GB2236560A - Seal or packing - Google Patents

Abstract

A static seal of packing comprises gasket or packing material in an envelope of PTFE. The envelope is made by slitting a strip or ring of PTFE to form a pair of arms 15 and then bending each arm backwards through 180 DEG . The resulting envelope can cheaply provide a thick seal or gasket with a relatively smooth surface adjacent the fluid to be sealed. <IMAGE>

Description

SEAL L OR PACKING This invention relates to static seals and packings, and in particular to protective envelopes and sheaths.

Flange joints, lids, covers and so on are commonly sealed by packings or gaskets made of a wide variety of materials for example compressed asbestos fibre, rubber, rubber-proofed cloth, millboard and so on. To protect the packing or gasket material from aggressive materials in the ambient atmosphere or within the space to be sealed, the packing or gasket is commonly provided with a protective sheath or envelope, usually made of an impervious plastics sheet material, in particular polytetrafluoroethylene (PTFE), fluorinated ethylene propylene (FEP) and perfluoroalkoxy (PFR).

Existing forms of PTFE envelope or sheath have practical disadvantages or are costly.

fin object of the present invention is to provide a sheath or envelope for a gasket or packing, which will provide improved performance and utility at moderate cost.

According to the present invention, the protective envelope or sheath comprises a strip of protective material, formed initially to have a slit or V-like or Y-like cross section defining a pair of arms adjacent one another, the arms being subsequently inverted or reversed to lie alongside and spaced from one another with the junction between them at one end, the gasket or packing material being accommodated in use between these inverted or reversed arms.

It will be understood that the junction region or "stem" will be short, at least in the finished configuration of the sheath or envelope, so as to leave as much room as possible between the arms to accommodate the gasket or packing.

The sheath or envelope can be made cheaply and simply, by starting with a ring or strip of PTFE or other protective material, slitting this through the greater part of its cross section to form a pair of arms, and then folding both arms back through approximately 2 right angles or folding one arm back through approximately 3600.

The invention will be further described with reference to the accompanying drawings, in which: Figures 1 and 2 show, in radial cross section, flange joints provided with gaskets having conventional envelopes, and Figure 3 is a similar view illustrating an envelope embodying the invention.

Each figure of the drawing shows a pair of adjacent pipe lengths 1, each provided with an end flange 2, these flanges being joined together by clamping bolts 3.

Compressed between the flanges is a gasket 4, made of any suitable material such as those mentioned aboue, in the form of an unbroken ring or of a strip cut to appropriate length to form a ring.

TO protect the gasket, in particular from the material within the pipeline, the inner sealing edge of the gasket and its side faces are protected by an envelope or sheath.

The envelope S shown in Figure 1 is a ring, parted off from a solid block of PTFE using a lathe, the ring being thin in relation to its radial extent. A radial slit is made, extending inwards from the outer edge of the ring to a position close to the inner edge of the ring, forming a pair of arms 6 joined by a short inner stem 7.

In use, the arms are spread apart so that the envelope has, in radial cross section, a form like a Y with long arms, or a tuning fork. The gasket is inserted between the separated arms 6.

This construction provides a cheap gasket and envelope, but the user is limited to thin gasket inserts, because the PTFE of the envelope can be spread apart only to a limited extent without tearing or otherwise damaging the envelope.

Another disadvantage is that the inner region of the envelope, around the stem 7, leaves voids 8 in the pipe system between the flanges, which can collect contamination and cause turbulence in the flow through the pipeline.

To overcome these problems, it is known to use an envelope 11 oF rectangular cross section, machined from solid PTFE. These envelopes substantially eliminate voids in the pipe system and can accommodate much thicker gaskets, but are very expensive and wasteful because they are machined from solid material.

Figure 3 shows an envelope embodying the invention which combines the practical advantages of that shown in Figure 2 with the cheapness of manufacture of that shown in Figure 1.

The envelope shown in Figure 3 is made, in a manner similar to that of Figure 1, by slitting a flat body, e.g. a ring or strip, of PTFE or other protective material. then, however, the arms 15 formed by this slitting operation are each turned back on itself through substantially 180 , so that the stem 16 by which the arms remain joined at their radially inner ends lies between the reversed arms and is connected to these by semicircular regions 17 which form the innermost surface of the envelope.

