Flange seals and gaskets
In petrochemical plants, flange is one of the most important and common forms of connecting pipes, pipe fittings, valves, instruments and equipment. Although this connection has the advantages of convenient disassembly and no hot work, because the sealing is realized by the cooperation of flange, bolt and gasket, the problem of any element will cause leakage, which will not only cause waste, but also pollute the environment, and even cause casualties and property loss. Therefore, today I will explain the relevant knowledge of flange sealing in detail.
Sealing of flange
Flange connection should be exactly bolt, flange and gasket connection, and its sealing is realized by the synergy of the three, in which gasket is the core component to realize sealing.
Sealing principle of flange
Sufficient pressure is generated between the gasket and the flange sealing surface through the preload of the bolt, so that the deformation on the gasket surface is enough to fill the micro unevenness of the flange sealing surface and achieve the purpose of sealing.
The minimum unit pressing force acting on the gasket to achieve the above purpose is called specific pressure. When the pipeline connected with the flange reaches the operating pressure, the two flanges tend to separate due to the axial force of internal pressure, while the bolt tends to elongate, and the compression force acting on the gasket will be reduced. When the pressing force acting on the effective section of the gasket drops to a certain critical value, the seal can still be maintained. At this time, the remaining pressing force on the gasket is the effective tightening force of the gasket. When the pressing force on the gasket is less than the effective tightening force, the flange will leak and even blow the gasket away. Therefore, the effective tightening force of the gasket must be greater than the operating pressure of the pipeline. The distance between the two flange sealing surfaces is greater in the operating state than in the initial state. At this time, the tightness between the gasket and the flange sealing surface is guaranteed by the rebound force of the gasket.
It can be said that in the initial sealing stage, the plastic deformation of the gasket surface plays a decisive role in filling the micro unevenness of the flange sealing surface; In the operating state, the sealing of the flange and the elastic recovery inside the gasket play a leading role.
Common forms of flange sealing surface
According to the working conditions, medium, pressure, specification and temperature, the following pipe flange sealing surface types are available:
- Flat Face (FF)
- Raised Face (RF)
- Female Face (FMF)
- Male Face (MF)
- Male-and-Female (M & F)
- Tongue Face (T)
- Groove Face (G)
- Tongue-and-Groove (T&G)
- Ring Joint Face （RTJ)
Flat Face (FF)
The flange surface is smooth, simple structure, easy processing, can be used in the occasion of not high pressure and temperature, but the contact area between sealing surface and gasket is large, the required compression force is large, the gasket should not be placed during installation, and the gasket is easy to extend or move to both sides after pre-tightening. When using lined flanges or non-metallic flanges, FF surface flanges can ensure that the sealing surface does not break when tightening, especially FF surface.
The sealing surface of this kind of flange is a smooth plane, sometimes there are two concentric circular grooves (commonly called waterline) with triangular interface on the sealing surface.
The gaskets suitable for flat flanges are various non-metallic flat gaskets, clad gaskets, metal clad gaskets, wound gaskets (can be with inner or outer ring or inner and outer ring at the same time). They can be used at nominal pressure p＜2.45MPa because of simple structure, easy processing and convenient construction of anti-corrosion lining. It is especially suitable in places where the pressure is below 0.588MPa and the temperature is not high, but this kind of sealing surface has a larger contact area with the gasket (especially wide face flanges for pipelines), the required compression force is large, and the gasket should not be positioned during installation. After pre-tightening, the gasket is easy to stretch or move to both sides. Therefore, gaskets with small friction coefficient such as PTFE are not suitable for this kind of sealing surface. In addition, if you use winding gasket, the triangular groove is not made on the sealing surface in order to reuse the gasket.
Advantages: simple structure, easy processing.
Disadvantages: It has a large contact area, requires a large preload ratio pressure, large bolt load, so the flange and other parts are demanding and bulky, the gasket is easy to extrude, and the sealing performance is poor. The use of pressure P ≤ 2.5Mpa, toxic, flammable, explosive media can not be used.
Raised Face (RF)
Simple structure, easy processing, can be used for less high pressure and less high temperature, but some people think it is possible to use gasket under high pressure. Due to the ease of installation of this flange, it is the more widely used form of sealing surface below PN 150.
It consists of male and female surfaces, and the gasket is placed on the concave surface. CoMPared with flat flanges, male and female flange gaskets are not easy to squeeze, easy to assemble, and have a larger working pressure range than flat flanges, and are suitable for strict sealing requirements. However, for equipment with high operating temperatures and large sealing diameters, it is believed that gaskets may still be extruded when using this sealing surface.
