Silicon carbide material mechanical seal

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The mechanical seal is an axial end face sealing device that relies on the elastic element to pre-tighten the sealing pair, and presses the sealing pair under the combined action of the medium and the elastic element to achieve sealing. Its structure is shown in the figure. There are two kinds of sealing ring, moving ring and static ring, which are the most important components of mechanical seal. The working condition of the sealing ring determines the performance and life of the mechanical seal, and the performance of the sealing ring material directly affects the working condition of the sealing ring. Therefore, understanding the performance requirements of the mechanical seal on the sealing material and selecting the appropriate sealing material are the keys to ensuring the safe operation of the mechanical seal.

1. fixing screw; 2. spring seat; 3. spring; 4. moving ring auxiliary seal; 5. moving ring; 6. static ring; 7. static ring auxiliary seal; 8. anti-rotation pin

 

Performance requirements of mechanical seals for sealing auxiliary materials

In order to ensure the normal operation of the sealing ring in the mechanical seal device, considering the aspects of reducing wear, corrosion resistance and preventing occlusion, the sealing ring is often configured as a pair of hard rings and soft rings with different hardness. The properties of the material have the following requirements:

1) Sufficient strength and rigidity will not damage the ring under working conditions (such as pressure, temperature and sliding speed, etc.), the deformation should be as small as possible, and the sealing can still be maintained when the working conditions fluctuate.

2) When running with good tribological performance, the end faces of the sealing ring are always in contact and rotating, so the material of the sealing ring is required to have good wear resistance, and maintain good lubrication performance of the fluid film and good boundary lubrication performance to ensure Satisfactory service life under working conditions.

3) When working with good thermal conductivity, heat resistance and thermal shock resistance, due to the poor lubrication conditions between the sealing surfaces and the temperature of the sealing fluid itself, the sealing ring is at a higher working temperature. Rapid changes in temperature, for this reason, require the material to have a high thermal conductivity and a small coefficient of linear expansion, so that it will not crack when subjected to thermal shock.

4) Good corrosion resistance The sealing ring with good corrosion resistance can resist the corrosion and corrosion wear of the fluid, so that it has a long working life.

In addition to the above properties, the hard ring should also have good forming and processing properties, small density and low permeability, and good self-lubricating properties. It is difficult for any material to fully meet all the above requirements. Usually, the main performance requirements of the sealing material are put forward according to the use environment, and the appropriate material is selected accordingly.

Since silicon carbide material has the good performance required by mechanical seals, it can be used in operating conditions such as high pressure, high speed, high temperature, corrosive media and media containing solid particles. Compared with tungsten carbide, silicon carbide has low density, good corrosion resistance and thermal performance, and low friction coefficient, which has obvious advantages in mechanical sealing materials. One of the most noteworthy excellent properties of silicon carbide material is its extremely high hardness, so it is widely used as a hard ring for mechanical seals.

At present, nearly half of the mechanical seal face materials used in industrial production use silicon carbide. There are many ways to manufacture silicon carbide, and its properties are also different. According to the different manufacturing methods, there are the following: normal pressure sintered silicon carbide, reaction sintered silicon carbide, hot-pressed sintered silicon carbide, chemical vapor reaction silicon carbide and chemical vapor deposition silicon carbide. As mechanical sealing materials, the first three are the most common.

1) Atmospheric pressure sintered silicon carbide

Using the powder metallurgy manufacturing method, the raw material is sintered with sub-micron silicon carbide particles without applying pressure, and the sintered density can reach 95% of the theoretical density. Sintering is carried out under the protection of inert gas or in a vacuum between 1900 and 2200 °C, and boron and carbon are generally added as sintering aids. After molding and sintering, the silicon carbide particles form a monolithic silicon carbide element. Atmospheric pressure sintered silicon carbide is the hardest and best corrosion-resistant sealing material of all silicon carbides, so it is the most widely used in mechanical seals.

2) Reaction Sintered Silicon Carbide

The porous carbon profile is reacted with molten silicon and sintered into monolithic silicon carbide. This silicon carbide contains about 10% to 12% of free silicon, which plays a good role in reducing friction and wear. The corrosion resistance of this silicon carbide material is worse than that of normal pressure sintered silicon carbide, but because it can form an excellent sealing pair with carbon graphite, API 682 specifies that this kind of seal is preferred.

3) Hot-pressed sintered silicon carbide is made of silicon carbide powder with a particle size of ≤1’m plus appropriate additives, put it into a graphite mold, and pressurized it in a hot-pressing furnace at 2000-2100 ℃ for 30-50 MPa. Hot-pressed sintered silicon carbide is one of the most chemically stable silicon carbides, and is used as a sealing surface material for corrosion-resistant sealing pairs.

Equipment selection: The RVS vacuum sintering furnace provided by SIMUWU is an excellent product for processing this kind of process. It has the characteristics of good temperature uniformity and high temperature control accuracy. SIMUWU provides a professional team of engineers who can solve various problems encountered in the production process and are committed to giving customers the most convenient and efficient experience.