Diverse types of pipe flanges and gasket selection

Pipe flanges play a vital role in various industrial applications. Due to their diverse types, the selection of gaskets has become particularly critical. This article aims to provide you with a guide on the types of pipe flanges and gasket selection to help you better understand and apply this important field.

Flange connection and gasket selection:

Flange connection, composed of flanges, gaskets, bolts and nuts, is a detachable connection method that is widely used in the connection between pipes and valves, pipes and pipes, and pipes and equipment. This connection method not only provides flexibility in installation and disassembly, but also ensures reliable sealing and high strength. Its structure is simple and low-cost, and it can be disassembled and used repeatedly. For this reason, flange connection is widely used in many basic engineering fields such as chemical industry, construction, water supply, drainage, petroleum, light and heavy industry, refrigeration, sanitation, plumbing, fire protection, electricity, aerospace, shipbuilding, etc.

Diverse types of flanges and gasket selection:

Pipe flanges, as key components in pipeline connections, have various types, and each type has its own unique application scenario. According to the connection method, pipe flanges can be divided into integral flanges, flat welding flanges, butt welding flanges, loose flanges and threaded flanges. Integral flanges are usually made in one piece with mechanical equipment such as pumps, valves, and machines. As part of the inlet and outlet of the equipment, they ensure the stability and sealing of the connection. Flat welding flanges, with their simple structure and affordable price, play an important role in low-pressure pipeline systems. The high-neck flange, through its unique design, effectively improves the problem of stress discontinuity and enhances the strength of the flange, which is particularly suitable for occasions with harsh working conditions.

In addition, loose flanges are also a unique connection method. It welds the auxiliary components connected to the pipe, and then uses bolts and gaskets for compression and sealing. This design makes the flange itself non-contact with the medium, flexible in rotation, and easy to center the bolt hole, which is very suitable for the installation of large-diameter pipelines and frequent cleaning and inspection needs. However, it should be noted that the pressure resistance of loose flanges is relatively low, so it is mainly suitable for the connection of low-pressure pipelines.

Characteristics and applications of threaded flanges:

The inner hole of the threaded flange is specially processed to form a pipe thread, which cooperates with the pipe with external thread to achieve a non-welding connection method. This type of flange is non-welded, so it is easy to install and maintain, especially suitable for occasions where welding cannot be performed on site. However, in environments where the temperature exceeds 260℃ or is below -45℃, it is recommended to avoid using threaded flanges to ensure the sealing of the connection. Its nominal diameter range is DN8 to DN100, and the working pressure is controlled between 0 and 5MPa.

Classification by sealing surface form:

The sealing surface form of threaded flanges is crucial to its performance and application. Common sealing surface forms include flat surface, concave and convex surface, and tongue and groove surface. Different forms of sealing surfaces have their own advantages and applicable scenarios, and the selection should be based on actual needs.

The selection of flange sealing surface needs to comprehensively consider process conditions, sealing caliber and gasket characteristics. The sealing surface of container flanges is diverse, including flat surface, concave and convex surface and tongue and groove surface, among which concave and convex surface and tongue and groove surface are favored due to their unique performance. The sealing surface of pipe flanges has six options: full flat surface, raised surface, concave and convex surface, tongue and groove surface, O-ring surface and ring connection surface.

The flat sealing surface has a simple structure, but it works best in situations where the pressure is not high and the medium is non-toxic, because its gasket is not fixed and it is difficult to press. The raised surface sealing surface is similar to it. Its smooth flat structure is convenient for anti-corrosion lining, but the gasket has a large contact area and is easy to extrude to both sides when pre-tightened, and the pressing effect is poor.

The concave and convex surface sealing surface is cleverly designed to facilitate centering and prevent gasket extrusion, and is suitable for slightly higher pressure situations. However, due to its large gasket width, the required pressing force is relatively high. The tongue and groove surface sealing surface has a unique tongue and groove surface design. The gasket is placed in the groove to effectively prevent extrusion, and the force is even and the sealing is reliable. Although it is difficult to replace the gasket and the cost is high, it is still an important sealing choice for flammable, explosive, toxic media and high pressure situations. However, the tongue surface part needs to be particularly careful during disassembly or transportation to avoid damage.

