What is the material substitute for the pressure vessel?

Note the following

Material Substitute Regulations

In the process of equipment design and manufacture, it is often difficult to purchase and select materials or to consider and analyze the lack of economics. Material substitution is widely used in the design of pressure vessels. "Stationary Pressure Vessel Safety Technical Supervision Regulations" has clearly stipulated in the substitution of pressure vessel materials. In its design and manufacture, the main requirement is that the material selection of the pressure-bearing parts of the pressure vessel and the material to be substituted are not only The shape and quality are similar, and the chemical composition, size standard, performance index and monitoring method of the material should be fully considered for reasonable substitution. The most basic principle of material substitution is: in the substitution process, it is necessary to eliminate the application of materials that do not meet the standards, strictly ensure the quality of materials and achieve absolute guarantees, and technically the technical requirements of substitute materials cannot be lower than the materials to be substituted. A material selection method to inspect and test materials.

The procedures for material substitution are as follows:

(1) The substitution of pressure-bearing parts of the container must be strictly carried out, and it must be approved by the technical department of the substitute unit and submitted to the re-inspection report or quality certificate of the substitute material, and the person in charge shall approve and reply;

(2) Substitute materials can be used in the manufacture of pressure vessels only after obtaining the permission of the original design unit and obtaining the certification documents;

(3) The design drawings, construction drawings and quality certificates of the pressure vessel shall be carefully marked with the specification parts, materials and specifications of the substitute materials.

Substitute good for bad

All metal materials used in pressure vessels should have excellent properties, including mechanical properties, corrosion resistance, high temperature resistance and manufacturing process of the material. The performance of each material is fixed. From the perspective of performance comparison, there are often problems of "superior" and "inferior" between materials. However, the requirements for material properties of each pressure vessel are also different in different situations. Therefore, the judgment of "superior" and "inferior" in material substitution is based on actual conditions, and specific problems are analyzed in detail. Below, based on my own work experience, the author mainly discusses several typical problems of "replacing the bad with the good".

2.1 In the manufacture of pressure vessels, in terms of mechanical properties such as strength and mechanical characteristics, although the commonly used low-alloy steels are obviously superior to carbon steels, their cold workability and weldability are inferior to those of carbon steels. Generally speaking, the higher the strength level, the poorer the cold workability and the weldability, and the two are negatively correlated. Therefore, the welding process should be adjusted accordingly, and there may be corresponding changes during the heat treatment, which should be given full attention.

2.2 Careful and comprehensive consideration should be taken when the material is replaced, otherwise there may be various safety hazards in the actual use of the pressure vessel. For example, in the wet hydrogen sulfide environment and the equipment with the risk of stress corrosion cracking, the sensitivity of the container to stress corrosion cracking increases with the increase of the strength level of the steel used in the container, and the two are positively correlated. At this time, if the 20R, Q235 and 20R series of steel are used as 16MnR and other low-alloy steels, it is very easy to cause problems. Therefore, this kind of "substituting the good for the bad" is not feasible in principle and should be prohibited. Killed steel has more advantages than boiling steel in many performance aspects, but in the manufacture of glass-lined containers, the enamel effect of killed steel is not as good as that of boiling steel.

2.3 Generally speaking, the corrosion resistance of stainless steel is excellent, but in the environment containing chloride ions, its corrosion resistance is not as good as that of low alloy steel and carbon steel.

2.4 Compared with ordinary stainless steel, although ultra-low carbon stainless steel has a price advantage and good corrosion resistance, the former has better high temperature thermal strength. In general, in order to improve corrosion resistance, it is necessary to reduce the carbon content, and in order to improve high temperature resistance, it is necessary to increase the carbon content. Therefore, in this case, the "superiority replaces the inferior", the equipment temperature should be designed especially accurately, and if necessary, it should be recalculated.

Substitute thick for thin

"Substituting thicker for thinness" often changes the stress state of the plane-stressed shell into a plane-strain state, which is harmful to the stress state of the vessel. Thin-walled vessels are more likely to generate three-dimensional tensile stress, which leads to plane strain brittle fracture.

3.1 For a vessel with equal thickness welding between the head and the cylinder in the original design, if the individual parts of the vessel shell are thickened instead of thinner, it is easy to increase the geometric discontinuity of the shell, so that the gap between the head and the simplified The local stress on the connection part increases, and at this time, it will cause great damage to the vessel that is prone to stress corrosion. Fatigue cracks may be caused, and fatigue fractures may be caused in severe cases.

3.2 When the thick plate replaces the thin plate, the connection structure is often changed accordingly. For example, when the simplified and thickened head is connected, the head usually needs to be chamfered. For equipment mainly composed of pipes, if the thickness of the cylinder wall is increased, the inner side of the simplified side must also be trimmed at the connection part between the head and the simplified. When the thickness increases greatly, it is often related to the change of the welding process.

3.3 "Substituting thick for thin" on the overall level of the container shell, although it will not increase the local stress of the simplified joint and the head, it will inevitably lead to adverse effects. (1) After the thickness is increased, the flaw detection method and welding process in the original shell design should also be changed accordingly, increasing the difficulty; (2) The increase in the thickness of the shell will inevitably increase the weight of the container. When the thickness is too large, it will inevitably have an adverse effect on the foundation and support of the container; (3) For the container that has a heat transfer effect on the shell at the same time, the increase in the thickness of the shell will definitely affect its heat transfer effect.

Other considerations

When substituting materials, the welding process should be adjusted appropriately according to the actual materials used. The general adjustment principle is: when replacing low-grade materials with high-grade materials, the standard of low-grade materials can still be used for experiments and acceptance without raising the standard; When the properties such as high temperature and toughness are different, the corresponding temperature may also change when the minimum water pressure test is carried out. At this time, the relevant provisions of GB150 should be strictly implemented; When the thickness of the steel plate reaches a certain level, ultrasonic flaw detection is also required, and if necessary, the pressure of the water test is increased.

5 Conclusions The design and manufacture of pressure vessels for the main material body just now is the basis of current vessel applications, and it is even more difficult and difficult for the performance requirements to be reasonable in the material substitution of pressure vessels. In terms of the mechanical properties of the material, the toughness should also be considered when the strength of the material is tested twice. If the toughness is satisfied, the strength should be improved as much as possible. From this point of view, it is necessary to correctly define "excellent" and "inferior" in the selection of pressure vessel materials, not simply from the thickness and strength of the material, but to conduct comprehensive analysis and consideration.