Stress-Induced Deformation and Prevention
1. Deformation Caused by Flame Cutting
(1) Shell Sections: When flame-cutting the edges of short shell sections for large-diameter vessels (where the raw material is relatively long and narrow), the cut edges at the ends are prone to deformation. This occurs because the high heat of cutting causes the processed edge to contract upon cooling, transforming the originally straight edge into a curved one. Consequently, after the shell section is rolled into a cylinder, its ends no longer lie within the same horizontal plane; if the resulting dimensional error is significant, it may fail to meet the requirements for fit-up and welding. Methods such as symmetrical cutting or mechanical machining should be employed to prevent such deformation.
(2) Heads: After the raw material for a formed head is flame-cut to size, the perimeter of the opening undergoes shrinkage, resulting in a reduction of the head's diameter. In severe cases, the diameter of the head after shrinkage may fail to meet the specified dimensional requirements. When processing the opening of a integrally formed head using flame cutting, the design of the forming die must account for the anticipated shrinkage after cutting. For segmented heads (assembled from multiple petals), if flame cutting is used to process the edges, the diameter of the head assembly should be slightly enlarged during fit-up to compensate for the shrinkage caused by cutting. Alternatively, mechanical machining methods can be utilized to prevent deformation.
(3) Machining Blanks (Primarily Steel Plate Blanks): These blanks are typically used for manufacturing large flanges, sealing rings, and similar components for pressure vessels. After flame cutting, uneven thermal expansion and contraction within the steel plate can cause the surface of the blank to become warped or uneven. In severe instances, this may result in insufficient machining allowance on the blank's surface. The steel plate blank should be flattened and straightened after cutting; for blanks that are difficult to straighten, the machining allowance should be appropriately increased.
2. Deformation Caused by Machining Instability
Deformation caused by machining instability typically occurs when cutting large openings (such as access ports or manholes) into already formed heads or shell sections. Because the structural stability of the opening zone-and the surrounding area-is compromised, the shell or component may undergo localized deformation. It is advisable to avoid cutting large openings directly into individual shell sections or heads whenever possible; instead, depending on the specific situation, the shell components should be assembled into a larger sub-assembly or a complete vessel before the large openings are cut. Prior to cutting a large opening, the designated opening zone should be reinforced using stiffening ribs or plates clamped tightly against the shell surface. These temporary reinforcements should remain in place until the nozzle has been welded in and the shell structure has regained its overall stability, at which point the reinforcing plates can be removed.
3. Welding Deformation
The welding procedure constitutes the technical requirements and operational regulations for vessel welding. It encompasses the specific welding method employed, joint preparation (groove design), electrode type and diameter, welding parameters, welding sequence, number of weld layers, pre- and post-weld treatments, environmental requirements for welding, as well as measures for preventing and counteracting deformation. The welding procedure must undergo and successfully pass a formal qualification process; furthermore, during actual welding operations, strict adherence to these procedural requirements is mandatory.
Based on the specific welding conditions and the volume of welding involved for pressure vessels and large-scale components, a preliminary analysis is conducted to predict the magnitude and pattern of anticipated welding deformation. Subsequently, targeted control measures are formulated, including:
(1) For large pressure vessels involving multiple weld passes-such as spherical vessels-the various sections should first be assembled and joined into a complete unit before welding commences. Welding operations must be performed symmetrically and strictly in accordance with the prescribed welding sequence.
(2) For large components involving multiple weld passes-such as composite petal-type heads or shell transition sections assembled from individual petals-in addition to complying with the aforementioned requirements, specialized "C-clamp" type fixtures should be installed at the welding site to provide rigid restraint.
(3) For pressure vessels of considerable length that are assembled from multiple cylindrical sections (cans), the dimensions of these sections must be cut with an appropriate allowance for anticipated welding shrinkage; this ensures that the finished shell does not end up shorter than specified after welding is complete.
