature control instruments on the heat treatment device; the heat treatment temperature is determined by the operator based on experience. To reduce costs, equipment manufacturers often purchase materials of low quality at low prices. Since the intrinsic quality of many materials cannot be determined merely by visual inspection, and since many material suppliers also take advantage of the information asymmetry between buyers and sellers to deceive customers, it is necessary to conduct random inspections of the quality management and control practices of those who supply the suppliers, during the supervision of chemical equipment production. 6. Welding and heat treatment control: The tasks that need to be carried out in terms of welding and heat treatment during the supervision of chemical equipment manufacturing should include reviewing and monitoring the welding process evaluations conducted by the manufacturing unit in accordance with relevant standard requirements ； Regularly conduct on-site spot checks on the drying, storage, and distribution of welding materials, as well as on the quality of weld grooves and the appearance of welds, and verify whether welders possess the required certifications ； Pay special attention to the rework process of defective welds ； Inspect and review the heat treatment timing, the temperature control range during heat treatment, the calibration of the temperature control instruments used, and the heat treatment curve ； A report on the mechanical properties of the welded test plates of the heat-treated products is prepared, along with random inspections of the non-destructive testing and hardness levels before and after heat treatment. For example, when supervising the production of a batch of heat exchange equipment for a certain factory, and requesting that they provide sample welding procedure qualification reports as well as related traceability documents for those used in our equipment, the factory stated that the procedure qualifications had been carried out but that the reports and other relevant documents had not yet been prepared ； On the same day, when requests were made to obtain the product welding test plate reports and related traceability documents for several devices that were about to be completed, the party in question stated that the mechanical and process performance tests on those test plates had not yet been carried out; the quality management practices of such an organization are truly concerning. For example, in the case of a medium-pressure steam generator manufactured by a certain company, severe misalignment beyond acceptable limits was found in the weld seam at the conical joint of the equipment. After this issue was pointed out, the company intended to carry out only simple local grinding and patch welding as repairs. Following further discussions with the supervision personnel, the company finally agreed to conduct magnetic particle testing (MT) on the welded areas after patch welding, and then proceed with heat treatment. For example, in a cold exchanger manufactured by a certain company, several cracks were found in the fillet welds between the support gaskets and the cylinder body after heat treatment of the equipment. After overall heat treatment of the equipment and pressure testing of the shell side, it was discovered that 38 heat exchange tubes were leaking. Appropriate arrangements should be made by the manufacturing company regarding the heat treatment of these welds that need to be repaired. 7. Non-destructive testing: For the non-destructive testing of materials used in chemical processing equipment as well as the welded joints in such equipment, in addition to conducting routine inspections of the testing methods, equipment, X-ray films, etc., in accordance with relevant standards, it is necessary to focus inspection efforts on 100% radiographic testing as specified in the equipment drawings or technical documents, supplemented by a certain proportion of ultrasonic testing; non-destructive testing after heat treatment for materials prone to reheat cracking; and non-destructive testing of welded joints that may develop delayed cracking. Special attention should be paid to areas that require repair or further testing, as well as those that could affect the manufacturing schedule. In addition to reviewing the non-destructive testing reports, it is equally important to examine the original records kept by the testers and the X-ray films. For example: A factory manufactured some equipment, and during inspection by the supervision personnel, it was found that the groove of the butt weld on one of the devices required flaw detection. Out of concern for extra hassle, the factory attempted to use PT instead of MT as specified in the design; however, PT can only detect surface cracks, while MT can detect both surface and near-surface cracks, whether they are open or not. At the insistence of the supervision personnel, the factory was forced to carry out flaw detection on the weld grooves in accordance with the original design requirements for non-destructive testing. When the supervision personnel reviewed the original records of the penetrant testing on the pipe sheet weld layer of a piece of equipment, they found entries stating “Penetrant II: 18/2 failed – 04” and “Too shaped to proceed with testing: 04.4.3”. However, the penetrant testing report indicated “JB4730, Grade I: Passed, February 18, 2014”. When the supervision personnel questioned them about this, they were forced to admit that they had falsified the records. 