Some Opinions on Material Change in Pressure Vessel Manufacturing

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How to select the correct materials is the first issue to be resolved in the design and manufacture of pressure vessels, and it is also a very important aspect. Once the material is selected incorrectly, it will leave a greater hidden danger to the safe operation of the container. Therefore, in the selection of pressure vessel materials, materials with appropriate mechanical, welding and corrosion resistance properties should be selected according to the operating parameters of the vessel, such as pressure and temperature, operating requirements, and medium characteristics. In addition, other relevant factors such as specific processing technology and technical and economic performance must be fully considered when selecting materials. Enough attention should be paid to the selection and modification of materials.

1. Specific regulations on material substitution

In the process of designing and manufacturing equipment, there are often difficulties in material procurement or due to economic considerations, material changes and substitutions often occur. “Stationary Pressure Vessel Safety Technical Supervision Regulations (TSGR0004-2009)” and “Stationary Pressure Vessel (GB150-2011)” have made relevant provisions for material substitution. Mainly include: “The pressure-bearing parts of the pressure vessel should have the same or similar appearance quality, chemical composition, dimensional tolerances, performance indicators, inspection items and inspection rates in the selection of substitute materials as the substitute materials.” The basic substitute of materials The principle is: to absolutely guarantee that, in terms of technical requirements, the substitute materials shall not be lower than the substitute materials. For individual substitute materials that are not strictly required in terms of detection rate or performance items, inspection and testing can be used to select suitable substitute materials.

2. Substituting the good for the bad

All metal materials used in pressure vessels must have excellent properties, including mechanical properties, corrosion resistance, high temperature resistance, and manufacturing processes. The performance of each material is fixed, and the judgment of “good” and “bad” in material substitution is based on reality, and specific problems are analyzed in detail. Based on my own work experience, I have expressed some insights on the following typical issues of “substituting good for bad”.

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 steel is significantly better than carbon steel, its cold workability and weldability are worse than carbon steel. Generally speaking, if the strength level is high, the cold workability and weldability are poor, and the two are negatively correlated. Therefore, when substituting this aspect, the welding process should be adjusted accordingly, and there may be corresponding changes during the heat treatment, and full attention should be given to the technical requirements.

2.2 When substituting materials, careful and thorough consideration should be taken to avoid various potential safety hazards in the actual use of pressure vessels. For example, in a wet hydrogen sulfide environment and in equipment with a 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 Q235 and Q245R series steels are substituted with low alloy steels such as Q345R, it is very easy to cause problems. Therefore, this kind of “substituting the good for the bad” principle is not recommended and should be avoided.

2.3 Compared with ordinary stainless steel, although ultra-low carbon stainless steel is more expensive and has good corrosion resistance, the former has better high-temperature thermal strength than the latter. In general, in order to improve the corrosion resistance, the carbon content needs to be reduced, and in order to improve the high temperature resistance, the carbon content must be increased. Therefore, in this case, the design temperature of the equipment should be carefully taken into account in the “replacement of the good with the bad”, and the calculation should be recalculated if necessary.

2.4 In principle, the selection of expansion joints, rupture discs, flexible tube sheets and such parts cannot simply be done to replace the inferior with the superior. In special circumstances, the substitute materials must be recalculated accurately and adjusted appropriately according to the results. The thickness of the part to prevent problems or failures between such parts and their adjacent parts.

For steel, small differences in its chemical composition may have a significant impact on its performance. Therefore, we must pay full attention to the problem of “substituting the good for the poor” of any type of pressure vessel steel, and be cautious to avoid causing product damage. The original design does not match, resulting in major undesirable consequences.

Three, replace thin with thick

“Substituting thickness for thinness” often causes the container shell to change from the stressed state of the plane stress state to the plane strain state, which is harmful but not beneficial to the stressed state of the container. Normally, thick-walled containers It is easier to produce three-way tensile stress than thin-walled containers, and then to produce plane strain brittle fracture.

3.1 For the container with equal thickness welded between the head and the cylinder in the original design, if the thickness of the individual parts of the container shell is replaced with thinner, it is easy to cause or increase the geometric discontinuity of the shell, so that the head and the cylinder The local stress at the connection part between the bodies increases. At this time, it will cause great damage to the container with stress corrosion tendency. It may cause fatigue cracks, which may cause fatigue fracture in severe cases.

3.2 When a thick plate replaces a thin plate, the connection structure often changes accordingly. For example, when the cylinder is connected to a thickened head, it is usually necessary to trim the head. For equipment with pipes as the main cylinder, if the thickness of the cylinder wall is increased, the inner side of the cylinder must also be trimmed at the connection part between the head and the cylinder. When the thickness increases greatly, the change of the welding process is often also taken into consideration.

3.3 The “thickness instead of thinness” on the overall level of the container shell will not increase the local stress at the connection of the cylinder and the head, but inevitably, it will still cause some adverse effects. 1) After the thickness is increased, the flaw detection method and welding process in the original shell design will also be changed accordingly, which increases the manufacturing difficulty; 2) The increase in the thickness of the shell will inevitably increase the weight of the container. When it is too large, it will inevitably have an adverse effect on the foundation and support of the container; 3) For containers with the shell also having a heat transfer function, the increase in the thickness of the shell will definitely affect its heat transfer effect.

3.4 The allowable application force of steel plate is closely related to its thickness. “Fixed Pressure Vessel (GB150-2011)” pointed out that the allowable application force of steel plate decreases with the increase of its plate thickness, and the two are negatively correlated. For example, under the environment of 20℃-150℃, when the thickness of Q345R plate changes from 16mm to 18mm, its allowable application force will decrease from 170MPa to 167MPa. At 150℃, when the plate thickness of Q245R changes from 16mm to 18mm, its allowable application force It is reduced from 135MPa to 125MPa. It can be seen that substituting thickness for thinness may cause the strength of the material to decrease under extreme conditions. Therefore, it is necessary to check the strength of substituting thickness for thinness in a critical state.

3.5 Because the thickness of the element is proportional to its rigidity, the greater the thickness, the stronger the rigidity, so in principle, it is not allowed to replace the thickness of the flexible thin tube sheet, bellows and expansion joints with thickness to prevent weakening and compensating deformation. ability.

In summary, the pros and cons of substituting thick for thin are very complicated. When substituting, the feasibility and impact of the substitution should be comprehensively considered before deciding whether it is feasible.

Fourth, conclusion

The substitution of materials in the manufacture of pressure vessels is not only a technical problem, but also a comprehensive technical and economic problem including the safety of the vessel, the economic benefits of investment, the cost of manufacturing and other economic and management issues. Therefore, no matter what kind of material is substituted, it is essentially a change in the design of the pressure vessel, and sufficient attention should be given to it, and corresponding design and production changes should be made.

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