This flange sealing surface consists of a combination of a concave surface and a convex surface, and the gasket is placed inside the concave surface. Its applicable gaskets are: various non-metallic flat gaskets, clad gaskets, metal clad gaskets, wound gaskets (basic type or with inner ring), metal waveform gaskets, metal flat gaskets, metal toothed gaskets.
Compared with flat flanges, gaskets are not easy to be extruded in concave and convex flanges, easy to be aligned during assembly, and the working pressure range is wider than that of flat flanges, which is used for occasions with tighter sealing requirements. However, for equipment with high operating temperatures and large seal diameters, there is still the possibility of gaskets being extruded when using this type of sealing surface. For example, a heat exchanger with a pressure of 2.45 MPa and a temperature of 250°C uses a pure aluminum flat gasket. Pure aluminum maximum use temperature of 425 ℃, its seal should be reliable. In fact, the heat exchanger into operation soon after the leakage, the second tightening is only maintained for a period of time. After stopping to check, the gasket was found to be significantly deformed inside diameter. The reason is that the plasticity of pure aluminum is good, the yield strength at 250 ℃ is about 15% at room temperature, elongation of up to 4-5 times, which means that at high temperatures aluminum gasket indentation, creep phenomenon is serious. Therefore, the gasket and the flange surface can not maintain the required sealing pressure, so the gasket material must be replaced or the use of tongue and groove surface flange and two stop with a concave and convex surface flange (such as high pressure sealing, metal flat gasket flange surface structure) to be resolved.
Advantages: easy to align, can prevent gasket extrusion, less scouring and corrosion by the medium. Suitable for high pressure applications.
Disadvantage: not easy to replace gaskets.
Tongue and Groove Face (TG)
The tongue and groove flange method consists of a tongue and groove face and a groove face with the gasket placed in the groove. As with the grooved flange, the tongue and groove flange does not extrude in the groove, so it has a small compression area and the gasket is evenly stressed. Since there is no direct contact between the gasket and the medium, the medium has little influence on the corrosion and pressure of the flange sealing surface, so it is mostly used in those occasions with strict sealing requirements such as high pressure, flammable, explosive and toxic media. This kind of sealing surface gasket is relatively simple and favorable in installation, but its processing and replacement will become more difficult.
The flange sealing surface consists of a tongue and groove surface and a groove surface to match, and the gasket is placed in the groove. Applicable gaskets are: metal and non-metal flat gaskets, metal clad gaskets, wound gaskets (basic type), etc. Like concave-convex flanges, tongue-and-groove flanges are not extruded in the groove, the compression area is minimal (only 52-68% of flat flanges and concave-convex flanges), and the gasket is evenly stressed. Since the gasket is not in direct contact with the medium, the corrosive influence of the medium and the penetration influence of the pressure mechanism are minimized, and it can be used in high pressure, flammable, explosive, toxic media and other sealing requirements of strict occasions. This kind of sealing surface gasket is well aligned during installation, and the processing of this sealing surface and gasket replacement are more difficult.
Advantages: Composed of tongue and groove surface, the gasket is placed in the groove and will not be squeezed out of the compression surface, which is less likely to be scoured and corroded by the medium. Due to the narrower sealing surface, the required bolt force is smaller, and it is suitable for higher pressure and strict sealing requirements, such as flammable, explosive and highly or extremely hazardous toxic media, and other important applications.
Disadvantages: complicated structure, gaskets are easily squeezed in the groove and not easy to remove, and it is difficult to replace gaskets. It should also be noted that the tongue and groove flange should be protected in transport to avoid collision deformation.
Ring-Type Joint (RTJ)
This flange sealing surface gasket is placed in the ring groove. By placing the gasket in the ring groove, the gasket will not be squeezed in the groove, the compression area is small, and the gasket is evenly stressed. Since the gasket is not in direct contact with the medium, the medium has little influence on the corrosion and pressure of the flange sealing surface, so it is mostly used for those occasions with strict sealing requirements such as high pressure, flammable, explosive and toxic media.
Advantages: ring connection surface is mainly used in the necked butt welding flange and the overall flange, good sealing performance, general installation requirements, use in high temperature and high pressure conditions.
Disadvantages: the processing accuracy of the sealing surface is extremely high.
