Flange leakage analysis and countermeasures

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1. Flange seal surface type: 1) Flat seal surface – The sealing surface is a smooth plane; usually, there are 2–3 concentric grooves on this plane. When the bolts are tightened, the gasket tends to be pushed outwards and inwards, making it difficult to achieve a proper seal. It is only suitable for applications with low pressure, non-toxic media, and materials that are not flammable or explosive. 2) Concave-convex sealing surface: It consists of a concave surface and a convex surface. This type of sealing gasket facilitates alignment, and it does not get squeezed out when compressed; it can be used in applications with high pressures. 3) The tenon-and- groove type sealing surface consists of a tenon surface and a groove surface; the gasket is placed in the groove and does not move as a result of compression. Since the gasket is narrow, the compressive force applied is relatively low. Its disadvantage is that the structure and manufacturing process are relatively complex, and replacing the gasket is also cumbersome. It is generally used in environments with flammable, explosive, or toxic media, as well as in situations with high pressure. 2. Requirements for the sealing surface: There are mainly two aspects to the requirements for the sealing surface. The first is surface roughness. It is required that the surface roughness of the flange be low; for the sealing surfaces of metal gaskets, this value is generally 1.6√ ； For sealing surfaces that use soft gaskets, too low a roughness is not desirable; it is generally around 3.2√, as this reduces the resistance to interfacial leakage and makes leaks more likely to occur. Several rounds of sealing lines (commonly known as water lines) are applied on the surface of the flange, also to prevent leaks. The other is stiffness. When the flange stiffness is insufficient, warping or wavy deformation may occur, leading to seal failure. 3. Gaskets 1) Performance of gaskets: Gaskets are an important component of flange sealing, and their quality directly affects the sealing performance. The gasket material should be dense, resistant to being impregnated by the medium, and able to withstand temperature and pressure fluctuations as well as medium corrosion. It should have appropriate deformation resilience and minimal permanent deformation. Generally, gaskets with high resilience can withstand fluctuations in pressure and temperature, while gaskets made of dense materials are less prone to leakage or corrosion and aging. 2) Difference in hardness between the flange and the gasket: Flange sealing relies primarily on the elastic or plastic deformation of the gasket, which fills in the minor irregularities on the flange surfaces and prevents leakage at the interface. Therefore, the hardness of the gasket should be lower than that of the flange, within acceptable limits; the greater the difference between the two, the easier it is to achieve sealing. 4. Bolt preload: Flange sealing is achieved by using bolts to compress the gasket. When the bolts are tightened, the gasket undergoes elastic or plastic deformation as a result of being compressed by the flanges, thereby filling in the minor irregularities on the flange surfaces and achieving sealing. During operation, it also allows the gasket to maintain a high sealing pressure, thus preserving a good sealing condition. The bolt preload must be applied to the gasket in a uniform and symmetrical manner. If the preload is too low, the gasket will not be compressed enough to prevent leaks; on the other hand, an excessive preload often causes excessive compressive deformation of the gasket, resulting in a loss of its resilience or even damage to it. 5. Operating conditions Operating conditions refer to the pressure and temperature of the system, as well as the physical and chemical properties of the medium. Pure pressure or medium factors have a minor impact on sealing; it is only when combined with temperature, especially under fluctuating high temperatures, that they will significantly affect the performance of the seal. At high temperatures, the dissolution and corrosion of the gasket by the medium intensify, increasing the likelihood of leakage. The flanges, bolts, and gaskets may experience creep and stress relaxation, which reduces the sealing pressure. If both temperature and pressure fluctuate, the gasket will become fatigued, leading to a failure in sealing. In seals used at low temperatures, the different cooling rates of the flange and bolts result in an uneven decrease in the preload on the gaskets. Additionally, the contraction of the gaskets and their reduced elasticity at low temperatures accelerate the occurrence of leaks. This requires that the operating conditions be stable, with efforts made to avoid sharp fluctuations in pressure and temperature; the bolts should be tightened again after some time of use, which is particularly important in applications involving high temperatures or cycles of heat and cold. Of course, there are many other factors that affect the sealing performance of flanges, such as mechanical damage or radial scratches on the flange surface, misalignment of the two flanges during installation, insufficient flexibility of the pipeline, and stress. 6 Installation: The surface of the flange must be kept clean; it must not have any mechanical damage, corrosion, or residual old gaskets. Check whether the surface roughness of the flanges is appropriate, whether the protrusions on the flange surfaces can be aligned, and whether the gap between the two flange surfaces is too large. The parallelism of the two sealing surfaces must meet the requirements specified for the use of gaskets; the bolt load should be used only to compress the gaskets, and not to pull the flanges together. Check whether the form, material, size of the gasket and the bolts meet the specified requirements. Place the gasket in the correct position, tighten the bolts evenly, and check whether the assembly is correct and proper. The bolts must be tightened symmetrically and gradually; if a certain bolt stress is required, a torque wrench should be used to achieve the correct bolt preload. The elongation of the bolts can also be measured using a hydraulic bolt tensioner on the **lan. In situations where the gasket requires a certain degree of compression, a feeler gauge must be used to check whether the gasket is compressed to the appropriate level. Depending on the usage conditions, in applications with high temperatures or cold and hot cycles, the gasket needs to be tightened again after being used for a period of time. Non-metallic gaskets must be stored in a dry and cool place, away from direct sunlight or areas with ozone. They should be placed flat rather than hung on hooks. If they are stored for more than 2 years, it is necessary to check whether they have deteriorated. The boxes in which the gaskets are stored must indicate technical details such as the material, type, size of the gasket, as well as the temperature and pressure ratings of the flanges, to prevent incorrect use. Reason 1: Unstable operating conditions, with fluctuations in temperature and pressure. 