When applying a fluid seal around a rising/falling or rising and rotating valve stem or shaft, care must be taken to ensure that the seal allows efficient operation of the shaft whilst maintaining the integrity of the seal around the shaft to prevent fluids transferring from one side of the seal to the other.

In all aspects of industry, action is taken to eradicate or at least reduce where possible leakage of liquids and gases from equipment. This is particularly difficult in the case of unanticipated leaks where the industrial equipment (often comprising valves, pumps or flanges) are sealed"notionally tight". During normal usage, such spurious leaks may occur.

Such leaks could lead to either emission or harmful products into the atmosphere or alternatively may lead directly to failure of the industrial equipment if the seal is compromised thus leading to loss of production, costly down time and repairs.

In the area providing a liquid tight seal around a shaft or valve stem, packing seals formed of a plurality of annular elements, which surround the shaft, have been commonly used. In these applications, the successive

annular elements are located around the shaft and co- operate to provide a fluid seal around the shaft as required. It is known to use textile materials such as ropes and braids to form the elements of such packing seals, which can then be impregnated with lubricant such as PTFE to enhance the operation of the seal.

In known examples of packing seals, braid rings are formed and packed into a chamber or"stuffing box", the rings being placed around the shaft to be sealed. A cap is then placed on the chamber, such as a screw-threaded adapter or follower, which is then retained by stud bolts and nuts. As the cap or follower is tightened in place pressure is applied to the rings within the chamber.

This axial pressure is then converted in the rings into radial pressure, which is applied against the shaft and the internal surface of the chamber. This conversion of axial to radial pressure depends on the nature of the material and its construction.

A traditional set of rings comprises braided headers at the top and bottom of the set with a plurality of graphite rings in between. As shown in Figure 1. The surface providing the seal around the stem or shaft is traditionally located at the bottom of the lower header of the set.

The braided headers are soft in comparison to the graphite rings and compress when loaded. The advantages of such headers include their anti-extrusion properties, spreading and high lubrication properties. However, they do not transfer load well through the set. As the set is loaded from above, high stresses are placed on the upper header and the loading transmitted through each

successive ring of the set down to the lower header falls off significantly. Figure 2 shows a schematic illustration of this fall of in loading through the set.

This can lead to a substantial waste of energy within the set in maintaining the seal and can lead to high frictional forces, high operational torques leading to poor control of the valve, particularly important in control valve applications where a tight seal coupled with excellent control is paramount.

It is well understood that the material from which the rings are formed and the geometry of the rings affects the radial pressure, which is generated within the rings within the chamber. In the past, various shapes such as wedges or triangles have been proposed for the rings. These shapes and designs go some way to eliminating loss of sealing pressure within the chamber however the pursuit of increasingly lower emission sealing is desirable.

Radial load on the packing set is achieved by applying an axial load to the top of the packing set.

This axial load results in a corresponding radial load in each ring of the seal set. However, this radial load has been shown to decrease exponentially down the packing set with the least load or stress resulting in the bottom- packing ring of the set. This can affect the integrity of the seal.

In an effort to overcome this problem, graphite rings have been used to form the elements of the packing set as it has been found that they transfer radial load far better than braided header rings, while still

offering improvements in terms of the quality of seal they can achieve.

Furthermore, it is also noted that fluid pressure also has an effect on the transfer of load through the packing set.

The present invention aims to overcome or at least migrate disadvantages of the above described sealing systems by providing an improved, low-emission sealing system which retains the advantages of the headers as noted but also provides improved transfer of load throughout the sealing system, incorporating the advantageous effect fluid pressure can have of the seal set. The invention aims to provide a more even spread of radial stress and hence sealing stress throughout the set but without sacrificing the integrity of the seal.

According to one aspect of the present invention there is provided a sealing system comprising a plurality of formed graphite rings which are adapted to co-operate in providing and maintaining a fluid seal around a rising/falling or rising/reciprocating shaft, each ring having an internal surface which in use contacts the shaft to maintain the seal, the density of the rings at the extreme ends being greater than that of the rings in the centre of the sealing system whereby the load is transferred through the upper and lower rings to the centre of the sealing system.

Preferably, the rings are manufactured of die-formed materials for example graphite and may advantageously be coated or incorporate PTFE or anti-corrosion inhibitors partly to enhance the sealing action of the rings.

Conveniently, the PTFE incorporation may be around 3% of the composition of the rings. Similarly inhibiting materials incorporated into the rings may be around 3-5%.

Preferably, the sealing system comprises upper and lower outer rings having a density of around 1. 8 g/cm3 containing an inter-locked header ring pressed into the centre of the rings. Upper and lower adaptor rings having a density of around 1. 6 g/cm3 and the centre rings, which also incorporate an inter-locked header ring between them having a density of 1. 4 g/cm3. This distribution of density within the rings of the sealing system allows the load to be spread over the entire sealing system rather than be concentrated at the upper header ring within a traditional set.

Advantageously, the centre rings comprises upper and lower rings of substantially triangular cross section, the lower ring being inverted with respect to the upper centre ring, the thickness of each of the centre rings decreasing from the internal circumference in contact with the shaft to be sealed to the outer circumference.

Preferably the upper and lower adaptor rings have sloping faces, which co-operate with the sloping faces of the upper and lower centre rings.

Additionally the centre rings have a slight angle difference from their respective adaptor rings. This allows an amount of additional sealing energy to be stored within the rings should providing additional sealing stress if required.

The interlocked rings pressed into the outer and centre rings provide adequate anti-extrusion, spreading and lubrication properties for the set while improving the overall load transfer properties of the system, each inter-locked ring being in contact with the surface of the shaft to be sealed.

