Rigidity, positioning and surface smoothness

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Rigidity_positioning and surface smoothnessž

A flange rigidity critically influences the required minimum surface pressure at mid-point between two bolts of a flanged joint. It also very much influences the type of a gasket suitable for the said joint. Where adequate bending rigidity is maintained, soft gasket materials and non-curing liquid gaskets can be used.

When deciding which of the gasketing options is suitable for your design, apart from flange rigidity, you must also look into the required / possible bolt spacing and positioning and the resulting stress distribution. Each of these factors are inevitably influencing each other and your selection of gasket material.

With conventional gaskets, surface finish or surface texture is an additional crucial factor, since the initial compressive load required to deform the gasket into the flange surface irregularities increases with rougher surface finish. Although it’s not to be completely neglected, the surface finish becomes much less important when using a liquid gasket material. For details you can consult the Gasketing design guide which I’m happy to share on request.

When talking about liquid gasket materials, there are several types depending on application method, curing method and timing or technology.

  • FIP (Formed-In-Place) Gaskets are formed by the application of a bead or by screen printing of sealant, which is then assembled in the uncured state.
  • CIP (Cured-In-Place) Gaskets are formed by the application of a bead of elastomer to one flange that is cured before the flanges are assembled. The gasket is then compressed by the mating flange to form the seal.
  • IIP (Injected-In-Place) Gaskets are liquid gaskets that are injected, after the assembly of the joint, into a groove between the two flange faces and then cured.
  • MIP (Moulded-In-Place) Gaskets are moulded directly onto one of the mating parts, usually into a groove.

I’ll go into the details of each and their advantages and recommended application areas in the next article.

Yes, the gasket can be a liquid

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gasketing header

As unorthodox as it may sound, applying some liquid material in between two rigid metal flanges in order to keep other liquids from seeping through the gap, actually does make sense. Yet, it’s the most common procedure to use pre-cut solid gaskets for this purpose.

That, however, means that the design of the flanges needs to be suitable to use such gaskets, which normally limits the designers’ freedom. On a more operational level, it means that the manufacturer must stock a huge number of as many different gasket types as there are types of flanges they are currently producing.

Both of these issues can be mitigated by the use of liquid gaskets. To better explain it, perhaps I should first go back to what a gasket is and why it’s needed in the first place. A gasket is a material positioned between two flanges which are held together by fasteners. Gaskets prevent leaking of fluids or gases by completely filling the space between the surfaces of the flanges. It is necessary for the seal to remain intact and leak-free for a prolonged time. The gasket must be resistant to the medium being sealed and able to withstand the application temperature, pressure, and micro-movements caused by vibration as well as thermal expansion/contraction.

With all this in mind, I can perfectly understand how liquid might not seem like an obvious choice. You’ll want something firm, something to really fill that gap there! But once you look at the surface of those flanges on the microscopic level, liquid makes perfect sense.

While we see a perfectly smooth surface looking at it with a naked eye, on microscopic level it looks something like this:

surface irregularities

 

And there’s nothing like a liquid gasket to get into every one of those little valleys and perfectly seal every gap once it cures into a solid.

Same as between solid gaskets, there are certainly variations between liquid gaskets as well. Your choice will depend on general design requirements.

Regardless of the sealing material, though, there are certain considerations that have to be taken into account, such as:

  • Rigidity of sealing flanges
  • Bolt preload
  • Potentially different thermal expansion
  • Stress and strain of the joint caused by external forces
  • Compressive stress distribution

A complete and comprehensive guide to gasketing design has been published by Loctite engineers recently, tackling all of the mentioned topics and much more. I’m happy to share it on request, so please contact me if you’d like to have a complete pdf.