Abstract:
A method is provided for forming a gasket for sealing opposing flange surfaces of a pipe having corrosive fluid flowing therethrough. The method comprises defining a gasket profile incorporating a serrated profile core having a flange extending radially inward therefrom. A deformable pillow extends radially inward from the serrated profile core, to define a gasket intermediate portion about the flange, and a gasket inner portion radially inward from the flange. The deformable pillow material and thickness are selected such that upon compression to a thickness no less than the core thickness, the gasket inner portion exhibits a stress level sufficient to preclude corrosive liquid flowing through the pipe from passing radially outward beyond the gasket inner portion, and the gasket intermediate portion exhibits a stress level sufficient to preclude gas and liquid flowing through the pipe from passing radially outward beyond the gasket intermediate portion.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of application Ser. No. 13/051,188 filed Mar. 18, 2011, which is herein incorporated in its entirety by reference. 
    
    
     STATEMENT RE: FEDERALLY SPONSORED RESEARCH/DEVELOPMENT 
     Not Applicable 
     BACKGROUND 
     The present invention is directed to a method for designing a gasket for fluid sealing for severe acid environments, e.g. in alkalization plants. 
     Sealing gaskets have been used in a variety of different applications. The construction of such gaskets is typically the function of the application, the environment in which the gasket will be utilized, the stress load and other factors. Where gaskets are intended to be used in severely corrosive environments, the gasket construction will preferably also take into account the location of the seal(s), the need for a plurality of seals to isolate the primary seal from the corrosive environment, and the need to seal the imperfections on opposing surfaces of the pipe flanges, caused by erosion from the corrosive material following through the pipe. Such imperfections are most common near the inner radius of the pipe flanges, which is closest to the corrosive material flowing through the pipe. Consequently, it is preferable to locate the primary sealing element away from the inner radius of the gasket, where the adjacent surface of the flanges is less likely to bear such imperfections. 
     The particular gasket material is also a factor that is affected by application and environment factors. A softer, more readily conformable material can be useful to seal imperfections in the flange surfaces near the flange inner radius. However, such softer material may be less suitable to provide a proper seal (of sufficient stress load) to isolate the primary seal from the corrosive material flowing through the pipe. 
     Further, many sealing elements operate most effectively at relatively high stress levels, e.g. 20,000 to 30,000 psi or more. However, the greater the load bearing surface of the gasket, the more distributed the gasket stress, and the lower the stress per square inch. 
     Accordingly, to achieve the optimum gasket design, consideration must be given to not only the general gasket architecture, bolt load, desired gasket stress, and environmental factors, but must also consider the need for the different gasket components to function cooperatively in a specific application. As such, the size, material, and functional characteristics of the individual gasket elements must be carefully engineered for cooperative interaction. Because different portions of a multi-element gasket may have different load bearing stress characteristics, sealing characteristics and compressibility, the gasket design is preferably optimized for a load specific application, allowing each element to contribute to the functionality of the gasket, without degrading the contribution of other elements. 
     The present invention is directed gasket design process, and a multi-zone gasket architecture, wherein each zone optimized for a specific function by a design process that produces a gasket that, under a normal load, can produce a consistent gasket sealing stress of 20,000 psi or more on the primary sealing element, which is effectively sealed from the corrosive environment, while also being sufficiently soft to seal imperfection in the flange surfaces. 
