Patent Application
20070176373
The present invention relates to spiral wound gaskets for sealing between pipe flanges. More particularly, the present invention relates to spiral wound gaskets that effect seals at low stress loads while resisting buckling of seal material during loading.
Spiral wound gaskets are well known for sealing between pipe flanges in high pressure flange joint applications. Typically such gaskets consist of an outer guide ring that is used as a compression limiter. The spiral winding or sealing element includes alternating layers of a metal band and a suitable filler material wound upon itself to form a laminated structure that is resilient in a direction perpendicular to the plane of the spiral. The outer guide ring attaches usually with a groove to the outer periphery of the wound sealing element. The outer guide ring centers the gasket within the bolt circle of the bolted flange connection, prevents over-compression of the wound sealing element, and contributes to an increase in radial strength. The outer guide rings are usually formed from carbon steel. Spiral wound gaskets install between opposed flanges of mating pipe ends. The pipe flanges clamp together with circumferentially spaced bolts or other suitable fastening arrangement.
By design, a spiral wound gasket can be compressed from its original manufactured thickness down to the outer guide ring thickness. For known spiral wound gaskets today, the original manufactured thickness is about 0.175 inches and the outer guide ring thickness is 0.125 inches. The outer guide ring functions as a mechanical stop and prevents over-compression of the sealing element. As the spiral wound gasket is compressed two things occur. The filler material compresses and as discussed below, the outer ring may become dished. First, depending upon the compressibility of the filler material, the filler itself compresses such that there is an overall reduction in the volume of the gasket element. Once the filler compresses to its “absolute density” there can be no further reduction in the sealing element volume. Further compression merely displaces the fixed volume of the sealing element.
Three predominate filler materials used in spiral wound gaskets today are mica-graphite, flexible graphite and PTFE. While both the mica-graphite and flexible graphite are compressible and allow some volume reduction within the gasket while being compressed, sintered PTFE is essentially uncompressible. The compression of a spiral wound gasket with sintered PTFE results mostly in a displacement of the original volume. However, due to the lack of control that exists with conventional gasket winding equipment, the potential compressibility that exists with the graphite filler materials is significantly reduced as the gasket is being produced. This results in the gasket being essentially incompressible even before installation in a flange.
To enhance the mechanical reliability and sealing performance of gaskets today, gaskets are installed using much higher bolt loads than were typically used in the past. These higher bolt loads overcome the resistance of the fully compressed filler/gasket element and force volume displacement as the gasket is compressed down to the thickness of the outer retaining ring. The increased loading and volume displacement can result in the gasket imploding at the inside diameter. This problem is referred to as inner buckling.
Inner buckling lends to substantial problems. First, inner buckling causes a loss of bolt load because of the stress relief that has occurred. Second, a protrusion of the gasket into the pipe bore not only creates turbulent flow, but the protrusion is also likely to break the gasket. A broken gasket may “unwind” into the flow stream and ultimately cause a total loss of seal. Further, objects called “pipe pigs” often are shot through pipes to clear scale or clogs. A pipe pig passing by a buckled gasket can break the gasket and cause the gasket to unwind and the seal to fail.
To prevent inner buckling, spiral wound gaskets include a separate inner retaining ring. Inner rings have become a requirement in national standards (ASME B16.20) on many sizes and filler styles of spiral wound gaskets to aid in resisting the distortion of the gasket in the radially inward direction. For instance, all spiral wound gaskets having PTFE as a filler material are required to have an inner ring. It is now recognized, however, that the inner ring does not prevent inward buckling. While inner rings impede the displacement or flow of the gasket into the inside diameter of the pipe, inner rings are physically unable to completely prevent this inward flow because of their narrow cross section. The inside diameter of the gasket remains as the weakest plane. Unfortunately, inner rings add considerably to the cost of the spiral wound gasket. These increased costs result from the cost of the metal itself (typically a stainless steel or exotic alloy), machining costs, labor costs to install it and finally the cost of inventorying a separate line item. Also, the fit of the inner ring within the spiral wound gasket inside diameter is often variable. Often times the inner ring falls out from the gasket during handling or shipping and that creates in persons seeking to seal flanges a sense of unreliability as to the gasket.
Another phenomena during compression is known as “dishing” of the outer guide ring. Dishing occurs when there are extreme radial forces developed during compression. The normally flat outer guide ring becomes dished, or forced into a convex or concave shape. As the ring becomes dished, still higher bolt loads must be exerted render the outer guide ring flat again so that the outer guide ring performs as a true compression stop.
As discussed above, buckling is a phenomena associated with compressible filler materials contained within the wound sealing element of traditional spiral wound gasket designs. However, buckling is necessary to establish a conformable seal within a bolted connection. The seal, however, is considered inferior to that of softer sealing elements that by nature are more conformable to flange irregularities and fill imperfections.
Expanded flexible graphite by nature is a soft conformable material that is considered one of the most advanced sealing elements due to its chemical inertness and ability to withstand elevated temperatures. When compressed or molded under high pressure, the porosity is extremely low, creating an excellent seal for applications requiring low fugitive emissions or leakage that permeates through the seal. Molded flexible graphite formed into a gasket shape, while highly conformable, lacks the rigidity or recovery associated with the spiral wound design.
Accordingly, there is a need in the art for a flange sealing gasket with the recovery performance of spiral would gaskets while providing a sealing surface readily conformable to flange irregularities. It is to such that the present invention is directed.
