Use superior materials for pressure vessels instead of inferior ones

The so-called principle of replacing inferior materials with superior ones means using materials of a higher grade or with better performance in place of those of a lower grade or with poorer performance； Thick-for-thin substitution involves using material of a thicker grade of the same steel type in place of material of a thinner grade. The aforementioned substitutes are often considered to be feasible casually, thus ignoring the existence of some related issues, which sometimes even turn into serious problems. 　　When thick plates are used in place of thin ones, it often requires changes to the connection structure; for example, in the connection between a thickened head and the cylinder, the head usually needs to be trimmed on the outside. For equipment with a cylinder made of pipes, when the cylinder wall is thickened, the junction between the cylinder and the end cap sometimes also requires internal chamfering on the side of the cylinder. 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 often occur 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. Another issue with using thicker plates instead of thinner ones is that it may lead to insufficient strength [17]. This is because as the plate thickness increases, the allowable stress of the material tends to decrease. For example, in the case of 16MnR, when the thickness increases from 16 mm to 18 mm, the allowable stress drops from 170 MPa to 163 MPa ； When 20R increases from 16 mm to 18 mm at 100 ℃, the allowable stress decreases from 132 MPa to 126 MPa. This requires special attention during the manufacturing of head covers, as a certain amount of extra thickness is often added to the raw material when cutting these head covers in order to ensure the minimum thickness after stamping; this can result in insufficient strength of the stamped head covers [17]. Therefore, when thick layers are over thin layers under these critical conditions, the strength must also be verified. For components such as expansion joints, bellows, flexible thin sheet plates, and thin sheet plates, it is generally not advisable to use a thicker version in place of a thinner one; as the thickness of the component increases, its rigidity also increases, thereby reducing the effectiveness of deformation compensation. The perforated reinforcement plate should not be made excessively thick either, as a too-thick reinforcement plate will cause high stress concentrations at the periphery where it connects to the cylinder, leading to cracking at the weld roots. It can be seen from this that using thicker layers instead of thinner ones is not always beneficial; therefore, special attention should be paid to the aforementioned issues when making such substitutions. 　　In general, selecting the better option over the worse is always beneficial, as it can increase the strength reserve of equipment and enhance its safety and reliability; however, this is not the case in some special processing environments. In wet H2S environments, it is evident that carbon steel offers better resistance to H2S-induced SCC compared to materials such as 15MnVR, 16MnR, and 20MnMo [18]. The same is true for liquid ammonia environments; since 16MnR also suffers from SCC in such environments, using low-alloy steels like 16MnR instead of steels from the 20R, 20g, and Q235 series in these conditions makes it more likely to encounter problems. Secondly, for cases where the design requires a low strength-to-yield ratio of σs/σb, attention should also be paid to the principle of substituting the better option for the worse one, such as large-opening reinforcement structures, reinforcement plate structures, and reinforcement structures designed using the limit method. Another issue to consider when using materials with a higher strength grade as substitutes is weldability, as generally the higher the strength grade, the worse the weldability. If a material with an even higher grade is used as a substitute in such cases, welding will become even more difficult. Furthermore, for components such as expansion joints, rupture discs, and flexible gaskets, it is not permissible in principle to use inferior materials in place of superior ones; otherwise, recalculation must be carried out using alternative materials, with their thicknesses appropriately reduced, as failing to do so could lead to the failure of these components and the areas surrounding them. 　　In addition to the issues mentioned above, those who choose alternatives should also consider the cost-effectiveness of the equipment, as such choices will undoubtedly increase the cost of the equipment; particularly when it comes to selecting materials for the alternative equipment, it is essential to weigh the pros and cons carefully.

The most important point is that the quality of a material is relative to the properties of the medium.

Advantages and disadvantages are relative; for example, the allowable stress of high-strength steel is higher than that of ordinary low-alloy steel, but it is not suitable in environments subject to stress corrosion. Another example is welding processes, external compressive elastic modulus, chemical composition, cost-effectiveness, corrosion resistance, flaw detection and heat treatment, classification, etc.; for these reasons, it is not possible to simply prefer one option over another.

Material substitution is a type of design change that requires a change order to be issued. 1. It must be approved by the original design agency, with the risks falling on that agency; 2. or it must be approved by a manufacturing unit with the appropriate design qualifications, in which case the risks fall on the manufacturing unit.

Therefore, it is essential to distinguish between what is good and what is poor. It cannot be assumed that a material is good just because it has high mechanical properties or a high price; a comprehensive analysis is required, with the decision being made based on specific processing conditions.

When using alternative materials, their brittleness must also be taken into account, especially in low-temperature containers, where the brittleness of thick-walled plates is greater than that of thin-walled plates.

When considering material substitutions, in addition to economic considerations and mechanical properties, it is also necessary to determine whether the substitute material is suitable for the operating conditions

It is necessary to explain why a change in material is needed. From an economic perspective, if inferior materials are used in place of better ones, or thicker materials are used instead of thinner ones, the costs will likely be higher. It should be clear what considerations drive the use of materials with higher costs