Seal Structure

This invention relates to seal structures, and methods of maintaining their effectiveness.

Elastomeric seals which in operation are exposed to low temperatures may become ineffective because the seal material loses its elastomeric properties upon which the sealing action depends. In particular, if an amorphous polymeric elastomer is exposed to a temperature below its glass transition temperature Tg it becomes hard and glassy. The level of elastic responsiveness reduces even as the elastomer approaches the Tg from a higher temperature. This problem is particularly important in well head or other drilling applications at high latitudes where the ambient ground and/or air temperature is below 0 ⁰ C for prolonged periods. The problem also arises in aerospace applications in which an aircraft or space vehicle is exposed to low temperatures. Such seals can be safety- critical components, failure of which can be (and indeed has been) catastrophic.

The present invention seeks to address this problem. It is in principle applicable to elastomeric seals in any field of use.

According to one aspect of the invention, there is provided a method of maintaining the effectiveness of an elastomeric seal comprising including in the elastomeric material of the seal a dispersed material susceptible to heating when subjected to a varying (preferably alternating) electrical, electromagnetic or magnetic field, and applying a said field to the seal whilst in use whereby to heat it.

According to another aspect of the invention there is provided a seal structure comprising an elastomeric seal made of an elastomeric material including a dispersed material, susceptible to heating when subjected to a varying (preferably alternating) electrical, electromagnetic or magnetic field and means for applying a said field to the seal whilst in use whereby to heat it.

The manner in which the field varies is not critical, provided it heats the seal. An alternating field may be the most convenient to generate, and may be

approximately sinusoidal. Other varying fields eg. pulsiform may be employed, and the variation may be superimposed on an unidirectional field so as to appear as a ripple rather than as a zero-crossing variation.

The particles may be magnetisable and the field may be magnetic or electromagnetic.

The selective internal heating of the elastomeric material of the seal can apply heat precisely where it is needed in an energy-efficient manner.

The particles may be one or more of ferromagnetic, paramagnetic or superparamagnetic

The particles may provide nanoscale magnetic domains in an oxide matrix. The particles preferably are nanoscale particles.

The majority of the particles preferably exhibit a particle size in the range 5 to 50 nm.

The oxide matrix may be one or more of SiO ₂ , CeO ₂ , ZrO ₂ , TiO ₂ and AL ₂ O ₃ . It may constitute between 3% and 20%, preferably about 5% by weight of the elastomeric material.

The particles may be of a material which is electrically conducting or has a dipole moment and the field may be electric or electromagnetic. Thus the particles may be of carbon, for example carbon black. The elastomer may be any thermoset or thermoplastic elastomer suitable (based on conventional criteria) for the sealing function envisaged in a particular application. Thus for oilfield applications it may comprise one or more of a fluoroelastomer (FKM), perfluoroelastomer (FFKM), tetrafluoroethylene-propylene copolymer (TFE-P) and hydrogenated acrylonitrile- butadiene rubber (HNBR). The elastomeric material may comprise a filler which has higher thermal conductivity than the elastomer of the elastomeric material Such fillers may improve electrical or thermal conductivity of the elastomeric material composition, and thus improve the heating efficiency of the matrix. Examples of such higher thermal conductivity materials are carbon black and

reinforcing clays (including nanoscale clays), or metal oxides such as zinc oxide or titanium oxide. Zinc oxide may also form part of the cure system for the elastomer, if the latter is a NBR, HNBR or CR rubber for example.

In some examples it is preferred for the elastomeric material to include carbon nanotubes (CNT). CNT are highly electrically and thermally conducting. The CNT may be single or multi-walled.

This feature is of particular benefit in some examples and is provided independently. Thus an aspect of the invention provides an elastomeric material including a dispersed material susceptible to heating when subjected to a varying (preferably alternating) electrical, electromagnetic or magnetic field, the elastomeric material including carbon nanotubes.

The material may optionally further include one or more of the features described herein of the material of other aspects. The material of this aspect may be incorporated into a seal structure comprising an elastomeric seal including the elastomeric material.

According to a further broad aspect of the invention, there is provided an elastomeric seal comprising an elastomeric material including carbon nanotubes. A further aspect provides a seal structure including such a seal and means to heat the seal material.

The temperature of the seal may be sensed and utilised as a feed back signal to control the field applied to the seal.

There may be means for thermally insulating the seal from its surroundings to retain heat therein.

The field-applying means may be within the insulating means. Thus, the field applying means may be embedded in the insulating means.

The seal may be annular and the field-applying means may be disposed around the seal.

The field-applying means may be configured to apply a toroidal field to the seal.

The magnetic field may be shaped to be concentrated at the seal.

The seal may be disposed or configured to be disposed between relatively rotatable parts. Thus, the elastomeric seal may be an O-ring or a lip seal and may be disposed or may be configured to be disposed in a wellhead.

In another embodiment the seal may be disposed or configured to be disposed between static parts, for example between jointed parts of fuel tanks or booster rocket casings of space vehicles.

