What is a Gasket?
A gasket is an essential element for creating a leak-proof seal in a piping system. It is a soft sealing material placed between two flanges to form a static seal. Gaskets are necessary to overcome misalignment of the flanges and fill the uneven surface of the flange in order to prevent leakage. They must be able to maintain the seal even under extreme temperature and pressure upsets.
Functions of a Gasket in a Piping Flanged Joint
Gaskets are used to create a static seal between two stationary members of a mechanical assembly. This seal helps to keep the system leak-proof and functioning effectively. Gaskets have to fill the uneven surface of the flange and overcome misalignment of the flanges in order to prevent leakage. Furthermore, they must be able to maintain the seal even under extreme temperature and pressure upsets.
Types of Gaskets
There are a variety of gasket materials available, each with its own unique properties and characteristics. Common gasket materials include rubber, PTFE, metal, graphite, and non-asbestos gaskets. The selection of the right gasket material for the job is important for creating an effective seal that is able to withstand the particular operating conditions.
Selection Methodology for Gaskets
When selecting a gasket, it is important to consider the operating conditions, such as temperature, pressure, shock, vibration, and chemical compatibility. The selection methodology should also take into account the type of material to be sealed, the type of joint, and any additional factors that could affect the performance of the gasket. Once these factors have been considered, the right gasket material can be selected and the proper installation techniques should be followed to ensure the best performance.
Working of a Gasket
Understanding the Three Major Forces
The three major forces that act on any gasket joint are the bolt load, the hydrostatic end force, and the internal pressure. The bolt load applies a compressive load that flows the gasket material into surface imperfections to form a seal. The hydrostatic end force tends to separate the flanges when the system is pressurized, while the internal pressure acts on the portion of the gasket exposed to the internal pressure, which can blow the gasket out of the joint or bypass the gasket under certain operating conditions. Additionally, shock forces and creep relaxation are factors that can come into play.
Achieving a Tight Joint
For a tight joint to be achieved, the initial compression force applied to the joint must serve several purposes. It must be sufficient to seat the gasket and flow the gasket into the imperfections on the gasket seating surfaces. It must also be able to compensate for the total hydrostatic end force that would be present during operating conditions, as well as maintain a residual load on the gasket/flange interface. To ensure this, a residual load on the gasket must be “X” times the internal pressure, with “X” being specified as the “m” factor in the ASME Pressure Vessel Code. The larger the value of “m”, the greater the assurance that a tight joint will be achieved.
Gasket Types Based on Gasket Materials
Gaskets are classified depending on the materials used in their production. These materials include non-metallic, semi-metallic or composite, and metallic.
Non-Metallic Gaskets
Non-metallic gaskets are made from composite sheet materials, which are commonly used with flat-face flanges in low-pressure applications. Non-asbestos materials such as aramid fiber, glass fiber, elastomer, Teflon (PTFE), and flexible graphite are used to produce non-metallic gaskets. Full-face gasket types are suitable for use with flat-face (FF) flanges, while flat-ring gasket types are suitable for use with raised-face (RF) flanges.
Semi-Metallic or Composite Gaskets
Semi-metallic or composite gaskets are produced by combining a metal core with a soft, non-metallic material, such as rubber or graphite. These gaskets are used in high-pressure applications and are resistant to high temperatures and pressures.
Metallic Gaskets
Metallic gaskets are made of solid metal, such as stainless steel, and are used in extreme high-pressure and high-temperature applications. They are also resistant to corrosion and chemicals.
Semi-Metallic Gaskets:
Semi-metallic gaskets are a combination of metal and non-metal materials. They provide strength, resiliency, conformability, and sealability for a wide range of temperatures and pressures. These gaskets are suitable for use on raised faces, male-and-female, and tongue and groove flanges.
Types of Semi-Metallic Gaskets:
Semi-metallic gaskets come in many varieties. Common types are spiral wound, metal jacketed, Cam profile, and metal-reinforced graphite gaskets. Each type of gasket is designed for different applications and offers unique benefits for different operating conditions.
Uses of Semi-Metallic Gaskets:
Semi-metallic gaskets are used in a variety of industries, including automotive, aerospace, chemical, and power generation. They are often used in high-pressure, high-temperature applications, where their combination of strength and resiliency make them a suitable choice.