     These and other features, objects and advantages of the invention are described below, in conjunction with the illustrated embodiments. 
     BRIEF SUMMARY 
     These and other features and advantages of the various embodiments disclosed herein will be better understood with respect to the following description and drawings, in which like numbers refer to like parts throughout. 
     A method is provided for forming a gasket for sealing opposing flange surfaces of a pipe having corrosive fluid flowing therethrough. The method comprises defining a gasket profile incorporating a serrated profile core, the core defining a core thickness. A flange is formed extending radially inward from the serrated profile core. A deformable pillow is formed extending radially inward from the serrate profile core, about the flange, to define a gasket intermediate portion. The deformable pillow further extends radially inward from the flange to define a gasket inner portion. The deformable pillow material and thickness are selected such that upon compression to a thickness no less than the core thickness, the gasket inner portion exhibits a stress level sufficient to preclude corrosive liquid flowing through the pipe from passing radially outward beyond the gasket inner portion, and the gasket intermediate portion exhibits a stress level sufficient to preclude gas and liquid flowing through the pipe from passing radially outward beyond the gasket intermediate portion. 
     The serrated profile sealing element may be defined as the primary gasket seal, i.e. the gasket portion bearing the highest portion stress level when a specified bolt load is applied to the gasket. 
     The characteristics of the intermediate portion may be varied by varying the flange thickness to provide a d desire gasket intermediate portion stress level when the gasket intermediate portion is compressed to a thickness substantially corresponding to the core thickness. The gasket intermediate portion may function as second primary seal, as it is preferably engineered to preclude the flow liquid and gas from passing radially outward beyond the gasket intermediate seal. Additionally, in another embodiment, the flange thickness can be regulated such that the minimum intermediate zone minimum stress level is no more than 21% of the gasket stress level. 
     The deformable pillow and pillow thickness may also be varied to provide a desired gasket inner portion stress level when the gasket inner portion is compressed to a thickness substantially corresponding to the core thickness. 
     The gasket core may be formed of fireproof or fireproof resistant material, upon application of a designated bolt load, results in a primary seal, which support approximately 80% of the total gasket stress level. 
     The distribution of stress levels on the gasket inner seal, and the gasket intermediate seal may be varied by regulating the various parameters such as the deformable pillow, thickness and length, the flange thickness and length and the core thickness and length. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features and advantages of the various embodiments disclosed herein will be better understood with respect to the following description and drawings, in which like numbers refer to like parts throughout. 
         FIG. 1  is a front perspective view of a fluid sealing gasket in accordance with the present invention; 
         FIG. 2  is a top view of the gasket shown in  FIG. 1 ; 
         FIG. 3  is a sectional view of the gasket; 
         FIG. 4  is an enlarged sectional view of a portion of the gasket shown in  FIG. 3 ; 
         FIG. 5  is a cut away view of the gasket; 
         FIG. 6  is an enlarged sectional view of the gasket, showing different zones, each having different stress characteristics; 
         FIG. 7  is a graph depicting the load characteristics of each gasket zone, for a 150 class gasket; 
         FIG. 8  is a graph depicting the load characteristics of each gasket zone, for a 300 class gasket; 
         FIG. 9  is a chart listing the gasket stress and distribution of stress for the graphs of  FIG. 7 ; 
         FIG. 10  is a chart listing the gasket stress characteristics of each gasket zone for the graph of  FIG. 8 ; 
         FIG. 11  is a graph depicting compression characteristics of the pillow material used in the gasket; and 
         FIG. 12  is a graph depicting the compression characteristics of different gaskets. 
     
