Seal for a joint or juncture – Seal between fixed parts or static contact against... – Contact seal for a pipe – conduit – or cable
Reexamination Certificate
1999-06-21
2001-03-13
Knight, Anthony (Department: 3626)
Seal for a joint or juncture
Seal between fixed parts or static contact against...
Contact seal for a pipe, conduit, or cable
C277S626000, C277S646000, C277S634000, C251S061000, C251S175000, C251S328000
Reexamination Certificate
active
06199870
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a seal for a shut-off member for translatory movement which is moved from a closed position to an open position or in the opposite direction, in order to shut off flows of a substance.
The seal comprises metal components which define a pressure chamber to which external pressure can be applied in order to prevent leakage. The exclusive use of metal components permits use for wide temperature ranges and extraordinarily wearing conditions caused by frequency of operation of the valve or the composition of the working medium.
For the operation of such a seal it is important that the leakage resistance of the fitting be achieved between the casing and the shut-off member by means of two sealing seats disposed in the casing. When there is pressure difference in a particular direction, a fixed sealing seat on the side facing away from the pressure difference is necessary. The side facing the pressure difference must be given an axially movable seal seat that is able to equalize even deformations of the casing by internal pressure and/or external stress. Mechanical point stresses from positions outside of the casing necessitate a rigidity of the movable seat ring directly proportional to the distance between these points of application of force, the seat ring being no longer able to compensate for deformations of the shut-off member and casing. Thus, the overall system is not assured against leakage in every state of operation. To achieve the basic requirement of allowing no components of the medium to get into the housing whether the fitting is in the closed or open position, a pressure can be applied within the casing by means of a blocking medium. The result is that, in the event of leakage, only components of the blocking medium can enter the working medium and the shut-off for manual work downstream is secure.
Basically, sealing systems are known which utilize expandable sealing elements in order to produce sufficient surface pressure on the shut-off member. Tubular sealing elements of resilient synthetic resin materials are usable only to a limited extent on account of temperature limits and poor resistance to wear. Particularly in the case of advanced wear, such sealing elements tend to stick in the gap they are sealing. Therefore either the expandability of the sealing element or the possibility of operating the valve is lost.
Known metal sealing systems for providing axial flexibility have the disadvantage that hybrid solutions are involved, of a combination of metal surrounding the pressure chamber that is to be expanded externally and the transmission of the force thereby produced to resilient sealing elements made of elastomers. The disadvantages described above are again encountered. The pressure chambers are defined, for reasons of elastic deformability, by a plurality of parallel membranes whose seal against one another cannot be tested either during manufacture or in operation. The multilayer character of the elastic membrane is necessary because it must have the ability to deform even at the maximum working temperature. As for the function of sealing the pressure chamber, the failure of a single membrane leads to a general failure. It is extremely difficult to use forming processes to manufacture a pack of membranes to accommodate the elastic sealing ring, especially to make them of approximately equal radial thickness. The retention of the elastic sealing ring in the membrane pack is inadequate, so that wear of the elastic sealing ring differs according to its circumferential position in relation to the direction of the translatory movement of the shut-off member.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a seal of the kind referred to above, which with relatively little expense achieves a sealing action and rate of wear that is uniform regardless of circumferential position, even if the working medium applies high temperatures and high pressures to the shut-off member and distortions of the shut-off member and/or external elements of the entire system occur, high frequencies of operation of the valve are to be expected, and intensified wear due to solid components of the medium is possible.
This object is achieved by the invention as described hereinafter. To understand the operation it is necessary to consider the pressure conditions in three different pressure chambers. When the valve is in the closed state, these are: the working pressure P
b
to be blocked off between the shut-off member and the connecting flange of the valve, the pressure within the casing P
g
, and the actuating pressure within the elastic seat P
s
for the production of a surface pressure on the shut-off member. All embodiments of the present invention use differing surfaces subjected to the pressure P
s
in a direction perpendicular to the direction of actuation of the shut-off member. For that purpose two membranes, optimized with regard to required deformability, are necessary, which are connected to the case in a pressure-tight manner, and are arranged substantially parallel to the shut-off member. This annular surface difference is sufficient of itself not only to close any space between the seat and the shut-off member, but also to produce the necessary surface pressure. At the same time the pressure P
s
can also remain below the pressure P
b
because the resultant force is applied to a very small contact surface of the seat. It is thereby as if it were transformed. It is thus possible to produce great surface pressures on the seat with relatively low pressure P
s
. This is also necessary, since the seal is to be exclusively metallic between the flexible seat and the shut-off member. Basically two different approaches to the solution are possible. For the first one, a membrane system can be used which, by means of the pressure P
s
, itself produces the contact with the shut-off member. For the second, the axial deformability of the membranes can be transferred to a relatively rigid seat body which then produces the contact with the shut-off member with a contact area which is independent of the pressure P
s
.
The first solution requires a wear-resistant coating on the entire radius of the membrane which faces the shut-off member. The line of contact between the membrane and the shut-off member is more thickly coated and raised. This configuration has the advantage of equalizing all process-related deformations of the components of the valve. The necessary stiffness, especially of the casing and of the shut-off member, and the requirements as to the departures of the sealing surfaces from the planar, can be reduced. This also affects the machining processes for the final working of the components of the valve which relate to the seal.
The second solution has the advantage of providing the membranes with less sensitivity to wear due to translatory movements of the shut-off member. However, two shaped membranes must be used.
Both configurations can be used with blocking gas pressure P
b
within the housing. However, the sealing effect is strengthened only if the average seal diameter is greater than the inside diameter of the membrane facing away from the shut-off member. If these two sizes are equal, then the surface pressure on the seat can be affected only by the pressure PS This opens up the possibility of minimizing wear during the opening or closing of the valve.
An additional embodiment contains an indirectly proportional ratio of the pressure P
s
to the surface pressure on the seat. This means that the seat pressure is produced only by bias at the time of installation and the working pressure P
b
, and before actuation of the valve, the pressure P
s
is increased in order to reduce the surface pressure on the seat and the wear. The retraction of the free area is slight.
In the case of fluctuating pressure direction, a design employing axially movable seats on both sides of the shut-off member is possible.
The production of the pressure P
s
and its partial or complete reduction during actuatio
Evenson, McKeown, Edwards & Lenahan P.L.L.C.
Friatec Aktiengesellschaft
Knight Anthony
Pickard Alison K.
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