The present invention meets the need in the art by providing a spiral wound gasket having a resilient core comprising an elongate band spirally wrapped with overlying turns having at least portions of adjacent turns in contacting relation and an outer guide ring mounted to an outer periphery of the resilient core. An intercalated graphite overlay covering at least a portion of opposing faces of the resilient core effects conforming seals of flanged pipe connections.
Objects, features, and advantages of the present invention will become apparent from a reading of the following detailed description of the invention and claims in view of the appended drawings.
With reference to the drawings, in which like parts have like identifiers,
The sealing element 12 further includes a conformable sealing material overlay 20 that provides a conformable sealing surface 22 for bearing contact with the face of the flange to be sealed. The overlay 20 covers at least a portion of the core 16. The sealing material overlay 20 in a first embodiment illustrated in exploded perspective view in
Each of the annular rings 23 is sized with an inner diameter and an outer diameter for being received on the core 16 of the sealing element 12. The rings attach to the opposing faces of the core. The rings 23 attach mechanically by being pressed into place and engaging the edges 18 of the metallic band forming the core 16. The sides 18 enter into the ring and portions of the sealing material fills the gaps 19 between adjacent sides 18. In alternate embodiments, the rings also attach with an adhesive 25 (illustrated on one of the rings 23 in
In another embodiment illustrated in
In this embodiment, the opposing overlay 20 are formed in a mold. A plurality of the intercalated graphite vermiform communicate into a first cavity of the mold. An intermediate gasket assembly made of the core 16 and the outer ring 14 is placed in the mold. Additional intercalated graphite vermiform communicate into the mold on the opposing side. The mold is then operated in order to compress the intercalated graphite vermiform together and sandwich the core 16. The overlay 20 is thereby molded at a first density but has remaining capacity to compress further during installation to a second density greater then the first density.
The molded overlay 20 mechanically engage the sides 18 with a portion of the intercalated graphite vermiform filling the gaps 19. The resilient material of the sealing element accordingly only partially fills the interstices between adjacent turns of the core 16. The spiral core 16 has contacts between adjacent turns of the elongate band. The resilient seal material does not extend transversely through the core 16 between the opposing faces defined by the edges of the sides 18 of the band.
The overlay 20 provided in sheet form as a ring (
The thickness of the bands 46, 48 can be the same or can differ. In the illustrated embodiment, the thickness of the first band 46 is less than the thickness of the second band 48. The thickness of the bands used for the core 16 and core 44 are typically about 0.007 inches; however, the thickness of the band ranges from about 0.005 inches to about 0.0125 inches thick. The width of the band is typically about 0.150 inches, although the width can range between about 0.125 inches to about 0.200 inches. Metal is preferred for the bands as providing a hard dense and non-compressible material for forming the spiral core.
A gasket made in accordance with the present invention was subjected to stress load testing to evaluate inner buckling. The test gasket was a 10-inch, Class 150 spiral wound gasket having an overlay 20 made by molding a plurality of intercalated graphite vermiform 27 as discussed above. For comparison purposes, a LEADER standard spiral wound gasket meeting ASME standard B16.20 was also tested. This gasket had sheet graphite filler material between the turns in the spiral core and as the overlay. The test evaluated the inner buckling of the gaskets after loading the bolts to three stress levels by measuring the deflection (in inches) at the bolt locations.
It was observed that the LEADER gasket experienced inner buckling occurred at several locations. In contrast, no buckling was measured or observed for the test gasket made in accordance with the present invention.
In addition to reduced or eliminated inner buckling, the present invention provides improved sealability during cycling of stress loads, based on tests that included a corrugated metal gasket with graphite jacketing layer, a LEADER standard spiral wound gasket, and other commercially available spiral wound gaskets. The corrugated metal gasket with graphite jacketing layer was tested because this product has been found to have superior recovery and sealing capability during gasket stress load cycles. Leakage from the seated flange connection was measured at the maximum psi load and at the minimum psi load in five cycles. The low-stress anti-buckling spiral wound gasket of the present invention had performance comparable to the corrugated metal gasket with graphite jacketing. The spiral wound gasket of the present invention had recovery performance superior to the other spiral wound gaskets in the tests.
The present invention accordingly combines the rigidity and recovery advantages of spiral wound gaskets with the conformability of soft sealing materials. The layer of flexible graphite over the outer faces of the spiral wound gasket sealing element (rather than layering them alternately with a filler or sealing material), creates a superior seal by eliminating the issues of non-conformity that is characteristic of traditional spiral wound gasket technologies. The layer of flexible graphite is extremely non-porous and creates a seal that has very low permeability. Eliminating the filler materials and winding only the band to form the core of the sealing element, greatly reduces or eliminates the possibility of inward buckling. The absence of a compressible sealing material that is subject to shifting prevents an extreme deformation of the sealing element or inward buckling. The volume reduction is consumed by the void or area between the two overlay 20 layers of sealing material.
The present invention accordingly provides an apparatus and method for forming improved spiral wound gaskets. The principles, preferred embodiments, and modes of operation of the present invention have been described in the foregoing specification. The invention is not to be construed as limited to the particular forms disclosed because these are regarded as illustrative rather than restrictive. Moreover, variations and changes may be made by those skilled in the art without departure from the spirit of the invention as described by the following claims.