    
    
     DETAILED DESCRIPTION 
     The description below is given by way of example, and not limitation. Given the disclosure, one skilled in the art could devise variations that are within the scope and spirit of the invention disclosed herein. Further, the various features of the embodiments disclosed herein can be used alone, or in varying combinations with each other and are not intended to be limited to the specific combination described herein. Thus, the scope of the claims is not to be limited by the illustrated embodiments. 
     Referring to the drawings, the presently preferred embodiment of a fluid sealing gasket is illustrated therein.  FIG. 1  illustrates the general architecture of fluid sealing gasket  10 , including an outer guide ring  11 , a serrated profile sealing element  13 , and a deformable inner barrier pillow  15 . The inside surface of the deformable inner barrier pillow  15  defines an inner gasket surface  25 . In use, the fluid sealing gasket  10  is disposed between a pair of opposing pipe flanges  12 ,  14 , as shown in  FIG. 3 , and seals the connection between the flanges. 
     Serrated profile gaskets are used in many seals due to exceptional sealability and reliable performance. These profiles work by having a solid serrated body, or core  17 , with a flexible covering layer or facing  21 . The serrations minimize lateral movement of the facing material, while the metal alloy core provides rigidity and blowout resistance. The arrangement allows for a very high compression and an extremely tight seal along the ridges of the gasket. 
     The serrated profile gasket offers a safe, effective seal under the most exacting conditions on both standard pipe work and specialized applications. It offers excellent sealability and recovery characteristics, allowing seal integrity under pressure and temperature fluctuation, temperature differentials across the flange face, flange rotation, bolt stress relaxation and creep. 
     The serrations concentrate load on a smaller area for a tight seal at lower stress. Under compression the sealing material fills the surface imperfections to form a tight connection that can withstand extreme fluctuations in temperatures and pressures. The serrated profile gasket commonly handles pressures from vacuum to Class 2500, and withstands temperatures from cryogenics to 2000° F. (1090° C.), depending on sealing material and metal. 
     More detail regarding the construction of the gasket  10  is illustrated at  FIGS. 4 and 5 . As shown therein, the deformable barrier pillow  15  defines a pillow thickness, t p , that is greater than the serrated profile sealing element  13  thickness, t sp , or the outer guide ring  11  thickness, t or . However, as explained in more detailed below, the pillow  15  is formed of a relatively soft, compressible material, such that the pillow material may be compressed to a thickness substantially equal to the thickness of the serrated profile sealing element  13 , t sp . 
     Graphite sealing facing  13  extends over/across the serrated core  17 . Upon compression, the graphite facing  13  is compressed against and fills in the recesses defined by the core serrations  21 . The flange, or inner attachment ring  19 , is engaged to and extends from the serrated core  17 . In the presently preferred embodiment, the serrated core  17  and flange  19  are formed from a single piece of metallic alloy, such as the family of alloys sold under the Monel trademark, but can be made of carbon steel or other metal compatible with the application. 
     Monel is a trademark of special metals corporation for a service of nickel alloys, primarily composed of nickel (up to 67%) and cooper, with some iron and other trace elements. Monel&#39;s good resistance against corrosion by acids and oxygen makes Monel a good material for use in highly corrosive environments. In the presently preferred embodiment Monel 400 is used in the construction of the serrated sealing element  17 . 
     The inner expanded PTFE barrier pillow  15  envelops the flange  19  and abuts against the core  17  and graphite facing  13 . 
     Upon compression of the gasket  10 , the inner expanded PTFE barrier pillow  15  and the graphite facing  13  are compressed to an orientation that is substantially coplanar with the thickness of the serrated profile sealing element  13 , and defines three sealing regions along the gasket  10 . 
     As explained further below, the particular material used form the barrier pillow  15 , and the particular thickness of the barrier  15  may be selected based upon the particular application, the bolt load applied to the gasket as the flanges are connected, the compression and sealing characteristics of the pillow material, and the construction and desired distribution of stress levels among the inner, intermediate and outer sealing zones of the gasket  10 . 
     As shown in  FIGS. 4 and 6 , flange  19  extends from core  17  into the pillow  15 . Flange  19  defines a flange thickness, t f  and a flange length, l f . As explained further below, the portion of pillow  15  through which flange  19  extends (intermediate Zone B) exhibits different characteristics in response to compression between the opposing pipe flanges  12 ,  14  (shown at  FIG. 3 ). Upon compression, the stress levels on the inner portion of pillow  15 , through which flange  19  extends (intermediate Zone B), are substantially greater than the stress levels on that portion of pillow  15  between flange  19  and the gasket inner surface  20  (inner Zone A). This is because the pillow material in intermediate Zone B experiences a higher percent compression than the pillow material in inner Zone A. The higher the compression, the higher the resulting stress level. 
