? Paper Keywords: substitution of pressure vessel materials with superior instead of inferior and thick instead of thin
? Abstract: One of the most important parts in the design of pressure vessels is the selection of materials, which is directly related to the quality and safety of pressure vessels. However, due to the difficulty in purchasing equipment during the manufacturing process, the phenomenon of material substitution is widespread. Common substitution problems are: shoddy, thick instead of thin, etc. These problems are directly related to the quality and safety of ships and the economic management of investment and construction parties, which deserve our attention.
How to choose suitable materials is the first step in the design and manufacture of pressure vessels, and it is also an intuitive and important step. In the design and manufacture of pressure vessels, once the materials are improperly selected, it will leave great hidden dangers for the safe use of the vessels. Therefore, when selecting materials for pressure vessels, materials with suitable mechanical properties, weldability and corrosion resistance should be selected according to the specific service conditions of the vessels, such as design pressure and temperature, operating characteristics and medium characteristics. In addition, other factors such as specific processing technology and economy should be fully considered when selecting materials.
1 Specific provisions on material substitution
In the process of equipment design and manufacture, due to the difficulty in purchasing materials or economic reasons, the phenomenon of material substitution often occurs in the design of pressure vessels. The Safety Technical Supervision Regulations for Fixed Pressure Vessels (TSG R0004-2009) and Steel Pressure Vessels (GB 150- 1998) have made relevant provisions on material substitution. Generally speaking, the main requirements are: in the selection of substitute materials, the pressure-bearing parts of pressure vessels should have the same or similar appearance quality, chemical composition, dimensional tolerance, performance index, inspection items and inspection rate as the substitute materials. The most basic principle of material substitution is to absolutely ensure that the substitute material is not lower than the substituted material in technical requirements. For some substitute materials that are not strict in detection rate or performance items, we can choose suitable substitute materials through inspection and testing. The procedural requirements for material substitution are as follows: (1) The substitution of pressure-bearing parts of containers should be strictly implemented, and it must be approved by the technical department of the substitution unit, and the reinspection report or quality certificate of substitute materials should be submitted and approved by the person in charge; (2) Material substitution can only be carried out in the manufacture of pressure vessels after obtaining the permission of the original design unit and the supporting documents; (3) The design drawings, construction drawings and quality certificates of pressure vessels shall be carefully marked with specifications, parts, materials and substitute materials.
Replace inferiority with superiority.
All metal materials used in pressure vessels should have excellent properties, including mechanical properties, corrosion resistance, high temperature resistance and manufacturing technology. The performance of each material is fixed. From the perspective of performance comparison, there are often "excellent" and "poor" problems between materials. However, under different conditions, each pressure vessel has different requirements for material properties, so the judgment of "excellent" and "poor" in material substitution is based on reality and specific problems are analyzed. Below, the author combines his own work experience and mainly discusses several typical problems of "shoddy".
2. 1 in the manufacture of pressure vessels, the commonly used low-alloy steel is obviously superior to carbon steel in mechanical properties such as strength and mechanical properties, but its cold working performance and weldability are not comparable to that of carbon steel. Generally speaking, the cold workability and weldability of high strength grade are poor, and they are negatively correlated. Therefore, when replacing in this respect, the welding process should also be adjusted accordingly, and there may be corresponding changes in the heat treatment process, which should be given full attention.
2.2 Material substitution should be carefully and comprehensively considered, otherwise various safety hazards may occur in the actual use of pressure vessels. For example, in the humid 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 grade of the steel used in the container, and the two are positively correlated. At this time, if 20R, Q235 and 20R series steels are made of low alloy steel such as 16MnR, problems will easily occur. Therefore, this kind of "shoddy" behavior is not feasible in principle and should be banned. Killed steel is superior to boiling steel in many aspects, but the enamel effect of killed steel is not as good as boiling steel in the manufacture of glass-lined containers.
2.3 Generally speaking, stainless steel has excellent corrosion resistance, but its corrosion resistance is not as good as that of low alloy steel and carbon steel in an environment containing chloride ions.
2.4 Compared with ordinary stainless steel, ultra-low carbon stainless steel has price advantage and good corrosion resistance, but its high-temperature thermal strength is better. Generally speaking, in order to improve corrosion resistance, the content must be reduced, while in order to improve high temperature resistance, the content of carbon must be increased. Therefore, in this case, it is necessary to accurately design the equipment temperature and recalculate it if necessary.
2.5 In principle, expansion joints, bursting discs, flexible pipe plates and other parts shall not be replaced by superior products. Under special circumstances, if replacement is necessary, the replacement materials should be used for recalculation, and the thickness of the parts should be adjusted appropriately according to the results, so as to prevent the failure of such parts and their adjacent parts.
2.6 For the tubesheet of heat exchanger, the overall performance of forgings is better than that of plates, so forgings are usually used, but when the thickness of tubesheet is less than 6cm, plates can also be used instead of forgings. However, it should be noted that even if the thickness, material and design temperature of forgings and plates are the same, their allowable forces are different, and the allowable forces of the former are slightly lower than those of the latter. Therefore, if it is necessary to replace the plate with forgings, the thickness of the tube plate should be re-approved.
For steel, the subtle differences in chemical composition may have a great impact on its performance, so we should pay full attention to the problem of "inferior quality" of steel for any type of pressure vessel, so as not to cause the product to be inconsistent with the original design.
3 thick instead of thin
"Replacing thickness with thickness" often changes the stress state of plane stress shell into plane strain state, which is harmful to the stress state of container. In general, thick-walled vessels are more likely to produce three-dimensional tensile stress than thin-walled vessels, and then plane strain brittle fracture occurs.
3. 1 used for equal thickness welding between the container cover and the cylinder in the original design, if the container shell
Replacing other members with thick members will easily increase the geometric discontinuity of the shell and increase the local stress at the joint between the head and the cylinder. At this time, it will cause great damage to containers with stress corrosion tendency. It may lead to fatigue crack and serious fatigue fracture.
3.2 When a thick plate replaces a thin plate, the connection structure often changes accordingly. For example, when a cylinder is connected with a thickened head, it is usually necessary to trim the head. For the equipment with pipes as the main cylinder, if the cylinder wall thickness is increased, the cylinder side must also be chamfered at the joint between the head and the cylinder. When the thickness is greatly increased, it is often related to the change of welding process.
3.3 "Replacing thickness with thickness" on the overall level of container shell will not increase the local stress at the joint between cylinder and head, but it will inevitably lead to the following adverse effects. After the thickness of 1) increases, the flaw detection method and welding process in the original shell design will also change accordingly, which increases the difficulty; 2) The increase of shell thickness will inevitably increase the weight of the container, and when the weight of the container increases too much, it will inevitably have an adverse impact on the foundation and support of the container; 3) For the container with heat transfer function, the increase of shell thickness will definitely affect its heat transfer effect.
3.4 The allowable force of steel plate is closely related to its thickness. Steel Pressure Vessels (GB 150- 1998) points out that the allowable force of steel decreases with the increase of its thickness, and the two are negatively correlated. For example, at 20℃- 150℃, when the thickness of 16MnR is changed from 16mm to 18mm, its allowable force is reduced from 170MPa to 167MPa, and at/kloc, It can be seen that replacing thin with thick is likely to lead to insufficient strength, so it is necessary to check the strength of replacing thin with thick in critical state.