Gaskets

Chapter 1: What is a Gasket?

Gaskets are a type of mechanical seal that inhibits leakage by filling the gap between static mating surfaces. It takes advantage of the compressive force that drives the gasket material to plastically flow between the two mating surfaces. Either polished or unpolished surfaces, particularly metal surfaces, have an inherent roughness or microscopic asperity that creates spaces where fluids can pass through. As the gasket is deformed by a compressive force, it conforms to the profile of the surface and fills the gaps between its peaks and troughs.

Gaskets are extensively utilized in every industry that deals with either pressurized or non-pressurized liquids and gases. They can be seen in most fluid-containing equipment such as pipes, tanks, heat exchangers, combustion engines, and so forth. Gaskets vary in form and rating to suit a particular application.

Chapter 2: Different Types of Gaskets

Material and form are important gasket specifications as it governs the gasket‘s properties against corrosive attacks, extreme temperatures and pressures, mechanical loading imparted by the mating surfaces, and dynamic operating conditions. Depending on the type of material used and construction, gaskets can be divided into three main categories: non-metallic, semi-metallic, and metallic gaskets.

• Non-metallic Gaskets: Non-metallic gaskets are typically used for low to medium fluid pressures. In terms of temperature, they can handle both low and high ranges depending on the material used. They are widely used because of their low cost except for special materials such as PTFE, vermiculite, and graphite. Also, they can easily be manufactured and supplied in the form of sheets which are cut to shape through cutting processes such as die-cutting. Suppliers and parts storage does not have to stock a particular form since the gasket can easily be cut according to the end requirements.
  
  

  

  
  

  Non-metallic gaskets are either made of homogeneous materials such as flexible graphite sheet and virgin PTFE, or a composite of fibers and granules embedded in an elastomer resin. As technology progresses, more advanced materials are being developed that are proprietary to select manufacturers. Common non-metallic gasket materials are enumerated below.

  • Flexible Graphite Gaskets: Flexible graphite gaskets are made by expanding graphite flakes through processes of intercalation, heating, and compression. High-quality particulate graphite flakes are mixed with acids such as nitric, phosphoric, and sulfuric acids. This chemical treatment creates graphite intercalation compounds that tend to exfoliate upon heating. The exfoliation process involves the expansion of the graphite by several magnitudes. The intercalated compounds diffuse within the graphite tend to vaporize forming gas pockets. After exfoliation, worm-like or vermiform structures with highly active, dendritic-like rough surfaces are produced. Compression results in a mechanical interlocking of these worm-like structures. After compression, a flexible graphite sheet is formed. Compared to other non-metallic gaskets, flexible graphite has poorer tensile strength. Reinforcements, laminates, and inserts are added to create a composite material that combines their strength to the pliability of graphite.
    
    

    

    
    
  • Phyllosilicate (Mica and Vermiculite Minerals) Gaskets: Phyllosilicates are a group of minerals based on the mica family which can be used to make non-oxidizing, high-temperature gasket materials. Their non-oxidizing properties solve the problem with graphite gaskets which is the tendency to oxidize or coke at high temperatures in environments containing oxygen or other oxidizing agents. The two main classifications of phyllosilicates used in gasket production are mica and vermiculite. Both of these minerals have the same temperature and chemical resistance properties. Mica gaskets are formed from sheets that are created by combining the mica mineral with polymer and subjecting them to high heat. Vermiculite, on the other hand, is technically a mica that is capable of expansion. Vermiculite is formed from the flash conversion of the water molecules present in between the layers or the crystal structure.

    

    
    
  • Polytetrafluoroethylene (PTFE) Gaskets: PTFE is currently a highly important material in chemical processing industries due to its high bonding energy making it resistant to chemical reactions, particularly corrosion. There are a few chemicals that can degrade PTFEs. Examples are fluorinating agents, magnesium, and molten alkali metals. Aside from its high chemical resistance, PTFE also has a low coefficient of friction, excellent insulating properties, and high toughness and impact strength. PTFE gaskets are available in three forms: virgin PTFE, filled PTFE, biaxially oriented PTFE, and expanded PTFE.
    
