A gasket is a sealing material placed between connecting flanges to create a static seal within a pipe. This seal will ensure the pipe remains leak-proof in all operating conditions. Different types of gaskets are used to achieve varying leak-proof seals between pipe flanges. Regardless of type, the primary function of gaskets is to seal the irregularities of each face of a flange such that there will be no leakage of the service fluid from the flange joint. In piping, there are three types of gaskets used: non-metallic, metallic, and composite. In this blog, we will discuss all three in detail.
The first type of gasket is the non-metallic gasket. The most common type of materials used for this gasket are graphite, rubber, teflon, PTFE, and Compressed Non-Asbestos Fiber (CNAF). Non-metallic gaskets are also known as soft gaskets and can be of the full face, inside bolt circle, or segmented types. A full face gasket is one that covers the entire flange. They are easily recognizable due to the fact that they feature holes cut into the gasket material for the bolts to pass through. Inside bolt circles, or IBC, sit within the circle of the bolts used to secure the joint. They are a continuous ring of material. Segmented gaskets are used when the flange is an uncommon shape, extremely large, or particularly expensive. These are formed by two or more gasket parts being arranged with an overlap, tab, or shaped joint arrangement so that, when compressed, it provides a continuous seal within the flange joint. Full face gasket types are only suitable for flat-face (FF) flanges. Flat ring gasket types are suitable for use with raised (RF) flanges.
Non-metallic gaskets can easily compress with low tension bolting; they are frequently used with low-pressure class flanges such as 150 and 300 class. They are also mainly used in low-temperature applications, although graphite gaskets can be used in conditions of up to 500 degrees centigrade. Rubber and elastomeric gaskets are not used in hydrocarbon services, but are popular in utility lines. Non-metallic gaskets are the cheapest and most widely-available type of gasket.
The next type of gasket, known as metallic gaskets, is made from materials including soft iron, low carbon steel, stainless steel, monel, and inconel. These gaskets are also known as ring joint gaskets or RTJ gaskets. Metallic gaskets are used in high-pressure class flanges, normally above 900 class, and can also be used in extremely-high temperatures. When using metallic gaskets, high tension bolting is required. Metallic gaskets are very strong but also the most expensive. Metallic gaskets fit in a groove machined on the flange face of both mating flanges. The two types of metallic gaskets used with metallic flanges are octagonal and oval; though they look similar, these gaskets have different cross-sections.
The final type of gasket is the composite gasket. Also known as semi-metallic gaskets, these are made from a combination of metal and non-metal materials. There are many different possible combinations depending on the service requirement. Spiral wound, metal jacketed, and kammprofile gaskets are among the most common types of composite gaskets. Composite gaskets are used in a broad range of pressures and temperatures. They are cost-effective compared to metal gaskets, but require very careful handling due to their less robust construction. Raised face, male-female, and tongue-and-groove flanges are the most commonly used flanges with composite gaskets.
When choosing the right type of gasket for your intended end use, there are three factors to consider: compatibility of the gasket material with the fluid, ability to withstand the pressure & temperature of the system, and the gasket’s service life. Because gaskets have the important role of maintaining a seal against all operation forces, it is important to understand the requirements of your particular application before making a selection.
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