Flange sealing generally relies on the preload generated by its connecting bolts to produce sufficient operating gasket contact pressure through various solid gaskets (such as rubber gaskets, asbestos rubber gaskets, plant-fiber gaskets, spiral-wound metal gaskets with asbestos filler, corrugated metal gaskets with asbestos filler, corrugated metal-jacketed gaskets with asbestos filler, corrugated metal gaskets, flat metal-jacketed gaskets with asbestos filler, grooved metal gaskets, flat metal gaskets with a raised center, metal ring gaskets, and metal octagonal gaskets) or liquid gaskets (which become solid gaskets of a certain shape after a certain time or under certain conditions). This prevents outward leakage of the sealed fluid medium. It falls within the category of forced sealing, as shown in Figure 2-1.
Common leaks of this sealing structure include the following.
1. Interface leak
The main causes include insufficient gasket compression, unsuitable roughness of the flange mating surfaces, thermal deformation of piping, and mechanical vibration, all of which can prevent a tight seal between the gasket and flange surfaces and cause leakage. After the flanges are connected, bolt deformation and elongation, plastic deformation and reduced resilience of gaskets after prolonged use, and aging, cracking, or deterioration of gasket material can also impair sealing and cause leakage, as shown in Figure 2-2. Leakage resulting from inadequate conformity at the interface between the metal surface and sealing gasket is therefore called 'interface leakage'.
Interface leakage will occur regardless of the type of gasket or the material the gasket is made of.
The vast majority of leakage accidents that occur at flange connection parts are interface leakage, and interface leakage accidents account for more than 80%~95% of all flange leaks.
2. Penetration leak
Plant fibers (cotton, hemp, and silk), animal fibers (wool, rabbit hair, etc.), mineral fibers (asbestos, graphite, glass, ceramics, etc.), and chemical fibers (nylon, polytetrafluoroethylene, and other plastic fibers) are common raw materials for sealing gaskets. Leather and cardboard are also frequently used. These gasket base materials have relatively loose, low-density structures, with countless microscopic gaps between the fibers that are easily penetrated by fluid media. Particularly under fluid pressure, the sealed medium can pass through the tiny gaps between fibers to the low-pressure side, as shown in Figure 2-3.
Therefore, we call the leakage phenomenon caused by the fact that there are certain gaps between the fibers of the gasket material and the fluid medium can pass through these gaps under certain conditions as "penetration leakage".
Permeation leakage is generally related to the operating pressure of the sealed fluid medium: the higher the pressure, the greater the leakage flow. It also depends on the fluid's physical properties. Low-viscosity media are more prone to permeation leakage, while high-viscosity media are less prone to it. Permeation leakage generally accounts for approximately 8%–12% of flange seal leakage incidents. Since the 90s, rapid advances in materials science have brought a steady stream of new sealing materials. These materials have very good compactness, and permeation leakage through gaskets made primarily from them has become increasingly uncommon.
With the further development of material science and technology, one day the leakage accident of sealing gaskets will be completely solved.
3. Destruction leak
Human factors account for a large proportion of damage-related leakage incidents. During installation, a sealing gasket can easily become misaligned, leading to insufficient local sealing contact pressure or excessive preload beyond the gasket's design limit, causing it to lose its ability to recover. Uneven tightening of flange bolts and misaligned flange centerlines can also produce these conditions as the flanges are tightened, as shown in Figure 2-4. Leakage caused by excessive gasket compression or insufficient sealing contact pressure resulting from poor installation quality is therefore called 'damage-related leakage'.
This leakage depends largely on human factors. Management of construction quality should be strengthened, such as selecting structural forms with strong sealing reliability. Generally speaking, low-pressure systems using wide-face flanges are easier to concentric and align than narrow-face flanges, as shown in Figure 2-5, and there are fewer leaks. In addition, the convex and concave flange sealing structure is better than the flat flange sealing structure. Adopting some effective methods on existing equipment and pipe flanges can also significantly improve the installation quality.
For example, during the installation process of flat flange, the application of positioning non-drying adhesive can effectively prevent the gasket from shifting and reduce the labor intensity of the operator. Damage and leakage accidents generally account for about 1%~5% of all leakage accidents.
The leakage volume of interface leakage and damage leakage will increase significantly with the passage of time, while the relationship between the leakage volume of penetration leakage and time is not very obvious. No matter what kind of leak it is, measures should be taken immediately as soon as it is discovered. First, you can use a wrench to check whether the connecting bolts are loose, and then tighten them evenly until the leakage disappears. If the leakage is not eliminated after tightening the bolts, you should consider using some method in dynamic sealing technology to solve the problem.
It is easier to eliminate leakage sooner than later by using dynamic sealing technology. Waiting until the leakage increases significantly before dealing with it will bring inconvenience to the dynamic sealing operation and virtually increase the difficulty of construction.
In addition to the three types described above, corrosion by the process medium can cause flange sealing-surface leakage. This is crevice corrosion, occurring mainly in tiny gaps at the flange joint. Insufficient oxygen supply in the medium inside the gap creates an electrical potential difference relative to the medium outside, producing electrochemical corrosion. This is known as concentration-cell corrosion. Corrosion leakage develops slowly. The sealed fluid can escape only after corrosion pits have joined to form a continuous channel.
During on-site maintenance, we often find that there are many spots on the flange sealing surface, and some have even formed obvious pits. These are the products of strong and light electrochemical corrosion, but no leakage occurs. Corrosion leakage is relatively rare. Even if leakage occurs, its form is very similar to interface leakage. It occurs at the contact interface between the flange sealing surface and the gasket. The form is similar to interface leakage and will not be described in detail here.
Source: Internet
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