Stock material or miscellaneous articles – Circular sheet or circular blank – Seal – gasket – or packing
Reexamination Certificate
1999-08-11
2002-11-26
Ahmad, Nasser (Department: 1772)
Stock material or miscellaneous articles
Circular sheet or circular blank
Seal, gasket, or packing
C277S407000, C277S532000, C277S540000, C277S626000, C428S064100, C428S066600, C428S066700, C428S068000, C428S071000, C428S076000, C428S316600, C428S421000, C428S422000
Reexamination Certificate
active
06485809
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to gaskets and, more particularly, to a gasket that forms a seal under less stress than required with existing gaskets.
BACKGROUND OF THE INVENTION
A wide variety of gaskets are known for use in sealing applications. Expanded polytetrafluoroethylene (PTFE) is widely used today as a gasket material. As disclosed in U.S. Pat. No. 3,953,566 to Gore, this material has numerous properties making it highly desirable as a gasket. These properties include being readily compressible and conformable, being chemically resistant, having relatively high strength, and being far less prone to creep and loss of sealing pressure than non-expanded full density PTFE alone.
In many sealing applications, the gasket is used to seal the junction between flanges, such as between pipes. In such applications, expanded PTFE is a desirable material for the gaskets because the expanded PTFE gasket can be placed between the flanges, and the flanges can then be pressed together with the application of force, such as by tightening of bolts. This application of force compresses the expanded PTFE. As the expanded PTFE is compressed, its initial pore volume is reduced, thus densifying the expanded PTFE. Particularly with metal-to-metal flanges, it is possible to apply sufficient force (or “stress”) to the flanges to fully densify the expanded PTFE. Thus, in at least part of the expanded PTFE gasket, the pore volume is reduced to substantially zero, such that a fluid contained within the pipes is prevented from leaking between the flanges by the densified, non-porous PTFE gasket, which seals the flanges.
In many applications, particularly when harsh chemicals are used which would readily break down the metal or the metal could contaminate the chemical which is being transported or housed, it is common to use glass-lined steel, glass, or fiberglass reinforced plastic (“FRP”) piping and vessels. Because this equipment is so often used with extremely harsh chemicals, there is great desire to use PTFE gaskets to seal the connecting flanges of this equipment because of the well known extraordinary chemical resistance of PTFE. Unfortunately, non-expanded full density PTFE gaskets are generally not conformable enough to effectively seal this type of equipment. In the case of glass-lined steel flanges, although there is a relatively smooth finish, there is often a large amount of unevenness or lack of flatness associated with the flanges. This unevenness or lack of flatness requires the gasket to have to conform to large variations around the perimeter as well as between the internal and external diameter of the flange in order to create an effective seal. Thus, a non-expanded full density PTFE gasket is not conformable enough to seal many of these applications.
Because expanded PTFE is so conformable, it would be desirable to use expanded PTFE to seal these commonly uneven flanges. Unfortunately, in many of these applications it is not possible to apply sufficient force to the flanges to create enough gasket stress to fully densify the expanded PTFE gasket to create an effective seal. For example, glass-lined steel piping flanges, glass flanges, or FRP piping flanges may deform, fracture, or break upon the application of a high amount of stress. Thus, in these applications, an expanded PTFE gasket may not be completely densified to reach a non-porous state, and therefore does not become leak proof, because the maximum stress that can be applied to the flanges without breaking them is not sufficient to so densify the gasket.
In many cases, it is not only necessary to be able to seal the actual fluid being housed or transported, but it is additionally necessary for the gasket to provide an air tight seal which can pass what is commonly known in the industry as a “bubble test”. It is common to run this type of test as a pre-start-up qualifying test for checking for leaks in piping systems before allowing the system to be used in production carrying the actual fluid for which it was intended. In this test, the gasketed piping systems are pressurized with air and then sprayed with soapy water. The pipe and flange assemblies are visually checked for bubbles appearing in the soapy water indicating air leakage. All leakage sites must be eliminated to pass the bubble test.
Thus, what has been desired for many years is an easy-to-use highly chemically resistant gasket, which can effectively conform and provide an air tight seal for this equipment with the low loads or stresses that are available to create the seal.
There have been many attempts to provide a gasket that can effectively seal these difficult applications. Most of these attempts involve a two-piece gasket. These gaskets are commonly referred to as envelope gaskets. In most envelope gaskets, an outer envelope of PTFE is formed and is then separately filled with a more compressible filler material such as compressed asbestos or other felted gasket filler, an elastomer or plastic material, or a corrugated ring of metal, usually stainless steel. The basic concept is the PTFE jackets for the envelope gaskets provide chemical resistance while conformability is provided by the filler material.
Unfortunately, as explained in U.S. Pat. No. 4,900,629 to Pitolaj, envelope gaskets are subject to a number of disadvantages. The envelope jacket often will fold over on itself during installation of the gasket, thereby creating creases in the gasket that cause leaks. Also, there may be pin hole leaks in the envelope itself, causing corrosive material to attack the envelope filler resulting in degradation of the filler. When the filler degrades, sealing stress can be diminished, causing a leak to occur. Another problem, which can result, is that the degraded filler material can contaminate the fluids that were contained within the pipe or vessel. In some instances, the envelope jacket of PTFE will separate from the conformable filler material and ripples or folds may occur merely from stretching the envelope over the filler, again causing leaks to occur. Also, if uneven flange torquing occurs, the jacket may become overstressed and burst, once again allowing the corrosive material to attack the filler resulting in degradation of the filler and loss of the seal. Another problem is that these envelope gaskets are also subject to cold flow or creep, which requires periodic bolt retorquing.
In U.S. Pat. No. 5,195,759 to Nicholson, an envelope gasket is employed with a PTFE envelope within which is an elaborate metal filling consisting of wound or nested turns of thin metal strips perforated to provide resilience in the direction of their width. Individual turns can move or collapse to different extents, thereby accommodating lack of flatness of the surfaces to be sealed. Turns of fluid-impervious material may be distributed among the turns of the perforated strips. Although the gasket has some advantages, it still suffers from many of the disadvantages mentioned above associated with envelope gaskets, such as chemical attack of the metal filling under certain conditions.
In U.S. Pat. No. 5,558,347 to Nicholson, a gasket is disclosed comprising an envelope of chemically resistant PTFE and a metallic packing ring within the envelope is shaped to form cells. The cells may be filled with an inert gas under pressure so that increased loads on the gasket may be cushioned. Although this gasket also has some advantages, it still suffers from many of the same disadvantages mentioned above associated with envelope gaskets.
In Japanese Laid-Open Patent Application Number 4-331876 to Ueda et al., another envelope (jacket) gasket is proposed in which the outer periphery of a core composed of low-density porous PTFE that has been fibrillated (expanded) and has a density of 1.8 g/cc or less is covered with a sheath composed of high-density sintered PTFE. Although this gasket has the benefit of being 100% PTFE, and therefore does not suffer the chemical attack problems resulting from pinhole leaks in the outer envelope, it can still suffer from the a
Dove Kevin Edward
Egres, Jr. Ronald G.
Hisano Hirokazu
Mills David John
Minor Raymond Bryant
Ahmad Nasser
W. L. Gore & Associates GmbH
Wheatcraft Allan M.
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