The invention now will be described merely by way of example with reference to the accompanying drawings wherein figures 1 , 2 and 3 respectively show sections through alternative forms of seal structure according to the invention. In the figures, parts common to more than one embodiment have the same reference numerals.

Referring to figure 1, a rotatable shaft 10 passes through fixed structure 12 of a well-head and is sealed thereto by an elastomeric seal structure 14 comprising O-ring seal 16. The seal 14 is contained in a groove (as known per se) in a moulded housing 18. The housing is made of a thermally insulating material such as a heat-resistant thermosetting plastics for example an epoxy or phenolic material, or a high performance thermoplastic such as PEEK. Reinforcing fillers may be added as necessary. Embedded within the housing 18 are magnetic field coils 20 which when energised by AC power provide a toroidal magnetic field passing through the O-ring 16.

The O-ring is made of an elastomer containing nanoscale magnetic particles, for example as described in WO-A 2006/024413, the disclosure of which is incorporated within this specification by reference. A suitable elastomer composition for a wellhead application is HNBR, TFE-P, FKM or FFKM compounded to suit the application requirements. Other elastomers such as NBR may be suitable for less arduous duties.

The magnetic field induces eddy currents in the magnetic particles which dissipate within the particles and the surrounding material with the generation of heat. The elastomeric material of the seal thus can be kept above its Tg temperature, and its effectiveness can be maintained.

In figure 2, the O-ring seal is replaced by a lip seal 22 supported (as known per se) by a rigid foundation ring 24.

In figure 3, the O-ring seal is replaced by a metal-encapsulated seal in which an elastomeric element 26 is contained in compression within a metal case 28. The field coils 20 are provided in a radially outer region of the case.

Also shown diagrammatically in figure 3, but applicable also to the other embodiments of the invention are an AC power source 30, a temperature sensor 32 (for example a resistance thermometer, thermocouple, thermistor or silicon bandgap temperature sensor) and a controller 34 which controls the output of the AC power source, based on feedback from the temperature sensor and a required temperature input 36.

Although elastomers with nanoscale magnetic particle fillers are preferred, for the reasons given in WO-A 2006/024413, satisfactory results can be obtained with magnetic particle fillers which are not nanoscale, for example the MagniF 10 filler quoted in the comparative examples in that specification. Power consumption may however be higher than it would have been with a nanoscale filler for the same heating effect.

In many embodiments of the invention it is possible to increase the heating efficiency by the use of ferromagnetic field-shaping elements to concentrate a larger proportion of the magnetic field to pass through the elastomeric seal. Techniques used in the formation of magnetic lenses or pole pieces (eg. in loudspeaker design) may be found appropriate. In particular the metal case of figure 3 may usefully be made at least partially of ferromagnetic, paramagnetic or super paramagnetic material and shaped to direct the magnetic field through the elastomeric seal.

Although described in the context of rotary seals, the invention also is applicable to static seals between parts which do not rotate or otherwise translate relative to each other. Thus an O-ring or other sealing packing between two parts of (say) a cryogenic storage tank or (in a specialist application) a solid rocket booster casing can be made of elastomeric material including a filler of magnetisable particles. It can be provided with an insulating surround including embedded field coils as described generally above. Appropriate AC power applied to the coils causes self-heating in the seal, maintaining its effectiveness and reducing the likelihood of potentially dangerous leaks.

Although the preferred embodiment uses an alternating magnetic field to generate heat within the seal, in principle it is possible instead to use other sources of AC energy which convert to heat within the seal material for example electrical (electrostatic) or electromagnetic (RF microwave) energy. Thus, in an alternative embodiment, conductive carbon black may be incorporated in an elastomeric seal material such as one or more of those already mentioned, and the seal can be subjected to RF microwave heating. Current flows in the carbon black particles and is dissipated as heat. Whilst this is often called dielectric heating, it strictly is a form of eddy current or inductive heating. Use of a polar elastomeric seal material or inclusion in a non-polar elastomeric seal material of a dispersed material having polar molecules or at least molecules having adequate dipole moment will result in true dielectric heating due to dipole rotation when an external electromagnetic RF field is applied. For example PEG (polyethylene glycol) may be added to EP (ethylene propylene) or EPDM (ethylene propylene diene monomer) rubber.

Each feature disclosed in this specification (which term includes the claims) and/or shown in the drawings may be incorporated in the invention independently of other disclosed and/or illustrated features. In particular but without limitation the features of any of the claims dependent from a particular independent claim may be introduced into that independent claim in any combination.

Statements in this specification of the "objects of the invention" relate to preferred embodiments of the invention, but not necessarily to all embodiments of the invention falling within the claims.

The text of the abstract filed herewith is repeated here as part of the specification.

A method of maintaining the effectiveness of an elastomeric seal comprising including in the elastomeric material of the seal a dispersed material susceptible to heating when subjected to a varying (preferably alternating) electrical, electromagnetic or magnetic field and applying a said field to the seal whilst in use whereby to heat it.