In summary, the flange sealing surface form is different, its characteristics are also different, the application range is different. Therefore, when we choose the flange, we must pay attention to its use occasions and performance requirements. For example: when the work is not harsh, choose RF sealing surface; when the working conditions are harsh, RTJ sealing surface selection fully meet the sealing requirements; in non-metallic or lined flange low pressure pipeline using FF surface is better. Of course, this is only a suggestion, we must specifically look at their actual needs.
Factors affecting flange sealing
The leakage of flange seal is mainly the leakage between sealing surfaces. The possibility of leakage caused by the capillary action of the gasket is very small. After the gasket is pressed by external force, the elastic deformation and plastic deformation generated by the gasket itself will fill the small uneven gap on the sealing surface, so that the sealing can be achieved when the resistance of the medium passing through the sealing surface is greater than the pressure difference on both sides of the sealing surface. However, when affected by external conditions, the seal is damaged, resulting in leakage. The factors affecting sealing mainly include the following aspects:
- (1) Effects of operating conditions. The operating conditions are pressure, temperature and physicochemical properties of the medium. In petrochemical plants, there are many low-pressure flanges, and the influence of simple pressure or medium factors on flange leakage is not the main factor. Only when combined with temperature, the problem becomes serious. When the temperature changes repeatedly, the possibility of seal failure is greater.
- (2) Influence of gasket coefficient and compression ratio in design parameters. At present, China selects the two parameters by looking up the table steel pressure vessel, but the gasket coefficient and compression ratio, even for the same material, the two coefficients are also related to factors such as gasket width, preload pressure, medium performance, flange sealing surface width and roughness. Therefore, there is no authoritative specification for how much the two coefficients can be selected.
- (3) Effect of bolt preload. Improving the bolt preload can increase the sealing performance of the gasket, but too large bolts and bolts will cause the gasket to lose elasticity, even crush the gasket or the foundation, which can not ensure sufficient elasticity under the working state.
- (4) Effect of gasket performance. Gasket deformation includes elastic deformation and plastic deformation. Gasket material is the main factor affecting gasket performance, so the quality of gasket will affect the sealing of flange.
- (5) Influence of flange stiffness. Insufficient flange stiffness will produce excessive warpage deformation, which is often one of the reasons leading to seal failure.
- (6) Influence of sealing surface. The shape and roughness of flange sealing surface shall match with the gasket. The flatness of the sealing surface and the perpendicularity between the sealing surface and the flange axis are the premise to ensure the uniform compression of the gasket. Therefore, the influence of reasoning or eccentric force caused by pipeline thermal expansion must be considered in design and installation. Otherwise, it will easily cause lax flange sealing and leakage.
Leakage of flanges generally occurs in two ways.
- The first is the penetration of the gasket itself.
- The 2nd kind is the leakage of the gap between the gasket and the sealing surface of the flange.
Due to the continuous emergence of new sealing materials, the first type of leakage can be solved basically. The vast majority of daily occurrences are the second type of leakage. In a particular operating conditions, the flange connection can achieve a leak rate lower than a specific indicator leak rate; or in the specified leak conditions, the flange connection can withstand specific operating conditions, to meet this indicator leak rate or specific operating conditions of the flange connection is considered not leak, or tight, and vice versa is considered to leak, or is not tight.
Flange and the iMPact of the bolt flange in the actual working conditions of the force situation is very complex, bolts, gaskets, internal pressure are directly on the flange force, these forces may lead to irregular deformation of the flange. These forces are not a problem in terms of strength, but the deformation may lead to flange leakage. Therefore, in the flange design not only to meet the strength requirements, but also to meet the minimum deformation required for the flange not to leak, that is, designed according to the stiffness theory.
It is known that tightening the bolts can improve the sealing effect and reduce the leakage rate, but this method is ineffective after reaching a certain level due to the strength and stiffness limitations of the flange and the bolts themselves. Improper bolt installation procedures can also produce leakage, the correct sequence of bolt tightening (cross cross isometric symmetry) to ensure the sealing of the flange connection point and shorten the tightening operation time is extremely important.
The most effective existing means to improve the sealing effect and reduce leakage is to improve the performance of the gasket.
Gasket is the main seal of the flange connection, so the correct choice of gasket is also the key to ensure that the flange connection does not leak. Gaskets can be divided into many types depending on the material used to make them. In addition to the type of gasket, the identification and selection of gaskets are also important!
Gasket structure type
The flat gasket used in industry generally consists of the sealing element and the inner and outer reinforcing rings. The sealing element or gasket body is the key part to stop the leakage. The sealing element or gasket body is the key part to stop the leakage. The common materials used are non-metallic materials such as flexible graphite, PTFE, fiber reinforced rubber based composite plates, etc. In addition, the sealing element material can also be rigid or flexible metal, and is usually used for higher pressure and temperature applications.
