Sealing and Leakage of Flange
Flange connection is one of the most common and important connection forms of pipeline, pipe fittings, valves, instruments and equipment in petrochemical plants. Connecting flange mainly relies on the pressure produced by its connecting bolt, through the flange gasket to achieve enough specific pressure of working seal to prevent the leakage of medium inside the pipeline and achieve sealing. Although this connection has the characteristics of easy disassembly and no fire, due to its own structural characteristics, improper treatment will lead to leakage, not only waste of resources, but also pollute the environment, and even cause casualties and property losses. Therefore, it is of great practical significance to discuss the leakage of pipe flange for design work.
1. Sealing and Leakage of Flange
The sealing principle of flange.
Flange connection, to be exact, should be bolt, flange, gasket connection, its sealing is achieved by the synergy of the three. The gasket is the core component of the seal.
1.1 Sealing Mechanism
The pre-tightening force of bolt is enough pressure between gasket and flange sealing surface to make the deformation of gasket surface enough to fill the micro-roughness of flange sealing surface and achieve the purpose of sealing. The minimum unit compressive force acting on the gasket for the above purpose is called specific pressure Y. When the pipe system reaches the operating pressure, under the action of the axial force of the internal pressure, the two flanges show the trend of separation, the bolts will produce elastic or plastic deformation, and the compressive force acting on the gasket will be reduced. When the pressure applied on the effective section of gasket is reduced to a certain critical value, the seal can still be maintained. At this time, the residual compressive force on the gasket is the effective tightening force of the gasket. When the effective tightening force is below a certain critical value, leakage will occur, and even the gasket can be dislocated. Therefore, the effective tightening force of the gasket must be greater than the operating pressure of the pipe system. The ratio of the effective tightening force of the gasket to the working pressure of the pipe system is called the gasket coefficient M. The distance between the sealing surfaces of two flanges is larger than the initial state in operation. At this time, the tightness between gaskets and the sealing surfaces of flanges is guaranteed by the resilience of gaskets. It can be said that in the initial sealing stage, the plastic deformation of gasket surface plays a decisive role in filling the micro-roughness of flange sealing surface; while in the operating state, the elastic recovery of gasket interior plays a leading role in flange sealing.
2. Thermal transient seal analysis of pipeline flange under internal pressure
High temperature will reduce the springback ability and actual sealing performance of gaskets. As far as the flange bolt structure is concerned, the deformation caused by temperature is one of the main factors affecting the sealing performance.
2.1 Transient Temperature Field
During the heating process, there is a certain temperature difference between the inside and the outside of the flange ring, because the heat is transferred from the inner wall of the flange to the outer wall, and the temperature change in the part near the outer wall of the flange lags behind the inner wall for a period of time. The heat transfer between bolts and flanges depends only on the contact surface. The temperature of bolts should lag behind that of flanges.
2.2 Flange Deformation Caused by Temperature
In the initial stage of heating, the flange nozzle is heated much faster than the flange ring. The difference of thermal expansion between the flange and the flange ring is large. The further expansion of the flange nozzle is constrained by the flange ring, so that the flange has a larger deflection angle.
When the flange temperature reaches a steady state, the temperature difference between flange and nozzle is the smallest, and the angle of rotation tends to decrease, but the linear expansion of flange and bolt increases in both axial and radial directions.
Effect of Deformation and Pressure Change Caused by 2.3 Warming on Sealing Performance of Flange Connection System
The effective tightening force of gaskets is smaller than that of pre-tightening force because of the system boosting. When the system warms up, the bolt temperature lags behind. The thermal expansion caused by the flange warming up faster makes the gaskets compress further and the gasket stress increases. Therefore, the stress variation of gaskets depends on the combined action of boosting pressure and warming in the process of the system boosting up temperature and pressure. On the one hand, the change of flange rotation angle restrains the further increase of gasket stress caused by the difference of expansion between flange and bolt; on the other hand, the rotational deformation of flange leads to the further compression of gasket measurement, resulting in plastic deformation, which reduces the resilience of gasket, and has a negative impact on gasket sealing.
