Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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This invention relates to a flexible ring seal which has
been specifically but not exclusively devised for sealing the
joints between the flanges of glass lined pressure vessels
and their associated piping systems.
The need for a flexible ring seal arises because after
glass powder has been sprayed on these pressure vessels and
their ancillary equipment a great deal of distortion occurs
when the glass is fused and becomes an integral part of the
vessel and/or equipment. Consequently the space to be sealed
between the flange connections varies considerably in
thickness and a successful seal must be able to adjust to the
variation.
Currently used ring seals are unsatisfactory. Firstly,
in view of lack of flexibility it is most difficult to obtain
an efficient sealing fit, and to achieve this many man-hours
of work may be needed, which is extremely expensive.
Secondly the known ring seals constitute a health hazard
because asbestos is extensively used in their construction.
In particular, asbestos millboard, which is one of the most
dangerous forms of asbestos is a commonly used ingredient.
The invention provides a more flexible and efficient
ring seal for this and other similar sealing purposes and
also one which is safe and easy to manufacture.
In accordance with the invention a flexible ring seal
comprises a coiled strip of flexible material enclosed within
an envelope made of an inert material having a low friction
co-efficient, the strip having at least one transverse
convolution and side margins of increased thickness such that
in the coiled structure the adjacent convoluted parts are
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separated by air spaces extending across substantially the
entire width of the convolutions.
The strip may be made of stainless steel or any other
ferrous or non-ferrous metal or alloy preferably capable of
imparting resilience to the strip. The convoluted base strip
may comprise layers laminated together.
Alternatively the coiled strip may be made of synthetic
plastics or elastomeric material which may have an embedded
metal reinforcing strip.
Also, for the purpose of preventing possible entrance
under pressure of part of the envelope into the groove formed
by the convoluted part of an adjacent coil of the strip, the
ring seal may further comprise a groove closure ring.
Some embodiments of the invention are hereinafter
described by reference to the accompanying drawings in which:
Fig. 1 is a cross-section of a metal-fabricated strip
preparatory to coiling;
Fig. 2, analogous to Fig. 1, illustrates an alternative
profile strip;
Fig. 3 is a radial cross-section of the main part of a
ring seal utilising the strip shown in Fig. 2;
Fig. 4 is a fragmentary radial cross-section of a
complete ring seal utilising the strip shown in Fig. 1;
Figs. 5, 6 and 7 are cross-sections of three typical
groove closure rings;
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Figs. 8 and 9 are fragmentary radial cross-sections of
two other ring seal embodiments;
Figs. 10 and 11 are radial cross-sections of the coiled
strip forming the main part of two other ring seal
embodiments;
Fig. 12 is a fragmentary plan view of a ring seal; and
Fig. 13 is a cross-section, analogous to Fig. 1,
illustrating a further strip profile.
The ring seals according to the invention which are to
be described comprise a coiled strip structure which is
enclosed in an envelope made of PTFE. When this structure is
made of metal it comprises, as shown in Fig. 1 or Fig. 2, an
assembly of three metal strips namely two rectangular section
strips 1 made of solid drawn metal (wire) and a roll-formed
strip 2 of uniform thickness, which is thin in relation to
its width and formed centrally with at least one arch-shaped
convolution 11 (a single convolution being illustrated). At
opposite sides of the central convolution 11 are respective
plane flanges 12, lying in a common plane, with the
convoluted intermediate region lying entirely on one side of
this plane.
As this formed strip 2 leaves the rolling machine it is
fed through two guide rollers onto which the two wires 1 are
fed accurately onto positions in contact with the respective
side flanges 12, with the outside edges of the rectangular
wires aligned with the outside edges of the flanges. The
wires are secured to the flanges by resistance welds 3 at
regular intervals, for example 10 millimeters. These wires
form side margins for the strip, of increased thickness. The
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convolution 11 and margins are of substantially equal height
in the illustrated strips.
As the compound strip leaves the machine, globules of
brazing paste 4 are deposited in the corners between the
convolution of the strip 2 and the wires 1.
After the compound strip leaves the machine, having been
welded and "pasted" it is fed onto the mandrel of a coiling
machine and wound upon it, the end of the strip being welded
at both sides to the coil when a first coil has been
established. The mandrel is then expanded so as to grip the
coiled strip firmly, whereupon further coiling occurs, the
inside diameter of the ring being controlled by the diameter
of the mandrel, until the desired outside diameter is
attained.
The strip 1, 2 is Argon-arc welded at the intended final
point of contact between the strip and its last wound coil
and is severed from the coil structure which is then removed
from the mandrel and transferred to a high temperature
furnace which is characterised by vacuum and an inert
atmosphere. This causes the brazing paste to become liquid
and to flow by capillary action to form a continuous brazed
fillet along the junction between the innermost sides of the
wires 1 and the convolution of the strip 2.
However, to improve the flexibility of the structure,
especially when the ring seal will be required to operate in
extremely severe environmental conditions it is preferable -
before the structure is brazed at high temperature - to apply
an adhesive, preferably by roller, to both end faces of the
ring, whereafter the ring is lowered into a fluidised bed of
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brazing powder and then brazed at high temperature. This
results in a complete, integral, flexible metal structure.
