Those jokes about substituting materials for pressure vessel materials

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I. General Provisions Due to limitations in supply as well as in the variety and specifications of materials used for pressure vessels, designers of such vessels often encounter requests from manufacturers for the use of alternative materials. TSG 21-2016 specifies that \"units responsible for the manufacturing, modification, and repair of pressure vessels must obtain written approval from the original design unit prior to using alternative materials for the pressure-bearing components, and must make detailed records thereof in the as-built drawings.\" ”Using alternative materials facilitates the smooth production of pressure vessels; it prevents delays in the schedule due to issues related to the procurement of small quantities of materials, and it also helps to reduce the inventory levels of manufacturers. However, the use of alternative materials raises concerns regarding strength, temperature, corrosion, manufacturing processes, and testing. Process engineers or designers should strengthen the management of such substitutions, exercise great caution when approving their use, and take into account all relevant failure modes. Provisions on material substitution in regulatory standards: 1. Article 2.1.5 of TSG 21-2016 stipulates that: \"Units responsible for the manufacturing, modification, or repair of pressure vessels must obtain written approval from the original design unit before substituting materials for the pressure-bearing components, and such substitution must be carefully documented in the as-built drawings.\" ” 2. Paragraph 3.14 of GB/T 150.2-2011 stipulates: “Units engaged in the manufacture or on-site assembly and welding of pressure vessels must obtain prior written approval from the original design unit before substituting steel materials for pressurized components, and must make detailed records thereof on the as-built drawings.” ” 3. Clause 4.3 of GB/T150.4-2011 stipulates that the manufacturing unit shall obtain written approval from the original design unit in advance for any modifications to the original design as well as for the substitution of steel materials used in pressure components, and such changes shall be carefully recorded in the as-built drawings. ” It can be seen that the requirements regarding material substitution in GB/T150-2011 and TSG 21-2016 are basically the same. II. General principles for material substitution 1. Compliance principle: Material substitution must meet the requirements of relevant safety technical specifications and material standards. When specified in the design documents, it shall also meet the requirements of those documents. 2. Safety principle: The use of substitute materials should not result in a reduction in the capacity of pressure vessels to withstand loads (including mechanical and thermal loads), nor should it lead to a decrease in the corrosion resistance of the equipment’s materials, or cause any other defects that could affect the safety performance of the equipment. 3. Principle of compliance: Material substitution must conform to the designer’s basic intent. The use of substitutes must not result in a decline in the equipment’s performance or affect the installation of its accessories ； Substitution should not lead to changes in the basic design principles, nor should it result in significant cost differences that could give rise to financial disputes. III. Replacing inferior materials with superior ones: All metal materials used in pressure vessels must possess excellent properties, including mechanical strength, corrosion resistance, high-temperature tolerance, and the suitability of their manufacturing processes. The properties of a material remain constant. From the perspective of performance comparison, the issue of which materials are \"better\" and which are \"worse\" often arises. However, the requirements for material properties vary for different types of pressure vessels under different conditions. Therefore, the judgment of what is \"good\" and what is \"bad\" in material substitution should be based on practical considerations, with each specific case being analyzed on its own. Below, based on practical work and relevant literature, several typical \"substituting the good for the bad\" problems are discussed. In the manufacture of pressure vessels, low-alloy steel is used in place of carbon steel. Although the low-alloy steels commonly employed possess superior mechanical properties such as strength and mechanical characteristics, their cold workability and weldability are not better than those of carbon steel. Generally speaking, the higher the strength grade, the worse its cold workability and weldability are, with an inverse relationship between the two. Therefore, when using substitutes in this regard, the welding process should be adjusted accordingly; changes may also occur during heat treatment, which requires due attention. 2. Compatibility between the medium and the material: When using a better material, it is important to pay attention to the compatibility between the medium and that material. Although carbon steel used for pressure vessels does not possess the same excellent strength properties as low-alloy steel, its resistance to SCC (stress corrosion) generally decreases as the strength level of the material increases. For example, in environments subjected to corrosion by liquid ammonia or wet hydrogen sulfide, using low-alloy steels such as Q345R instead of carbon steels from the Q245R and Q235 series makes it more likely to encounter problems. Another example is that, under certain operating conditions, austenitic stainless steel has a lower corrosion resistance than carbon steel, such as in environments containing chloride ions. 3. Ultra-low carbon stainless steel replaces ordinary stainless steel. Ultra-low carbon stainless steels (such as S30403) have lower high-temperature resistance compared to ordinary austenitic stainless steels (such as S30408). Although the price and corrosion resistance of the former are superior to those of the latter, Article 3.3.3 of GB/T 150.2-2011 stipulates that when austenitic stainless steel is used at temperatures above 525°C, the carbon content in the steel must be no less than 0.04%. 