Instantaneous excessive temperature rise will also lead to adverse consequences, which is because gaskets will be compressed too much in a short period of time, losing a greater resilience, while making the flange produce greater deformation and reduce to sealing capacity. In addition, the flange will also produce greater transient thermal stress which is not conducive to sealing, so the speed of temperature rise must be controlled; the magnitude of instantaneous cooling will also make the flange deform greatly in the instant, and produce larger transient thermal stress which will cause leakage, so it is necessary to control the temperature drop.
3. Analysis of Leakage
Flange leakage is absolute, and no leakage is relative. It is impossible to require flange seals to be absolutely leaky. What we should do is to determine the corresponding zero leakage standard according to the importance, danger and safety and economic requirements of the production process. The limit of zero leakage in general industry is 1.6 x 10-3 cm 3/s volume leakage rate.
The main causes of leakage of pipeline connecting flange are usually as follows:
- (1) Due to insufficient pressure of sealing gasket, offset installation of sealing gasket, different bolt tightness and offset of center line of two flanges lead to leakage of pipeline after pressure lifting.
- (2) The unreasonable design of pipeline technology, inadequate vibration reduction measures or external factors cause pipeline vibration, resulting in loose bolts and leakage.
- (3) The pipeline is deformed or leaked due to unequal force on the flange due to settlement.
- (4) The expansion and deformation of the connecting bolt are caused by the heat expansion and cold contraction of the pipeline, and leakage occurs in the alternating seasons.
- (5) Sealing gaskets are used for a long time, resulting in leakage due to plastic deformation, reduction of resilience and aging of gasket materials.
- (6) Leakage due to quality problems of gaskets and flanges.
There are two ways to analyse the leakage of blue: the first is the penetration of gasket itself; the second is the leakage of the gap between gasket and flange sealing surface. Because of the emergence of new sealing materials, the first kind of leakage is no longer a problem. The vast majority of daily leaks are the second. The second kind of leakage is analyzed below.
Under a certain operating condition, the leakage rate of flange connection is lower than that of a specified index; or under the specified leakage condition, the flange connection can withstand a specific operating condition, and the flange connection meeting the leakage rate of this index or the specific operating condition is not leaking or tight, otherwise, it will be able to withstand a specific operating condition. Leakage, or not tight.
The analysis of flange tightness can start from two aspects. The first is the prediction of leakage rate: if the pre-tightening load and operating pressure of bolts are known, the analysis of whether the Blue Connection structure meets the requirements of specified index leakage rate; the second is the control of leakage rate: under the given working conditions, how much bolt pre-tightening force is needed to make the flange connection reach a certain index leakage rate.
4. Improving the Effect of Sealing Elements on Leakage
The stress state of flange is very complex in work. Bolts, gaskets and internal pressure all exert direct force on flange, which may lead to irregular deformation of flange. These forces are not problematic in terms of strength, but these deformations may lead to flange leakage. Therefore, the design of flange not only meets its strength requirements, but also meets the minimum deformation required for flange not to leak, that is, the flange should be designed according to the stiffness theory.
It is well known that tightening bolts can improve sealing effect and reduce leakage rate, but due to the strength and stiffness limitations of flanges and bolts themselves, this method is ineffective to a certain extent.
Improper installation sequence will also lead to leakage. Correct bolt fastening sequence (cross equidistant symmetry) is very important to ensure the sealing of flange connection points and shorten tightening operation time.
Flange gaskets are generally divided into metal gaskets and non-metal gaskets.
Non-metallic gaskets are gaskets made of asbestos, rubber, synthetic resin and other non-metallic materials.
Sealing rings of various rubber materials
Types of Non-metallic Flat Gaskets
1. Graphite gasket
It is cut or stamped by pure graphite plate or metal (tooth plate, flat plate, mesh) to enhance graphite plate. It has many excellent sealing properties, such as thermal stability, self-lubrication, corrosion resistance, non-aging, non-brittle, etc. It can be used stably for a long time in harsh working conditions and needs little maintenance. Different metal sheets can be selected for lining materials. Type can be chosen without edge, inner edge, outer edge, inner and outer edge. Graphite cutting gaskets are made from pure graphite plates by drilling or cutting. They have good corrosion resistance, high/low temperature resistance, good compression resilience and high strength. Various round and complex geometric gaskets are widely used in pipelines, valves, pumps, pressure vessels, heat exchangers, condensers, generators and air. Compressors, exhaust pipes, refrigerators, etc.
