Seal for a joint or juncture – Seal between fixed parts or static contact against... – Contact seal for a pipe – conduit – or cable
Reexamination Certificate
2001-10-30
2004-01-27
Knight, Anthony (Department: 3676)
Seal for a joint or juncture
Seal between fixed parts or static contact against...
Contact seal for a pipe, conduit, or cable
C277S602000, C277S608000, C277S612000
Reexamination Certificate
active
06682081
ABSTRACT:
This invention relates to gaskets that are installed between pipes typically in industrial applications. Specifically, the invention is directed to reduced area gaskets for use in plastic piping systems.
BACKGROUND OF THE INVENTION
Plastic piping systems are prevalent throughout the chemical processing, waste treatment, pulp and paper and other industries. While the plastic pipe and flanges (FRP, PVC, CPVC, etc.) are chemically resistant to the media passing through the pipes, at a fraction of the cost of alloy metallic piping systems, the inherent mechanical properties of the plastic create sealing challenges beyond those inherent with metallic flanges/piping.
Plastic flanges are brittle and exhibit low allowable compressive stresses. As a result, the manufacturers of these piping and flange systems specify extremely low maximum allowable assembly bolt loads. Assembly bolt loads in excess of these limits can damage the flanges either through a compressive stress failure or brittle failure. The flanges are typically flat (with no raised face as with metallic flanges) with a wide variety of surface profiles across the flat face (grooves, serrations, smooth, etc.). To prevent cantilever loads and resulting brittle (bending) failure of the flanges, full face design gaskets are specified. A full face gasket extends from the inside diameter (“ID”) of the flange to the outside diameter (“OD”) of the flange with bolt holes matching those of the flange. The combination of the resulting large surface area gasket and low permissible bolt load creates a low compressive stress applied to the gasket material. Table 1 shows the surface stress developed on several, conventional full face gaskets when compressed between plastic pipe flanges at the manufacturer's specified assembly bolt load:
TABLE 1
Gasket Stress Developed
Flange
Full Face Gasket
Allowable Bolt
Assembly Stress
Size
Area
Torque
Developed (f = .18)
NPS 2 ×
22.2 in2
20 ft-lb
385 psi
150
NPS 3 ×
33.0 in2
20 ft-lb
260 psi
150
NPS 8 ×
80.4 in2
40 ft-lb
350 psi
150
Table 2 shows the minimum compressive stress required for various gasket materials to provide an impermeable seal:
TABLE 2
Compressive Stress Required For Various Gasket Materials
Minimum Required
Material
Assembly Stress
40-60 durometer Elastomers (EPDM,
300 psi
Viton, Red Rubber, etc.)
⅛″ thick expanded PTFE (GORE-TEX ®,
2,800 psi
Inertex ®, etc.)
⅛″ thick virgin or mechanical grade
3,500 psi
PTFE
⅛″ thick filled PTFE
4,800 psi
The cause of poor sealing performance of plastic flanges is evident upon review of the two tables. Of all of the available, conventional gasket materials, only elastomers will receive the necessary compressive stress in plastic piping applications to effect a seal. All other materials require compressive stresses approximately ten times what is available with plastic flange and bolting specifications.
Wherever possible, elastomer type gaskets are typically used in plastic piping systems. They are inexpensive and they seal well with the low available bolt loads. The sealing challenges that are the focus of this invention are applications with plastic piping systems where elastomer gaskets are not chemically or thermally compatible with the process media. In these applications, conventional elastomer gaskets would be quickly destroyed, so PTFE based (or similarly inert) gaskets are necessary. This is a common occurrence within chemical processing and pulp and paper applications where strong caustic or acidic solutions are transported through plastic piping systems.
Currently, there a variety of means that attempt to solve this problem:
(1) Various PTFE based gaskets (expanded or filled), fabricated as full-face design, are installed with bolt loads far in excess of the manufacturers specifications, but still below that required by the gaskets for effective sealing. Flange breakage is a common result and, where the flanges are not broken during assembly, the plants learn to live with leakage and seepage of the gaskets.
(2) These same PTFE materials are fabricated into ring type gaskets instead of full face design. Use of ring gaskets requires lower bolt torque proportional to the reduced contact area of the gaskets. Ring gaskets fit and seal inside of the flange bolt circle. Their use with plastic flanges is discouraged because of the bending moment created as the bolts are tightened. The incidence of flange breakage increases with the use of ring gaskets, and leakage/seepage typically still occurs as the gaskets are still not able to receive sufficient compressive stress.
(3) Full face elastomer gaskets with a protective PTFE envelope, or coating, have been developed for use with plastic pipe flanges and corrosive media. The PTFE coating on these gaskets increases the bolt load necessary for the elastomers to seal and, as a result, the manufacturers of these gaskets specify minimum assembly bolt torques that exceed the allowable limits of the plastic flanges. The result of this mis-match is, again, high incidence of flange breakage and leakage/seepage resulting from under-stressed gaskets. Another limitation preventing widespread use of this concept is concerns and incidents of permeation of the chemicals through the thin PTFE coating, resulting in rapid chemical deterioration of the elastomer base gasket.
(4) Modified full face gasket designs have been developed where sections of the gasket between the ID and OD are removed, thus yielding reduced compressive areas (See FIG.
1
). Unfortunately, the widespread usage of these gaskets as a solution to the plastic flange sealing problems is hampered by several limitations of this concept.
a. Expanded PTFE is a preferred material for use with this design. Extremely poor rigidity results from removing large sections from the gasket while leaving a ring at the OD. This poor rigidity (floppiness) makes it very difficult to install these gaskets on larger diameter pipe flanges (~>4″ NPS).
b. The minimum assembly bolt torque required to provide proper compression to these types of gaskets during assembly still exceeds the levels specified by the flange manufacturers.
As a result of the foregoing failed attempts to solve the problems, users of plastic piping systems have learned to live with and accept the poor sealing performance of PTFE based gaskets and the high incidence of flange breakage during assembly. Recently, however, with the strict recording and procedural requirements of OSHA Process Safety Management Rules, users of plastic piping systems within PSM critical areas (plant processes or areas that are subject to the OSHA rules) cannot tolerate the poor sealing performance of these joints, and more importantly they are not able to knowingly deviate from manufacturer specifications in these processes without doing engineering analyses. Specifically, plastic piping systems that specify 20 ft-lb maximum assembly bolt torque must be installed with 20 ft-lb assembly bolt torque. For plastic piping systems in PSM critical services, a better PTFE based gasket design is required that is easily installed, compatible with the processes, not subject to concerns with permeation, and finally that seals as required with bolt loads no higher than those specified by the equipment manufacturer.
A solution to this problem is obtained by “reverse engineering” a gasket. The contact area of the flanges is fixed. The minimum stress required by various suitable gasket materials is fixed. The maximum allowable bolt torques are also fixed and cannot be changed. The only variable remaining is the dimensions of the gasket and the resulting surface area. The solution lies in reduced area gasket designs that have their contact area reduced sufficiently such that the load developed by the bolts when torqued to the maximum specified value produces a compressive stress on the reduced area portion of the gasket that exceeds the minimum stress required for that material to achieve a tight seal. This must be done without creat
Burton Brian Douglas
Frew James E. B.
Waterland III Alfred F.
Inertech, Inc.
Knight Anthony
Peavey Enoch E
Thomas P.C. John H.
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