Types of Metallic Gaskets
Metallic gaskets are an ideal choice for high-pressure and temperature applications. They are typically fabricated from one or a combination of metals and can be manufactured to the desired shape and size. Some of the most common metallic gaskets include ring-joint gaskets and lens rings.
Gasket Configurations
Gaskets come in various configurations, each suited to different applications. Graphite foil gaskets are particularly useful in oxidizing and reducing environments, as they are capable of withstanding temperatures between –200 and 2,000°C. Moreover, they have excellent chemical resistance, making them suitable for the majority of commercially used common chemicals.
Benefits of Graphite Foil Gaskets
Graphite foil gaskets offer many advantages due to their physical and chemical properties. They are capable of withstanding extreme temperatures and have excellent stress-relaxation properties. Additionally, they are free of binder materials, making them highly resistant to chemical degradation.
Advantages of Metallic Gaskets
Metallic gaskets are ideal for high-pressure and temperature applications due to their strength and durability. They are also capable of withstanding extreme temperatures and require a high bolt load to seal effectively. Furthermore, they are available in a variety of shapes and sizes, making them suitable for a wide range of applications.
Spiral-Wound Gasket: Overview
A spiral-wound gasket is composed of a preformed metal strip and a filler, wound on the periphery of a metal mandrel. It is furnished with a centering ring to control compression, and to ensure the gasket is properly centered in the bolt circle. An inner ring may be used to prevent buckling or the buildup of solids, as well as to protect against damage under vacuum conditions. These gaskets can be used at temperatures ranging from -250 to 1000°C, and pressures from vacuum to 350 bar.
Spiral-Wound Gasket: Compression Requirements
For spiral-wound gaskets up to 1-inch in diameter and class number 600, a uniform bolt stress of 25,000 psi is required to compress the gasket. Gaskets larger in size and higher in class number require a compression of 30,000 psi.
Ring-Joint Gaskets
Ring-joint gaskets are used in grooved flanges for high-pressure-piping systems and vessels. They have an applicable pressure range of 1,000 to 15,000 psi and are designed to provide a high gasket pressure with a moderate bolt load. The hardness must be lower than the flange material so that the material can flow without damaging the flange surfaces.
Types of Ring-Joint Gaskets
The two most commonly used ring-joint gaskets are oval and octagonal types. Oval gaskets contact the flange face at the curved surface and provide a reliable seal, yet the curved shape makes it difficult to achieve accurate dimensioning and surface finishing. Octagonal gaskets, on the other hand, are more cost-effective and are easier to dimension and surface finish than oval gaskets. However, a greater torque load is required for octagonal gaskets to flow into any imperfections on the flange faces. Octagonal gaskets can be used more than once.
Corrugated-metal Gaskets
Corrugated-metal gaskets are a versatile type of gasket used in a variety of applications. They are available in a wide range of metals, including brass, copper, copper-nickel alloys, steel, Monel, and aluminum, and can be manufactured to nearly any shape and size.
Material and Manufacturing Specifications
Corrugated-metal gaskets typically have a thickness of 0.25 or 0.3 mm, and the corrugations have a pitch of 1.6, 3.2, or 6.4 mm. The gaskets are manufactured to exact specifications in order to ensure a reliable seal between the peaks of the corrugations and the mating flanges.
Sealing Mechanism
The sealing mechanism of corrugated-metal gaskets is based on point contact between the peaks of the corrugations and the flanges. This ensures a reliable and secure seal that can withstand high pressures and temperatures.
Types of Gaskets Based on Face
Gaskets are used in many industries and come in two varieties based on the face of the gasket. These two types of gaskets are commonly known as ring gaskets and full-face gaskets. Ring gaskets extend to the inside of the flange bolt holes and are typically used with raised face or lap joint flanges. Full-face gaskets, as the name implies, cover the entire flange face and are pierced by the bolt holes. They are intended for use with flat-face flanges.
Flat Ring Gaskets
Flat-ring gaskets are a popular option when service conditions allow. They are easily cut from a flat sheet and installed. They are constructed from materials such as rubber, paper, cloth, asbestos, plastics, copper, lead, aluminum, nickel, Monel, and soft iron. The thickness of these gaskets range from 1/64 to 1/8 inches. Paper, cloth, and rubber gaskets are not suitable for temperatures above 120° C. Asbestos-composition gaskets can be used up to 350° C, while ferrous and nickel-alloy gaskets can be used up to the flange’s maximum temperature rating.