     As shown in the embodiment illustrated at  FIGS. 3 and 4 , the fluid sealing gasket  10  defines a gasket width, of 1.1525 inches. The outer guide ring  11  extends approximately 30% of the gasket width. The serrated profile core  17  extends approximately 38% of the gasket width. The flange  19  extends approximately 32% of the gasket width. Consequently, in accordance with the present invention the main sealing element of the gasket, i.e. the core, extends over less than half of the gasket area. The size and location of the serrated core mitigates buckling problems and breaking associated with the conventional spiral-wound gaskets that contain Teflon inner rings, and enhances the ability of the gasket to compensate for relaxation that may occur. 
     In the same embodiment the exposed portion of the outer guide ring extends approximately 0.3437 inches. The unexposed portion of the outer guide ring extends approximately 0.0625 inches into the serrated profile core. The outer guide ring has a thickness that is also approximately 0.0625 inches. 
     The serrated profile core has a length, l f , of approximately 0.4375 inches, and a thickness, t c , of approximately 0.125 inches. The serrated profile core is preferably formed of a Monel metallic alloy. The serrated profile core flexible covering layer or facing has a thickness of approximately 0.020 inches and is preferably formed of APX2 Graphite, i.e., a flexible graphite material. The facing extends along the serrated profile core  17 . 
     The length of the flange, or inner guide ring  19  extends approximately 16% of the gasket width, i.e. approximately 0.18565 inches. The flange is preferably formed to have a width, w f , of approximately 0.03125 inches and a thickness t f  of approximately 0.069 inches. 
     In the preferred embodiment, the flange  19  and the serrated profile core  17  may be formed as from a single, uninterrupted piece of Monel alloy material.  FIG. 5  illustrates a construction wherein the serrated profile core and the flange are formed of a single piece of material  20 . However in other constructions, the flange and the serrated profile core may be separately formed and connected. 
     In one preferred embodiment, the inner barrier pillow  15  preferably is formed to extend to approximately 32% of the width of the gasket, i.e., approximately 0.3713 inches. The barrier pillow has a thickness, t p , of approximately 0.250 inches and defines a slot to receive the inner guide ring. The inner barrier pillow is formed preferably of expanded PTFE, which is machined and attached to the flange/serrated profile body, by a process in which the barrier pillow is first machined and then attached to the flange in a manner to assure that the barrier pillow resists separation or dislodging from the serrated sealing element inner attachment ring  19 . More particularly, as described in relation to  FIG. 4 , a groove or slot is cut in the outer diameter of the PTFE barrier pillow ring. To do this, the barrier pillow is clamped between two machined metal tools that make up a clamping device that holds the barrier pillow by its inner diameter. The metal tool clamping device, with the barrier pillow clamped in it, is secured in a lathe, allowing a torque to be applied. While the metal clamping device and the barrier pillow are rotating about its center in the lathe, a sharp tool that cuts a groove into the barrier pillow&#39;s outer surface, shown in  FIG. 4  as slot  23 , that is approximately the same as the flange  19 . After the groove has reached the desired depth, the sharp tool is backed out of the groove. The barrier pillow, with outer diameter groove, is then removed from the tooling. The flange  19  is then inserted within the slot formed in the barrier pillow. 
       FIG. 6  illustrates the separate sealing zones of a serrated profile sealing element in accordance with the present invention. As shown therein, inner Zone A and intermediate Zone B encompass different portions of the barrier pillow. Inner Zone A defines an inner portion of the barrier pillow extending from the inner surface  25  of the gasket to the inner surface  23  of the flange  19 . Intermediate Zone B defines that portion of the barrier pillow through which flange  19  extends, i.e., from the inner surface of the flange  19  to the inner surface  27  of the serrate profile core  17 . Outer Zone C encompasses the core portion extending from the core inner surface  27  to the core outer surface  29 . 
     Described in more detail below, inner Zone A constitutes the softest portion of the serrated profile sealing element which, when compressed between the flanges  12 ,  14  (shown at  FIG. 2 ) will seal against imperfections  16 ,  18  formed on the flanges  12 ,  14  respectively. The goal of this inner seal is simply to fill the area between the flanges and stop the process (caustic liquids) from being able to pass outwardly through the gasket to intermediate Zone B. The stress is low in this area, so it does not consume a high percentage of gasket stress that can be used more effectively to seal intermediate Zone B and outer Zone C. The stress load exhibited in inner Zone A is a function of characteristics of the material used to form inner Zone A, including the compressibility of the material, and the characteristic load associated with the compression of the inner Zone A material to a level substantially coplanar with the surface of the core  17 . 
