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What is a Gas Seal?

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  • Written By: N. Kalu
  • Edited By: Allegra J. Lingo
  • Last Modified Date: 31 October 2016
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A gas seal, often referred to as a dry gas seal, is used to close the shafts of centrifugal compressors. Its main advantage is that it does not require lubricating oil to run and so is less likely to contaminate the sealing process. Dry gas seals are used in tough industrial environments where workers are exposed to many chemicals, such as in petrochemical processing, extraction and refining, oil drilling, and gas transmission. Gas seal failure can occur due to a variety of reasons including compressor flooding, shaft misalignment, or particle contamination.

The design of the gas seal is crucial to its function. It is created to keep harmful odors, chemicals, and gases within centrifugal compressors, carrying tubes which transmit gases from one processing site to another. Spiral grooves on the outside of the seal make it easier to lift and manipulate it during industrial use. The grooved part of the seal leads gas toward the non-grooved part of the seal. This action traps the gas between the two parts, which increases internal pressure.

Such pressure leaves a small gap between the two faces of the seal, the grooved and the non-grooved, improving the resistance of the fluid film. The resistance allows the most protection against any contaminants touching the faces of the gas seal. Fluid film resistance can be adjusted according to the industrial project or nature of the compressor system.

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Dry gas seals have an interesting and varied history. John Crane Inc., a British turbomachinery manufacturing and maintenance company, was the first to register the dry gas seal patent in 1968. The company patented the aforementioned spiral groove model, which has become very popular in oil and petrochemical applications. Before the company introduced the new model, the industry relied on lubricated seals to close and maintain centrifugal compressors. Lubrication was messy, made the seals more difficult to manage, and sometimes led to contamination of the compressor's contents.

Despite its disadvantages, lubrication is necessary to reducing the friction between gas seal faces. John Crane Inc.'s innovation was in their use of a dry gas film, which reduced friction without the need for liquid lubrication. There are two other types of lubrication used in these operations: full liquid film, which carries various types of oils and must be used in light processing operation, and partial liquid and gas film, which utilizes a one part liquid and one part gas balance to maintain the integrity of the seal.

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