8. Pressure test: The supervision personnel shall conduct on-site inspections and verifications of the test pressure, test temperature, holding time, test procedure, and pressure gauges used for the test, and evaluate the test results. For pressure vessels whose specified minimum tensile strength of the main material is 540 MPa or higher, after hydraulic or pneumatic testing, the surface must undergo non-destructive testing to ensure the absence of cracks before they can be deemed acceptable. For pressure vessels equipped with expansion joints, attention should be paid to the requirements for tightening the bolts of these expansion joints before and after the pressure test, as well as to the actual procedures to be followed. For pressure vessels that require a tightness test, the results of the tightness tests on the main body of the equipment, the joints, or the reinforcement rings must be inspected and approved on-site in accordance with the provisions of the design drawings. It is stipulated that when using ammonia for leak detection, the requirements of relevant standards such as HG20584 \"Technical Requirements for the Manufacturing of Steel Chemical Storage Vessels\" and HG/T 3176 \"Manufacturing and Inspection Methods for High-Pressure Urea Equipment – Ammonia Leak Testing Method for High-Pressure Urea Equipment\" must be complied with. 9. Inspection and control: In accordance with relevant regulations, standards, design drawings, process documents, and equipment manufacturing supervision plans, random inspections should be carried out regarding the inspection contents, methods, and measuring instruments used in the manufacture of chemical engineering equipment. A comprehensive inspection should be conducted on the overall structure, geometric dimensions, pipe connection positions, installation dimensions, components and safety accessories of the equipment, as well as its external and internal anti-corrosion properties after cold and hot processing. The water cooler equipment manufactured by a certain factory is of the U-tube type heat exchanger, and there is a bypass baffle in the middle of the tube bundle; this baffle must be welded in place first before the heat exchange tubes can be installed. The factory’s operators failed to notice the presence of this bypass baffle while installing the heat exchange tubes. Fortunately, the quality inspection personnel detected it in time; otherwise, it would have led to rework and severely affected the equipment manufacturing schedule ； During the inspection prior to the assembly of the boiler feedwater preheater’s vessel and tube bundle, it was found that there was extensive rust on the inner wall of the vessel, which was made of stainless steel composite plates (especially at the weld areas). The acid washing and passivation process had not been effective, so it became necessary to carry out acid washing and passivation on the inner wall of the vessel again. Since the various components of chemical processing equipment are mostly formed by riveting and welding rather than machining, the tolerances are relatively large. For such equipment where the internal components can only be assembled after the equipment itself has been installed, it is essential to pre-assemble and trial-assemble these components with the equipment at the manufacturer’s site, while strictly controlling the fitting dimensions between the components and the equipment. This should be specified in the technical annexes of the contract, and the on-site supervision personnel must ensure its implementation. One company learned from this experience during a large nitrogen fertilizer project: the internal components of the ammonia synthesis tower, the grating plates of the shift converter, and the trays of the purification and absorption towers had to be temporarily modified so that they could be fitted in place during the actual assembly on site, which delayed the project timeline. 10. Factory documents: For the factory documents of chemical equipment that will be delivered to users in the future, they should be reviewed in accordance with standards such as TSG 21 \"Regulations on Safety Supervision of Pressure Vessels\", GB/T150 \"Steel Pressure Vessels\", and JB4710 \"Steel Tower Vessels\", as well as the design drawings and technical appendices specified in the contract. Special attention should be paid to their completeness and consistency with the actual conditions; any omissions found should be requested to be corrected by the supplier, while any errors identified should prompt the supplier to make corrections. Many manufacturers provide as-built drawings of chemical processing equipment that include only assembly diagrams, without detailed drawings of the individual components. This is extremely inconvenient for the company’s future equipment management; it is therefore necessary to specify this requirement in the technical annexes of the contract, so that the supervision personnel can review the drawings accordingly. 11. Related considerations: In addition to the aspects mentioned above, supervision of chemical processing equipment should also take the following points into account: a) The personnel responsible for supervision should not be dispatched by the procurement department; they should not merely act as representatives of the procurement department to push for progress. Instead, they should be assigned by the company’s comprehensive technical management department and work in the interest of the entire company. b) In addition to having a solid foundation in equipment, equipment supervision personnel should be eager to learn, capable of learning, diligent, and self-disciplined; it is also desirable that they have experience in chemical equipment manufacturing technology and quality management. c) The duties and authorities of the supervision personnel should be defined: under what circumstances they have the authority to handle matters independently, under what circumstances they must report, to whom they must report, and whether interim or periodic reports are required.