     Intermediate Zone B exhibits a higher stress load then inner Zone A, due to the presence of flange  19  such that the PTFE material in intermediate Zone B is more highly compressed than the PTFE material in inner Zone A. In addition to the stress load characteristics of the pillow material, the stress load characteristics of intermediate Zone B are also a function of the thickness of the flange  19 . As such, the thickness of the inner flange may be regulated to set the desired gasket stress in this zone. The thicker the inner flange, the thinner the pillow material, causing greater compression of the pillow material resulting in higher gasket stress. In the presently preferred embodiment, the flange thickness is set at 0.069 inches, resulting in 3,620 psi gasket stress, which is above the 2,800 psi needed to establish a gas and liquid seal for the PTFE pillow material. As such, intermediate Zone B acts as an initial primary seal. 
     Ideally, the pillow material, pillow dimensions, and dimensions of the flange  19  are selected such that, upon compression of intermediate Zone B to a level substantially coplanar with the core  21 , the stress load level will be at a level sufficient to provide an effective seal, precluding the flow of the caustic liquids and gasses to the primary seal, i.e. the core. The pillow material used in the presently preferred embodiment forms a liquid and gas seal at a stress level of approximately 2,800 psi. 
     The outer Zone C gasket stresses are generally even across all gasket sizes and pressure classes. In the presently preferred embodiment, the outer Zone C gasket stresses average approximately 21,000 psi, after the load on the pillow is deducted. Consequently, even if the pillow leaks, the serrated core would keep the flange connection tight. 
       FIG. 7  is a graph illustrating the gasket seating stress, by zone, for class 150 gaskets of different pipe sizes. As shown, the stress in inner Zone A is consistently 680 psi, the stress in intermediate Zone B is consistently 3,600 psi, and the stress in outer Zone C varies between 13,774 psi and 22,063 psi. The average outer Zone C seating stress for this class of gaskets is 19,922 psi. 
       FIG. 8  is a graph illustrating the gasket seating stress, by zone, for class 300 gaskets of different pipe sizes. As shown, the stress in inner Zone A is consistently 680 psi, the stress in intermediate Zone B is consistently 3,600 psi, and the stress in outer Zone C varies between 20,609 psi and 24,405 psi. The average outer Zone C seating stress for this class of gaskets is 22,371 psi. 
       FIGS. 9 and 10  are tables of data points used to prepare  FIGS. 7 and 8 , and also show the total stud stress applied to the gaskets, the amount of the stud stress delivered to each zone, and the percentage of the total stud stress on each zone. It is shown that only a small part of the stud load is required to generate the gasket stresses in inner Zone A and intermediate Zone B, while the largest fraction of stud load is delivered to the primary seal (outer Zone C), amounting to an average of 86% for class 150 gaskets ( FIG. 9 ), and 90% for class 300 gaskets ( FIG. 10 ). 
       FIG. 10  illustrates the compression characteristics of the material used to form the pillow in one presently preferred embodiment. As shown therein, a PTFE pillow, having an initial thickness of 0.250″, exhibits a gasket stress of 680 psi when compressed 44%. This compression is sufficient to establish basic fluid sealability in inner Zone A. 
       FIG. 11  further demonstrates the compression needed to obtain the desired gasket stress levels for PTFE material in intermediate Zone B. For example, a 62% compression is needed to obtain 3,600 psi gasket stress in intermediate Zone B, when the material is 0.096 inches. This level of compression is obtained in intermediate Zone B both through the method of grooving the outer diameter of the PTFE barrier pillow and by the selected thickness of flange  19 , and ensures that intermediate Zone B has sufficient seating stress to comfortably exceed the stress required to seal the PTFE material against both liquids and gasses. Where higher stress levels are required to form a seal for the selected PTFE material, the thickness of the flange  19  may be increased (or the thickness of the pillow material may be altered) to obtain higher intermediate Zone B stress levels. Ideally the architecture of the serrated profile sealing element components will be such that inner Zone A exhibits only a relatively low stress needed to prevent liquid intrusion, intermediate Zone B exhibits only a stress level sufficient to reliably form a seal that is impervious to liquids and gasses, and the highest stress levels occur in outer Zone C, where the primary sealing resides. 
     Moreover, the distribution of the available stud stress is also dependent upon the area of each Zone that is being stressed. For example, the shortening of flange  19  (while maintain the same length of pillow material) will both increase the load required to compress inner Zone A and decrease the load required to compress intermediate Zone B. Consequently, by appropriate engineering of the components of gasket  10 , as described herein, the appropriate loads can be further distributed between Zones A, B and C. Ideally the distribution of the load and the overall load itself will remain relatively constant. 
       FIG. 12  provides a comparison of the compression characteristics of ALKY-ONE gasket, with the compression characteristics of a spiral wound gasket having a serrated core inner ring. As shown in  FIG. 12 , over a range of bolt loads, the ALKY-ONE gasket is more compressible, comes to full compression with half the bolt load, and generates much higher sealing stresses in its primary seal. 
     Table 1 (below) shows the calculation for determining the percentage compression of the PTFE pillow material when the load is applied to the gasket. Once the percent compression is computed, the stress load applied the zone may be determined by referencing the graph depicting the compression characteristics of the pillow material used, as shown at  FIG. 10 . 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Row 
                   
