Classification and structural forms of pipe flange gaskets

What is a pipe flange gasket?

Flange gaskets are used in pipe flange connections and are seals between two flanges. They include non-metallic gaskets, semi-metallic gaskets, etc. Different media, pressures and temperatures directly affect the selection of flange gaskets.

Classification of pipe flange gaskets:

Flange gaskets are mainly divided into two types according to materials: non-metallic gaskets and semi-metallic gaskets.

Non-metallic gasket is a gasket made of non-metallic materials such as asbestos, rubber, and synthetic resin. Non-metallic jacket gasket is a non-metallic gasket wrapped with a layer of synthetic resin, etc.

Semimetallic gasket is a gasket made of metal and non-metallic materials, such as wound gaskets and metal-coated gaskets.

a. Spiral wound gasket is a gasket made of a V-shaped or W-shaped cross-section metal belt clipped with a non-metallic belt, and is spirally wound. Because of its good elasticity, it is widely used in flange sealing structures in petrochemical, chemical, electric power and other industries. Depending on the specific location, a steel ring can be added to the inner or outer layer of the gasket for positioning or reinforcement.

(1) The inner ring is a metal ring set on the inner ring of the wound gasket.
(2) The outer ring is a metal ring set on the outer ring of the wound gasket.

b. Metal cladding
Metallic gasket made of metal such as steel, aluminum, copper, nickel or Monel alloy.

Structural type of pipe flange gasket:

Flat gaskets used in industry generally consist of sealing elements and inner and outer reinforcing rings. The sealing element or gasket body is the key part to prevent leakage. Commonly used materials include non-metallic materials, such as flexible graphite, polytetrafluoroethylene, fiber-reinforced rubber-based composite panels, etc. In addition, the sealing element material can also be rigid or flexible metal, which is usually used in situations with high pressure and temperature.

For sealing elements made of non-metallic materials, metal materials are usually inserted to strengthen them, which also facilitates the manufacturing and processing of sealing elements made of easily breakable materials such as graphite. The reinforcing material can be a metal sheet or a wire mesh. The metal sheet is often punched with holes to improve the reinforcement effect and increase elasticity, and they are bonded together through adhesive and roller pressing. The sealing element can also be provided with a surface layer or anti-adhesion treatment layer to increase the sealing effect and prevent the flange sealing surface from adhering.

The external reinforcing ring or outer ring material is solid metal, and its functions are:

a. Help the sealing components to be centered when installed;
b. Prevent sealing elements from being damaged due to excessive compression; prevent gaskets from blowing out and reduce flange rotation, etc.
c. The external reinforcing ring is not in contact with the sealing medium, so it is not required to be resistant to medium corrosion, so it is often made of carbon steel. The external reinforcing ring can also be integrated with the sealing element, such as metal toothed gaskets and corrugated toothed composite gaskets.

d. If the inner reinforcing ring or inner ring is in contact with the fluid, its material should be able to resist the corrosion of the sealing medium.

The function of the inner reinforcing ring is to prevent the gap between the sealing element and the container or pipe flange, so as to avoid the gap interfering with the flow of the fluid and the resulting impact of the fluid on the gasket.

Types of pipe flange gaskets:

1. Non-metallic flat gasket - PTFE coated gasket

PTFE coated gasket is a non-metallic composite soft gasket, generally composed of two parts: a covering skin and an insert. The envelope mainly plays an anti-corrosion role and is usually made of polytetrafluoroethylene. The insert (filler) is a non-metallic material with or without metal reinforcement, usually made of asbestos rubber sheet.

PTFE coated gaskets are mainly suitable for fully flat and raised surface steel pipe flange connections. They are suitable for corrosive media with a nominal pressure PN of 0.6~5.0MPa and a working temperature of 0~150°C or where cleanliness is affected. media with higher requirements.

Gasket form:

PTFE-coated gaskets for pipe flanges can be divided into three types according to their manufacturing methods: cut type (S type), machined type (M type), and folded type (F type).

For machined encapsulation gaskets, the inner diameter of the encapsulation layer can be consistent with the inner diameter of the flange to prevent the fluid from generating eddy currents at the flange. For the folding type packing gasket, it is made of a strip-shaped polytetrafluoroethylene film with a thickness of 0.4~0.8mm. The two ends are heat-sealed using a heat sealing method and then wrapped around the annular core material to form a U shape. It is easy to manufacture. , suitable for flange sealing with larger sizes.

2. Metal composite gasket

1) Metal spiral wound gasket

Metal spiral wound gasket is a semi-metallic flat gasket made of spiral composite winding of metal strips and non-metallic strips.

