June 2016

Non-Stick Graphite Gaskets Reduce Downtime

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!

Buna-N (Nitrile) Gaskets and Oil

When working with a gearbox, a pump, or part of a fuel system, it’s essential to select a gasket material with resistance to hydrocarbons. Mineral and vegetable oils, along with diesel, gasoline and similar fuels, attack many materials, causing them to swell or break down. The notable exception is nitrile rubber. Nitrile rubber gaskets, often referred to as Buna-N or NBR gaskets, offer excellent resistance to oil.

Nitrile Rubber Chemistry

Nitrile rubber is made from a blend of acrylonitrile (chemical formula C3H3N) and butadiene (C4H6.) Each component is mixed with water as an emulsion before being combined in a polymerization reaction. This makes the carbon (C) and hydrogen (H) atoms form into long chains that tangle round another to form a rubbery compound.

Increasing the proportion of acrylonitrile makes the rubber stronger and reduces its gas permeability. The trade-off is that it results in a rubber that’s stiffer at low temperatures.

Rubber and Swelling

Most rubber absorbs oil and swells up in size. This is a problem in gaskets used in gearboxes and engines because it can lead to leaks. However, acrylonitrile reduces this tendency. So the greater the proportion of acrylonitrile used in the nitrile rubber formulation the less it will swell.

Swelling is covered in the ASTM D2000 specification. Most nitrile gasket materials are classed as ‘BF,’ ‘BG’ or ‘BK.’ The ‘B’ indicates an upper temperature limit of 100°C and the second letter shows how much the rubber will swell. ‘F’ is swell of 60% by volume, (under specified conditions,) while ‘G’ is 40% and ‘K’ 10%.

When and When Not to use Nitrile Gaskets

Good applications are those where peak temperatures are moderate and there’s exposure to oils. However, nitrile rubber does not hold up well to ozone and oxygen, so should not be used in places where these are present. Water purification equipment is one such location.

The Application Dictates the Gasket Material

If there’s any question about the properties of various gasket materials, consult the specialists at Hennig Gasket. They can explain the material choices available for your application.