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Reverse Pressure Capable Finger Seal

Finger seals are a contacting air-to-air seal technology that allows for low leakage in high-speed and high-temperature operating conditions.

Each finger seal consists of a number of multiple thin metal laminates, each with a multitude of flexible projections that are referred to as “fingers.” To form a functional seal, multiple laminates are formed into a stack, with each laminate oriented so that the slots between its fingers cover the slots of the neighboring laminates. For protection and support of the laminates, forward and aft cover plates are placed on each side of the finger seal stack. A high pressure exists on one side of the seal, providing a driving force to push the air from one side of the seal to the other.

In the Reverse Pressure Capable Finger Seal, two stacks of finger seals are installed in line with each other. At either end, cover plates with their own pressure balance cavities and dams are installed. When a cavity on one side of the seal is pressurized higher than the other, the laminates on that side will be deflected away from the dam by the pressure-induced axial load, allowing air to flow through the pressure balance circuit to the cavity between the seals. This pressure then acts on the second set of laminates, which is pushed into its cover plate, limiting leakage through the pressure balance circuit.
Each finger of the seal remains in light contact with the rotating shaft, limiting leakage that can pass between the seal and shaft. The pressure acting on the face of the upstream laminate can cause axial loading of the fingers against the downstream plate. In some seals, this axial loading is high enough to result in a high amount of friction between the downstream laminate and aft plate, as well as between the laminates themselves. This friction restricts the free motion of the fingers, preventing them from remaining in contact with the shaft, and harming the seal’s performance. To reduce this pressure-induced friction, a design has been created that includes a pressure balance circuit.

As pressure is applied to the finger seal, the laminates are pressed back into the dam near the inner diameter of the aft (downstream) plate. This creates a sealed cavity between the aft plate and downstream-most laminate, where air from the high-pressure side can be routed, which reduces the axial loading that acts on the laminate stack.

While the inclusion of this pressure balance circuit has improved the finger seal, it has caused the seal to be functional in only a single direction. During pressure loading in the design direction, the laminates of the finger seal are pressed into the dam by the pressure-induced axial load, sealing the pressure balance cavity, except for a small flow that leaks down between the finger slots. The overall leakage of the seal is low, and the majority of air passing through the seal occurs between the shaft and laminates. For reverse pressure loading, the pressure-induced axial load pushes the laminates away from the dam on the aft plate, opening an area for leakage. This allows a large amount of leakage to flow into the pressure balance cavity, through the pressure balance circuit, and to the low-pressure side. The overall leakage for a reverse pressured seal will be high, with the majority of leakage passing through the pressure balance circuit. To prevent this high leakage when pressure is reversed, the pressure-balanced finger seal design can only be used in applications where the pressure will never reverse.

To extend the benefits of finger seals to applications that may have reversing pressures, a new design is required that prevents air from traveling directly from the low side to the high side by passing through the pressure balance circuit. This can be accomplished by designing a seal that includes two separate stacks of laminates, oriented so that air passing between the two cavities on either side of the seal must pass through both sets of laminates. A cavity exists between the two sets of laminates and a plate may be installed between them. This intermediate plate between each stack of laminates is optional, and omitting it does not change the overall function of the seal. Cover plates are installed at either end of this seal, which each contains a pressure balance cavity and dam.

When pressure is applied to the reverse capable finger seal design, the upstream set of laminates (the set of laminates on the side of the seal facing the high pressure) is pushed away from the dam on its cover plate. As air leaks past the upstream laminates, it flows into the pressure balance cavity, through the pressure balance circuit, and into the cavity between the two sets of laminates, causing it to pressurize. Due to the rising pressure in the cavity between the sets of laminates, the downstream set is axially loaded into the dam on the downstream cover plate.

The direction of pressure applied to the seal can be reversed, and the two sets of laminates will also reverse roles, with the set formerly acting as the upstream laminates becoming the downstream, and vice versa. In this reversed pressure case, the one set again will be pushed away from its dam, allowing leakage to occur, while the other set will be pressed into the dam on its cover plate. This allows the seal to function equally well in either direction.

This work was done by Nathan Gibson and Joe Yanof of Honeywell International for the Air Force Research Laboratory. AFRL-0215

Topics:
Design processes Machinery Mechanical Components Metals Parts Pressure Research and development Seals and gaskets
This Brief includes a Technical Support Package (TSP).
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Reverse Pressure Capable Finger Seal

(reference AFRL-0215) is currently available for download from the TSP library.

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    Defense Tech Briefs Magazine

    This article first appeared in the June, 2012 issue of Defense Tech Briefs Magazine (Vol. 6 No. 3).

    Read more articles from this issue here.

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    Overview

    The document titled "Reverse Pressure Capable Finger Seal" discusses advancements in finger seal technology, particularly focusing on a design that allows for effective sealing in both directions of pressure. Authored by Nathan Gibson and Joe Yanof from Honeywell International, Inc., the paper addresses the limitations of traditional finger seals, which are typically designed to operate with a unidirectional axial pressure differential. This limitation restricts their application in environments where the pressure may reverse, such as in certain gas turbine engines.

    Finger seals are a type of contacting air-to-air seal technology that utilizes multiple thin metal laminates, each featuring flexible projections known as "fingers." These seals are designed to minimize leakage in high-speed and high-temperature conditions. The document outlines how a stack of laminates is arranged so that the slots between the fingers of one laminate cover the fingers of adjacent laminates, creating a functional seal. The design includes a pressure balance circuit that helps reduce friction caused by pressure-induced axial loading, which can impair the seal's performance.

    The authors propose a novel design that incorporates two separate stacks of finger seal laminates oriented in opposing directions. This configuration allows one set of laminates to act as upstream while the other serves as downstream, effectively enabling the seal to function regardless of the direction of pressure. The paper also discusses potential design considerations for reverse pressure capable finger seals, such as varying the diameters of the laminates, omitting intermediate plates, and using laminates with different stiffness characteristics.

    The research highlights the advantages of finger seals over traditional labyrinth seals, which tend to exhibit higher leakage rates and more wear due to rubbing against the seal land. By improving the design of finger seals to accommodate reverse pressure, the authors aim to enhance their applicability in various engineering contexts, particularly in aerospace applications where reliability and efficiency are critical.

    Overall, the document presents a significant step forward in sealing technology, offering insights into the design and functionality of reverse pressure capable finger seals, which could lead to improved performance in gas turbine engines and other high-demand applications.

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