Pipe joints or couplings – With assembly means or feature – Guide and support
Reexamination Certificate
1999-02-12
2001-07-31
Barrett, Suzanne Dino (Department: 3627)
Pipe joints or couplings
With assembly means or feature
Guide and support
C285S350000, C277S603000, C277S608000
Reexamination Certificate
active
06267414
ABSTRACT:
This invention relates to gaskets, and in particular to a gasket for a demountable pipe joint of an hygienic food processing system.
Systems and apparatus for the processing of liquid food inevitably require joints between adjacent pipes of the apparatus. Although pipes can be joined by welding, they can not then be readily disassembled for cleaning and other purposes.
In view of the foregoing, demountable pipe joints are preferred in many applications, although it is recognised that such joints (i.e. joints that are not welded together but can be assembled and disassembled) are unsatisfactory from a hygienic point of view. Problems associated with demountable pipe joints are discussed in a paper by the European Hygienic Equipment Design Group entitled “Hygienic Design of Closed Equipment for the Processing of Liquid Food” by G. J. Curiel, Dr. G. Hauser, P. Peschel and D. A. Timperley dated October, 1993. In this document, various types of pipe joint incorporating elastomeric gaskets are discussed. As a result, in general, it is clear that gaskets, whether in the form of sheets, moldings or “O” rings, which are usually made of elastomer such as Neoprene rubber, EPDM or Viton, are not ideal. These known prior art gaskets are used because they deform to take the shape of the mating surfaces when relatively small forces are applied. However, these gasket materials “creep”, particularly at high temperatures, and compress or move readily during use in response to the pressure of liquid food in the pipe of the processing apparatus.
If a gasket exhibits “creep” (—creep means a time dependent change of relaxed dimensions after having been subject to heat and/or pressure), the pressure between the gasket and a mating face of a pipe end at a joint can be lost and crevices can open up. As a result, micro organisms can become established in the crevices, thereby preventing the pipe joint remaining sterile. Furthermore, problems with micro organisms can be especially bad when using elastomeric gaskets because the food processing equipment usually goes through cycles of heating and cooling during long production runs and the micro organisms can easily flourish in crevices as the soft elastomeric gasket moves in its seat.
Another problem associated with elastomeric gaskets is that, when a pipe needs to be sterilised before a new production run is commenced, temperatures between 122° and 140° (for example) need to be applied, often by means of superheated pressurised water or steam to kill any micro-organisms present. If there are crevices present then the sterilising agent may not contact the entire surface. Heating elastomers to this sort of temperature, however, results in degradation of the material, thereby shortening the life of the gasket.
U.S. Pat No. 5,518,257 discloses a seal device which incorporates an inner seal member made from engineering plastics and resiliently biased towards a flow path. The inner seal member is designed to slide relative to pipe ends of a joint and includes a sharp internal corner, both of which are inherently unsatisfactory in hygienic applications due to their ability to harbour micro-organisms.
Although metal gaskets are known in certain applications, these are not considered appropriate in food processing apparatus because they can result in scars and scratches being produced on the pipe ends which, when a new gasket is applied, will prevent a satisfactory seal being produced.
Although the problems described above have been known for many years in the field of hygienic food processing equipment, as far as the applicant is aware no one has provided a satisfactory answer until now.
With the foregoing in mind, the present applicant has invented a new demountable pipe joint and gasket suitable therefor which overcome the problems associated with the prior art in a simple and efficient manner. Furthermore, although the present invention appears to go against all teachings of the prior art in this particular specialised field, it provides a significant improvement over the prior art.
According to the present invention, there is provided a gasket for a demountable joint of an hygienic food processing system, the gasket comprising a surface for providing a smooth transition from a first joint part to a second joint part at a joint, the sides of the gasket each defining a plateau adjacent the surface for abutting the joint parts, in use, to produce an hygienic seal, wherein the gasket is made from a plastics material having sufficient structural strength (i.e. engineering plastics) when not fully supported to withstand pressures and/or temperatures within the pipes, during use, without significantly reducing the hygienic performance of the joint.
As will be appreciated, a gasket according to the present invention is a significant departure away from known prior art gaskets of the relevant type, which in general were elastomeric and soft. Furthermore, although particularly applicable to hygienic food processing systems, a gasket according to the present invention may be used in any demountable joint or other appropriate application. For example, the invention may find significant uses in the biotechnology or pharmaceutical industries where cleanliness is required.
Although those skilled in the art will understand the significance of the present invention, in particular the difference between elastomers and engineering plastics, the distinction can be clearly recognised from the differences in relative tensile strength/modulus. In particular, a typical elastomer has a tensile strength of 20-40 MPa and an elongation of 300-700% before breaking. This implies a tensile modulus of less than 10-20 MPa. In contrast, a typical engineering plastic has a tensile strength of 100-200 MPa and an elongation of 3-100%, thereby giving a tensile modulus in the range of 2400-10,000 MPa at room temperature. Further, a typical elastomer exhibits between 2-3 times as much linear thermal expansion (and hence between 8-27 times as much volume expansion) as an engineering plastics such as polyetheretherketone.
Preferably each plateau (or seal surface) is positioned on a nib or platform which extends from a body of the gasket. In a particular embodiment, wherein the gasket is substantially annular, each nib is also substantially annular.
In an alternative embodiment, each pipe end may include a raised platform portion for gripping a simply shaped gasket, such that the raised platforms of the pipe ends replace the nibs of the gasket.
The nibs are preferably shaped and sized to exhibit creep during assembly of a joint, such that deformation of the nibs results in an extremely good seal being produced between the gasket and the adjacent joint part.
The body of the gasket is preferably shaped and sized to remain resilient throughout the lifetime of the gasket. With this in mind, provided that the compression force applied to the gasket does not exceed the critical stress (in this specification, “critical stress” means the maximum stress at which no relatively rapid permanent deformation occurs) of the particular plastics material forming the gasket, the body of the gasket can remain resilient such that the nibs of the gasket are continually biased into contact with the joint parts. This preferably applies at all times and at all working temperatures experienced by the gasket.
The radial width of each plateau of the gasket may be less than 50%, preferably less than 22%, more preferably about 20%, of the radial width of the body of the gasket. Further, the axial length of each nib is preferably less than 5%, more preferably less than 2%, of the total axial length of the gasket. By using these preferred relative dimensions, normal usage of the gasket results in the nibs experiencing creep during assembly of a joint and the body of the gasket remaining resilient throughout the assembly and subsequent lifetime of the joint. A markedly improved seal is thereby provided which should not require any subsequent tightening of the joint.
The gasket may include a stop for defining the maximum compressi
Barrett Suzanne Dino
Beckswift Limited
Fulbright & Jaworski L.L.P.
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