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John Crane's Type 28 Dry Gas Seals: How Does It Work?

How Does It Work?

Highest Pressure Non-Contacting, Dry-Running Gas Seal

Type 28 compressor dry-running gas seals have been the industry standard since the early 1980s for gas-handling turbomachinery. Supported by John Crane's patented design features, these seals are non-contacting in operation. During dynamic operation, the mating ring/seat and primary ring/face maintain a sealing gap of approximately 0.0002 in./5 microns, thereby eliminating wear.

These seals eliminate seal oil contamination and reduce maintenance costs and downtime.

John Crane's highly engineered Type 28 series gas seals incorporate patented spiral-groove technology, which provides the most efficient method for lifting and maintaining separation of seal faces during dynamic operation. Grooves on one side of the seal face direct gas inward toward a non-grooved portion of the face. The gas flowing across the face generates a pressure that maintains a minute gap between the faces, optimizing fluid film stiffness and providing the highest possible degree of protection against face contact. Riding only on the load support of a cushion of gas, the seal's film stiffness compensates for varying operating conditions by adjusting the gap and pressure generated to maintain face stability.
John Crane Type 28 seal designs include:
  • Type 28AT Advanced Technology: Designed to excel in low pressure applications, the 28AT is today's most widely used non-contacting, dry running gas seal. Its design pedigree stretches back over 30 years and it has been proven in use on many thousands of installations around the world. The versatility offered by the 28AT makes it ideal for a very wide variety of gas compressor applications.
  • Type 28XP High-Pressure: Offers all the features and benefits of the 28AT, but is optimized to provide reliable performance on higher pressure duties. The 28XP's increased chemical resistance and ability to operate over a wider temperature range has established it as an industry benchmark for reliability.
  • Type 28EXP Extreme High Pressure: John Crane's highly effective solution for sealing gas installations that involve extreme pressures. It has been proven in use in the most challenging and hostile environments and delivers rugged and reliable performance around the world.
  • Type 28ST Steam Turbine: The non-contacting, steam lubricated Type 28ST is an effective sealing solution for small to medium-sized general steam turbine applications. By combining proven rotating groove technology with a high-temperature capability that virtually eliminates leakage of steam along the shaft, it creates an excellent alternative to traditional sealing devices such as segmented carbon bushes and labyrinths.
  • Type 28VL Vaporizing Liquids: Uses the energy of the rotating equipment to vaporize process fluid at a controlled rate, creating a gas film that lubricates the seal faces in applications of extremely low temperature.

What Sets The Type 28 Series Apart?
  • Accurate centering and balancing: The Type 28 seal mating ring and sleeve use tolerance rings to provide precise centering and cushioning. The seal runs true with a dynamically balanced rotor assembly that eliminates vibration.
  • High-pressure support capability: A metal carrier in the Type 28XP seal controls distortions that can occur under high-pressure loads.
  • Low-drag ring assembly: A custom-fit o-ring and thrust ring set provides extremely low drag and uniform loading to the primary ring surface.
  • Reduced-stress drive points: Low-stress drive flats on the Type 28 seal eliminate stress riser pin holes found in other designs.


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