For non-metallic sealing elements, metal materials are usually inserted to enhance, while also facilitating the manufacturing process of sealing elements such as graphite and other fragile materials. The reinforcing material can be sheet metal or wire mesh, which is often perforated to improve reinforcement and increase flexibility, and is bonded and rolled to fit together. The sealing element can also be provided with a surface layer or anti-bonding treatment layer to increase the sealing effect and prevent the flange sealing surface from bonding.
The outer reinforcement ring or outer ring material is solid metal and serves to:
- To assist in the alignment of the sealing element during installation.
- Prevent the sealing element from being damaged by excessive compression; prevent the gasket from blowing out and reduce flange rotation, etc.
The outer reinforcement ring does not come into contact with the sealing medium, so it is not required to resist medium corrosion and is often made of carbon steel. The outer ring can also be made into one with the sealing element, such as metal tooth gasket, wave tooth composite gasket.
The inner reinforcing ring or inner ring is in contact with the fluid and its material should be able to resist the corrosion of the sealing medium. The role of the inner reinforcing ring is to prevent the gap between the sealing element and the flange of the vessel or pipe so that this gap does not interfere with the flow of fluid and the resulting iMPact of the fluid on the gasket.
Non metallic flat gasket – PTFE encapsulated gasket
PTFE encapsulated gasket is a kind of non-metallic composite cushion sheet, which is generally composed of envelope and insert. The envelope is mainly anti-corrosion, usually made of polytetrafluoroethylene, and the insert (filler) is a non-metallic material with or without metal stiffeners, usually made of asbestos rubber plate.
Teflon coated gasket is mainly applicable to the flange connection of full plane and raised surface steel pipes. It is applicable to corrosive media with nominal pressure PN of 0.6 ~ 5.0MPa and working temperature of 0 ~ 150 ℃ or media with high requirements for cleanliness.
Form of PTFE encapsulated gasket
PTFE encapsulated gaskets for pipe flanges can be divided into three types according to the manufacturing method: sectioning type (s type), machining type (M type) and folding type (F type).
For machined encapsulated gaskets, the inner diameter of the cladding layer can be consistent with the inner diameter of the flange to prevent fluid vortex at the flange. For the folding type shoe gasket, it is made of banded polytetrafluoroethylene film with a thickness of 0.4 ~ 0.8mm. After heat sealing at both ends, it is wrapped outside the annular core material in a U shape. It is easy to manufacture and is suitable for flange sealing with large size.
Metal composite gasket
(1) Spiral wound gasket
Metal spiral wound gasket is a kind of semi metal flat gasket which is spirally wound by metal belt and non-metal belt.
The characteristics of spiral wound gaskets are:
- Good compression and resilience;
- It has multi-channel sealing and certain self tightening function;
- It is not sensitive to the surface defects of the flange pressing surface and does not adhere to the flange sealing surface, which is easy to align, so it is convenient to disassemble;
- It can partially eliminate the influence of pressure, temperature change and mechanical vibration;
- It can maintain its excellent sealing performance under various harsh conditions such as high temperature, low pressure, high vacuum, iMPact and vibration.
Form of metal spiral wound gasket
Spiral wound gaskets can be divided into four types according to their different structures: basic type, with inner ring type, with inner ring type, with inner ring type and with ring type.
Selection and application of spiral wound gasket:
- a. Basic type gaskets are suitable for tenon face flanges;
- b. Inner ring gasket is suitable for concave convex flange;
- c. Ring gaskets are suitable for flat and raised face flanges;
- d. The inner ring and ring gasket are suitable for flat and raised face flanges.
(2) Metal clad gasket
Structure of metal clad gasket
Metal clad gasket is a composite gasket with non-metallic material as the core material, cut into the required shape, and coated with metal sheet with thickness of 0.25 ~ 0.5mm. According to the covering state, it is generally divided into plane type and corrugated type.
According to the structural type, it is divided into plane type (F type) and corrugated type (C type). The structural type of metal clad gasket for pressure vessel flange is plane type.
(3) Metal punching plate flexible graphite composite gasket
This kind of gasket, also known as flexible graphite metal reinforced composite gasket, is a sealing gasket made of punched metal tooth plate or punched metal core plate and flexible graphite particles. It is usually cut from flexible graphite composite reinforced plate.
As required, flexible graphite metal reinforced composite gasket can be made with stainless steel or carbon steel inner edge or inner and outer edge.