The characteristics of metal spiral wound gaskets are: a. Good compression and rebound performance; b. Multi-channel sealing and certain self-tightening function; c. Not very sensitive to surface defects on the flange pressing surface, non-adhesive method The blue sealing surface is easy to center, so it is convenient to disassemble; d. It can partially eliminate the effects of pressure, temperature changes and mechanical vibration; e. It can be used under various harsh conditions such as high temperature, low pressure, high vacuum, shock and vibration, etc. , maintaining its excellent sealing performance.

Gasket form:

Wound gaskets can be divided into four types according to their different structures: basic type, with inner ring type, with fixed ring type, with inner ring and fixed ring type.

2) Metal-clad gasket

Metal-clad gasket is a composite gasket composed of a non-metallic material as the core material, cut into the required shape, and covered with a metal sheet with a thickness of 0.25~0.5mm. According to the state of the package, it is generally divided into two types: flat type and corrugated type.

Gasket form:
According to the structural type, it is divided into two types: flat type (F type) and corrugated type (C type). The structure type of the metal-clad gasket used for the pressure vessel flange is a flat type.

3) Metal punched tooth plate flexible graphite composite gasket

Flexible graphite composite gasket is also called flexible graphite metal-reinforced composite gasket. It is a sealing gasket composed of a punched metal tooth plate or a punched metal core plate and flexible graphite particles. It is usually composed of flexible graphite composite Cut from reinforced board.

Flexible graphite metal reinforced composite gaskets can be made with stainless steel or carbon steel inner or outer edges as required.

Main features:
It has good high and low temperature resistance, corrosion resistance, radiation resistance and other properties. It has high strength and can be used under high pressure conditions.

The required preload force is smaller than using metal gaskets or metal spiral wound gaskets. Since flexible graphite has plasticity and good filling properties, the matching flange sealing surface does not need to be precision processed when using this gasket. It can also be adapted to the "waterline" groove formed on the flange sealing surface. Relatively speaking, Relatively economical.

Flexible graphite composite gasket types are divided into RF type, MFM type and TG type, which are suitable for raised surface, concave and convex surface and tongue and groove surface flanges respectively. As required, RF type gaskets can be equipped with stainless steel or carbon steel inner edges, which is the RF-E type.

4) Metal corrugated composite gasket

The metal corrugated composite gasket made of flexible graphite is a composite gasket in which a metal plate machined into a corrugated shape is covered with flexible graphite on both sides. This gasket has the strength of metal and the elasticity of corrugation. Features: maximum temperature 650℃, maximum pressure up to 26MPa.

Gasket type:
Flexible graphite metal corrugated composite gaskets are divided into three types according to their structures: basic type, type with positioning and replacement, and type with spacers.

3. Metal gasket

1) Metal ring gasket
Metal ring gaskets are solid metal gaskets processed from metal materials into an octagonal or oval cross-section, and have a radial self-tightening sealing effect. Therefore, the metal ring gasket is in contact with the inner and outer sides (mainly the outer side) of the flange ladder groove and is pressed to form a seal.

The octagonal annular gasket matches the flange groove and is mainly in surface contact. Compared with the oval ring gasket, although it is not easy to achieve close contact with the flange groove, it can be used again, and because the cross section is composed of a straight line, it is easy to process. The oval ring gasket is in line contact with the flange groove and has good sealing performance, but requires high processing accuracy, which increases the manufacturing cost. At the same time, the oval ring gasket cannot be reused.

The applicable nominal pressure of the metal ring gasket is 2.0~42.0MPa.

2) Metal toothed gasket
Metal toothed gasket is a solid metal gasket with a zigzag cross-section, tooth pitch t=1.5~2mm, tooth height h=0.65~0.85mm, and tooth tip width C=0.2~0.3mm. Several concentric circles are turned on the sealing surface, and the number of teeth is 7 to 16, depending on the size of the gasket.

The structural form and advantages of gaskets:

There are four structural types of metal toothed gaskets: basic type, outer ring type, inner ring type and inner and outer ring type. The more common structure is the basic type.

Since the V-shaped ribs in the contact area of the sealing surface of the metal toothed gasket form many spatial line contacts with pressure differences, the sealing is reliable and the service life is long. Compared with ordinary metal gaskets, this gasket requires less pressing force.

The disadvantage is that every time the gasket is replaced, the sealing surfaces of the two pipe flanges must be processed, which is time-consuming and labor-intensive. In addition, gaskets tend to leave indentations on the flange sealing surface after use, so they are generally used in parts that are rarely disassembled. The applicable nominal pressure for metal toothed gaskets is 1.6~25.0MPa.

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