(4) For pressure vessels-particularly those with complex structural designs-the assembly process must follow a logical sequence, and appropriate measures to prevent welding deformation must be implemented to ensure that the vessel retains its intended shape throughout the manufacturing process.
(5) Counter-Deformation Measures: Based on practical experience or engineering calculations, a deliberate "counter-deformation" (pre-deformation) is applied to the workpiece in a direction opposite to that of the anticipated welding deformation. The magnitude of this pre-deformation is calculated such that it is precisely offset by the subsequent welding deformation. Specific implementation methods include: when striking the arc at the ends of the longitudinal butt joints on pressure vessel shells, a specific allowance for counter-deformation is established in the direction opposite to where welding deformation is expected to occur; similarly, for composite petal-type heads and transition sections, the dimensions of the forming dies or molds are designed to incorporate the necessary counter-deformation allowance to offset the effects of welding deformation.
4. Preventive Measures Against Heat Treatment Deformation
(1) Heat treatment furnaces must strictly comply with relevant code and standard requirements. The temperature within the furnace must be uniform and accurately controlled. Firewalls or baffles should be installed near the flame burners on the furnace walls to prevent direct contact-or even close proximity-between the open flame and the components undergoing heat treatment.
(2) When pressure vessels of considerable length are loaded into the furnace for heat treatment, temporary support blocks or fixtures must be added to provide adequate support. The number of such supports required should be determined based on the specific dimensions and geometry of the vessel in question. (3) Shells with a relatively large diameter and thin wall thickness should generally undergo internal reinforcement.
(4) For pressure vessels prefabricated in sections, reinforcing supports should be installed at the section interfaces.
(5) Pressure vessel components that are susceptible to instability under high-temperature conditions should also be reinforced and strengthened as appropriate, based on specific circumstances.
Deformation Caused by Processing Errors
1. Deformation Caused by Cutting Errors
Inaccurate cutting dimensions result in the shape of the formed component deviating beyond the limits specified by relevant standards. Inaccurate cutting dimensions are primarily caused by errors in calculation or in the preparation of full-scale templates. In addition to improving the technical proficiency of the cutting personnel, a system for verifying cutting dimensions should be implemented, and-whenever possible-computer software should be utilized to manage and control cutting dimensions.
2. Deformation Caused by Forming Errors
Deformation in pressure vessel components during the forming process can occur due to improper operation or the use of non-standard molds. For hot-formed heads, specific requirements exist regarding the demolding temperature; premature demolding while the temperature remains high can lead to significant shrinkage of the head, potentially causing its geometric dimensions to exceed standard tolerances in severe cases. Components formed via mechanical rolling or pressing may also undergo deformation due to improper operational techniques. Furthermore, inadequate or erroneous mold design can result in the geometric dimensions of the formed pressure vessel components failing to meet specified requirements. The primary preventive measures include:
(1) Forming operations must be conducted in strict accordance with the prescribed technical process requirements.
(2) Inspection templates must be utilized to strictly control the shape of the processed components.
(3) Mold design should be based on the theoretical dimensions and shape of the component, while fully accounting for the dimensional changes that occur both during and after the forming process. Specifically, cold-forming molds must account for the springback of the formed component, whereas hot-forming molds must account for the shrinkage of the component upon cooling.
3. Deformation Caused by Assembly Errors
Deformation that occurs during the assembly of pressure vessel shells-resulting from excessive misalignment (mismatch) at joints or deviations in straightness-is referred to as assembly deformation. Preventive measures include:
(1) Positioning fixtures should be utilized during shell assembly. For shells with a relatively large diameter and thin wall thickness, additional internal supports should be installed within the shell sections during assembly to strictly limit any misalignment at the shell's butt joints.
(2) Horizontal assembly of pressure vessel shells should be performed on roller supports, and a straightedge or reference line should be used to verify and control the straightness of the shell. (3) For pressure vessels fabricated in sections, positioning fixtures shall be installed during assembly, and a theodolite shall be used to verify their straightness.