2. Metal wound gaskets
The metal winding gasket is made of high quality SUS304, SUS316 (“V” or “W”) metal strip and other alloy materials alternately and superposedly spirally wound with graphite, asbestos, polytetrafluoroethylene, asbestos-free and other soft materials. The metal strip is fixed by spot welding at the beginning and end. The metal winding gasket is the most resilient gasket in the semi-metal sealing gasket. The structure density of the metal winding gasket can be made according to different locking force requirements, and the maximum tightness can be controlled by the inner and outer steel rings. The surface accuracy of the flange sealing surface contacted by the winding gasket is not high. Metal wound gaskets are especially suitable for occasions of uneven load, easy relaxation of bonding force, periodic variation of temperature and pressure, shock or vibration. Winding gasket is the original static seal for flange connection processing of valves, pumps, heat exchangers, towers, manholes, hand holes, etc.
Application scope of winding gaskets: sealing at flange joints of pipelines, valves, pressure vessels, condensers, heat exchangers, towers, manholes and hand holes in petroleum, chemical, metallurgical, electric power, ship and machinery industries.
There are four types of winding gaskets: basic winding gaskets, inner ring winding gaskets, outer ring winding gaskets and inner and outer ring winding gaskets.
3. Metal-clad gaskets
Metal-clad gaskets refer to composite gaskets with non-metallic materials (flexible graphite polytetrafluoroethylene, asbestos rubber sheet, ceramic fibers) inside and metal sheet coated with special cold working process outside. According to the cross section of gaskets, they are usually divided into two types: plane-clad gaskets and corrugated-clad gaskets. Seal on heat exchanger, pressure vessel, pump, valve and flange. But the resilience is limited, flange surface flatness and high tightening force are required to achieve good results. According to different materials, 304 stainless steel asbestos-coated gaskets and stainless steel Graphite-Coated gaskets can be combined.
Material of cladding gasket: tinplate, stainless steel 304, 316, copper, aluminium, etc.
Coated gasket fillers: asbestos, flexible graphite, polytetrafluoroethylene, asbestos-free, ceramic fibers, etc.
Coated gaskets are mainly used for sealing flanges of pressure vessels with larger diameters (such as heat exchangers, reactors, etc.).
Seals for pumps, valves, pipes and flanges of various media. Suitable for medium and low pressure working conditions. Suitable for medium and low pressure working conditions.
4. Tooth-Shaped Composite Gasket
Tooth gasket is a kind of metal soft gasket. Its sealing effect is better than that of metal flat gasket. Generally, a precise lathe is used to process a concentric zigzag groove with 90 degree angle waveform on both sides of metal stainless steel flat gasket. According to different media, flexible graphite, PTFE, asbestos-free board or some other soft metals can be selected and pasted on the gasket. On the other hand, using the advantages of soft cover, strength and elasticity of metal and labyrinth sealing, the same sealing effect can be achieved, and the gasket compression force is smaller. Tooth-shaped composite gaskets can also be directly used without covering sealing layer, and can achieve good sealing effect, but in high-pressure situations, it is easy to cause damage to the surface of flanges. Tooth-shaped composite gaskets are mainly used for sealing medium, high pressure and high temperature pressure vessels and pipeline flanges.
5. Waveform Composite Gasket
The corrugated composite gaskets are processed into concentric wavy rings by moulding, rolling or turning processes. Non-metallic materials such as expanded graphite or tetrafluoroethylene are pasted on the surface. The wavy gaskets have excellent thermal conductivity, which can meet the high temperature, high pressure and corrosive occasions. Any inch, metal material is optional. Corrugated composite gaskets are basically linear contact gaskets, which are provided with mechanical support by corrugation. Their resilience can be determined by the type, metal thickness, corrugated spacing and corrugated height. Generally, graphite layer is pasted on both sides of corrugated gaskets, which can be used for uneven sealing surface. This product can be used renewably in general applications and is quite economical and effective. It can directly replace the winding gasket and the cladding gasket. Suitable for high temperature and high pressure occasions such as pressure vessels, heat exchangers, valves, pipes and other flanged joints of sealing parts.
