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Mechanical face seals are a complex combination of materials and design that form a system whose prime objective is maintaining the integrity of the pumping system, keeping what is inside where it belongs and preventing contamination from the outside.
From the simplest design to the most complex, the system must operate across a multitude of conditions (and often beyond what the original design intended) in terms of speed, contact loads and environment. Every component in the system is a vital link contributing to the system’s success or failure.
These systems enable relative motion between stationary and rotating components while simultaneously accommodating some level of axial or radial movement. The technology that has been developed and implemented in these systems has become so reliable that mechanical seals are often taken for granted. The vast majority of failures of mechanical seals can be attributed to the system in which the seal is installed, not the seal itself.
One of the materials used in many mechanical seal systems is based on the fourth most common element and the basis of all life—carbon. This article will explore how this common element plays such a critical role in these complex systems.
Because mechanical sealing systems must maintain tightly controlled contact between one rotating and one stationary face, interface stability must be maintained across a potentially wide spectrum of conditions. Face flatness is critical and is measured in millionths of an inch; unexpected distortion will change the interface dynamics significantly. With the proximity of the two face materials, contact is inevitable and the materials must be able to operate with some level of self-lubricity so they do not damage each other, which would create a path across that interface that enables system leakage.
Figure 1. Five of the eight carbon allotropes. (Graphics courtesy of the author)Figure 1. Five of the eight carbon allotropes. (Graphics courtesy of the author)
There are eight allotropes of the carbon element (see Figure 1), based on how the carbon molecules align in the lattice, that result in a range of properties from the softest version, graphite (often used in pencil “lead”) through amorphous carbon to diamond, the hardest material known to man.
Included in this listing are Buckyballs and Buckytubes, which are more exotic (and expensive) forms whose value has yet to be realized. Diamond is a high-end option and is typically applied to the hardface for use in the most demanding applications where the associated premium for this option can be tolerated.
The primary allotropes used in mechanical seal materials are graphite (both synthetic and natural) and amorphous.
Mechanical carbon used in seals can be classified in three categories
The process from the raw materials to a finished part is complex. There is mixing and milling of the raw materials, molding of the powder followed by baking and impregnation.
There is then machining and final impregnation before lapping and packaging. Figure 2 depicts the many steps required.
Figure 2. Mechanical carbon process overviewFigure 2. Mechanical carbon process overview
Unlike metals such as 316 stainless steel, there is no standard offering from manufacturers.
Each company creates a unique recipe intended to address specific conditions that maintain the interface stability; from low temperatures (cryogenic), to water, to methane, to oil, to higher temperatures and beyond, materials exist to provide the needed capability. These materials are designed for specific conditions. One recipe cannot perform in every condition, and seal companies work with material manufacturers to provide the optimum candidate for the application.
The ratio of carbon to graphite in the formulation will have a significant impact on the performance characteristics of the material.
Figure 3 shows the variation in physical properties, thermal conductivity and chemical resistance of the part according to the relative amount of carbon and graphite in the finished product.
Figure 3. Effect of composition on physical, thermal and chemical propertiesFigure 3. Effect of composition on physical, thermal and chemical properties
The attributes of the allotropes, or the various forms of carbon, can be combined in specific ratios and leveraged to achieve desired responses in a system where there is relative motion between two components in terms of friction and wear, (a.k.a. tribology), along with a large temperature range and the need for corrosion resistance. Carbon is an ideal seal face material for the range of conditions within a mechanical sealing system, and is widely used.
In the same way a pencil transfers graphite to a piece of paper, mechanical carbon can transfer material to the mating face—the level of that transfer is where the technology comes into play.
Manufacturers of carbon-based materials for mechanical seal faces achieve specific tribology through formulation of different elemental forms of the carbon atom and the use of other proprietary elements. Processing these recipes results in a structure that is literally held together by atomic carbon bonds.
A mechanical seal’s mating pair is the heart of the system and the two components are designed to work together. There exist optimum pairs for specific conditions.
The components of the recipe enable the proper tribology—too much or too little film transfer would result in shortened life or performance of the mechanical seal.
The structures can be enhanced through the introduction of various resins (which provide strength and impermeability) or metals (antimony, copper, or silver, which can improve strength and stability). See Table 1 for applications of various impregnation materials.
Table 1. Applications for various formulationsTable 1. Applications for various formulations
Non-contacting mechanical seal designs offer improved life over the traditional contacting mechanical seals and also provide for higher duty.
The role of the mechanical carbon differs with this high-end mechanical seal and the resulting recipe is unique for these types of seals.
Carbon is also a critical component in some of the hard face materials used in mechanical seals because it combines with metals at high temperatures to form metallic carbides. These materials include tungsten carbide and silicon carbide.
These carbides provide superior hardness and stiffness. They are not self-lubricating and therefore cannot run “dry,” but they run well against mechanical carbons.
If you are looking for more details, kindly visit metallurgy filed using graphite.
Some material manufacturers have even developed methods to create hybrid structures, specifically for silicon carbide, that contain free graphite.
These materials, while not self-lubricating in the classic sense, offer enhanced capability for some of the most demanding applications by creating a stable and hard material that can run in marginally lubricated conditions.
Next Month: Packing friction and valve actuation
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.
