Graphite Gaskets

Graphite is something of a wonder material for gasket applications. Not much will attack it and it seals over a very wide temperature range. Graphite gasket material is sold mostly in sheet form and has a dark gray, silver-ish color that looks a lot like lead. This is one reason people sometimes assume it’s a metal. The other reason is that graphite conducts electricity, which is something associated with metals.

In this blog we’ll set the record straight.

What You Should Know About Graphite

Graphite is a form of carbon. Carbon atoms can arrange themselves in several ways, which determines its properties. In graphite the atoms are arranged in layers. These can slide over one another, which is what makes graphite slippery, but at the same time, the layers are surprisingly strong.

Carbon is found on the top right side of the periodic table, between boron and nitrogen. This tells us it’s not a metal, because those elements are grouped in the center of the table.

Useful Properties for Gaskets

Graphite is soft and conforms to match uneven surfaces, a key requirement for a gasket material. It also retains its properties over a temperature range from -400°F to 950°F in air (and higher in non-oxidizing atmospheres.)

Another requirement for gaskets is chemical compatibility, and here graphite excels. It resists almost everything except for a few highly oxidizing chemicals.

Graphite Reinforcement

One weakness of graphite is a tendency to creep under load. This is remedied by mounting it to a stainless steel carrier via little hooks or tangs stamped into the metal. Alternatively, it’s often bonded to stainless foil for increased creep resistance.

Not for Food and Beverage

Unfortunately, graphite is unsuitable for applications where it would come into contact with food or drink. This is because carbon particles can detach and enter into the media. PTFE is usually an acceptable alternative.

Graphite for Gasket Applications

Graphite, particularly when reinforced with stainless steel, makes an excellent gasket material for many applications. Hennig Gasket carries a wide range of types, widths and thicknesses. Contact us to learn more.

Study our website and you’ll see gaskets can be made from a wide range of materials. One of the most versatile yet under-appreciated of these is graphite. Graphite gasket temperature range handles temperatures from -400°F to +875°F, provide excellent dimensional stability and resist chemical attack. On top of that, they have an additional advantage that’s not obvious: they don’t stick to flanges.

The Cost of Flange Cleaning

When replacing a gasket it’s essential to clean flanges thoroughly. Many gasket materials stick to these flanges and have to be scraped off. That’s a problem, for two reasons. First, there’s a risk of scratching the faces while scraping, and that can create leak paths. Second, doing the job properly, (and avoiding early replacement,) takes time.

As explored in “Focus on the Cost of Sealing,” (December 2015,) gasket replacement almost always means taking equipment, or even an entire plant, offline. Each application is different, but the cost of downtime usually dwarfs the price of even the most expensive gasket. The less time that’s needed to swap out the old gasket for the new one, the faster the plant gets back into production, so anything that reduces down time is worth considering.

Just Carbon

Graphite is one form of carbon. That’s the same element that forms diamonds. It’s strong, inert and resists temperature and pressure extremes, but what makes graphite special is its slipperiness. In graphite the carbon atoms are formed into layers that can slide over one another. That helps when closing up a gasketed joint because the material moves easily to fill uneven gaps and take up flange deformation or surface imperfections.

Graphite Gasket Materials

Here at Hennig Gasket we carry graphite material in a range of sheet thicknesses and widths as well as flexible homogenous graphite rolls. These can be cut to practically any desired shape and provide excellent sealing in many different applications. For especially challenging applications ask about the graphite-sided kammprofile gasket. Whichever graphite gasket you choose though, you can be sure of one thing; you won’t waste time scraping old material off the flanges!

High temperatures challenge many gasket materials. Nitrile gaskets will go up to about 95^oC, silicone gaskets to 200^oC and PTFE to 260^oC, but what if you need to go higher? One option is to go with a metal gasket. A better one is to ask about graphite. Graphite seals and gaskets retain their properties at temperatures as high as 450^oC, and have some other very useful sealing characteristics.

Properties of graphite

Graphite is a form of carbon where the atoms are arranged in layers or sheets. That lets them slide over one another easily, which translates to a slippery feel when rubbed between finger and thumb. (It’s also what lets a “lead” pencil write – graphite actually rubs off onto the paper.)

This slipperiness or low coefficient of friction is useful in gasketing or sealing. As mating surfaces are brought together graphite allows some slip. That accommodates any rotation or sideways movement as the clamping load goes on without damaging the sealing material. Used as a dynamic or shaft seal the low friction properties of graphite minimize energy losses and heat build-up while maintaining an effective barrier.

Graphite is also a very soft material, (which might seem odd when you consider it’s a cousin of diamond,) but that lets it flow into surface irregularities, which is what provides the sealing function.

Like PTFE, graphite is quite inert. It resists attack from most corrosive chemicals, even at high temperatures, and likewise doesn’t contaminate them.

Graphite seals and gaskets

It’s possible to buy graphite for use as a gasket or seal. It’s also used as a coating for some metal gaskets, (Kammprofile gaskets are an example,) where it provides excellent sealing performance along with temperature and chemical resistance. If you’re looking for gasket material for a high temperature application, ask Hennig Gasket if graphite seals might be right for you.