Characteristics of flexible graphite composite gasket with metal punching plate
The main features are: good high and low temperature resistance, corrosion resistance, radiation resistance and other properties, high strength, and can be used under high pressure conditions.
The required preload is less than that of metal gasket or metal spiral wound gasket. Due to the plasticity and good filling property of flexible graphite, when using this gasket, there is no need for precision machining on the matching flange sealing surface, and it can also adapt to the “water line” groove made on the flange sealing surface, which is relatively economical.
Flexible graphite composite gaskets are divided into RF type, MFM type and TG type, which are applicable to raised face, concave convex face and tenon groove face flanges respectively. As required, RF type gasket can be provided with stainless steel or carbon steel inner edge, i.e. rf-e type.
(4) Metal wave tooth composite gasket
The metal corrugated composite gasket made of flexible graphite is a composite gasket covered with flexible graphite on both sides of the metal plate machined into corrugated teeth. This gasket has the characteristics of both metal strength and corrugated elasticity. The maximum temperature is 650 ℃ and the maximum pressure can reach 26MPa.
Types of metal wave tooth composite gaskets
Flexible graphite metal wave tooth composite gasket can be divided into three types according to different structures: basic type, type with positioning replacement and type with spacer.
Metal ring gasket is a solid metal gasket with octagonal or oval section processed from metal materials, which has the function of radial self tightening sealing. Therefore, the metal ring gasket is in contact with the inner and outer sides (mainly the outer sides) of the flange ladder groove and forms a seal by pressing.
The octagonal ring gasket is matched with the flange groove, mainly in face contact. CoMPared with the oval ring gasket, although it is not easy to close with the flange groove, it can be used again, and it is easy to process because the section is straight. The oval ring gasket is in line contact with the flange groove and has good sealing performance, but the processing accuracy is required to be high, which increases the manufacturing cost. At the same time, the oval ring gasket can not be reused.
The applicable nominal pressure of metal ring gasket is 2.0 ~ 42.0MPa.
(2) Metal toothed gasket
The metal toothed gasket is a solid metal gasket. The section surface of the gasket is serrated, the tooth pitch t = 1.5 ~ 2mm, the tooth height h = 0.65 ~ 0.85mm, and the tooth top width C = 0.2 ~ 0.3mm. Several concentric circles are turned on the sealing surface, and the number of teeth is 7 ~ 16, depending on the specification of the gasket.
Structure and advantages of metal toothed gasket
There are four structural types of metal toothed gasket: basic type, with outer ring type, with inner ring type and with inner and outer ring type. The more common structure is basic type.
Because the V-shaped ribs in the contact area of the sealing surface of the metal toothed gasket form many spatial line contacts with differential pressure, the sealing is reliable and the service cycle is long. CoMPared with the general metal gasket, this gasket requires less pressing force.
The disadvantage is that each time the gasket is replaced, the two French sealing faces must be processed, which is time-consuming and laborious. In addition, the gasket is easy to leave indentation on the flange sealing surface after use, so it is generally used for parts with less disassembly. The applicable nominal pressure of metal toothed gasket is 1.6 ~ 25.0MPa.
Basic principles of gasket selection
(1) When selecting or ordering gaskets, you should know the following basic data:
a. Sealing surface type and size of matching flange.
For different flange sealing surface types, the corresponding gasket types are also different. For example, the spiral wound gasket type corresponding to raised face flange (RF) is spiral wound gasket with inner and outer ring, the spiral wound gasket type corresponding to tenon and grooved face (TG) flange is basic spiral wound gasket, and the spiral wound gasket type corresponding to concave convex face (MFM) flange is spiral wound gasket with inner ring only.
When the flange roughness and unevenness conditions are met, the thin gasket shall be selected as much as possible. The thickness of gasket is related to its type, material, diameter, processing condition of sealing surface and sealing medium. For most non-metallic sheet gaskets, thin gaskets also have great ability to resist stress relaxation. Because the contact area between the inner side of the thin gasket and the medium is also small, the leakage along the gasket body is also reduced. In this case, the blowing force borne by the gasket is also small, and the gasket is not easy to be blown out. The anti blowing ability of the thin gasket is stronger than that of the thick gasket. Of course, you can also refer to the gasket standard to select the gasket thickness and other dimensions.
b. Nominal diameter of flange and gasket.
c. Nominal pressure of flange and gasket.
