Gasket

Gasket

[gas-kit]

A gasket is a mechanical seal that fills the space between two objects, generally to prevent leakage between the two objects while under compression. Gaskets save money by allowing less precise mating surfaces on machine parts which can use a gasket to fill irregularities. Gaskets are commonly produced by cutting from sheet materials, such as gasket paper, rubber, silicone, metal, cork, felt, neoprene, nitrile rubber, fiberglass, or a plastic polymer (such as polychlorotrifluoroethylene). Gaskets for specific applications may contain asbestos. It is usually desirable that the gasket be made from a material that is to some degree yielding such that it is able to deform and tightly fills the space it is designed for, including any slight irregularities. Many gaskets require an application of sealant directly to the gasket surface to function properly.

Properties

One of the more desirable properties of an effective gasket in industrial applications for compressed fiber gasket material is the ability to withstand high compressive loads. Most industrial gasket applications involve bolts exerting compression well into the 14 MPa (2000 psi) range or higher. Generally speaking, there are several truisms that allow for best gasket performance. One of the more tried and tested is: "The more compressive load exerted on the gasket, the longer it will last". There are several ways to measure a gasket material's ability to withstand compressive loading. The "hot compression test" is probably the most accepted of these. Most manufacturers of gasket materials will provide or publish these results.

Gasket Design

Gaskets come in many different designs based on industrial usage, budget, chemical contact and physical parameters:

Sheet Gaskets

The premise is simple in that a sheet of material (in older situations the material would be compressed asbestos, but now generally a fibrous material such as graphite is used) has the gasket shape "punched out" of it. This leads to a very crude, fast and cheap gasket. These gaskets can fill many chemical requirements based on the inertness of the material used and fit many budgetary restraints. Common practice prevents these gaskets from being used in many industrial processes based on temperature and pressure concerns.

Solid Material Gaskets

The idea behind solid material is to use metals which cannot be punched out of sheets but are still cheap to produce. These gaskets generally have a much higher level of quality control than sheet gaskets and generally can withstand much higher temperatures and pressures. The key downside is that a solid metal must be greatly compressed in order to become flush with the flange head and prevent leakage. The material choice is more difficult; because metals are primarily used, process contamination and oxidation are risks. An additional downside is that the metal used must be softer than the flange -- in order to ensure that the flange does not warp and thereby prevent sealing with future gaskets. Even so, these gaskets have found a place in industry, although not a large one.

Spiral Wound Gaskets

Spiral wound gasket utilizes a mix of metallic material and "filler material" generally the gasket has a chosen metal, normally a carbon rich or stainless steel, wound (hence the name) outwards in a circle (although other shapes are possible this is the primary) with the filler material, generally a flexible graphite, starting at the opposite side of the circle and winding in the same direction. This leads to a growing circle of alternating layers of filler and metal. These gaskets have proven to be reliable in most applications and although more expensive than solid material they do not require as high of bolt forces to be effective. This is possible mainly because the graphite makes the primary seal with the flange and the metal only acts to keep the gasket structurally sound.

Double Jacketed Gaskets

Double Jacketed (hence forth called DJ) gaskets are another combination of filler material and metallic materials. In this application, a tube with ends that resemble a "C" is made of the metal with an additional piece made to fit inside of the "C" making the tube thickest at the meeting points. The filler is pumped between the shell and piece. When in use the compressed gasket has a larger amount of metal at the two tips where contact is made (due to the shell/piece interaction) and these two places bear the burden of sealing the process. Since all that is needed is a shell and piece, these gaskets can be made from almost any material that can be made into a sheet and a filler can than be inserted. This is an effective option for most applications.

Kammprofile Gaskets

Kammprofile gaskets are used in many older seals since they have both a flexible nature and are great sealers. Kammprofile's work by having a solid corrugated (many equally spaced bumps) core with a flexible covering layer. This arrangement allows for very high compression and an extremely tight seal along the ridges of the gasket. Since generally the graphite will fail instead of the metal core, Kammprofile can be repaired when the seal is not needed such as during a shutdown of some kind. Kammprofile has a high initial cost for most applications but this can be justified both by long term savings and increased reliability.

Improvements

Many gaskets contain minor improvements to increase lifespan or acceptable operating conditions:

  1. A common improvement is an inner compression ring. A compression ring allows for higher flange compression while preventing gasket failure. The effects of a compression ring are minimal and generally are just used when the standard design experiences a high rate of failure.
  2. A common improvement is an outer guiding ring. A guiding ring allows for easier installation and serves as a minor compression inhibitor. In some alkylation uses these can be modified on Double Jacketed gaskets to show when the first seal has failed through an inner lining system coupled with alkylation paint.

See also

Sources

  1. Bickford, John H.: An Introduction to the Design and Behavior of Bolted Joints, 3rd ed., Marcel Dekker, 1995, pg. 5
  2. Latte, Dr. Jorge and Rossi, Claudio: High Temperature Behavior of Compressed Fiber Gasket Materials, and an Alternative Approach to the Prediction of Gasket Life, FSA presented Paper, 1995, pg. 16

Helpful Links

ThomasNet Gasket Resources Gasket Fabricators Association Fluid Sealing Association

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