Another expedient which may be employed to increase
flexibility is to form perforations spaced along the strip
prior to form rolling, these perforations preferably having
the form of slots which are elongated transversely across the
strip. Typically for a strip which is 9.83 mm wide, the
slots measure 1.57 mm by 6.65 mm, the interval between
adjacent sides of the slots being 1.57 mm. Also greater
flexibility can be achieved if the strip 2 is of laminated
formation and formed preferably of three, four or five equal
thickness layers.
After the coil has cooled and has been removed from the
furnace, it is highly lapped to remove any nodules of brazing
material and the inner start and outer finish of the coil are
dressed to meet final specification requirements.
In Figs. 1 and 2 typical dimensions of the wires 1 and
the strip 2 are indicated for two compound strips whereof the
central convolutions are somewhat differently shaped,
different radii A, B and C being indicated.
Fig. 3 illustrates the coiled structure which is formed
by coiling the compound strip which is formed as shown in
Fig. 2.
The coiled structure is enclosed within an envelope 5
made of inert low friction co-efficient material such as
PTFE. This envelope may be made either from a ring which has
been slit radially inwards or two rings fused or bonded
together at its inside diameter, as shown in Fig. 4, or it
may be milled instead of slit as shown in Figs. 8 and 9.
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Slit envelopes are usually used for lower pressure
applications, and milled envelopes for higher pressure ones.
Whichever envelope 5 is used, and although it is
unnecessary from the operational point of view, it is
desirable to fit, around the inside diameter of the coiled
structure, a closure ring 6 as shown for instance in Fig. 4.
This ring 6 may typically be dimensioned and have one annular
rib 6A as also shown in Fig. 5 or Fig. 6 or it may
alternatively be shaped as shown in Fig. 7 which again shows
typical relative dimensions, or Fig. 9 and have two annular
ribs 6A which face the grooves of the innermost coil.
Note that Fig. 9 shows a strip with three convolutions
11, all lying on the same side of the plane of the flanges 12
and not exceeding the height of the margins 1 when relaxed.
The purpose of the closure ring 6 is to prevent any
possibility of the envelope 5 being forced under pressure
into the adjacent groove or grooves formed by the or each
convolution with a consequent risk of splitting of the
envelope.
Suitable alloys which can be employed in the fabrication
of the coiled structure~ described above~are for ins~tance,
C the Nimonic~and Inconel alloys, Waspalloy, Hastelloy and
- similar other alloys of which the resilience characteristic
can be greatly enhanced by an ageing process.
If desired, there may be welded to one face of the ring
seal one or more radial metal ribs 7 (Fig. 12). Conveniently
there are four such tabs equally spaced around the outer
circumference of the ring seal.
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The coiled structure need not necessarily wholly consist
of metallic material but may instead comprise a coiled
transversely convoluted strip of synthetic plastic or
elastomeric material having side margins M of increased
thickness with adjoining coils adhesively secured to one
another and preferably incorporating an embedded metallic
reinforcing strip R extending into the thicker margins, as
illustrated in Figs. 10 and 11.
In cross section the strip may have one or more
convolutions, in general an odd number of convolutions, as
can be seen for example in Figures 9 and 11.
Figure 13 shows a further metal ring seal with three
convolutions in the strip cross section. Each convolution is
semicircular in cross section and the convolutions are
interconnected by straight regions. The outermost
convolutions are connected to the side flanges by outward
joggled or dog-leg regions 10. Typical dimensions in
millimeters are shown, by way of example only, in Figure 13.
In this seal, the convolutions all lie on one side of the
plane of the flanges, but the two outer convolutions are
higher than the margins and nest in the convolutions of the
adjacent layer of the coil. This seal is made in a manner
similar to that described with reference to the seal shown in
Figure 1 and is provided with a low-friction envelope as
already described.
As a background to the invention it should be mentioned
that it is a recognised fact that as the diameter of a flange
connection increases so does the distortion of the flange
faces and this increases the problem of obtaining an adequate
seal. To cope with this problem when utilising the metallic
mode of construction which has been described with reference
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to Figs. 1 to 7 inclusive, variations can be made in the
width, thickness and other dimensions of the roll-formed
strip and the wires, as well as in its convoluted shape and -
in the number of coil turns. Similar considerations apply tothe dimensions, shapes and coil turns of the embodiments
which are made of synthetic plastic or elastomeric material
and enable the characteristics of the ring seal to be
adjusted to almost all the requirements of the operational
environment.
The success of the ring seal derives principally from
the air space which is available between the or each
convolution and the adjacent parts of the coil and it is the
availability of this space which allows the ring seal when
under compression to conform with the utmost flexibility to
the changing dimension between the flanges to be sealed. In
this connection it should be realised that the ring seal does
not act in the same manner as a block of inert cellular
material which can readily be compressed. In fact it is a
very complex resilient structure such that under compression
the convolutions become flat and this has the effect of
forcibly increasing the diameter of the apex of each of the
convolutions. However this compressing effect is resisted by
the natural hoop strength of the material with the
consequence that when the compression mode is released it
tends to return very closely to its original dimension.
In addition to the hoop strength which is inherent in
the convolutions there is also a double parabolic spring
effect on either side of the apex of each convolution which
also reacts to the compression mode, thereby ensuring that
the low friction co-efficient material (PTFE) is forced into
the mating faces to guarantee a perfect seal. This is of
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critical importance when the ring seal is to be used in
highly corrosive environments.
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