Seal for a joint or juncture – Seal between relatively movable parts – Circumferential contact seal for other than piston
Reexamination Certificate
1999-11-15
2001-08-14
Knight, Anthony (Department: 3626)
Seal for a joint or juncture
Seal between relatively movable parts
Circumferential contact seal for other than piston
C277S536000, C277S539000
Reexamination Certificate
active
06273431
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to stuffing box packings, and more particularly to a three-ring stuffing box packing set for forged valves.
2. Background of the Related Art
Stuffing box packing rings for sealing moving shafts are well known in the art. In general, they are used both as a fixed bearing and to prevent leakage in a stuffing box. Stuffing boxes are commonly employed with reciprocating shafts such as piston rods and rotating shafts such as valve stems.
Packing rings are typically formed from materials that have a capacity to prevent leakage, are non-abrasive and have a low coefficient of friction. A particularly effective material of construction for stuffing box packing rings is flexible graphite tape as it satisfies each of these criteria. Flexible graphite tape has one negative attribute however, in that it tends to extrude through fine clearances as a result of heavy compressive forces that are applied during packing installation. In the past, to control and prevent extrusion, packing assemblies have included anti-extrusion end rings which are often formed from braided packing materials, or the like.
An example of an extremely effective prior art stuffing box packing system for sealing the stuffing box of a valve stem is disclosed in U.S. Pat. No. 4,328,974 to White et al. the disclosure of which is herein incorporated by reference in its entirety. The packing system comprises first and second sealing assemblies separated from one another and supported between flat rigid spacer rings. Each sealing assembly includes a low density graphite preform ring and a higher density die-forned graphite adapter ring. Each preform ring has a density of about 1.1 g/cc to about 0.7 g/cc and each adapter ring has a density of about 1.4 g/cc to about 1.7 g/cc. The facing surfaces of the rings are each oriented at an acute angle to the longitudinal axis of the packing, with the angle in one surface being about 45° and the other being about 60°.
During packing installation, the differing densities and the difference in angles between the facing surfaces of the high density adapter rings and the low density preform rings provide a predictable stress, that causes the material to flow and expand in a controlled manner through deformation, forming an effective seal around the valve stem. The packing assembly disclosed in the White et al. patent also includes anti-extrusion braided packing rings which act as wipers to strip graphite particles from the valve stem and absorb them into the braid.
A more recent example of a stuffing box packing assembly for valve stems, shafts and piston rods is disclosed in U.S. Pat. No. 5,806,858 to Harrelson, III, the disclosure of which is also herein incorporated by reference in its entirety. The packing is a five-ring assembly consisting of three nested graphite rings of equal size rigidly supported between two braided anti-extrusion end rings. The graphite rings are die-formned from flexible graphite tape and have a density of about 0.5 g/cc to about 1.4 g/cc, and the braided end rings are die-formed from reinforced braided-stock and have a density in excess of 1.8 g/cc. The lower density graphite rings have facing surfaces oriented at a 45° angle, and the higher density end rings have facing surfaces oriented at 60° angles, so as to cause the lower density material to flow and expand during installation. Advantageously, the braided end rings, being relatively rigid, are unaffected by the compressive forces applied during packing. They serve as wiper rings to remove particles of graphite from the shaft or valve stem and provide extrusion protection for the packing.
Industrial piping systems generally employ several different types of valves for performing various fluid control tasks. Some industrial valves are relatively expensive and are often reconditioned after a certain period of service. Other valves, such as forged valves, are less expensive, and are generally discarded after a certain period of service due to wear. In such instances, the use of relatively expensive valve packing sets, such as the eleven and seven-ring packing sets disclosed in U.S. Pat. No. 4,328,974 to White et al. or the five-ring packing set disclosed in U.S. Pat. No. 5,806,858 to Harrelson, III, are not economical. Indeed, in such instances such complex valve packing sets may cost as much or more than the valve itself.
Therefore, it would be beneficial to provide an inexpensive stuffing box packing set for use in conjunction with a forged valve stem, or the like. Such a packing set would not require the use of anti-extrusion rings, wiper rings, resiliency rings, spacer rings or the like.
SUMMARY OF THE INVENTION
The subject invention is directed to a stuffing box packing set for a forged valve stem which does not require specialized anti-extrusion components. Accordingly, the valve stem packing set of the subject invention does not make use of anti-extrusion end rings, wiper rings, resiliency rings, or the like. This simplifies the design of the packing set and significantly reduces its cost as compared to prior art stuffing box packing sets.
The stuffing box packing set of the subject invention preferably includes three packing rings including an upper adapter ring formed from reinforced graphite tape, having a first density and a first axial thickness, an intermediate preform ring formed from graphite tape, having a second density less than the density of the upper adapter ring and a second axial thickness less than the axial thickness of the upper adapter ring, and a lower adapter ring formed from reinforced graphite tape, having a density equal to the density of the upper adapter ring and having an axial thickness equal to the axial thickness of the upper adapter ring.
Preferably, the upper and lower adapter rings each have a density of about approximately 1.1 g/cc to about approximately 1.92 g/cc, and the intermediate preform ring has a density of about approximately 0.7 g/cc to about approximately 1.4 g/cc. In a preferred embodiment of the subject invention, the intermediate preform ring has opposed upper convex and lower concave facing surfaces that are inclined at acute angles with respect to the axis of the stuffing box. Likewise, the upper adapter ring has a lower concave facing surface inclined at an acute angle with respect to the axis of the stuffing box and the lower adapter ring has an upper convex facing surface inclined at an acute angle with respect to the axis of the stuffing box.
Preferably, the angle of inclination of the facing surfaces of the intermediate preform ring differ from the facing surfaces of the upper and lower adapter rings. In particular, the lower facing surface of the upper adapter ring and the upper facing surface of the lower adapter ring are inclined at an angle of about approximately 30°. Furthermore, the upper and lower facing surfaces of the intermediate packing ring are inclined at an angle of about approximately 45°.
In accordance with a preferred embodiment of the subject invention, the upper and lower adapter rings are formed from reinforced graphite tape, wherein the tape is reinforced with fibers or filaments (strands) selected from the group consisting of metallic filaments or fibers, inorganic fibers or filaments, and organic filaments or fibers. In instances wherein the graphite tape is reinforced with metallic fibers or filaments, they may be stainless steel fibers or filaments, such as, for example, Inconel® fibers or filaments. In instances wherein the graphite tape is reinforced with inorganic fibers or filaments, they may be glass fibers or filaments. In instances wherein the graphite tape is reinforced with organic fibers or filaments, they may be aramid fibers or filaments. In each case, the reinforcing fibers or filaments perform an anti-extrusion function.
These and other features of the forged valve packing set of the subject invention will become more readily apparent to those having ordinary skill in the art from the following detailed description of the inventio
Cummings & Lockwood
Garlock Inc
Knight Anthony
Pickard Alison K.
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