4. Use bolts with higher strength in place of those with lower strength. When higher-strength bolt materials are used instead of lower-strength ones, the thickness of the flange is determined by the preload; as a result, the flange thickness may be insufficient to meet the strength requirements, and the flange needs to be recalculated. 5. Using forgings of the same material as a substitute for plates: For forgings and plates made of the same material, the overall performance of forgings is better than that of plates. However, under identical conditions, the allowable stress for forgings is slightly lower than that for plates. When using forgings to replace plates, it is necessary to determine whether their strength meets the requirements; recalculation is required in such cases. For example: heat exchanger tube sheet. 6. For components such as expansion joints, rupture discs, and flexible pipe sections, it is generally not allowed to use inferior alternatives; otherwise, recalculation must be carried out using alternative materials ； Its thickness should be appropriately reduced; otherwise, it may lead to the failure of these components and adjacent areas. IV. Using thickness over thinness: When manufacturing pressure vessels, due to manufacturing-related requirements, we can choose to opt for higher quality materials instead of lower-quality ones, as long as this does not violate the principles mentioned earlier. And compared to replacing the inferior with the superior, replacing the thin with the thick seems even more unreasonable. From the perspective of stress analysis of the shell under load, using thicker materials in place of thinner ones is detrimental rather than beneficial to the structural integrity of the container. Yet this practice does occur in the actual manufacturing process of pressure vessels. Therefore, when using thicker materials instead of thinner ones under the same standards and with the same material grade, we should pay attention to the following issues: 1. Structural discontinuity – When thick plates are used in place of thin ones, it often leads to changes in the connection structures; for example, when connecting a thickened head to the cylinder, it is usually necessary to trim the edges of the head. For equipment with a steel tube shell, when the shell wall is thickened, the junction between the shell and the end cap sometimes also requires internal chamfering on the side of the shell. These same problems exist in the butt-welded joint structure of the cylinder with the tube sheet and flat cover. When the thickness increases significantly, changes in the welding structure are often required as well; for example, the welds between the nozzle and the shell, as well as the butt welds, may change from a single V-groove to an X-groove, along with corresponding adjustments to the welding processes. 2. Increase testing requirements: Using thicker materials for components under stress instead of thinner ones can give rise to requirements such as those related to the condition of the supplied material, ultrasonic testing of steel plates, heat treatment during manufacturing (including heat treatment to restore the material’s properties), and welding test pieces. 3. Decrease in allowable stress: As the material thickness increases, its allowable stress may decrease, which can result in insufficient strength of the equipment. For example, at 100°C, the allowable stress for Q245R steel plates with a thickness of 16 mm is 147 MPa, while it drops to 140 MPa for plates with a thickness of 18 mm. Due to this decrease in allowable stress, an increased thickness can actually result in insufficient strength. This requires special attention during the manufacturing of heads, as a certain additional thickness is often added to the blank when cutting the heads in order to ensure the minimum thickness of the head after stamping; this can result in insufficient strength of the stamped head. Therefore, when thick layers are over thin layers under these critical conditions, the strength must also be verified. 4. It is not recommended to use a thicker reinforcement ring instead of a thinner one. Generally, it is not advisable to use a thicker reinforcement ring; if the thickness is increased, it will lead to higher stress concentrations at the weld joint between the reinforcement ring and the housing, which can cause cracking at the weld area. 5. Changing welding and heat treatment processes: An increase in material thickness may require changes to the welding process, which could mean that equipment that previously did not need preheating before welding now does require it ； Equipment that originally did not require post-weld heat treatment now needs to undergo it. 6. Increase in pipe thickness: An increase in pipe thickness may increase the flow velocity of the fluid, and it may also cause problems with the installation of insert-type accessories such as radar level gauges and thermometers. An increase in the thickness of the heat exchange tubes may also reduce the heat transfer efficiency. 7. Increased thickness of the main components of the heat exchanger: An increase in the thickness of these components alters the stress conditions throughout the heat exchanger, so it is necessary to recalculate the design of the heat exchanger. 