2 The clamping force of the flange is insufficient at high temperatures. The bolt material is 1Cr18Ni9Ti, which exhibits good performance at temperatures below 600°C. Above 600°C, the strength of the bolts decreases, and plastic deformation as well as oxidation increase under tensile stress. In particular, after multiple startups and shutdowns, the clamping force of the bolts on the flange significantly declines, which is one of the reasons for flange leakage. (Thermal tightening) 3. Expansion graphite gaskets fail at high temperatures. The sealing of flange, bolt, and gasket assemblies is achieved, essentially, by increasing the leakage resistance of fluids through the deformation of the gaskets. Therefore, the sealing performance of the gasket at high temperatures depends on its compression properties. 4 Oxidation failure: During operation, under the influence of medium pressure and high temperatures of 600°C, the graphite contained in the wound graphite gasket reacts with the O2 present in the environment as follows: C (graphite) + O2 (g) → CO2 (g). Although this reaction is not very intense at around 600°C, over time the graphite in the gasket is gradually oxidized from the outside in, thereby destroying the gasket’s original compressive properties and causing it to lose its sealing function. 5 Failure of the water-gas reaction: When the temperature exceeds 640°C, the expanded graphite in the gasket reacts with water vapor as follows: C (graphite) + H2O (g) → CO (g) + H2 (g). This reaction starts by breaking out at a certain point along the radial direction of the gasket, and then spreads in a circumferential direction, thereby causing the gasket to fail rapidly. Based on the inspection and replacement of gaskets, it was found that the removed graphite-wound gaskets no longer contained any graphite; only the outer ring and the sealing stainless steel strip remained. This further confirms that at high temperatures, the expanded graphite in the gaskets underwent a water-gas reaction, resulting in the failure of those gaskets. The solution involves using a new type of gasket: a copper wave-tooth composite gasket. During on-site maintenance, it was found that neither the gasket nor the flange face was damaged; it can be inferred that the leakage was caused by a failure in the proper compression of the flange gasket. We first analyzed the original design data for the sealing surface of the carburetor tube box flange. We found that although the bolt tension in the original design met the design requirements, due to the high operating pressure of the medium (3.7 Mpa), the specific pressure of the gaskets used in the original design was low, resulting in a large difference in the minimum compression force required for the gaskets. As a result, the bolt tension acting on the gaskets under pre-tensioning conditions was as high as 3.5×10^6 N, while the actual compression force of the gaskets during operation would be reduced by 2.7×10^5 N. The original gasket for the carburetor’s tube box is a steel-metal-encased asbestos gasket; due to the difference in elastic moduli between the metal surface and the asbestos inside, its rebound capacity is **lower than that of asbestos gaskets. All these factors result in the gasket having a limited performance under conditions of up to 3. A large plastic deformation occurs under a preloading force of 5×10N, reducing the resilience of the gasket. As the compressive force on the gasket decreases, it is no longer able to provide sufficient compensation, resulting in leakage. During startup and shutdown, the speed of operation is high, resulting in frequent changes in the operating temperature and pressure of the sealing surfaces. The water temperature at the inlet of the shell side of the waste heat boiler is 50–60°C, while the temperature of the raw gas at the inlet of the tube side is 210°C. This results in a large temperature difference, leading to uneven temperature distribution across the components. The temperature is higher in the upper part of the waste heat boiler than in its lower part. This causes the flanges and bolts at various parts of the sealing surface to expand and contract differently due to thermal changes, generating significant temperature differential stresses. As a result, the local gaps at the sealing surface increase, which in turn reduces the compression force on the gaskets. When this compression force becomes less than that required by the operating pressure, leakage occurs at the sealing surface. To address the causes of carburetor leakage, we improved the original carburetor from two aspects. (1) Wave-tooth composite gaskets are used to replace the original steel ladle asbestos gaskets. Wave-tooth composite gaskets are a new type of sealing gasket, composed of a metal framework with a special structure combined with expanded graphite material. As shown in Figure 2 of its cross-sectional view, the upper and lower surfaces of the metal frame feature concentric circular grooves of a special shape that are offset from each other, thereby granting it good elasticity. Composite gaskets on the upper and lower surfaces of the metal frame. Wave-shaped gaskets combine the advantages of both metallic and non-metallic expanded graphite surfaces. Thanks to the good resilience of their metallic framework structure, they maintain excellent sealing properties and resilience even under conditions of pressure and temperature changes, making them particularly suitable for high-temperature, high-pressure environments as well as those with fluctuating pressure and temperature. Based on the operating pressure and temperature of the carburetor, we selected BCH–0–MF–950–4. The 0–A–Ⅱ type composite wave-shaped gasket uses a metal framework made of 0Crl3 stainless steel combined with expanded graphite material; its design pressure is 4 Mpa, and it can operate properly within the temperature range of –60 to 200°C. This type of wave-shaped gasket not only has strong self-compensation capabilities, but its high specific pressure also increases the minimum compressive force required of the gasket, thereby improving the stress conditions on it and ensuring the reliability of the sealing surface. (2) Disc springs are used in place of the conventional bolt washers. These disc springs are made from highly elastic materials, and they are called disc springs due to their disc-shaped design. It features a low stroke and high compensation. It can absorb the stress relaxation caused by temperature fluctuations, maintaining a constant preload on the bolts, while also eliminating the effects of pressure changes and compensating for the deformation of the flange sealing material. Based on the operating parameters of the carburetor, we selected the 693A–71 type disc spring.