The gasket must be able to withstand the maximum pressure, which may be the test pressure, which may be 1.25-1.5 times the normal working pressure. For non-metallic gaskets, the maximum pressure is also related to the maximum working temperature. Generally, the value of the maximum temperature multiplied by the maximum pressure (i.e. pxt value) has a limit value. Therefore, when selecting the maximum working pressure, the maximum pxt value that the gasket can withstand should also be considered. The following figure is the pxt value diagram of common pressed gaskets of Garlock. Different types of gaskets can bear different pressures.
d. Temperature of fluid medium.
The selected gasket should have a reasonable service life at the highest or lowest working temperature. The maximum working temperature of different gasket materials varies greatly.
In addition to the maximum and minimum operating temperatures that can be tolerated in the short term, the allowable continuous operating temperature shall also be considered. The gasket material shall be able to resist creep to reduce the stress relaxation of the gasket, so as to ensure the sealing of the gasket under working conditions. Most gasket materials will creep seriously with the increase of temperature. Therefore, an important index of gasket quality is the creep relaxation performance of the gasket at a certain temperature.
e. Properties of fluid medium.
The gasket shall not be affected by the sealing medium under the whole working conditions, including high-temperature oxidation resistance, chemical corrosion resistance, solvent resistance, permeability, etc. obviously, the chemical corrosion resistance of gasket material to the medium is the primary condition for selecting the gasket. For non-metallic gaskets or non-metallic materials in composite gaskets, the chemical corrosion resistance of materials can refer to the chemical resistance table of non-metallic materials provided by the gasket manufacturer. The metal materials in direct contact with the medium in the gasket shall have the same or higher corrosion resistance as the flange material.
It mainly refers to the information of the connection system where the gasket is located. It is necessary to select the appropriate gasket material and type according to the flange material, flange sealing surface type, flange roughness and bolt information.
For non-metallic flanges, the gasket with relatively small preload must be selected, otherwise it is likely that the gasket has not been pressed and the flange has been crushed during flange tightening.
The following factors shall also be considered when selecting gaskets:
- a. Good compression and rebound performance, able to adapt to temperature and pressure fluctuations;
- b. It has good plasticity and can fit well with the flange sealing surface;
- c. For some metal flanges with stress corrosion cracking tendency (such as austenitic stainless acid resistant steel), it shall be ensured that the gasket material does not contain excessive impurities causing various corrosion, such as controlling the chloride ion content of the gasket to corrode the flange;
- d. Non polluting medium;
- e. For sealing highly toxic chemicals, the gasket is required to have greater safety; For the pipeline system conveying flammable liquid, the maximum service pressure and maximum service temperature of the gasket used on the flange are required to be within the limit;
- f. Not easy to harden at low temperature, small shrinkage, not easy to soften at high temperature, good creep resistance;
- g. Good processing performance, convenient installation and compression;
- h. The flange sealing surface is not bonded, so it is easy to disassemble.
Selection of standard gasket
1. The type and scope of application of the gasket shall comply with the provisions of the design documents and the current effective standards of the state and relevant industries. The changes of gasket material and service standards shall be approved.
2. There are many factors affecting the selection of gaskets. Generally, on the premise of ensuring safe operation, gaskets with low price, easy manufacture, simple installation and replacement should be selected as far as possible.
3. The thickness of the gasket depends on the case. Generally speaking, if the sealing surface is well processed and the pressure is not too high, thin gasket should be selected. However, when the internal pressure is high, the gasket is too thin, which corresponds to the elongation of the bolt, and the rebound of the gasket is too small to achieve the necessary recovery, which is prone to leakage. Therefore, when the pressure is high, a thicker gasket should be selected.
4. In order to ensure the interchangeability of gaskets, the specifications and materials shall be unified as far as possible during type selection. While considering meeting the use requirements, the specifications and materials shall be merged as far as possible, and unnecessary diversification shall be avoided.
5. For occasions with high gas medium and high pressure, high-strength graphite gasket without edge wrapping is not allowed, and corrugated gasket is not allowed for tenon and grooved flange.
6. Special requirements shall be taken into account. For example, asbestos rubber plate and other fibrous gaskets shall not be selected if trace fibers are not allowed to be mixed in a certain medium. If some parts vibrate greatly, gaskets with strong vibration resistance, large resilience and durability shall be selected.
7. Asbestos rubber gaskets are not allowed for highly toxic, explosive, strongly corrosive and polluting media and harmful gases.
8. The gaskets of valves and flanges at the bottom of the liquefied spherical tank shall be spiral wound gaskets with metal protective rings (the gaskets of convex and concave flanges shall be equipped with inner reinforcing rings);
9. When selecting metal gasket, it shall be used in the fully annealed state, and soft metal materials shall be selected as far as possible. The hardness of gasket shall be hb30 ~ 40 lower than that of flange sealing surface.
