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Mechanical seals serve an important purpose—to seal fluid within a vessel where a rotating shaft passes through a housing. The engineering that goes into mechanical seal design can be complex. Since mechanical seals must be able to endure a wide range of application conditions, they must be designed with meticulous attention to detail. One of the boundary conditions some mechanical seals face is a lack of lubrication in the contacting interface between the sealing faces. Some application conditions have limited or intermittent lubrication available while others may have no lubrication whatsoever.
IMAGE 1: Carbon/graphite seal faces are highly engineered components of a mechanical seal and the material selection for these parts is critical. (Images courtesy of Metallized Carbon Corporation)IMAGE 1: Carbon/graphite seal faces are highly engineered components of a mechanical seal and the material selection for these parts is critical. (Images courtesy of Metallized Carbon Corporation)
In these instances, seal face material selection is critical. Over many decades, carbon/graphite material grades have been developed to withstand some of the harshest dry running conditions, including high speeds and high temperatures. This article delves into some of the science behind the development of these carbon/graphite grades and the considerations that must be taken into account when selecting seal face materials for dry running applications.
In order to understand how carbon/graphite materials are being developed to withstand harsher conditions, it is important to get a fundamental knowledge of how carbon/graphite self-lubricates. Carbon/graphite’s self-lubrication is attributable to its molecular structure. Plain carbon graphite consists of graphite grains bound together by strong amorphous carbon. While the amorphous carbon provides much of the material’s strength, it is the graphite grains that allow the material to self-lubricate.
Graphite grains have a layered structure comprised of strong graphene layers stacked on top of each other, bonded by weak van der Waals forces between the layers. These layers slide over each other when rubbing occurs. This sliding action is the basis of carbon/graphite’s self-lubricating ability.
On a macroscopic scale, this action results in a burnished film being deposited on the mating surface against which the carbon/graphite runs. Thus, the carbon/graphite is able to effectively fill in the surface roughness of the counter face and run against a graphite film as opposed to the counter face itself. This is critical in dry running conditions, where there is no fluid lubrication to mitigate the harmful effects of friction.
Two important considerations that must be taken into account when selecting a counter face are surface finish and hardness. The burnished film deposited by carbon/graphite can only be so thick. If this thickness is less than that of the counter face’s surface roughness, a burnished film will not be sustained and the carbon/graphite will wear rapidly. It is typically recommended that the counter face roughness is 16 µ·inch (0.4 micrometer [μm]) or less. Having a sufficiently hard counter face is also important. If the hardness is too low, the counter face could score, the burnished film may not be sustained, and leak paths can form.
Graphite grains are the key to carbon/graphite’s self-lubricating ability. Thus, by increasing the graphite content in a seal face, its dry running abilities can be augmented. Carbon/graphite manufacturers are able to do this using a process called graphitization. During this process, carbon graphite is heated in a controlled atmosphere to temperatures greater than 4,000 F for an extended period of time. During this time, amorphous carbon reorganizes into the graphene-layered graphite matrix. This material is now referred to as electrographite, or graphite for short.
There are a number of benefits to using graphite for dry running seal faces over carbon graphite. In addition to improved self-lubrication in dry running conditions, graphite has improved thermal properties. After graphitization, temperature resistance in oxidizing conditions can increase by more than 200 F (110 C), which is crucial in high-temperature applications. On top of this, graphite’s thermal conductivity is greater than five times that of carbon graphite. In high-speed applications, this helps to facilitate heat dissipation away from the sealing interface where frictional heat is typically generated. Coupled with graphite’s low coefficient of friction, its high-thermal conductivity considerably reduces excessive heat buildup at the sealing interface, which can ultimately deteriorate the seal face, coke hydrocarbons and introduce leak paths.
There are certain restrictions when it comes to using graphite over carbon graphite. When amorphous carbon is converted to graphite, the material becomes softer and loses some of its strength. This can be problematic in high-load applications where hardness and strength of the seal face are key properties. In certain high-speed applications, flexure of the seal face must be minimized, so the stiffness of carbon graphite is often preferred over graphite. Regardless, it is always best to consult with a carbon/graphite manufacturer when determining the proper material grade for a specific application.
Plain graphite and plain carbon graphite are porous as a result of the initial baking process. During this process, molded “green” carbon/graphite components (i.e., graphite powders held together by a pitch binder) are baked in a controlled atmosphere baking oven. This causes pitch to be converted into amorphous carbon, forming plain carbon graphite. Noncarbon elements outgas from within the material as it is baked, which results in an interconnected network of porosity.