Axial and Radial Gasket Flow: How Compression Affects Performance
When a gasket is subjected to initial compression, it will experience both axial and radial flow. Axial flow is necessary to fill any imperfections in the flange facing and ensure proper sealing. Radial flow serves no purpose unless the gasket is confined. When a flange joint is heated, the increased gasket pressure and the softening of the gasket material due to the heat can cause additional axial and radial gasket flow. To counteract this, it is common to re-tighten the flange bolts after the joint has reached its normal operating temperature.
Thin Gaskets vs. Thick Gaskets: Understanding the Difference
Thin gaskets experience significantly less radial flow than thick gaskets, even at high unit pressures. This makes thin gaskets a better choice for high temperature applications, as they will maintain their permanent thickness, while thick gaskets may become compressed and leak over time. However, thin gaskets may not be thick enough to adequately seal the flange faces.
Spiral-Wound Asbestos-Metallic Gaskets: Combining the Benefits of Thick and Thin Gaskets
Spiral-wound asbestos-metallic gaskets are a good option for those looking to benefit from both thick and thin gaskets. These gaskets are thick (most common types are 0.175” thick) but their spiral construction prevents radial flow. This allows them to remain resilient and adjust to varying conditions. They can be made with different filler materials such as Teflon or Grafoil for compatibility with particular fluids. When used with raised face flanges, these gaskets usually have an inner metal ring and an outer centering ring.
Laminated Gaskets
Laminated gaskets are constructed with a metal jacket and a soft filler, usually of asbestos. They can be used at temperatures up to 450°C and require less bolt load than solid metal flat ring gaskets to stay in place.
Serrated Metal Gasket
Serrated metal gaskets are a type of gasket fabricated from solid metal, featuring concentric grooves machined into the faces. The grooves reduce the contact area on initial tightening, thus reducing the bolt load. The gasket is deformed as the contact surface area increases.
What are Serrated Metal Gaskets Used For?
Serrated metal gaskets are used in situations where soft gaskets or laminated gaskets are deemed unsuitable and when the bolt load is too great for a flat-ring metal gasket. Smooth-finished flange faces should be used with serrated gaskets.
Corrugated Gaskets with Asbestos Filling
Corrugated gaskets with asbestos filling are a type of gasket that is widely used in low-pressure liquid and gas services. They are similar to laminated gaskets but feature a rigid surface with concentric rings, instead of being serrated. Furthermore, these gaskets require less seating force than other types of gaskets.
Ring-Joint Gaskets
Ring-joint gaskets are designed for high-pressure services and come in two standard types. The first type has an oval cross-section, while the other has an octagonal cross-section. These rings are typically made of soft iron, soft steel, Monel, 4-6% chrome, and stainless steel, and the alloy-steel rings should be heat treated to soften them. It is recommended that the ring joint gasket be used for class 150 flanged joints, and that line flanges be of the welding neck type.
Parameters affecting Gasket performance
Effect of Flange Load on Gasket Performance
The performance of a gasket is directly affected by the amount of flange pressure applied to it. This is known as the “y” factor, and it is important for the flange designer to take into account the uneven distribution of force around the gasket that occurs in actual service. The greatest force is exerted on the area directly surrounding the bolts, and the lowest force occurs mid-way between two bolts.
Impact of Internal Pressure on Gasket Performance
The internal pressure of the system puts a strain on the gasket, reducing the initial compression. To compensate for this, an additional preload must be applied to the gasket material, the “m” factor. This factor is established by ASME and defines how many times the residual load must exceed the internal pressure. It is important to take both the normal pressure and the test pressure into account.
Temperature’s Effect on Gasket Performance
Temperature affects the gasket material, the flanges, and the bolts, resulting in a reduction of the flange load. The higher the operating temperature, the more care needs to be taken with the gasket material selection. Different coefficients of expansion between the bolts, the flanges, and the pipe can result in forces that can affect the gasket.
Role of Fluid in Gasket Performance
The media being sealed, usually a liquid or gas, affects the sealing performance of the gasket. The higher the temperature, the more aggressive the fluid can be. Therefore, the gasket material must be resistant to corrosive attack from the fluid and should chemically resist the system fluid to prevent serious impairment of its physical properties.