An embodiment of the present invention will now be described with reference to and as shown in the accompanying figures in which: Figure 1 is a schematic representation of traditional packing seal; Figure 2 is a schematic graph illustrating the fall off in loading transfer through a traditional packing seal; Figure 3 is an exploded perspective view of a packing seal according to one aspect of the present invention; Figure 4 is a cross-sectional view of the packing seal of figure 3, and Figure 5 is a view of the packing seal of Figure 3 in assembled state.

Turning now to the figures, there is shown in Figures 3 and 4 a sealing system 1 according to one aspect of the present invention. The sealing system comprises a plurality of sealing elements, which are intended to be placed within a chamber (not shown)

surrounding a rising/falling or rising/rotating shaft S around which a seal is to be maintained.

The first element of the sealing system 1, which would be the top element of the system and therefore introduced last into the chamber, is an upper outer ring 2. This comprises of an annular member 3, which has an outer diameter, which is slightly smaller than the internal diameter of the chamber and an internal diameter, which is slightly larger than the diameter of the shaft to be sealed.

The first member is formed of die-formed graphite having a density of around 1. 8 g/cm3. A coating of PTFE is applied to the surface of the first member in addition to anti-corrosion inhibiting agents. The PTFE coating is around 3%, similarly the inhibitors are around 3%. These additions have been found to improve the overall performance and sealability of the set by acting as an aid to improve break-in and aid even distribution of graphite to minimise stem pick up, while removing concern over corrosion.

The outer ring has an outer shoulder 4, which is integral with the body of the ring 3. An interlocked header ring 5 is incorporated into the outer ring. The interlocked ring has a height, which is equivalent to the height of the outer shoulder 4 of the header ring. The outer diameter of the interlocked ring is slightly smaller than the internal diameter of the shoulder of the outer ring and the inner diameter of the interlocked ring matches the internal diameter of the outer ring.

The interlocked ring 5 sits within the shoulder 4 of the outer ring.

Situated below the outer ring of the sealing system is a die-formed graphite adaptor ring 6, which has a density of around 1. 6 g/cm3. The density of the adaptor ring is reduced relative to the outer ring 2 to allow it to be distorted by a lower axial load yet still effect a suitable seal.

The adaptor ring 6 comprises a generally annular body 7, the outer diameter of which matches the outer diameter of the outer ring. The inner diameter of the adaptor ring matches the inner diameter of the outer ring. The adaptor ring again incorporates suitable PTFE coating and inhibitors.

The thickness of the adaptor ring 6 increases from the centre of the ring to the outside of the ring. This provides a sloping face 8 between the inner and outer surfaces of the adapter ring.

The main sealing element comprises a centre ring unit 9 formed of die-formed graphite elements having a density of around 1. 4 g/cm3 again each ring has a PTFE, inhibitor inclusion as described previously. The centre element has the lowest density of the sealing unit to allow it to seal with the axial load, which will be transferred from the gland follower and also allows a positive sealing effect to be included from the media pressure.

The central unit 9 comprises a first annular element 10 having internal and external diameters, which match

those of the adaptor, ring 6. A shoulder 11 is formed in the lower surface of the first annular element 10 to receive an interlocked header ring 12. This ring 12 has the same general dimensions as the first interlocked ring 5. The thickness of the first annular element 10 decreases from the centre of the element to the outside of the element to provide a co-operating sloping face 13 for the face 8 of the adaptor ring 6 located immediately above it. This sloping face 13 however is at a slightly sharper angle than the adaptor face 8. This allows a degree of sealing energy to be stored within the set once axially loaded should there be a requirement for additional loading during service.

A further annular element 10'inverted from that of the first is provided as the lower part of the central sealing element 9. Similarly, a further adaptor ring 6' is provides below the central sealing unit, the second adaptor ring having the same dimensions and features as the first adaptor ring but being provided in the inverted position with respect to the first adaptor ring 6.

Finally a lower outer ring 2'identical to the outer ring with a similar incorporated header ring 5'is provided, again in an inverted position with relation to the first outer ring 2.

Figure 5 shows the successive elements of the packing seal in position around the shaft. This clearly shows the co-operation between the sloping faces of the upper and lower adaptor rings and the central sealing unit.

This figure also clearly shows the symmetry of the packing seal as described. This ensures quick and simple installation of the various elements of the packing seal whilst eliminating errors that can occur in the correct placement of successive elements of prior art packing seals.

As described above, the density of the graphite elements making up the packing seal decreased form the extreme ends of the packing seal i. e. the outer rings 2, 2'to the centre of the packing seal i. e. the centre element 9. This change in density over the length of the packing seal allows the seal to utilise the media pressure within the seal to improve the sealing effect of the various elements. As the density of the element increase, the sealing factor of the elements also increases i. e. the transfer of axial to radial stress increases.

In addition, by incorporating a slight variation in angles between the centre element 9 and the two adjacent adaptor rings, the set has an included stored energy once loaded. This stored energy allows for additional load to be placed upon the packing set without compromising the integrity of the set.

Furthermore, the use of high-density elements at the extreme ends of the packing seal assists in eliminating extrusion of the packing during application.

The interlocked rings described above may be formed of any suitable material such as for example, graphite, braided graphite, carbon, braided carbon, PTFE or combinations of the above.

The elements of the sealing system can be manufactured by the traditional 3-piece tool method either on semi or fully automated equipment. The symmetry of the various elements also allows the sizes of the elements to be easily scaled up or down according to the dimensions of the stem to which the seal is to be applied. Another feature of the set is its suitability for automatic manufacturing and assembly within its required equipment.