In industries such as chemical processing, hydrocarbon refining, and power generation, leakage from extreme temperature process streams can result in loss of efficiency and production as well as adverse environmental impacts and compromised employee safety. One of the most commonly used sealing products in systems subject to high pressures and temperatures is a spiral-wound gasket. These gaskets typically consist of filler and winding materials selected on the basis of application requirements and end-user preference. Proper selection of these materials is critical to achieving the desired performance in all applications.
Sealing at temperatures above 850oF (454oC) is particularly challenging because of the limited number of filler materials that can resist thermal degradation at extreme temperatures – these temperatures affect both the sealing material and metal components. For instance, the yield strength of fasteners decreases as the temperature is increased. In addition, certain chemicals can become more volatile and aggressive in high-temperature reaction processes.
The two most common filler materials in spiral-wound gaskets are graphite (can withstand temperatures up to 850ºF) and polytetrafluoroethylene (PTFE; tolerance up to 500ºF). Other filler materials are used mainly for their thermal insulating properties, not for sealability, include mica, exfoliated mica and ceramics. While graphite and PTFE perform satisfactorily in terms of temperature and chemical resistance, they have limitations. Graphite is not compatible with heavily oxidizing media at any temperature; nor can it withstand continuous operating temperatures above 850oF. Beyond 850ºF, volume loss through oxidation becomes excessive and sealing effectiveness is compromised.
Many high-temperature systems, such as exhaust manifolds and flanged piping connections in exhaust systems, are oxidizing. Other services are oxidizing because of the operating temperature and media involved.
For example, molten salts have become popular as heat-transfer media for high-temperature processes and solar power generation. The temperature at which molten salts provide optimal heat-transfer performance is 1,049ºF (565ºC), creating a perfect storm in terms of oxidizing capability.
The only other filler that performs well in oxidizing media is PTFE, which offers exceptional chemical resistance in both oxidizing and nonoxidizing environments. However, it is limited to a maximum operating temperature of 500oF (260oC).
Binders and bonding agents
The addition of binders makes filler materials flexible enough to be manipulated in the manufacturing process and robust enough to withstand rough handling in the field. Graphite can be exfoliated and recompressed and PTFE can be sintered together, but nongraphite materials such as mica, vermiculite, ceramics and talc require a bonding agent to hold the particles and fibers together. Once the gasket is assembled and properly installed, the bonding agent has served its purpose. At this point, it will carbonize or burn off completely, depending on the service temperature. The loss of the bonding agent correlates to loss of mass, which can adversely impact sealability.
One method for measuring this impact is to check the sealability of a material under ambient conditions, expose it to elevated temperatures (i.e., 850ºF -1,000ºF) for an extended period of time, and repeat the initial sealability test. Loss of organic bonding agents would likely translate into a loss of sealing performance.
As noted, the metal components of spiral-wound gaskets likewise must be able to withstand elevated temperatures and be compatible with the service. Common winding metals include but are not limited to 304, 304L, and 316L. Depending on their size, spiral-wound gaskets can be manufactured horizontally or vertically. Spot welds secure the metal windings for structural integrity and prevent them from unraveling. In addition, inner and outer rings can be installed with the windings to guard against damage during handling, installation and use.
In manufacturing spiral-wound gaskets, continuous lengths of filler material have to be produced in a way that allows the material to be wound in plies between alternating layers of preformed metallic wire.
Converting the raw filler material into usable solid sheets typically involves a sheet or roll forming process, which precludes many high-temperature materials from being used in the spiral winding process.
The ability to exfoliate or expand and densify graphite enables production of rolled sheets of high-purity (>99%); these can be slit to varying widths depending on the required thickness of the filler material. However, the quality of the raw graphite can significantly affect how well it processes and performs in service. Lower-grade graphite can contain higher levels of impurities that will oxidize at lower temperatures, resulting in mass loss and compromised sealability.
The most widely specified standard for metallic gaskets such as spiral wounds is ASME B16.20-2012, Metallic Gaskets for Pipe Flanges. This is an industry-accepted standard that specifies dimensions, tolerances, marking requirements and method of construction for spiral-wound gaskets, as well as grooved metal gaskets (kamm profiles), ring-type joint gaskets and double-jacketed (DJ) gaskets. Due diligence must be performed to confirm that gaskets with different fillers comply with such manufacturing standards.
As noted, finished gaskets need to maintain their integrity during shipping, handling, storage, and installation. This “logistical integrity” is critical. A commonly overlooked factor that affects gasket performance is moisture absorption. Some filler materials are more hydrophilic or susceptible to absorbing moisture, which can pose a major problem if they are exposed to chemicals that react with water or elevated temperatures that could cause the moisture to flash off.