Surface Finish of Gasket and Performance
The surface finish of a gasket governs the thickness and compressibility required by the gasket material to form a physical barrier in the clearance gap between the flanges. A finish that is too fine or shallow is undesirable, and a finish that is too deep will yield a gasket that requires a higher bolt load. Flange faces with non-slip grooves that are approximately 0.125 mm deep are recommended for gaskets more than 0.5 mm thick.
Impact of Gasket Thickness on Performance
Thinner gaskets are able to handle higher compressive stress than a thicker one. However, thinner materials require a higher surface finish quality. The gasket must be thick enough to occupy the shape of the flange faces and still compress under the bolt load.
Gasket Width and Performance
A raised face flange with a narrow gasket will require less preload, and thus less flange strength than a full-face gasket. High-pressure gaskets tend to be narrow.
Stress Relaxation and Performance
Gasket material will lose some resiliency over time due to the flow or thinning of the material caused by the applied pressure. After some initial relaxation, the residual stress should remain constant for the gasket.
Effect of Gasket Outer Diameter on Performance
For two gaskets made of the same material and having the same width, the one with a larger outer diameter will withstand higher pressure. Therefore, it is advisable to use a gasket with an external diameter that is as large as possible.
Types of Gasket Standards for Design
There are various standards adopted for specifying gaskets for different applications. These include:
ASME B16.21: Non-Metallic Flat Gaskets for Pipe Flanges
ASME B16.20 offers standards for metallic gaskets for steel pipe flanges, Ring Joints, Spiral Wounds, and Jacketed.
IS2712: Compressed Asbestos Fiber Jointing Specification
BS 3381: Spiral Wound Gaskets to Suit BS 1560 Flanges
These standards are necessary to ensure the quality and safety of the gaskets used in various applications.
Selecting the Right Gasket Material
When selecting a gasket material, it is important to consider the physical and chemical effects of the service conditions on the gasket. The material should be able to withstand the pressure, temperature and service life of the system, and should also be corrosion-resistant against the service fluid. Additionally, the gasket should be readily available and affordable.
Flange Construction
The thickness of the flange can affect the bolt load and seating stress, so when selecting a gasket, it is important to consider the flange construction. Thin and deformed flanges typically require softer gaskets.
Standardization and Quality Studbolts
Standardization plays an important role in the selection of gaskets, and it is important to ensure that quality studbolts are used for installation.
Emission Parameters
When selecting a gasket, it is important to consider the emission parameters of the gasket. Each type of gasket has different emission parameters, and these should be taken into account.
Flange Misalignment
It is important to take into account the maximum misalignment of the flanges, which should not exceed 0.4 mm.
Gasket Materials
| Metallic | Non-Metallic | Winding Strips of Spiral Wound Gaskets | Filler Material for Winding |
|---|---|---|---|
| Low Carbon Steel | PTFE | Stainless Steel | PTFE |
| Stainless Steel | Compressed Asbestos | Duplex Steel | Graphite |
| Soft Iron | Rubber | Monel | |
| Chrome-Moly | Ceramic Fiber | Titanium |
Gasket Identification: Spiral Wound Gaskets
Spiral wound gaskets are one of the most common types of gaskets used in process industries. They are composed of a stainless steel core with a filler material of either PTFE or graphite. To help facilitate the inspection and identification of such gaskets, the American Society of Mechanical Engineers (ASME) B16.20 provides a Color Coding Chart. This chart is illustrated in Fig. below.
Characteristics of a Gasket
Characteristics of a Gasket
A gasket material must have adequate softness to compress and fill in the irregularities of the flanges.
Sealing Ability:
A gasket must be gas-liquid tight to avoid leakage and emissions. It should also be capable of achieving a seal at elevated temperatures.
Resilience:
The gasket must be resilient to static and dynamic pressures, temperatures, and stresses. It should be able to withstand internal pressure without being blown out.
Chemical Resistance:
The gasket must be resistant to the chemical attack of the medium without contaminating it. It should also possess anti-stick properties to make disassembly of the flange easier.
Installation:
The gasket should be stiff enough to make installation easy. This ensures that the bolts are under sufficient stress and that there is no leakage.
Why do Gaskets Leak?
