Hollow mechanical seal rings
According to a first aspect of the invention a mechanical seal ring has a hollow cross section which is open at its radially innermost side and has limbs generally directed towards the radially innermost side. The radially outermost region of the cross section is provided with reinforcement so as to locally increase its hoop strength.
One known form of sealing ring has a radial cross section of C shape, with the open side of the C facing the center of the ring. Another known seal is that known as the “Ellipseal” (Trade Mark), described in British patent specification 2187805, comprising a radial cross section of modified parabolic form with convergent margins. A further known seal is that of GB 2038961, in which the limbs of the seal cross section have out-turned lips, forming an Ω-shaped cross section.
The above-mentioned seals have been very successful in numerous static sealing applications, but are not always entirely successful in meeting the demands of sealing equipment and pipelines used in natural gas fields, where pressures are commonly in the region of 10000 PSI, and may exceed 30000 PSI.
A reason for the lack of success with the known seals when sealing extremely high pressures, is their lack of hoop strength. Because of this, the seal rings expand under the applied internal fluid pressure until they can expand no further because of the restriction imposed by the recesses in which the seals sit. During this change of diameter of the seal ring, the areas of the seal ring surface in contact with the mating faces to be sealed are subjected to a galling action which roughens the surfaces, and in most cases it becomes impossible to establish a satisfactory seal. If the thickness of the seal ring metal is increased, to increase the hoop strength, the flexibility of the seal is substantially reduced, as the seals are made of metal of constant thickness. This then requires larger bolts and increased torque to compress the seal, and makes the seal less able to cope with rotation of the flanges to be sealed, i.e. loss of paralellism, which can occur under the action of the pressure of a contained fluid.
An object of the present invention is to provide a self-energizing metal seal capable of overcoming the described shortcomings of the known seals, and in particular capable of providing reliable sealing against a fluid at very high pressure.
According to a first aspect of the present invention a mechanical seal ring has a cross section which is hollow and open on its radially inner side and has convergent limb regions at this side, and the radially outermost region is provided with reinforcement locally increasing its hoop strength. Preferably the reinforcement of this outermost region is provided by a greater thickness of material than radially inner limb regions which in use engage the surfaces to be sealed.
The increased material thickness in the radially outermost region and resulting increased hoop strength resist expansion of the seal in use, whereas the smaller thickness of inner regions, comprising the flexible limbs which contact the surfaces to be sealed, provides ample flexibility. Relative movement of the seal and sealed surfaces is therefore reduced or eliminated, so that the seal is not subjected to galling and roughening, but the load required to compress the seal is not substantially increased and the seal remains sufficiently flexible to accommodate misalignment and lack of parallelism of the surfaces to be sealed.
In a mechanical seal ring of the kind generally according to the foregoing, the reinforcement of the radially outermost region is provided by an increase of the thickness of the ring material in this region; this thicker region is shaped to provide an axially broad outer rim or heel surface generally parallel to the axis of the ring; the radially innermost free ends of the limb regions have axially outwardly projecting tips which provide the maximum axial dimension of the ring in its relaxed state; and the limb regions have, radially external to the said projections, regions of reduced axial thickness providing points of preferential flexing of the inner ends of the limb regions during axial compression.
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