                 Zone A 
                 Zone B 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 Original PTFE 
                 0.250 
                 0.250 
                   
               
               
                   
                 Thickness (in) 
               
               
                 2 
                 Cut in PTFE (in) 
                   
                 0.063 
               
               
                 3 
                 Net PTFE Thickness (in) 
                 0.250 
                 0.187 
                 [Row 1 − Row 2] 
               
               
                 4 
                 Ultimate Compressed 
                 0.140 
                 0.140 
               
               
                   
                 Thickness 
               
               
                 5 
                 Thickness of ID Flange 
                   
                 0.069 
               
               
                   
                 (in) 
               
               
                 6 
                 Space into which PTFE 
                 0.140 
                 0.071 
                 [Row 4 − Row 5] 
               
               
                   
                 is Compressed 
               
               
                 7 
                 Compression of PTFE 
                 0.110 
                 0.116 
                 [Row 1 − Row 6] 
               
               
                   
                 (in) 
               
               
                 8 
                 % Compression of PTFE 
                 44.0% 
                 62.0% 
                 [1 − (Row 6/ 
               
               
                   
                   
                   
                   
                 Row 3)] 
               
               
                 9 
                 Gasket Stress @ 
                 680 
                 3,600 
               
               
                   
                 Above % (psi) 
               
               
                   
               
             
          
         
       
     
     As shown at Table 1, in one presently preferred embodiment the thickness of the barrier pillow, t p , was selected to be 0.25 inches. In inner Zone A this material is compressed down to a thickness of 0.140 inches, a level substantially coplanar with the serrated core and core facing. In intermediate Zone B, the cut in the pillow reduces the effective thickness of the PTFE pillow to 0.187 inches, and the added presence of the flange  19  requires that the pillow be compressed into a gap of only 0.071 inches. The compression of the PTFE in Zones A and B are then determined and the percentage of compression of the PTFE may be calculated. At that point, the stress level associated with the percentage compression can be derived from the PTFE pillow compression characteristics, such as shown at  FIG. 10 . In the presently preferred embodiment, the resulting gasket stress level in inner Zone A is 680 psi, whereas the gasket stress level in intermediate Zone B is 3,600 psi. As noted above, for the particular PTFE material selected, the gasket stress level in intermediate Zone B is sufficient to form an effective seal, precluding the flow of acids to the primary seal (i.e., the core). As such, the gasket is effective to provide dual, independent sealing zones when constructed to the referenced dimensions. In the event that the resulting stress levels in the gasket, e.g. at intermediate Zone B, were determined to be insufficient to form an effective seal for the selected pillow material, the gasket construction could be modified to enhance the stress levels in different portions of the gasket. For example, the thickness of the flange could be increased, the thickness of the pillow material could be decreased, or different pillow material could be selected. 
     As one of ordinary skill in the art would recognize, various enhancements, and substitutions may be made in relation to the materials, and dimensions used for a particular implementation, without departing from the broader scope and spirit of the present invention. As such, those modifications, enhancements and substitutions are intended to be encompassed within the scope of the present invention.