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Page Intro
GENERAL
Does Chesterton Stationary Equipment Solutions have installation instructions for its products?
We have installation instructions for all our products and in a variety languages. This information is contained on this website on each specific product page. Contact the
Chesterton Application team for more assistance
Do I need to use anti-seize? If so, whose and which one?
Using anti-seize on your hardware such as the bolts, nuts and springs assures consistent, even loads are being applied. Anti-seizes are made up of various materials that affect the k nut factor coefficient. These coefficients can range from as low as .11 and increase beyond a dry nut factor value of .20. Chesterton uses their own anti-seizes such as the 772, 783ACR, and 787 for testing to ensure the results are consistent and accurate.
PACKING
What is the difference between valve packing and pump packing?
Specific packing is designed and manufactured to fit equipment operating parameters. Valve packing is designed to not have any visible leakage and effect a tighter seal between the valve stem and the stationary packing. In the case of pump sealing, the packing is generally designed to allow some leakage between the sleeve and packing to provide lubrication and heat removal due to the rotating sleeve. Some packing is fit for both services.
Can pump and valve packings be used interchangeably?
In general, you can use a pump packing in a valve, but not a valve packing in a pump application. Some packing is specifically designed to be used in either type of equipment. Please
Ask an Expert to confirm a non-standard use for a packing.
Are all black packings the same?
No, though they may look alike externally, there can be a wide difference in the material properties and performance.The black material may be carbon, graphite or a graphite coated PTFE material, and within each material there are different impurities and properties associated with performance. Braid styles can also vary and effect performance.
PACKING RINGS
How many rings make up a Style 5800 and 5800E sets? Also, how tall do these sets measure?
The 5800 set contains 5 graphite die- formed wedge style rings (dfr). The 5800E sets have 5 dfr rings plus 2 additional braided end rings of 477-1 for a combined total set quantity of 7 rings. The 5800 sets will have a stack height measurement of 4 cross-sections tall. The 5800E sets will have a total stack height of 6 cross-sections tall.
How do I calculate the application's cross-sectional (c/s) size and the quantity of rings?
You will need to know a few dimensions such as the shaft diameter or sleeve diameter (I.D.), the box diameter (O.D.), and the stuffing box depth: (c/s) = (O.D - I.D.)/ 2. To calculate the required number of packing rings you will need the stuffing box depth divide by the packing C/S and round to the nearest whole number.
What is the difference between butt and skive cut rings?
A butt cut joint is typically a straight on a piece of packing when the ends are joined.These cuts or joints are primarily used in rotating applications.Skive cut ring joints are cut at a 45 degree angle, these ends will meet up and lay over each other tightly. These type of ring joints are used mostly in valve applications.
Are more packing rings effective in containing process fluids?
Actually the use of more than 5/6 rings in a stuffing box does not provide additional sealing. Testing and performance recognizes 5 rings as the ideal recommendation where possible.
I unpacked 10 rings of packing from the stuffing box, and should only put 5 back. How should I deal with the excess space?
In any equipment with a box depth that exceeds the 5/6 ring recommendation, use a sleeve or bushing to fill up the excess space.
VALVE PACKING KITS
Does Chesterton offer replacement valve packing kits for Original Equipment Manufacturers (OEM)?
Yes, we do. Chesterton has designed a variety of
packing sets using for control valves manufacturers that include Masoneilan, Fisher, Valtek, and Leslie.
EPA COMPLIANCE
I'm concerned about the EPA showing up at my plant looking at valve fugitive emissions. Is there a packing that meets the EPA's limits on fugitive emissions?
Chesterton has developed a single spool Low E Mechanical Packing qualified to the API 622 Test Standard protocol for Emissions Packings (Chesterton 1622 Low E Packing). This packing will meet the Low E emissions requirement of sealing to less than100ppm for 5 years. To learn more
Ask an Expert.
FDA COMPLIANCE
Does Chesterton have FDA-compliant packings?
Yes, we do. Chesterton offers three FDA compliant mechanical packings that meet Title 21 CFR: 1725A, 425, and CMS2000FP. Please note: the FDA does not certify food grade mechanical packings.The FDA does have the right to enter the company at any time to audit the processes and the materials to ensure these products are compliant.
GASKETS
Does Chesterton offer sheet gaskets?
Chesterton offers a variety of sheet gasketing that ranges from standard rubber material to the compressed fiber, PTFE's, and graphite types. Contact Chesterton's Applications department for your specific recommendation.
Which sheet gasket thickness is best to use?
Based on our product testing, we advise that thinner is always better. However, the reality of equipment conditions and visual perceptions lead most people use a sheet thickness of a 1/16". You may want to contact the Chesterton Applications department to discuss your particular application.
Does Chesterton offer rigid type gaskets? Chesterton offers three options specialty gaskets that meet this description the Spiral Wound, Camprofile, and Steel Trap gaskets. Each gasket offers distinct benefits for a variety of applications. Contact the Chesterton application's department to discuss your particular application needs.
Does Chesterton have a shelf life on gaskets?
Most all gaskets have an expected shelf-life placed on their materials and proper storage conditions impact these limits. Contact Chesterton Applications team for your particular request.
For more information, please visit Continuous Casting in Graphite Materials.