    

    

    
    
  • Elastomer (Rubber) Gaskets: Elastomers are classes of polymers that have a highly elastic nature created by cross-linking long polymer chains into amorphous structures. The intermolecular forces between the polymer chains are relatively weak, allowing them to be reconfigured upon application of stress. Because of this property, elastomer gaskets can easily conform to the profile of the surfaces creating a tight seal. Also, there is a wide range of elastomer formulations available in the market to meet specific requirements. Their chemical and temperature resistance, however, is inferior to PTFE. Depending on the type of curing reaction or vulcanization, elastomers can degrade under the presence of water, ultraviolet light, oils, and certain solvents. High temperatures can easily expand and melt elastomers, while low temperatures can make them brittle. Types of elastomers used for gasket manufacture are nitrile (NBR), ethylene propylene diene monomer (EPDM), neoprene, silicone, and fluoroelastomer (FKM).
    
    

    

    
    
  • Compressed Fiber Gaskets: As the name suggests, this type is composed of naturally occurring mineral fibers or synthetic polymer fibers. One of the earliest known compressed fiber gaskets used for industrial applications is the asbestos gasket. Asbestos is a naturally occurring silicate mineral and is described as having long and thin fibrous crystals. However, the manufacturing of asbestos gaskets is now being discontinued due to associated health hazards such as asbestosis and cancer. Substitutes for asbestos are carbon, graphite, glass, aramid, and other fibers.
    
    

    

    
    

    Compressed fiber gaskets are produced by a process called beater addition. Beater addition is generally a proprietary process. In this process, the minerals are beaten fibrillating the main fiber into tiny fibrils. This causes the fibers to spread. elastomer resins are added to bind the fibers together. Typical elastomers used are styrene-butadiene rubber (SBR), NBR, neoprene, and EPDM.

  • Cork Gaskets: Cork is a viable material for low temperature and pressure applications. A gasket is made by compressing granulated cork bark and binding it with an elastomer resin. Cork gaskets are characterized as being lightweight, flexible, and impermeable to water, oil, and other petrochemicals.
    
    

    

• Special Types of Non-Metallic Gaskets:
  • Santoprene Gaskets: This type of gasket is made of rubber material that belongs to thermoplastic vulcanizates (TPV) or thermoplastic polymers (TPE). Santoprene is a patented material composed of a dynamically vulcanized EPDM dispersed in a polypropylene matrix. Because of its thermoplastic property, it can be melted and recycled. Regarding sealing characteristics, Santoprene has good resistance against degrading factors such as ultraviolet light and ozone.
  • Poron Gaskets: Poron is a patented gasket material made from multicellular polyurethane. Because of its porous structure, it is well suited for thermal insulation, vibration dampening, acoustic dampening, and shock absorption. The inherent resilience and rebound properties of polyurethane make it suitable for sealing because of the reduced effect of creep relaxation.

    

    
    
    • RN-8011 Gaskets: This is another patented gasket made from a composite of low-density cellulose fiber material with high rubber filler content dispersed in a nitrile rubber matrix. It is well suited for sealing oil and water even at low bolt loads. They are commonly used in engine heads, transmission pans, water pumps, and environment seals.
• Semi-metallic Gaskets: These types of gaskets are composites of metallic and non-metallic materials. The metallic component provides structural strength and increased toughness, while the non-metallic part offers enhanced sealing. The myriad number of possible metal and non-metal component combinations, as well as the different styles available, enables semi-metallic gaskets to be suitable in almost every condition. The downside of using semi-metallic gaskets is that they are supplied in distinct sizes and shapes. They cannot be cut and shaped the same way as non-metallic gaskets. Semi-metallic gaskets must be dimensionally suitable with the mating surfaces, usually flange faces for piping. The different types of semi-metallic gaskets are as follows.
  
  
  • Spiral-wound Gaskets: This type of semi-metallic gasket is composed of V-shaped metal strips wound alternately with a filler material. The winding is supported by an inner and outer ring. The inner ring is the part in contact with the process liquid, apart from the windings. Thus, its material has higher requirements than the outer ring which is typically made of carbon steel. The inner ring and metal windings are usually made from stainless steel while the filler can be PTFE, non-asbestos fibers, or graphite.
    
    

    

    
    
  • Jacketed Gaskets: In this type, the filler material is partially or entirely enclosed in a metal jacket. There are several forms and configurations available such as single, double, and corrugated jackets. Sealing is achieved by the deformation of the metal overlap which is thicker than the rest of the envelope. This thicker section bears more of the compressive load creating the seal.
    
    

    

    
    
  • Corrugated Gaskets: Corrugated gaskets are composed of a thin metal ring with a wave or corrugated pattern and are coated with a soft layer of a non-metallic material such as graphite, PTFE, and ceramic layers. Its working principle is based on the conformity of the soft layer to the surface irregularities. Corrugated gaskets are suitable for uneven flanges or flanges with surface imperfections.
    
