This invention relates to a flexible ring seal uihich 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 pouαder 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 unsatisf ctory . Firstly, in vieω of lack of flexibility it is most difficult to obtain an efficient sealing fit, and to achieve this many man-hours of uiork 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 general ob e'ct of the invention is to provide 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 separated by air spaces .

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-f bricated 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;

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-f rmed 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 convolution 11 and margins are of substantially equal height in the illustrated strips.

As the compound strip leaves the machine, globules of brazing paste 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 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. 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 structures described above are for instance, 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 tabs 7 (Fig. 12). Conveniently there are four such tabs equally spaced around the outer circumf rence of the ring seal.

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 referenc 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 to the. 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 spac 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 critical importance when the ring seal is to be used in highly corrosive environments.