In the modern world of extreme temperature and pressure applications, the qualities of flexible graphite have made it the almost perfect choice for gasket materials. Its ability to withstand heat, pressure and aggressive media, whilst maintaining its shape and flexibility to seal irregular surfaces, makes it the superior material for seals across multiple industries.
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Structurally, flexible graphite far exceeds the temperature and pressure requirements that accompany pressure seal applications. Regardless of temperature extremes, flexible graphite springs back into shape, so steam cycling does not affect it. Its non-stick surface also makes the pressure seal as easy to remove as it is to install and in fact, hides most imperfections left when replacing a metal seal. Inert to almost every media, it is practically unbeatable for fluid sealing applications.
It&#;s hard to imagine that a material made from a mineral could be flexible, but such is the wonder of flexible graphite. Much like a diamond, graphite is a native form of the element carbon but while they share this common building block, their properties are vastly different. While diamonds are famously hard enough to cut glass, graphite is actually soft and flexible. This is what makes graphite the engineer's best friend.
Flexible graphite is not in itself a naturally occurring material, but the result of a chemical and heat expansion process first introduced in . Graphite, which is pure carbon, has layers of carbon atoms which form strong hexagonal bonds. Weaker bonds exist between each of the layers. Flexible graphite is created by submerging natural graphite flakes into hydrofluoric and sulfuric acid. This weakens the bonds between each of the carbon layers, so the flake graphite becomes expandable.
The treated graphite is then exposed to intense heat, expanding the graphite further, and then rolled to form a continuous strip without the need for a binder material. Finally, the now-flexible graphite is formed through mechanical compression into the desired shape, such as sheets for gaskets.
Graphite may well be in the public consciousness the most for its use in Number 2 pencils - in Greek, graphite means &#;to write&#;. The vast majority of us have benefitted from this instrument (and the ability to erase mistakes) since mass production first began in Nuremberg, Germany in . But graphite has changed the game for multiple sectors over the years, not least in naval applications.
The remains of the Borrowdale Mine in the Lake District, Cumbria
Graphite was first discovered in the Parish of Borrowdale, situated in the Cumbrian Lake District, during the s. Legend has it that some shepherds discovered a huge deposit of the soft mineral, exposed to the open after a tree was uprooted, and realised they could use it to mark their flock. Monks in a nearby monastery then began using it as a writing implement.
It was during the reign of Queen Elizabeth I (-) that graphite made its first significant contribution to world history. Due to its ability to withstand extreme temperatures without losing its shape, graphite was used as a refractory material to line moulds for cannon balls. This meant the ammunition could be made smoother and rounder, increasing its aerodynamic properties. Cannons could now be fired with greater range and accuracy, giving the Royal Navy a major advantage in battle.
A cannon on the Golden Hind (left) and an Elizabethan-era cannonball (right)
As a result, the English grew in superiority, becoming the world&#;s largest navy and building a worldwide empire. Consequentially for the Parish of Borrowdale, the Borrowdale Mine became closely guarded with the graphite becoming a precious commodity. Such was the size and purity of the deposit that Borrowdale graphite was exported all around Europe for nearly 300 years.
By the time flexible graphite gasket material had entered the market, having gone into production in the late s as a replacement for asbestos, the United States of America had become the world&#;s largest navy. Coming out of the Cold War at the beginning of the s, the 550+ active fleet was a huge expense, none more so than the nuclear vessels. The problem with the nuclear fleet was that to successfully seal the imperfect surfaces of high-pressure valves, a relatively soft silver coating was used. While it was the best material to fabricate these vital seals from, it had become prohibitively expensive, given the volume of seals needed.
A flexible graphite seal can adapt to surface imperfections while maintaining enough friction to stay in place under load and system pressure. Metallic seals require a perfectly clean surface when replacing - if the surface is pitted or scratched in any way, metals won&#;t create the seal. Conversely, flexible graphite is easy and quick to replace as it can cover these imperfections. Furthermore, flexible graphite doesn't lose its properties when exposed to nuclear radiation. As such, it was an ideal solution to the U.S. Navy's valve-sealing issues.
USS George Washington, a modern nuclear vessel of the US Navy
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There are various ways to buy Flexible Graphite sheet materials, with a range of inserts available, often known as Graphite Laminate. Each variation has differing properties and should be carefully selected according to the parameters of the application being sealed.
Dobson Gaskets supplies Flexible Gasket products for any application, ranging from standard premade gasket and brand-name sheets, to a wide variety of inserted materials for highly customised use. Although the choice of insert can alter the properties of the material, there are many consitencies across the range that help decide whether they are right for the application.
No-one could be blamed for considering Flexible Graphite the ultimate gasket and sealing material. Nothing is perfect, however, and there are some important factors to be taken into consideration when selecting it as a gasket or sealing material.
Raw, or homogenised, flexible graphite, which contains no metallic insert, does have high temperature and pressure tolerances compared to other materials, but not at the upper end of the scale. Furthermore, it has a low tensile strength of around 700psi. In modern applications, especially within heavy industry such as the manufacturing and petrochemical sectors, there is a greater need to combine its tolerance to aggressive media and flexibility with higher temperature and pressure ranges. Strength can also be a key factor in these situations.
Laminating graphite onto material such as stainless steel, can significantly enhance these properties. Depending on the inserted material, the temperature and pressure tolerances can increase dramatically, while tensile strength can increase to a far more practical 5,000psi
While this in itself is not an issue, the downside to combining with metals is a slight reduction in flexibility and recovery. When they are relieved of thermal and pressure loads, they may not snap back into shape in the way homogenous flexible graphite seals would. Due care must therefore be taken when considering how well the gasket will retain its shape after loads are removed.
For some sectors, flexible graphite is not suitable at all. The nature of the material&#;s flexibility is inherent in its flaky structure. As a result, there is the potential to release carbon particles into the fluid. For this reason, it should not be used in the food, medical or pharmaceutical sectors due to risk of contamination.
Most importantly, the use of low-grade graphite in the sheet material can result in seal failure due to the potential presence of impurities within the material. High-grade graphite must be used for high-temperature, high-pressure seals.
An example of Planichem brand flexible graphite sheet material at Dobson Gaskets
The sheer range of flexible graphite materials, and choice of insertion, can make the process of selecting the right combination a complicated one. Our highly skilled technicians are available to discuss your sealing requirements and provide you with the best and most cost-effective solutions. We can then manufacture your gaskets at any volume, quickly and efficiently.
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With over 60 years experience in manufacturing gasket and seals, our technicians are ready to help you find the right material for your application. Visit our Graphite Gaskets page to find out more and get in touch with one of our expert technicians.
FIND OUT MOREFlexible graphite is manufactured from mineral graphite (expandable flake graphite). It is also called graphite foil, or referred to by a brand name such as Grafoil®. As its name indicates, flexible graphite is flexible and conformable, an ideal property as a gasket material. Flexible graphite can also be used in many other thermal, electronic or electrochemical applications.
Chemical Composition: C 99% or higher
Density: 62.4 or 70 lb/ft3
Thermal Conductivity:
Through Thickness ("c") 36 BTU.in/ft2.h.F (5 W/m.K)
Parallel to Surface("a") 960 BTU.in/ft2.h.F (140 W/m.K)
Compatible pH Range: 1-14
Heat and chemical resistant gasket, seal, liner; sheet lubricant; thermal conductive seal; heat barrier; EMI (electromagnetic interference) shielding and gasketing, heat dissipation, stress sensing, vibration damping, spark plasma sintering(SPS), and other thermal, electronic or electrochemical applications.
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