    

    

    
    
  • Camprofile Gaskets: This type of semi-metallic gasket is composed of a grooved metal ring with a non-metallic facing material covering the grooves. Sealing is achieved in a similar way as corrugated gaskets. Compressing the gaskets forces the soft material to conform to the surface of the flange, while the grooved metal faces create concentric rings that further stress the soft material. The grooves enhance the sealing capabilities as well as providing structural support to the gasket.
    
    

    

    
    
• Metallic Gaskets: Extremely high temperatures and pressure cause non-metallic gaskets and filler sealing materials to fail. In this case, solid metallic gaskets are the only option. They are normally used in boiler and heat exchanger sealing. Similar to semi-metallic gaskets, metallic gaskets are supplied in standard shapes and sizes and must be dimensionally compatible with the mating surfaces. The types of metallic gaskets are ring-joint, flat metal, and grooved metal gaskets.
  
  
  • Ring-type Joint (RTJ) Gaskets: These are gaskets with thick cross-sections that are suitable for high temperature and high-pressure applications. This gasket works by tightly compressing and eventually crushing it between the mating surfaces. This forcefully flows the metal into the surface imperfections and leak paths. The materials used are softer than the flange material. Examples of RTJ gasket materials are soft iron, low carbon steel, stainless steel, and special alloys such as Inconel and Hastelloy.
    
    

    

    
    
  • Flat Metal Gasket: Unlike the RTJ, this metal gasket has a thinner cross-section. These gaskets are cut from sheet metal; thus, they can be made to match any surface‘s shape and size. They operate the same way as non-metallic gaskets, but they are more suitable for higher temperature applications. Also, they can only be used for applications with high bolt loads.
    
    

    

    
    
  • Grooved Metal Gaskets: This is similar to the flat metal gasket; however, serrations or grooves are present on the surface. The peaks of the concentric rings experience higher stresses when loaded. Sealing is achieved by creating a labyrinth seal effect along the grooved surface.
    
    

    

    
    
  • Welded Gaskets: This type of gasket does not primarily rely on the compression of the gasket along the mating surfaces. Rather, they achieve sealing by having a permanent welded connection between the surfaces.
    
    

    

    
    

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Chapter 3: Factors to Consider in Selecting a Gasket

Like any other component, gaskets perform well when all process parameters are considered. In selecting a gasket, the first thing to evaluate is the process fluid. The external environment rarely has an effect on the gasket specifications. The pressure and temperature are the two main parameters needed. The pressure rating of the gasket specifies its tensile strength; while the temperature rating is for determining if the gasket material can sustain its performance through heat and cold. Aside from these two, the gasket‘s chemical composition and thickness are also required, especially for non-metallic gaskets.

• Pressure: In normal operation, a gasket experiences three forces or loads. These are the bolt or flange load, the hydrostatic end force, and the blowout or the force from the internal pressure. The pressure inside the vessel or pipe directly affects the hydrostatic end force and the blow out force. When these forces exceed the tensile strength of the gasket, rupture of leakage will occur. The gasket must be able to resist the maximum internal pressure that can occur within the vessel. This is typically the test pressure, or the pressure used to check the integrity of the equipment. The test pressure is commonly 1.5 times the working pressure. Gaskets are usually specified with a pressure rating or a pressure class number standardized by engineering associations such as the ASME and DIN.
  
  
• Temperature: Temperature affects the mechanical and chemical properties of the gasket. Two mechanical properties are affected by temperature—creep and relaxation. Creep is the loss of gasket thickness when subjected to a constant load, while relaxation is the loss of compressive stress under constant deformation. Both of these effects are augmented by rising temperatures resulting in a decreased sealing characteristic of the gasket. Regarding the chemical properties, the effects are particularly evident in graphite gaskets and gaskets with elastomer resins. Graphite tends to oxidize at high temperatures. As oxidation consumes the graphite material, the gasket loses its sealing potential. For elastomer resins present in full elastomer gaskets or elastomer binders, high temperature can further cure or vulcanize the gasket making it more brittle and lose tensile strength. When purchasing gaskets, always look for the pressure-temperature curves or the maximum operating limits of the gaskets and verify if it suits the intended application.
  
  

  

  
  
• Process Fluid: The previous two parameters, pressure, and temperature, are mostly imparted by the process fluid wherein the effects of the external environment are almost negligible. The process fluid also has other properties that can dictate the compatibility of the gasket. The presence of oxidizing agents, acids, alkalis, oil, water, and abrasive media can degrade the material directly in contact with the fluid. That is why composites are widely popular since a highly chemical resistant inner ring can be used to absorb the attacks of the process fluid while maintaining the desired sealing and structural characteristics of the other components.
  
