Brush seals for steam turbine applications

Seal for a joint or juncture – Seal between relatively movable parts – Brush seal

Reexamination Certificate

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Details

C277S303000, C277S416000

Reexamination Certificate

active

06250640

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a brush seal assembly for use in steam turbine applications which enables the use of brush seals between the stator and the rotor without affecting the dynamic behavior of the rotor.
BACKGROUND OF THE INVENTION
In aircraft and industrial gas turbine applications, brush seals are generally used to seal between rotating and static components. Generally, brush seals are assembled with an interference fit to the rotor in order to allow the bristles to wear in to a line-on-line condition vis-a-vis the rotor. In aircraft engine applications, the rotor is typically coated with a hard coating such as chrome carbide. The rotary components in aircraft and industrial gas turbines are generally stiff. Also, the dynamic behavior of the rotary component is generally not affected by the contact between the rotary component and the brush seal, i.e., the sealing contact between the free ends or tips of the brush seal and the rotary component.
Steam turbines, however, contrary to aircraft and industrial gas turbines, are designed and run above the rotor's first bending critical frequency and often run near the second bending critical frequency. It has been found that application of brush seals in steam turbines in accordance with conventional seal design theory, i.e., with contact between the tips of the bristles and the rotary component, causes the brush seals to impart vibrations to the rotor. It is believed that this is caused by sustained rubbing between the brush seals and the rotor, particularly where the rotor has a bow. Friction from the rubbing leads to uneven temperature distribution about the circumference of the rotor, with the high spot on the rotor increasingly becoming hotter. That is, if the rotor has a bow, the proud part of the rotor is preferentially heated by increasing friction-generated heat which, in turn, further increases the bowing of the rotor with resultant increased vibration.
It has also been found that brush seals do not provide thermal relief during start-up of a steam rotor turbine. At start-up, there is very a little steam flow through the turbine and essentially no steam flow past the brush seal. Without flow, the heat generated by rubbing between the brush seal and the rotor is not dissipated, exacerbating the build-up of heat in the rotor at the seal and particularly in the proud parts. It has been discovered that contact of the brush seals with a steam turbine rotor which operates near or above critical bending frequencies generates sufficient heat to affect rotor dynamic behavior in a manner which causes serious deleterious rotor vibrations.
Another phenomena which is believed to affect the dynamic behavior of the rotor caused by contact with the brush seal is the blow-down effect. Blow-down is the radial inward flow of upstream fluid passing through the brush seal, causing the bristles to tend to move inward toward the rotor. The blow-down effect increases bristle contact pressure and rotor heating when there is initial contact between the bristles and the rotor. It reduces or eliminates initial clearance where provided, tending to increase heating. In any effort to alleviate the rubbing heat-generating frictional contact between the brush seal and the rotary component, the phenomena of blow-down must be accommodated.
BRIEF SUMMARY OF THE INVENTION
In accordance with the present invention, a brush seal is located and sized to provide a clearance between the bristle tips and the rotary component throughout the entire range of operation of the turbomachinery, particularly accommodating the effects of blow-own and dimensional changes caused by temperature changes. Thus, in all operating conditions of the turbomachinery in accordance with the present invention, the brush seal is designed to provide a minimum clearance between the tips of the bristles and the rotary component. For example, even for the highest pressure drop across the seal and the highest flow across the seal, and accommodating the phenomena of blow-down and the thermal changes in dimension of the rotor and stator, the present invention always provides a minimum running clearance between the bristle tips and the rotor. In this manner, the vibrational problems alluded to above are minimized or eliminated. For example, for a given maximum pressure difference and maximum flow across the brush seal, if the rotor has a diameter of x, and the brush seal has a radial maximum of 10 mils of blow-down at that pressure and flow, and the maximum radial excursion of the shaft is 5 mils, then the inner radius of the bristle tips at start-up should be designed to be in excess of {fraction (x/2+L )}+15 mils to maintain some clearance between the bristle tips and the rotor throughout the operating range. The decrease in sealing performance caused by the brush seal clearance as described above is minimal, while the benefits of precluding or minimizing vibrations are substantial.
In a preferred embodiment according to the present invention, there is provided turbomachinery comprising a rotary component rotatable about an axis and a stationary component about the rotary component and the axis, the rotary component having a maximum radial excursion relative to the axis throughout the entire range of operation of the turbomachinery, including from a start condition of the turbomachinery, a brush seal about the axis carried by the stationary component and having a plurality of bristles terminating in free ends spaced radially outwardly from the rotary component a radial distance relative to the axis in excess of the maximum radial excursion of the rotary component relative to the axis to maintain a radial clearance between the rotary component and the free ends of the bristles throughout the entire range of operation of the turbomachinery whereby the dynamic behavior of the rotary component is not affected by contact between the bristles and the rotary component.
In a further preferred embodiment according to the present invention, there is provided a method of installing a seal in turbomachinery having a rotary component rotatable about an axis, a stationary component about the rotary component and the axis, and a brush seal carried by the stationary component, comprising the step of providing bristles in the brush seal located to afford a clearance between tips of the bristles and the rotary component throughout the entire range of operation of the turbomachinery, including blow-down of the bristles and excursions of the turbomachinery responsive to temperature changes to prevent dynamic behavior of the rotary component from being affected by contact between the bristles and the rotary component.
In a still further preferred embodiment according to the present invention, there is provided in turbomachinery having a rotary component rotatable about an axis, a stationary component about the rotary component and the axis and a brush seal carried by the stationary component, a method of operating the turbomachinery comprising the step of operating the turbomachinery with a clearance between the bristle tips and the rotary component throughout the entire operating range of the turbomachinery to prevent dynamic behavior of the rotary component from being affected by contact between the bristles and the rotary component.
Accordingly, it is a primary object of the present invention to enable use of brush seals on turbine rotor components without affect on the dynamic behavior of the rotor whereby the turbine rotors may operate near critical bending frequencies without increased risk of rotor thermal bowing and consequent vibrations.


REFERENCES:
patent: 768593 (1904-08-01), Emmet et al.
patent: 779785 (1905-01-01), Junggren
patent: 910472 (1909-01-01), Junggren
patent: 957887 (1910-05-01), Junggren
patent: 980282 (1911-01-01), Junggren
patent: 1352277 (1920-09-01), Junggren
patent: 1352278 (1920-09-01), Junggren
patent: 1505647 (1924-08-01), Junggren
patent: 1505924 (1924-08-01), Warren
patent: 1536014 (1925-04-01), Junggren
patent: 1594838 (1926-08-01), K&eacut

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