10. The replacement principle of gasket on imported equipment (including instruments and meters) is to replace the type and material as is. If the gasket form or material needs to be replaced due to special reasons, it shall be approved.
11. The type and material of gasket shall be selected according to the fluid, service conditions (pressure and temperature) and the sealing requirements of flange joint. The type and surface roughness of flange sealing surface shall be coMPatible with the type and material of gasket.
12. When asbestos or flexible graphite gasket is used for stainless steel or nickel base alloy flange, the chloride ion content in gasket material shall not exceed 50PPm.
13. When flexible graphite material is used in oxidizing medium, the maximum service temperature shall not exceed 450 ℃.
14. For flanges with nominal pressure less than or equal to 1.6Mpa, when semi-metallic gaskets or metal ring gaskets such as spiral wound gaskets and metal clad gaskets are used, flange structures with high rigidity such as neck butt welding flanges shall be selected (very important).
15. When selecting gasket size, attention shall be paid to ensure that the inner diameter of non-metallic flat gasket or inner ring of spiral wound gasket shall not be lower than the inner diameter of flange. If the inner diameter of gasket (or inner ring) is required to be flush with the inner diameter of flange, the corresponding flange inner diameter shall be provided to the gasket manufacturer as the inner diameter of gasket, but sufficient bearing width of gasket shall be ensured.
16. Gaskets with good rebound characteristics, such as wave tooth composite gasket or spiral wound gasket (with inner and outer rings), shall be used for occasions with temperature and pressure changes or occasions with vibration and iMPact.
17. Asbestos rubber plate gasket: it is recommended to be generally only used in non important equipment occasions with low temperature and pressure. Asbestos rubber as gasket should be considered comprehensively and selected carefully. Asbestos gasket shall not be used in extremely or highly hazardous media and high vacuum sealing occasions. The thickness of asbestos gasket for pipeline is generally 1.5 ~ 3mm. The use pressure of thin gasket is high, but the sealing pressure is large. The resilience of thick gasket is better than thin gasket, which is easier to seal. The specific pressure value of gasket is also low, but the thickness exceeds 3mm, which has little change on the leakage rate, and it is easy to extrude when the pressure is high.
18. PTFE gasket
Polytetrafluoroethylene gasket can resist the corrosion of various chemical media including acid, alkali and solvent, and is clean and pollution-free. Due to its cold flow performance and thermal creep characteristics, the service pressure of pure polytetrafluoroethylene gasket is generally no more than 4.0Mpa and the service temperature is generally no more than 150 ℃. The sealing surface type should be full plane, concave, convex or tenon surface and groove surface. However, PTFE coated gasket can overcome the disadvantages of inelastic and easy cold flow of general PTFE gasket, so its performance is better. PTFE coated gasket shall not be used in occasions where vacuum or its embedded layer material is easy to be corroded by medium. Generally, PMF type and PMS type are used to reduce liquid retention in the pipe. PFT type is used for occasions with nominal size greater than or equal to DN350.
19. Spiral wound gasket:
(1) Application scope of spiral wound gasket
- The chemical industry standard hg20160-2009 stipulates that the spiral wound gasket is applicable to the nominal pressure of pn1.6 ~ 16.0MPa, and the temperature range is: the maximum temperature of the spiral wound gasket with stainless steel tape and special asbestos tape is 500 ℃;
- The maximum temperature of stainless steel tape and flexible graphite tape wound gasket is 650 ℃ (oxidizing medium ≤ 450 ℃);
- The maximum temperature of stainless steel tape and PTFE tape wound gasket is 200 ℃.
(2) The hardness of the metal strip used for the spiral wound gasket is recommended to be HV ≤ 150. When used for austenitic stainless steel flange sealing, the chloride ion content in the non-metallic filler in the spiral wound gasket shall be controlled within 100PPm.
(3) The corresponding gasket type shall be selected according to the flange type. The spiral wound gasket with inner ring, outer ring or outer ring is recommended for plane and convex flanges, and the spiral wound gasket with inner ring is recommended for concave convex flanges.
(4) The spiral wound gasket is generally only applicable to the flange above the medium pressure. If it is used on the low-pressure flange, the due sealing effect may not be obtained due to insufficient bolt force. It is recommended to preferentially use expanded graphite spiral wound gasket.