6. Tetrafluoro gasket
PTFE gaskets or Teflon gaskets abroad are flat gaskets, V-shaped gaskets, piston rings, ball valve gaskets made of PTFE rods, tubes and plates by mechanical turning or cutting, which have good characteristics of corrosion resistance, aging resistance and non-conductivity. The pH value of PTFE is 0-14 (except for molten alkali metals and fluorine at high temperature and high pressure). It has good mechanical strength between – 100 C and 100 C. Pressure 10 Mpa.
Note: When the temperature is below – 185 C. PTFE gaskets will become brittle. Pure polytetrafluoroethylene gasket is a kind of clean sealing product. It does not pollute any substance it touches. It can be widely used in food, medicine and other industries.
7. Asbestos Rubber Gaskets
Asbestos rubber gaskets are made of asbestos fibers and rubber as main raw materials, rubber compounding agents and fillers, through mixing and stirring, hot roll forming, vulcanization and other processes. Asbestos rubber gaskets can be divided into ordinary asbestos rubber gaskets and oil-resistant asbestos rubber gaskets according to their formulation, technological properties and uses. According to the temperature and pressure used, it can be divided into low pressure asbestos rubber gasket, medium pressure asbestos rubber gasket and high pressure asbestos rubber gasket. It is suitable for sealing water, steam, oil, solvent, medium acid and alkali. Asbestos gasket is mainly used in sealing of medium and low pressure flange connection.
8. Metal elliptical cushion and octagonal cushion
Metal elliptical gaskets and metal octagonal gaskets are made by forging, heat treatment and mechanical processing. They are metal sealing gaskets used in oil and gas pipe flanges, pressure vessels, high-speed joints, high-temperature and high-pressure valve caps, etc. They can still maintain good sealing performance especially when pressure and temperature fluctuate.
Installation of gaskets
To ensure the sealing of the system, besides good sealing materials, the following correct installation methods should be followed:
- 1. Gaskets must be placed in the center of the flange, especially on the protruding flange.
- 2. Ensure the smoothness and processing accuracy of the sealing surface.
- 3. The bolts must be tightened symmetrically and evenly.
- 4. Spring washer must be used to ensure uniform load, and torque wrench should be used to tighten bolts.
- 5. Check and recalibrate the torque of the connecting bolt after running the system for one day.
- 6. In order to ensure the service life of gaskets, do not use anti-adhesive or lubricant of liquid or metal matrix.
The most effective way to improve the sealing effect and reduce leakage is to update the core component – gasket. The material and structure of the existing gasket are various, so long as it is selected correctly according to the actual situation, it can basically meet the requirements. Of course, with the development of science and technology, new materials as gaskets are constantly emerging, gaskets with more perfect structures are constantly appearing, and the research on sealing mechanism is deepening, and sealing technology will continue to improve.
5. Leakage Problems Should Be Noticed in Pipeline Specialty
5.1 The determination of flange grade should take full account of the influence on flange seal.
In order to determine the flange grade, besides satisfying the strength condition under operation conditions, the rigidity requirement under self-designed temperature should also be considered. Otherwise, under the action of force and moment, the flange will deform too much and leak easily. At this time, the flange grade should be improved.
5.2 Minimize the force and moment acting on the flange
The force separating flange will inevitably reduce the residual compressive force between gasket and flange. When this force is too large and the residual compressive force of gasket is less than the effective compressive force of gasket, leakage will inevitably occur.
Torque makes the force of gasket uneven. The effective compaction force of one part of gasket is larger than that of no moment, while the effective compaction force of another part is smaller than that of no moment. When the residual compaction force of the smallest part is smaller than that of gasket, leakage occurs here.
5.3 Cold tightening can also reduce leakage under certain conditions
Cold-tightening is to cause plastic deformation in a part of the pipe system by external forces during the installation and construction of the pipeline. Under operating conditions, when the pipe system is heated and expanded, the internal stress in the opposite direction will be generated, so as to reduce the stress and moment of the pipe system. Reducing leakage by cold tightening is actually achieved by reducing the force and moment on flange in hot state.
6. Concluding remarks
There are many examples of leakage hidden danger caused by improper layout of pipeline direction, which are not listed here because of space limitation. In a word, the force and moment acting on the flange should be reduced as much as possible, especially when the operating pressure of the flange is close to the maximum non-impact pressure allowed by the flange at the operating temperature.
Source: China Flanges Manufacturer – Yaang Pipe Industry Co., Limited (www.metallicsteel.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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