The O-ring face type is a new flange connection method, which is characterized by the matching convex and groove surface design, in which the O-ring is cleverly embedded in the groove. This O-ring relies on its elastic deformation to generate contact pressure to achieve the sealing effect. When the contact pressure exceeds the internal pressure of the medium, the seal is maintained, otherwise leakage will occur. This process of using the medium itself to adjust the contact state of the O-ring to achieve sealing is called "self-sealing effect". Compared with traditional flat gaskets, O-rings have better sealing performance. In addition, they have small cross-sectional dimensions, light weight, low material consumption, and are easy to install and disassemble. It is particularly worth mentioning that O-rings have excellent sealing capabilities and a wide range of applications. The static sealing working pressure can be as high as 100MPa or more, and the applicable temperature range is -60~200℃, which can meet the application requirements of various media.

Ring joint face flange is a type of narrow face flange, and its sealing surface is designed as an annular trapezoidal groove located on the raised surface of the flange. Similar to the tongue and groove face flange, this flange needs to be separated axially during installation and removal. It is designed for use with metal solid gaskets, which are usually octagonal or elliptical in cross-sectional shape. Since the selection of metal ring gaskets can be based on the inherent characteristics of the metal, this type of sealing surface has superior performance, is not strict on installation requirements, and is very suitable for high temperature and high pressure working environments. However, it should be noted that the processing accuracy of its sealing surface is relatively high.

Flange gasket:

Gaskets, as a circular ring material that can produce plastic deformation and has a certain strength, are widely used in sealing connections between pipelines and sealing connections between machine equipment components. Most of these gaskets are cut from non-metallic plates or made to size by professional factories. Their materials include asbestos rubber sheets, asbestos sheets, PTFE sheets, polyethylene sheets, etc. In addition, there are metal-clad gaskets, which are made of thin metal plates (such as tinplate, stainless steel) wrapped with non-metallic materials such as asbestos. Another common gasket is a spiral wound gasket, which is made of thin steel strips and asbestos strips. The combination of stainless steel strips and graphite is the most common, but depending on the use environment, the spiral wound strip can also be asbestos.

Ordinary rubber gaskets are suitable for environments with temperatures below 120°C; while asbestos rubber gaskets are suitable for environments with water vapor temperatures below 450°C, oil temperatures below 350°C, and pressures below 5MPa. For corrosive media, acid-resistant asbestos sheets are the most commonly used choice. In high-pressure equipment and pipelines, lens-shaped or other shaped gaskets made of metal materials such as copper, aluminum, No. 10 steel, and stainless steel are usually used. The contact width between these high-pressure gaskets and the sealing surface is very narrow, usually in line contact, and the processing finish of the gasket and the sealing surface is required to be high.

Pipe flange selection:
When selecting a pipe flange, technical parameters such as medium, temperature and pressure need to be considered. Generally speaking, low-pressure systems (PN<2.5MPa) are suitable for flat welding or plate flanges with ordinary raised face (RF) seals; medium-pressure systems (2.5-64MPa) recommend the use of butt welding flanges with raised face (RF) or concave and convex face (FM/M) seals; high-pressure systems (above 10.0MPa) usually also use butt welding flanges, and tongue-and-groove (TG) and ring joint (RJ) sealing surfaces can be considered. In stainless steel low-pressure systems, in order to save costs and facilitate maintenance, loose flanges or loose ring flanges are sometimes selected. In addition, the integral flange integrates the flange with equipment, pipes, etc. This form is more common on equipment and valves, but is less commonly used in pipes and pipe fittings.

Read more: Classification and structural forms of pipe flange gaskets or How to choose pipe flange material?