IMAGE 2: Carbon exists in three forms in nature—diamond, graphite and amorphous carbon. Diamond: Four bonded valence electrons. Highly ordered lattice structure. Graphite: Three bonded valence electrons. Strong graphene layers bound together by weak van der Waals forces. Amorphous Carbon: Two bonded valence electrons. Strong “entanglement” of carbons.
Carbon/graphite manufacturers take advantage of this porosity by filling it with various impregnation materials, including metals, resins and even carbon. The type of impregnation used has a considerable effect on the ultimate physical properties of the material. As a result, it is important to carefully select the impregnation type to ensure optimal performance in a specific application.
One impregnation material often used in dry running applications is salt. Salts can serve to make the burnished film more robust and able to handle even rougher surfaces without the presence of fluid lubrication. Various types of salts are often used to achieve this effect, as these salts can increase film robustness more than any other type of impregnation.
Salt impregnations have other benefits as well. In addition to fortifying graphite’s burnished film, salts can help limit oxidation at high temperatures. They do so by bonding to active sites within the material that would otherwise react with oxygen at high temperatures and be driven away from the material in the form of carbon dioxide. By blocking these active sites and limiting oxidation, salt impregnations can increase temperature resistance of seal faces by more than 100 F (55 C).
Certain salt impregnated graphite grades have some degree of porosity, whereas others are entirely impervious. There are benefits and drawbacks to
each of these types that must be taken into account during grade selection. As a general rule of thumb, an impervious salt impregnation is used when maintaining an aerodynamic film is a concern or when zero leakage is permissible.
Graphitization and salt impregnations have enabled carbon/graphite manufacturers to produce material grades that can withstand incredibly harsh dry running conditions. However, even with these advancements, there are some mechanical seal environments that are so dry that even these highly engineered grades cannot properly self-lubricate. This is because carbon/graphite requires some small amount of moisture in the environment for self-lubrication to take place.
In cases where absolutely no moisture is present, engineers often turn to molybdenum disulfide-based materials. Molybdenum disulfide self-lubricates comparably to graphite, but can do so in vacuum-dry conditions.
A basic rule of thumb that can indicate whether the use of moly grades may be necessary is the dew point of the sealing environment—if the dew point is less than -30 F (-35 C), the atmosphere is likely so dry that molybdenum disulfide-based grades are the only feasible seal face material option. This can occur in seals used in deep space exploration, dry mixer seals, vacuum seals, nitrogen-purged dry gas seals, etc.
Carbon/graphite manufacturers are putting a large focus on developing advanced processing techniques that enhance the dry running ability of their materials. As more extreme mechanical sealing applications are developed, the materials used in these applications must be developed at the same rate.
We invite your suggestions for article topics as well as questions on sealing issues so we can better respond to the needs of the industry. Please direct your suggestions and questions to sealingsensequestions@fluidsealing.com.
Read more FSA Sealing Sense articles by clicking here.
Thread starter
maqadir
Start date
Jan 9, 2009Tags
In summary: The material goes in at one end and comes out the other. The seal can be between the stationary and rotating parts, or it could be part of the rotating part.In summary, a rotary seal is needed between the blue dots and the rotating glass tube in order to allow material to be fed into the tube.
maqadir
I am a mechanical engineering student and have a desin project application question.
In the picture, as you can see, the working tube, is a glass cylindrical tube, mounted onto main body of the furnace using a motor that rotates the tube at certain RPM.
This is an open ended tube, and the blue holes on the end of the tube, show, where the seal, that i need to select should be mounted, onto the ends A,B of the tube, with ideally 2 holes in it for feeding of the material, that goes into the tube. Hence the seal needs to seal the ends, and be rotary so that the outer layer of the seal rotates with the tube, while the middle, which should have 2 ideal holes in it, stay stationary, so materilas that are to be added to the furnace tube can be fed through easily. for the process:
Tungsten Oxide, Wo3+H2S-----> WS2 (Tungsten+H20)
The two holes at end B, show one hole to let the gas product out in the gas chamber which sucks it away, and the other hole to collect the powdered material in a base, or likewise. These two holes will have pipes, or otherwise likewise material connected for feeding purposes.
Any one know what such seals are called, and where they can be found or bought??
thanks , would be great help!
