SELECTING THE CARBON/ GRAPHITE FACE SA003
The most common face combination you will be selecting is a good grade of carbon-graphite running against a corrosion resistant hard face. The seal face we refer to as a carbon is really a compound of carbon and graphite. We use graphite for its lubricating qualities and good heat conductivity, and carbon for its strength.
With few exceptions mechanical seal companies purchase carbon-graphite molded faces from one of several carbon manufacturers. The seal companies pay for the necessary molds and then retain the exclusive use of them. A really good seal face would be a mixture of carbon, graphite and nothing else.
The carbon is purchased as a by-product of a manufacturing process while the graphite is mined with the main sources being in Canada and Madagascar. Two things determine the cost of these elements:
- How finely is the product milled? A fine talc is desirable.
- How pure is the product? There will always be some impurities, but the fewer the better because these impurities could possibly present a chemical compatibility problem.
A good carbon-graphite mixture would be about 80% carbon and 20% graphite. Graphite is a good conductor of heat, a natural lubricant and has a laminar grain structure similar to a deck of playing cards, allowing the individual grains to slide over one another. It is this laminar structure that allows the graphite to release from the carbon/ graphite face and deposit on the hard face in the same manner a graphite pencil will write on a sheet of paper.
Carbon is a very different element. It is manufactured by heating an organic material (it once was alive) to 2000 degrees Fahrenheit (1000°C). It is not a very good conductor of heat and is a poor lubricant because of its crystal structure. If carbon is heated to 4000 degrees Fahrenheit (2000°C) under pressure, it will convert to graphite.
To manufacture the finished product we place this carbon-graphite mixture in an oversized mold using a hydrocarbon as the glue to hold the powder together. The mixture is then compressed and placed in an oven at 2000° Fahrenheit (1000° C) for a period of thirty to sixty days. The hydrocarbon will convert to carbon at this temperature. The piece must be heated slowly or otherwise the carbon will combine with oxygen to form carbon monoxide or carbon dioxide, which will, in either case ruin it. At the end of this time the piece has shrunk a small amount but still resembles a real carbon face. The problem is:
- It has poor tensile strength
- It has low heat conductivity because the mixture is very porous.
- It has low density that would be a problem in vacuum applications, along with pharmaceutical and food products because of the difficulties in cleaning the lapped seal faces..
At this point any inorganic (it never lived) material can be imbedded into the carbon/graphite shape. If you should use such an impregnation you would have to be concerned about the chemical compatibility of the filler material with the product you are trying to seal.
If you want a serious carbon you must place the component into an autoclave where a vacuum will remove impurities that may have imbedded into the porous face. The autoclave will then be filled with a hydrocarbon and pressurized to force the hydrocarbon into the porous face under high pressure. In the old days the hydrocarbon was “pitch” from a tree but in modern times a variety of hydrocarbons are available.
This first impregnation will penetrate approximately 25 mm. (one inch) meaning that 50 mm (2 inches) will be impregnated if the hydrocarbon can penetrate from all sides of the shape. The face is then placed back into the oven and fired at 2000° Fahrenheit (1000 C.) for an additional 30 to 60 days where the impregnate is converted to carbon. There is also a certain amount of shrinking that takes place during this converting process.
You now have a denser carbon/graphite, but you are a long way from a good one. Two more impregnations at 3,0 mm. (0.125 inches) and 0,5 mm (0.020 inches) will complete the impregnations, each taking 30 to 60 days in the oven.
About this time you hit a point of diminishing returns, so the third impregnation is pushed into the carbon/graphite, but not fired in the furnace. This type of seal face is referred to as an “unfilled carbon and is available from several manufacturers both in the United States and abroad.
- C = 25,0 mm (1 inch) impregnation
- B = 3,0 mm (0.125 inches) impregnation
- A = 0,5 mm (0.020 inches) impregnation
As shown in the diagram, the last impregnate will wear away from the seal face, but will remain on the outside and inside diameters providing the density the face needs to hold vacuum and provide the surface needed to prevent bacteria and other un-desirable elements from penetrating into the composite.
If a seal manufacturer needs a only a few seal faces for test purposes he can machine them out of a good grade of unfilled carbon and then send them to the carbon manufacturer for the final impregnations. Small batch applications are handled like this also.
Carbon-graphite is the face that should be the standard in all of your mechanical seals. It can be used in any chemical or combination of chemicals except an oxidizing agent, a halogen and some special applications.
As mentioned, the oxidizing agents will combine with the carbon to form carbon dioxide and carbon monoxide. Here is a list of some of the common oxidizers:
- Aqua Regia (a combination of nitric and hydrochloric acid) used for dissolving metals.
- Chloric acid ignites organic material on contact.
- Chlorous acid, over 200 degrees Fahrenheit (100 C).
- Ferric chloride used in sewage treatment photography, medicine and feed additives.
- Hot sulfuric acid, the most widely used industrial chemical.
- Hydrofluoric acid used for etching, cleaning castings and fermentation.
- Methyl Ethyl Ketone (MEK) a common solvent.
- Nitric acid used in fertilizer, dyeing, explosives, drugs, etching and medicine.
- Oleum used in the manufacture of detergents and explosives.
- Perchloric Acid – 2N
- Perchloric acid used in the manufacture of medicine, explosives, and esters.
- Sodium hypochlorite, used in bleaching paper pulp, textiles, and tanning textiles.
- Sulfur trioxide used to manufacture sulfuric acid.
Additionally look for any chemical whose name contains the word:
The Halogens are another group of chemicals that will attack carbon. They are easy to identify because their chemical name ends in the letters “ine”:
The oxidizer’s chemical concentration and temperature will affect the degree of attack. If you are handling any of these chemicals or any chemical you suspect might attack carbon, it would pay to test an unfilled carbon for compatibility prior to installing a mechanical seal.
Recent experience shows that all grades of carbon are no longer being recommended in the following applications:
- If there is a possibility of color contamination of the product. Some paper, pharmaceutical and paint applications have this problem.
- If you are sealing hot oil and have to meet fugitive emission standards.
- Some de-ionized water applications can attack carbon.
Original equipment manufacturers (OEM) use filled carbon in their seals, and as a result you end up with a spare parts problem. It is not unusual to find five similar seals, with five different part numbers and the only difference between them is the grades of carbon/ graphite.
Cryogenic service uses a special carbon that has some inorganic compounds added to compensate for the fact that adsorbable gases or vapors are not present to weaken the interlacing bonding forces between the carbon and the graphite. It is these adsorbable gases and/ or vapors that allow the graphite to release from the compound and coat the hard surface with a low friction-lubricating layer.
Children recognize this when they lick the end of a graphite pencil so the writing will be darker.
Most sealing applications can be satisfied with an unfilled carbon running against one of several hard faces. You should contact the carbon manufacturers for their catalog showing you the grades they have available and the physicals (specifications) of their unfilled carbon. You can then check with your seal supplier to be sure he is using the proper unfilled grade in your mechanical seals.
A carbon company can provide several unfilled grades depending upon the number of impregnations (density) and special characteristics, such as the ability to fracture without producing many dust particles. This is an important characteristic in some split seal designs.
I have included a typical specification chart for you. It is a reproduction of a page from the advertising literature of the Pure Carbon Company of St. Marys, Pennsylvania, USA. Their grade P658RC would be a typical unfilled carbon.
You can locate these carbon companies on the “Web” or find them in various technical directories such as the Thomas Register in the United States.