More specifically, a block of sintered PTFE is set up in a lathe, and the bore and outside diameter are machined to the required dimensions. Then, using a tool in the form of a knife, a slit is formed parallel to the end of the block, extending from the outer circumfery inwards to a position close to the bore. The slitted end region oF the block is then parted of by a further cut parallel to and spaced from the slit, extending through the entire radial thickness of the block. This produces a ring with a pair of arms extending radially outwardly from a junction at their radially inner ends. With these arms slightly separated, the ring has a V cross section with its apex directed radially inwards.This V section ring is then turned inside out, for example by passing one oF the limbs through the bore, bending it through substantially 1800 while leaving the other limb in substantially its original orientation. This produces the cross section shown in Figure 3.

The PTFE is very flexible but not very elastic. The flexibility permits the manipulation and deformation just described which transforms the envelope from an initial cross section resembling that shown in Figure 1 to the cross section shown in Figure 3. Because of the limited elasticity of the material, the material may become creased or wrinkled when it is turned inside out and fitted.

Typically, the thickness of the envelope wall is about O.Smm, 0.75mum at most, and consequently hoop stresses in the inverted envelop will not normally be a problem.

With the thicknesses and radial dimensions of typical envelopes, the material of the envelope is at all times stressed well within its elastic limits.

The gasket material is placed between the reversed arms 15. The type of envelope shown in Figure 3 can accommodate thicker gasket material than that shown in Figure 1. The radiused inner surfaces 17 can lie essentially flush with the internal surfaces of the pipes, and allow essentially smooth flow in the pipe system. The radii produced in these regions by the reversal of the arms causes surprisingly little stress in the envelope material.

The production of the envelope shown in Figure 3 is slightly more complex than that of Figure 1, as dimension control is more critical, and it is desirable to hot press the envelope e.g. for 10 minutes at 2000C after fitting, to smooth out wrinkles caused by inverting the envelope and fitting it to the gasket and ensure that the envelope retains its proper shape.

Despite this, for a given size of gasket, the envelope shown in Figure 3 is substantially cheaper and simpler to produce, with essentially no waste material, than that shown in Figure 2 whose performance is more comparable to that of Figure 3.

As a general rule, the PTFE block from which the envelope is to be made will be provided with a smaller bore than a block used for the production of conventional V type envelopes to fit a corresponding pipe diameter. Typically, a block from which the present invert" envelope is to be formed will have a bore about 3rnm smaller than the bore of a block for producing conventional V type envelopes.

Although the invention has been particularly described with reference to the sealing of pipe flange joints using annular gaskets and envelopes, it is to be understood that the invention is applicable to the sealing of compression joints between opposed surfaces of substantially any nature, purpose and configuration.

Claims (12)

Claims

1. A static seal or packing, comprising a body of gasket or tracking material within a protective sheath or envelope, and in which the protective envelope or sheath comprises a strip of protective material, formed initially to have a slit or V-like or Y-like cross section defining a pair of arms adjacent one another, the arms being subsequently inverted or reversed to lie alongside and spaced from one another with the junction between them at one end, the gasket or packing material being accommodated in use between these inverted or reversed arms.

2. A static seal or packing, comprising a body of gasket or tracking material within a protective sheath or envelope, and in which the sheath or envelope comprises, in cross section, a pair of spaced arms having the said material between them, these arms being joined to one another at one end and at lest one arm having a bend, away from the other arm, between the junction and the remainder of the arm, the said bend or bends having a total angle of substantially 360-.

3. A seal or packing as claimed in Claim 2 in which each arm has a said bend, through substantially 180 each.

4. A seal or packing as claimed in Claim 1, 2 or 3 in which the sheath or envelope is of PTFE.

5. A seal or packing as claimed in any preceding claim in the form of a ring.

6. A seal or packing as claimed in any of Claims 1 to 4 in the form of an elongate strip.

7. A method of making a static seal or packing, comprising a body of gasket or tracking material within a protective sheath or envelope, and in which a strip-like body of protective material is slit to define a pair of arms with a junction between the arms at one end of the arms, bending at least one arm adjacent the junction, in a direction away from the other arm, the total angle of bending of the arm or both arms being approximately 360 whereby the arms lie alongside and spaced from one another with the junction between them at one end, and placing the gasket or packing material between the arms.

8. A method as claimed in Claim 7 in which each arm is bent through -substantially 180 relative to the junction.

9. A method as claimed in Claim 7 or 8 in which the sheath or envelope is hot pressed after the gasket or packing material has been placed between the arms.

10. A method of making a gasket or packing, substantially as herein described with reference to Figure 3 of the drawings.

11. A gasket or packing when made by the method claimed in Claim 7, 8, 9 or 10.

12. A gasket or packing substantially as herein described with reference to Figure 3 of the drawings.