AS-C443 Customized single Cartridge Mechanical Seal

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How to choose carbon graphite as friction pair material

There are dozens of types of carbon graphite available, with different properties. It is necessary to understand the grades, properties and sources of supply of various carbon graphites, and select them reasonably. The following deals with how to choose such materials based on introduced mechanical seal failure form——high wear.

At present, there is a general tendency in selection of materials, that is, harder carbon graphite is used regardless of working conditions of mechanical seal, because it is believed that the higher the hardness, the more wear-resistant the carbon graphite.

However, this is not the case under some conditions. For example, for media with poor lubricity, no lubricity, or other conditiojns that are prone to dry friction, such as light hydrocarbon media, hard M106k as material of friction pair wears more, while soft high-purity electrochemical graphite has less wear.

Soft graphite has excellent low-friction properties. It is due to the good self-lubricating properties of graphite crystals：During operation of the said mechanical seal, a very thin graphite film is transferred to surface of mating part, so that friction surface is lubricated, friction coefficient is reduced, and wear is reduced.

On another occasion，when the paired material is sprayed ceramics, it is advisable to choose medium-hard graphite.

If content of solid particles in medium exceeds 5%, carbon graphite should not be used as a paired material for single-end mechanical seals, nor should it be used as a main seal ring in a tandem mechanical seal.Otherwise,the said seals will experience high wear.

In hot oil pumps, pits and scars are often found on the end face of the carbon graphite ring of the mechanical seal. This is because when the carbon graphite material is selected, the temperature at which the impregnant and the carbon graphite material are combined has not been fully studied. Due to the difference of the impregnation, the applicable temperature range of the carbon graphite seal is also different. For example, with graphite impregnated with phenolic resin, when the surface temperature of the friction pair exceeds 180°C, the resin will decompose to form hard particles and precipitate volatiles on the sealing end surface, forming scars, thereby greatly increasing friction and causing surface damage and leakage. When conditions permit, reducing the contact width of the friction pair is effective in reducing the temperature rise caused by friction heat. Therefore, appropriately reducing the friction width of the carbon graphite ring is beneficial to prevent the occurrence of blisters.

The poor quality carbon graphite ring has many internal pores, which expand during use, blowing carbon particles and resin volatiles out to form blisters, and will cause adhesion of the sealing end surface of the mechanical seal. Therefore, copper immersion or antimony graphite is recommended for high temperature conditions.

In high temperature situations, it is advantageous to choose impregnated metal carbon graphite.In high temperature situations, it is advantageous to choose impregnated metal carbon graphite. Because both the contact end surface temperature and thermal conductivity are much higher than condition using impregnated resin.However, during the operation of the mechanical seal, if dry friction occurs, the seal is more likely to fail, and the end face temperature instantaneously rises high , thus molten droplets will appear on the metal impregnation , causing the end face to weld and the seal breakage.Carbon graphite rings impregnated with babbitt alloy, copper alloy or antimony are not suitable for high temperature dry friction conditions, under which impregnated sliver should be used.This is because silver has good self-lubricating properties and can effectively prevent occlusal fracture of mechanical seal from occurring.

This article deals with mechanical seal failure forms——fracture and deformation and how to choose carbon graphite as friction pair material to avoid such occasions

Fracture

Graphite cracks are caused by the vibration of the transmission parts, the expansion of the sealing ring of the mechanical seal, and the stress concentration at the sharp corners of the graphite ring itself.When the carbon graphite ring is used in a high viscosity fluid, especially as a floating non-compensation ring of mechanical seal, due to viscous resistance of the fluid, the carbon graphite seal ring will be bumped back and forth and broken.Therefore, carbon graphite sealing ring should use fluid whose viscosity is below 30Pa.s；for high viscosity fluid, copper alloy ring can be used instead of carbon graphite ring in the mechanical seal.

Deformation

Floating carbon graphite non-compensation ring is prone to deformation under high pressure, so contact installation is recommended for mechanical seals under high pressure conditions

For non-compensation rings with large shaft diameters, more attention should be paid to the design of their shape and dimensions, and the radial and axial dimensions should be increased to prevent deformation of the mechanical seal.

For the carbon graphite ring with clamping structure, when the shaft diameter is more than 100mm, it is clamped with a PTFE O-ring, which is easy to cause deformation of the carbon graphite ring. Therefore, it is recommended to use a rubber O-ring.