Abbreviation:

Face Material:

Density:

Hardness:

Modulus:

Transverse:

CAB

Carbon

Resin Impreg.

1.8 g/cc

95 shore

3.5 x 106  psi

11,500 psi

A1.CAB

Carbon

Antimony Impreg.

2.4 g/cc

90 shore

3.6 x 106  psi

12,500 psi

CER

Ceramic

Alumina Oxide

3.89 g/cc

~1800 vickers

55 x 106  psi

47,000 psi

SiC

Silicon Carbide

Reaction Bonded

3.08 g/cc

2400 vickers

50 x 106  psi

40,000 psi

S.SiC

Silicon Carbide

Self Sintered

3.1 g/cc

3,000 vickers

88 x 106  psi

65,500 psi

TC

Tungsten Carbide

Nickel Bound

1.8 g/cc

95 shore

3.5 x 106  psi

11,500 psi

General Duty:

Dry Running:

Abrasive Service:

Blister Resist:

Chemical Resist:

Impact Resist:

Temp. Shock Resist:

Hydrocarbons:

Oils:

Acids:

Bases:

Overall Chem. Res.:

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CARBON

One of the most abundant elements on earth, carbon is an inert, stable material that can be self-lubricating.

MANUFACTURING PROCESS

Mechanical carbons used in seal faces are a mixture of amorphous carbon and graphite. The percentages of each help determine the physical properties on the final grade of carbon. They are manufactured by blending together types of amorphous carbon (i.e., lampblack, charcoal, or coke) and carbon graphite with a carbonaceous binder (i.e., pitch or resins). The mixture is then pressured into shapes and heated in an inert environment. At a high temperature, most of the binder decomposes into carbon while a small amount volatises and leaves the part. This leaves the carbon porous and soft.

The carbon is then placed into a vacuum chamber to remove any air from the porous carbon. While under a vacuum, the carbon is immersed into liquid impregnant. The vessel is then pressurised to drive the impregnant into the carbon. This effectively fills the pores of the carbon making it impermeable and greatly increasing the strength of the final material. The most common impregnant for mechanical seal faces are thermoset resins and antimony metal.

RESIN IMPREGNATED CARBON

As the name implies, resin impregnated carbon is a mixture of amorphous carbon / graphite that has been impregnated with a thermoset resin. This is by far the most common type of carbon for mechanical seals used in industry today. Most resin impregnated carbons are capable of operating in a wide range of chemicals from strong bases to strong acids. They possess good frictional properties and an adequate modulus to help control pressure distortions.

ANTIMONY IMPREGNATED CARBON

Metallised carbons are available in a variety of metal impregnants and of these, antimony has proven to be the most successful in seal applications for two reasons.

Firstly, the addition of a metal impregnant increases the strength and modulus of the material. This is beneficial for high pressure applications when a stronger and stiffer material is needed. Secondly, antimony impregnated carbons are more resistant to blistering in high viscosity fluids or light hydrocarbons. This has made it the standard grade for many refinery applications.

CERAMIC

Ceramics can be defined as nonmetallic, nonorganic materials that usually require high temperature processing. The most widely used oxide ceramic is alumina oxide or alumina.

OVERVIEW

Alumina is a white or ivory colour ceramic that can be produced by direct sintering. This results in a dense homogenous material. It shares many of the favourable attributed of other ceramics such as high hardness, high strength and high stiffness. It also has excellent dielectric properties. It is commonly used for electrical insulator, wear resistant components, grinding media, and high temperature components.

In high purities, alumina has excellent chemical resistance to most process fluids other than some strong acids. This led to its widespread use in many mechanical seal applications. Alumina does have one weakness that has restricted its use in seals; it can fracture easily if thermally shocked. Thermal shock may result from rapid heating in the process fluid or from dry-running operation. Even with this concern, it continues to be used where thermal shock is not considered an issue.

SILICON

CARBIDE

In the simplest sense, silicon carbide (SiC) consists of one atom of silicon bonded to one atom of carbon. This results in a tenacious bond that is extremely stable over a wide range of temperatures and chemical environments. It also has other desirable properties such as a high hardness and a high modulus.

MANUFACTURING PROCESS

Silicon carbide almost never exists in nature, and is unfortunately a material that is difficult to manufacture in shapes suitable for component design. For many years, a reaction bonding process has been used to manufacture components. More recently, sintering processes have been used.

The vast majority of SiC material is solid material free from any significant pores, voids or imperfections. This results in seal faces that are lapped very flat and smooth. Under certain conditions, faces that are too flat or too smooth can prevent fluid from migrating across the seal faces, which can prevent proper lubrication of the seal faces.

REACTION BONDED SILICON CARBIDE

Reaction bonded silicon carbide (RBSiC), as its name implies, is a material formed by bonding SiC particles to each other in a reaction process. SiC particles and carbon are mixed together with a binding agent and pressed into the desired shape. This green state material is placed into a furnace with an inert atmosphere and exposed to molten silicon metal. The silicon metal wicks its way throughout the material reacting with the free carbon to form additional SiC. The reaction literally bonds the original SiC particles together by forming additional SiC material that acts as a "glue."

While the silicon metal does not significantly affect most of the physical and thermal properties of the material, it does limit the chemical resistance of the material. Anything that can chemically attack elemental silicon can attack the interconnected passages of silicon in RBSiC. This can weaken the material and case seal failures. The most common chemical that will attack RBSiC are caustics (and other high pH chemicals) and strong acids. RBSiC should not be used with these applications.

SELF-SINTERED SILICON CARBIDE

Researchers discovered that it was possible to sinter SiC particles directly together using non-oxide sintering aids (such as C, B, or Al) in an inert environment at temperatures over 2000°C. The resulting material consists entirely of SiC with a very small volume of unconnected voids. Due to the lack of a secondary material (such as silicon), the direct sintered material is chemically resistant to almost any fluid and process condition likely to be seen in a centrifugal pump.

TUNGSTEN

CARBIDE

Tungsten Carbide (WC) is a carbide ceramic that is used in many products requiring high hardness and toughness.

MANUFACTURING PROCESS

WC shares many of the same difficulties in manufacturing shapes with other ceramics. WC is ready available in powder form but must be processed into a final shapes. Tungsten carbides are most often manufactured as cemented carbides. As a cemented carbide there is no attempt to bond WC to itself. Rather a secondary metal is added to bind or cement the WC particles together. This results in a material that has the combined properties of both the WC and the metal binder. This has been used to an advantage by providing greater toughness and impact strength than possible with WC along. One of the primary weaknesses of cemented WC is its high density.

COBALT-BOUND TUNGSTEN CARBIDE

Cobalt-bound tungsten carbide (Co-WC) proved to be successful earlier on, however as the mechanical seal developed over the years, cobalt did not exhibit the range of chemical compatibilities required by industry and was gradually replaced by nickel-bound tungsten carbide (Ni-WC). applications.

NICKEL-BOUND TUNGSTEN CARBIDE

Current NI-WC grades range from 6 to 10 percent free nickel. Ni-WC is still widely used as a seal face material especially where its high strength and high toughness properties are beneficial. It has good chemical compatibility generally limited by the free nickel.