  
• Required Thickness: Gasket thickness is not a critical factor for metal and semi-metallic gaskets as the thickness is already standardized for a given pressure rating. The effect of gasket thickness is only evident on non-metallic gaskets. Thicker non-metallic gaskets generally have lower pressure and temperature ratings. In order to achieve the required sealing, thicker gaskets must be compressed with greater force. Thinner gaskets offer better blow out resistance, lower creep relaxation properties, and better compressive strength. It is best to select the thinnest non-metallic gasket material that can conform to the flange irregularities.

Chapter 4: Common Causes of Gasket Failures

The system safety safeguards are intentionally designed to fail during process abnormalities. The next weakest point in the system is the gaskets. During overpressure of pipes or pressure vessels, gaskets are usually the first to burst. Even if the gasket has the right specifications, there is still plenty of reason for these components to fail. Enumerated below are the most common causes of gasket failures.

• Uneven Compression: Uneven compression of gaskets creates areas of low and high compressions. High compression areas have a higher resistance to blow-out. Low compression areas have lower gasket conformity against the mating surface making it prone to leaks and blow-out. Gaskets perform well when the surfaces are parallel with each other. In flanges, parallel surfaces can be achieved by tightening opposite bolts with equal turns until gasket compression or using torque wrenches to ensure that all bolts exert equal forces. For gaskets clamped by asymmetric bolting, take note of the bolt arrangement. Areas with closer bolts can create higher compression.
  
  
• Over Compression: Over compression can lead to permanent gasket failure. Signs of over-compression are bulging or extrusion of the gasket material, or inward buckling for spiral wound gaskets. Non-metallic gaskets are typically limited to 15,000 psi. Metallic gaskets are designed to be crushed at higher pressures up to 30,000 psi.

  

  
  
• Under Compression: It was mentioned before that there are three forces acting on a gasket. These are the bolt compression force, hydrostatic end force, and the blowout or force from internal pressure. Hydrostatic end force is caused by the internal pressure of the vessel which acts to break the flange or sealing surfaces apart. This is countered by the bolt compression force. Not enough bolt compression can lead to less gasket conformity which makes it easier to blow out to happen.
  
  
• Overheating: Temperature has significant effects on the performance of gaskets. In case of process or operation upsets, higher than normal temperatures can occur. This can hasten creep and relaxation effects which leads to torque loss. As tension gradually decreases, leakage will eventually occur.
  
  
• Gasket Reuse: It is important to never reuse gaskets. This may be a general rule for most industrial plants. Equipment switch-overs and quick inspections may tempt operations or maintenance personnel to reuse gaskets especially if the vessel or pipe undergoes frequent opening. Gaskets when used plastically deform according to the irregularities of the mating surface. They do not fully rebound to their original thickness. Thus, after use, most of its sealing characteristics are already gone.
  
  
• Chemical Attack: Gaskets are made from various polymers and minerals that can oxidize or react to certain process fluids. Composites composed of fillers, binders, inserts, and laminations must be noted regarding their individual chemical resistances. Some signs of chemical attack on gaskets are brittle cracking, softening, tearing, erosion, and discoloration. If these signs are found after service, it is best to first eliminate the possible effect of fluctuating temperature. If overheating is not the case, the next course of action is to change the type of gasket material.

Conclusion:

• Gaskets are a type of mechanical seal that inhibits leakage by filling the gap between static mating surfaces. As the gasket is deformed by a compressive force, it conforms to the profile of the surface and fills the gaps between its peaks and troughs.
• Depending on the type of material used and construction, gaskets can be divided into three main categories: non-metallic, semi-metallic, and metallic gaskets.
• Non-metallic gaskets are either made of homogeneous materials such as flexible graphite sheet and virgin PTFE, or a composite of fibers and granules embedded in an elastomer resin. These types of gaskets are suited for low to medium pressure applications.
• Sem-metallic gaskets are composites of metallic and non-metallic materials. The metallic component provides structural strength and increased toughness, while the non-metallic part offers enhanced sealing.
• Metallic gaskets are used for extremely high pressure and temperature applications.
• In selecting a gasket, it is important to evaluate the process fluid‘s pressure, temperature, and chemistry. For non-metallic gaskets, take note of the thickness. Thinner gaskets with the same ratings have better sealing performance.
• Improper compression, overheating, gasket reuse, and chemical attack are the most common reasons why gaskets fail.

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