(5) The inner ring material shall have the same or higher corrosion resistance as the metal strip material, and the stainless steel material shall comply with the provisions of GB / t3280 or GB / t4237. Unless otherwise specified, carbon steel can be used for the middle ring, and the carbon steel can adopt the provisions of GB / t11253 or GB/T 912, and be sprayed, electroplated or other surface treatment.
(6) Basic spiral wound gasket is only applicable to tenon flange. Inner ring shall be used for CLASS600, class900, class1500, class2500 and gaskets filled with PTFE.
20. Flexible graphite composite gasket
The maximum working pressure of flexible graphite composite gasket is 6.3MPa, and the maximum working temperature depends on the metal core plate material. For low carbon steel and stainless steel core plates, it is 450 ℃ and 650 ℃ respectively (450 ℃ for oxidizing medium).
The flexible graphite composite gasket is applicable to the following media: water, oil, solvent, acid, liquid hydrocarbon, low-temperature liquefied gas, etc; Gases include air, hydrogen, oil and gas, high-temperature flue gas, steam, hydrocarbons and various gases with strong permeability.
Flexible graphite composite gasket has low requirements for flange surface processing, and can be used in corrosive occasions. It is suitable for low-pressure, high-temperature convex and concave convex welded steel flanges, especially to replace the asbestos rubber plate used in general parameter occasions in the past.
21. Wave tooth composite gasket
Wave tooth composite gasket is suitable for high and low temperature (- 200 ℃ ~ + 700 ℃), high and low pressure, vacuum (vacuum ~ 25MPa) and other occasions. It can maintain high sealing performance under the condition of low compressive stress. It can be applied to the pipe flange of dnl0 to the container flange of dn2500. It can also maintain excellent sealing performance for a long time in the case of easy fluctuation of temperature and pressure.
22. Metal clad gasket
The maximum service temperature of metal clad gasket is 450 ℃, and the maximum design pressure is 6.4Mpa. The maximum hardness value Hb of clad metal shall not exceed the following values: L2 ＜ 40,08 steel ＜ 90, tinned steel sheet ＜ 90, 0Cr13 ＜ 183,0cr19ni9, 0cr18ni11ti and 00Cr17Ni14Mo2 ＜ 187.
When asbestos board is used as the filling material, splicing of the filled asbestos board shall be prohibited, and it must be clearly indicated when the purchase demand is put forward to each supplier of metal clad gasket of our coMPany.
23. Metal gasket
(1) Metal ring gasket
Metal ring gasket is mainly used for high-pressure and high-temperature equipment and pipeline flanges. National standards and petrochemical industry standards are applicable to nominal pressure of 2.0 ~ 42.0MPa, chemical industry standards are applicable to 2.5 ~ 16.0MPa, and JB / t89-94 is applicable to 6.3, 10, 16 and 20MPa.
The service temperature of metal ring gasket depends on the metal material used. The hardness of metal ring gasket material shall be hb30 ~ 40 lower than that of flange sealing surface material, and its maximum value shall comply with the provisions of relevant standards.
The sealing surface of the metal ring gasket shall be free of scratches, bumps, cracks and defects, and the surface roughness shall not be greater than Ra1.6 μm.
The metal ring gasket of No. 10 steel or 08 steel shall be used, and the finished product shall be coated with antirust oil after inspection.
(2) Metal toothed gasket
Metal toothed gasket can be used for concave convex integral cast steel pipe flange and butt welded steel pipe flange with nominal pressure of 4.0 ~ 16.0MPa. The maximum service temperature of 08 or 10 or 022cr17ni12mo2 is 450 ℃, the maximum service temperature of 0Cr13 is 540 ℃, and the maximum service temperature of 06Cr19Ni9 is 600 ℃.
The limit deviation of parallelism of the two end faces of the metal toothed pad shall not exceed ± 0.05mm per 100mm diameter length. The surface roughness Ra of the tooth top plane of the metal toothed pad is 1.6 μ m. The metal toothed pad shall be made of a whole steel plate, and butt welding is not allowed.
Because the gasket usually adopts the bolt flange connection structure, the bolt needs to be pre tightened enough to meet the sealing requirements during assembly. In addition, the gasket in this structure is a sealing element affected by many factors, so how to control the pre tightening force is a very difficult problem. The compression loads required by various gaskets are also different, and the gasket sealing with special requirements , they do not have standard connection dimensions, such as flange thickness, bolt size, bolt spacing, etc., which requires special design.
Source: China Gaskets Manufacturer – Yaang Pipe Industry (www.steeljrv.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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