8. Increase in the wall thickness of the container shell: An increase in the wall thickness of the container shell will inevitably increase the weight of the container, which is detrimental to the supports or foundation of the container. 9. For components such as expansion joints, bellows, flexible tube sheets, and thin tube sheets, in principle, thicker ones should not be used to replace thinner ones. This is because as the component thickens, its stiffness increases accordingly, thereby reducing the effect of compensating for deformation. 10. Increased thickness of the saddle or support base plate or cover plate: An increase in the thickness of the saddle or support base plate, as well as the saddle steel plate, will inevitably lead to an upward movement of the equipment’s pipe outlet location if the height of the reinforcing plates is not reduced accordingly. At this point, the height of the rib plate must be reduced to ensure the elevation dimension of the pipe opening. 11. Increased thickness of the heat exchanger baffle: Sometimes the thickness of the baffle increases; in such cases, the spacing tubes need to be shortened accordingly, otherwise the length of the tie rods may not be sufficient to allow the nuts to be tightened. Therefore, the more the baffle thickness increases, the more the spacing tube needs to be shortened. V. Updates to material standards 1. GB713 standard: When GB/T150.2-2011 was issued, the GB713 standard for plates referenced was the 2008 version; the current latest version of GB713 is GB/T713-2014. Pressure vessels designed in accordance with GB/T150 should use plates that comply with this newer standard. 2. When GB/T150.2-2011 was issued, the GB9948 standard referenced was the 2006 version; the material 1Cr5Mo is no longer included in the revised GB/T9948-2013. For raw materials, the corresponding materials specified in GB/T9948-2013 should be used in accordance with the updated standards, and it is not permissible to use 12Cr5MoNT from GB/T9948-2013 as a substitute for 1Cr5Mo from GB9948-2006. 3. GB6479 standard: Since GB/T150.2-2011 restricts the reference to the GB6479 standard to only its 2000 edition, for pressure vessels designed in accordance with GB/T150, it is generally required that steel pipes supplied in compliance with GB6479-2000 be used in their design and manufacture. When steel pipes specified in GB6479-2013 are used in the construction of pressure vessels due to market procurement reasons, it shall be ensured that all performance indicators and relevant technical requirements are not lower than those stipulated in GB6479-2000. During the construction process, it is necessary to carry out data comparison, supplement technical documents related to material procurement, and implement quality control measures such as acceptance verification; furthermore, procedures for the proper use of materials must be followed in accordance with relevant regulations. VI. Use of low-temperature steel in place of normal-temperature steel: When low-temperature steel with the same strength grade is used in place of normal-temperature steel, it is necessary to consider whether the allowable stress at the design temperature can meet the requirements of the original design. For example, when the design temperature of a pressure vessel is 200 ℃, material 16MnDR with the same thickness can be used as a substitute for Q345R. Since the allowable stress of 16MnDR at 200 ℃ is t = 167 MPa, whereas that of Q345R at 200 ℃ is t = 183 MPa, this leads to a problem of insufficient strength. VII. Interchangeability of austenitic stainless steels: When austenitic stainless steel materials are used interchangeably, insufficient strength may also occur. For example, when the design temperature of a pressure vessel is 200 °C, using austenitic stainless steel material S30403 with the same thickness as S30408 may lead to problems; this is because the allowable stress of S30403 at 200 °C is t = 110 MPa, while that of S30408 at 200 °C is t = 130 MPa. It is therefore necessary to increase the thickness of the vessel shell in order to meet the strength requirements. At this point, it is necessary to consider redesigning the equipment rather than dealing with the issue through simple material substitutions. When austenitic stainless steel materials are used interchangeably, compatibility issues with the medium may also arise. For example, although S31603 has a higher corrosion resistance rating than S30403, it is more prone to corrosion when used in nitric acid environments. Furthermore, the high-temperature service limit of ultra-low carbon stainless steel is lower than that of low carbon stainless steel, which is also something to keep in mind when substituting materials. VIII. Substitution of materials under external pressure: If within the range of elastic instability, the critical pressure for instability is independent of the material’s yield limit; it depends only on the elastic modulus and Poisson’s ratio, as well as on the diameter, thickness, and length of the device. In this range, using a material with higher stress capacity instead of one with lower stress capacity is ineffective. If, within the range of inelastic instability, the critical pressure for instability is related to the yield limit of the material, then high-stress materials can be used as substitutes for low-stress materials. IX. Conclusion Essentially, material substitution involves continuous changes to the design of pressure vessels; whenever alternative materials are used, it may be necessary to recalculate the data and redesign the structure. The emergence of surrogate problems poses not only a technical challenge to the manufacturing of pressure vessels, but also relates to the safety and cost-effectiveness of these vessels. Therefore, the use of alternative materials in pressure vessel manufacturing is a quite important aspect; process engineers or designers must consider all relevant factors before giving their approval.