For very high-speed turbomachinery, including gas turbines, seal runner speeds may reach speeds greater than 1300 fps, requiring novel seal arrangements to overcome wear and pressure limitations of conventional face seals. Two classes of seals are used that rely on a thin film of air to separate the seal faces. There are two types of seals as below for aeospace.
Labyrinth Seals :
By their nature, labyrinth seals are clearance seals that also permit shaft excursions without potentially catastrophic rub-induced rotor instability problems. By design, labyrinth seals restrict leakage by dissipating the kinetic energy of fluid flow through a series of flow constrictions and cavities that sequentially accelerate and decelerate the fluid flow or change its direction abruptly to create the maximum flow friction and turbulence. The ideal labyrinth seal would transform all kinetic energy at each throttling into internal energy (heat) in each cavity. However, in practical labyrinth seals, a considerable amount of kinetic energy is transferred from one passage to the next. The advantage of labyrinth seals is that the speed and pressure capability is limited only by the structural design. One disadvantage, however, is a relatively high leakage rate. Labyrinth seals are used in so many gas sealing applications because of their very high running speed (1500 ft/s), pressure (250 psi), and temperature (>1300°F), and the need to accommodate shaft excursions caused by transient loads. Labyrinth seal leakage rates have been reduced over the years through novel design concepts but are still higher than desired because labyrinth seal leakage is clearance-dependent, and this clearance opens due to periodic transient rubs.
Applications of Labyrinth seals :
There are innumerable applications of labyrinth seals in the field. They are used to seal rolling element bearings, machine spindles, and other applications where some leakage can be tolerated. Since the development of the gas turbine engine, the labyrinth seal has been perhaps the most common seal applied to sealing both primary and secondary airflow.
Brush Seals :
The brush seal is the first simple, practical alternative to the finned labyrinth seal that offers extensive performance improvements. Basic brush seal construction is quite simple. A dense pack of fine-diameter wire bristles is sandwiched and welded between a backing ring (downstream side) and a side plate (upstream side). The wire bristles protrude radially inward and are machined to form a brush bore fit around a mating rotor, with a slight interference.
Applications of brush seal :
Brush seals are seeing extensive service in both commercial and military turbine engines. Allison Engines has implemented brush seals in engines for the Saab 2000, Cessna Citation-X, and V-22 Osprey. GE has implemented a number of brush seals in the balance piston region of the GE90 engines.
Pusher Seals :
This is a general term that is applied to mechanical seals that employ an O-ring or other elastomer or plastic seal as the floating secondary seal. They can vary from simple unbalanced designs that would be used in applications comparable to those discussed for elastomer-bellows seals, It has very specialist high-pressure, high-duty seals. The term pusher is derived from the fact that the spring force pushes the seal along the shaft or sleeve to provide the sealing contact. The most common type of secondary seal for the floating component is an elastomer O-ring
Standard O-rings for aerospace: :
1) AS 568 B: Aerospace Size Standard for O-Rings.
2) AS 5782: Aerospace Standard, Retainer, Backup Ring, Hydraulic, Pneumatic, Polytetrafluoroethylene, Resin, Uncut.
3) ISO 16032-1: Aerospace fluid systems – O-rings, inch series: Inside diameters and cross sections, tolerances and size-identification codes. Part 1: Close tolerances for hydraulic systems.
4) ISO 16032-2: Aerospace fluid systems – O-rings, inch series: Inside diameters and cross sections, tolerances and size-identification codes. Part 2: Standard tolerances for non-hydraulic systems.