Damage During Assembly
Gaskets can leak due to damage during assembly, such as due to improper installation or inadequate protection of the gasket during installation. Damage can also occur when the bolts are tightened too much, or when the gasket is not properly aligned during installation.
Poor Gasket Selection
Leaking gaskets can also be caused by poor gasket selection. Choosing the wrong material for the application or the wrong size can lead to a gasket not being able to effectively seal the joint. Additionally, using a gasket that is not rated for the pressure or temperature of the application can lead to a leaking gasket.
Excessive Flange Rotation
Gaskets can become damaged due to excessive flange rotation. This can occur when the flange is not properly aligned before the bolts are tightened, or when the bolts are tightened too much. This can cause the gasket to become misaligned and can lead to leakage.
Gasket Damage or Relaxation Due to Flange Rotation
If the flange is not properly aligned during assembly and the bolts are tightened too much, the gasket may become misaligned and may suffer damage or relaxation due to the excessive flange rotation. This can lead to the gasket not being able to effectively seal the joint and can lead to leakage.
Gasket Damage Due to Differential Thermal Expansion
Differential thermal expansion is a phenomenon which occurs when different materials expand and contract at different rates due to changes in temperature. This can lead to gasket damage, as the gaskets may not be able to withstand the changes in temperature and may become misaligned or suffer damage due to the expansion and contraction.
Incorrect Assembly Bolt Load
Incorrect assembly bolt loads can also lead to gasket failure. If the bolts are not tightened to the specified torque, the gasket may not be able to effectively seal the joint and may lead to leakage.
Load Loss Due to Thermal Fluctuation
Gaskets can also suffer from load loss due to thermal fluctuations. When temperatures change, the gasket may not be able to maintain its shape and can become misaligned or suffer damage. This can lead to leakage.
Gasket Load Loss Due to Pressure and/or Piping Loads
Gaskets can suffer from load loss due to pressure and/or piping loads. If the gasket is not able to withstand the pressure or loads, it may become misaligned or suffer damage. This can lead to leakage.
Excessive Gasket Relaxation
Excessive gasket relaxation can also lead to gasket leakage. If the gasket is not able to maintain its shape and becomes too relaxed, it may not be able to effectively seal the joint and can lead to leakage.
Excessive Gasket Load
Gaskets can also fail due to excessive gasket load. If the gasket is overloaded, it may become misaligned or suffer damage and may not be able to effectively seal the joint. This can lead to leakage.
Financial Impact of Gasket Failure
A gasket failure can greatly reduce profits due to the leakage of materials. Additionally, the cost of online sealing with clamps, re-matching, replacement equipment, and engineering and maintenance hours spent addressing leakage can add up quickly.
Paperwork and Incident Reports
Whenever a gasket failure occurs, there is often a need to file incident reports and additional paperwork. This can be a time consuming and costly process.
Environmental Impact
In addition to financial losses, gasket failure can also lead to environmental pollution. Cleaning up the mess can require further costs and resources.
Personal Injury Risks
In some cases, a gasket failure can lead to serious personal injuries. Therefore, it’s important to take precautions to ensure gaskets are properly maintained and replaced when necessary.
Repair and Maintenance
If a gasket fails, it can require expensive disassembly, repair, and machining. This further adds to the cost of a gasket failure.
What could engineering do to prevent leakage through Gaskets?
Proper Bolt Stress for Gasket Leakage Prevention
Selecting the Correct Location, Constraint and Width of the Sealing
Bending Loads and Misalignment Considerations
Gasket Creep/Relaxation Effects on Leak Prevention
Temperature and Pressure Effects on Leak Prevention
Determining Maximum Permissible Assembly Load
Gasket Selection for Leak Prevention
Root Cause Analysis for Gasket Leak Prevention
Basic calculations for Gasket Selection
- Gasket Factor of Safety:
Gasket Factor of Safety = Maximum Allowable Load / Design Load
- Bolt Stress:
Bolt Stress = Bolt Area x Applied Load / Bolt Area
- Gasket Stress:
Gasket Stress = Gasket Compressibility x Applied Load / Gasket Area
- Compression Ratio:
Compression Ratio = Applied Load / Initial Thickness
- Maximum Allowable Load:
Maximum Allowable Load = Gasket Factor of Safety x Design Load