If you want to learn more, please visit our website China Custom Graphite Shaped Parts Factory.
turbo
Gold Member
You may be able to use seals made of compressed graphite. These are sold as "pump packing" to seal pump shafts. I'm pretty sure Garlock has them.
maqadir
turbo-1 said:
Thankyou for your reply, however, wot do i do about the 2 holes needed for feeding?
Science Advisor
So the blue tubes remain stationary and the glass tube rotates? The seal needs to be between the blue tubes and the rotating glass tube? Am I understanding you correctly?
Also, you need to specify the speed of rotation, the ID of the glass tube and the expected temperature the seal will see.
maqadir
FredGarvin said:
Umm, you misunderstood me there, the blue dots, does not mean a separate tube, the glass tube rotates yes, and it is hollow, the blue dots on the diagram, is where i need the seal to fit, ( i.e at the hollow end of the glass tube) and the blue dots describe, the holes that should be in the seal to allow feeding of the material into the glass tube (its a gas and powder ) so that a reaction can be obtained in the glass tube. For this reasons, the blue dots, which is essential the seal, should remain stationary. The blue dots, are just my requirments of a whole in a seal, that allows material to be fed into the rotating tube, that's allIts controlled by a electric motor, supplied by a.c current alternator, so the RPM, is not going to be very high, maybe 60-100 rpm, while temperatures in furnace will conduct the glass tube to be working at 200 odd degrees celcius.So in summary, i needa seal, to seal the hollow ends of the tube that is drawnblue dots indicate, that the sea i fit onto both ends, remains stationary, while the tube rotates and has holes for as said earlier feeding of the metrialI hope this helps explain what the seal is required for,Thanks a lot for your reply
coreyh
Wow, confusing problem... Good job mentioning temperature, which is pretty much the most important consideration when it comes to seals. 200C is pretty reasonable for high temperature plastics, so you should have lots of options.
I'm still a bit confused about the two blue dots. So these are inlets for material on one side and outlets on the other side? If this is the case, can you make a small manifold, say out of stainless steel, which has two fittings on it for supplying material and have the seal on the outer rim of the glass tube? This would simplify things significantly! A great company for high temp seals is:
http://www.tss.trelleborg.com/com/www/en/homepage.jsp
I would suggest to write them a request with a less confusing diagram;)
hope this helps!
maqadir
The blue dots , forget about them
Just consider a normal rotary seal. Now in this type os seal, only the outer part rotates, and the inner part stays stationary right?
Now, the stationary part of this seal, needs to have 2 holes ( which are the 2 blue dots) that are required for feeding.
Thanks for the website though, its just that i have no idea what to do about the holes.
coreyh
Sounds like you need more than just a seal, but rather a mechanical part which holds the seal and also provides holes for feeding in material and supports the tube/furnace.Usually a seal is just a single part, like an 0-ring for example, which is inserted into a groove cut into a fixed part. The rotating part then inserts into the fixed part and the seal is compressed between the two surfaces. Check out this link for a few nice diagrams:I would recommend doing some google searching and trying to find a similar problem to yours and see how it was solved.Good luck!
andy taylor
if your looking fora vacuum /rotary seal.. there is also the possibility of choosinga feroofluidic seal and or a magnetic driven feed . eliminating any machining
The main purpose of a seal in a rotating furnace is to prevent any leakage or loss of material from the furnace. This is important for maintaining the furnace's efficiency and preventing any potential hazards.
When choosing the right seal for a rotating furnace, factors such as the temperature and pressure of the furnace, the type of material being processed, and the rotation speed of the furnace should be taken into consideration. Other factors may include the availability and cost of the seal, as well as its compatibility with other furnace components.
There are several types of seals that can be used for a rotating furnace, including mechanical seals, gland seals, and labyrinth seals. Each type has its own advantages and disadvantages, and the best choice will depend on the specific needs and requirements of the furnace.
To ensure the seal will be effective and long-lasting, it is important to properly install and maintain it. Regular inspections and replacement of worn or damaged components can also help to extend the lifespan of the seal. It may also be helpful to consult with a seal manufacturer or expert for guidance on selecting and maintaining the seal.
Yes, seals can be customized for specific rotating furnaces. This may be necessary if the furnace has unique specifications or if a standard seal is not suitable. It is important to work with a reputable seal manufacturer who can provide customized solutions and ensure the seal will meet the specific needs of the furnace.
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