What is Flange Sealing?
Flange connection becomes the most common connection method during the industrial pipeline system due to its reliability and removability. And the flange sealing surface, as the core component of flange connection, the sealing performance of it related directly to the safe operation, sealing performance and service life of the whole pipeline system. The flange sealing system consists of three parts: flange plate, sealing gasket and Fasteners.
Flange sealing surface is always located on the central area of the flange plate, and the holes arranged in a circular pattern around the bolt are the surfaces that come into contact with the gasket. According to different application scenario and pressure rating, the width, surface treatment method and geometric shape of the sealing surface are all different. The design of the sealing surface needs to take many factors into consideration, such as, working pressure, temperature, medium character and flange material. All these factors jointly determine the optimal design of the sealing surface.
During the industrial application, the failure of flange sealing surface is the direct reason which leads to the pipeline leakage. According to statistical data, about 35% industrial pipeline system leakage dues to incorrect flange sealing surface selection. Therefore, we need to have a thorough understanding of what kind of flange seals should be chosen under different working conditions, to make sure that the whole pipeline can be safe working.
Flange Sealing Types Classification
The sealing surface of the flat seal is a flat surface. This is the simplest sealing form. This kind of sealing is usually used to low pressure pipeline system (PN≤1.6MPa), for example, such as water supply pipeline, low-pressure steam system, and the transportation of non-hazardous media.
Advantage: low price cots, east installation
Disadvantage: poor sealing performance, not available for high pressure and dangerous liquid transfer
Applicable Standards: ASME B16.5（Class 125/150）/ EN 1092-1（PN6/PN10）
Raised Face Sealing (RF) is the most applied sealing type at present. For this type of sealing, the sealing surface is higher then the flange surface, the shape of the raised part is usually concentric circle or spiral groove. This kind of design enables the preload of the bolts to be more concentrated on the sealing area, thereby enhancing the sealing effect.
Raised Face Sealing is suitable for medium and high pressure application, and always applied to industry like oil, chemical industry and electricity.
Advantage: superior sealing performance; wide application range
Disadvantage: high requirement for gasket compression control
Applicable Standards: ASME B16.5（Class 150-2500）/ EN 1092-1（PN10-PN400）
The flange with concave-convex sealing are respectively machined into concave surface and convex surface. This kind of flange is mainly used to medium and high pressure application (PN≤16.0MPa), and is suitable for refinery pipelines and high-temperature steam system. The advantage of this kind of flange is that, it can be used to medium and high pressure working situation, it is more easy for installation and remove then RTJ sealing, but with disadvantage of high processing accuracy.
Applicable Standards: ASME B16.5（Class 150-900）/ EN 1092-1（PN16-PN100）
The sealing surface of the ring connection is equipped with precisely machined annular grooves (usually elliptical or octagonal). It is used in conjunction with metal ring gaskets (such as soft iron, stainless steel, Inconel), and its sealing performance is extremely reliable. It is mainly applied to high-pressure oil and gas pipeline, high-pressure steam systemsand offshore drilling platform. Due to its metal-metal sealing design, it has extremely high pressure resistance reliability. Therefore, it is suitable for ultra-high pressure (>Class 900), high temperature or highly-vibrating working condition.
Advantage: can be used to ultra high pressure and high temperature environment
Disadvantage: high price coat; strict installation accuracy; hard to remove, not suitable for situation which require frequent maintenance.
Applicable Standards: ASME B16.5（Class 150-2500）/ API 6A
This sealing form consists of two parts: tenons (protrusions) and grooves (cavities), which are used in pairs. Usually, non-metallic or semi-metallic gasket are adopted. It is mainly used for the connection of compressor pipeline and chemical reaction vessel. The advantage is that the gasket has precise positioning, prevents extrusion, and is suitable for high pressure and high vibration environment. The disadvantage are is the processing is complex, the cost is high, and the paired flange must be strictly matched.
Applicable Standards: ASME B16.5 (special working condition) / DIN 2696
Flange Sealing Type Selection Guide
Low Pressure (PN16): FF/RF flange sealing
High Pressure (>PN100): RTJ flange sealing
Corrosive Medium: stainless steel RF
Gas: RTJ
High Temperature (>400°C): RTJ/M&F flange sealing
Frequent Vibration: T&G/RTJ
Frequent Remove: RF (easy maintenance)
Permanent Connection: RTJ (long-term sealing)
Flat face sealing (FF) is suitable for low pressure pipeline system, and raised face sealing (RF) is always used for medium and high pressure working application. As for ultra high pressure and extreme environment, Ring Connection Sealing (Ring-Type Joint, RTJ) and screw Slot Sealing (Tongue & Groove, T&G ) can supply better reliability.
Key Technical Parameters of the Flange Sealing Surface
The performance of flange sealing face is influenced by many technical factor, among them, the most important factor is flange surface roughness. Suitable roughness can increase the friction between the sealing surface and the gasket, thus to lead to better sealing result. Always, the surface roughness Ra value of the sealing surface is ideally controlled within the range of 3.2 - 6.3μm. Too low a roughness may cause the gasket to slide, while too high a roughness may damage the gasket material.
The flatness and parallelism of the sealing surfaces are equally crucial. Deviations in flatness can cause uneven force on the gasket, creating leakage channels. The flatness of high-standard flange sealing surfaces is typically required to be no more than 0.05mm. At the same time, the parallelism of the two paired flange sealing surfaces must also be strictly controlled to ensure uniform distribution of the bolt preload force.
The choice of materials has a decisive influence on the performance of the sealing surface. Commonly used flange materials include carbon steel, stainless steel, alloy steel, etc. When choosing, factors such as the corrosiveness of the medium, temperature and pressure need to be taken into consideration.
