Patent Application
20050012063
Not Applicable
Not Applicable
1. Field of the Invention
The present invention is in the field of gate valves.
2. Background Art
Certain valves are required to exhibit a specific ability to seal against leakage around the stem when the valve is in a given position. For example, API specification 6A requires all gate valves to be provided with a back seat, or other means of repacking the stuffing box while the valve is in service and at the maximum pressure for which the valve is rated. This specification also specifies a test wherein the “gas back seat” is tested at 5% to 10% of the working pressure of the valve with nitrogen, with no leakage being allowed.
Typically, the stem and the backseat of a valve have abutting or mating shoulders with matching angles. When the low pressure gas backseat capability is required, during assembly of a valve, the angles on these shoulders are carefully measured and matched, to ensure the greatest probability of passing the low pressure backseat test. This measurement and matching of angles is based largely on the experience of the assembly personnel. When a valve fails the low pressure gas backseat test, it is typically disassembled, and the mating surfaces of the stem and backseat are reworked. The valve is then reassembled and retested. It is sometimes necessary to repeat this process several times, and even repeated reworking sometimes fails.
One object of the present invention is to enhance the ability of the stem to seal reliably within the bonnet, while being sufficiently robust to withstand repeated tests of the gas back seat.
The present invention solves the backseat testing problem by allowing a portion of the stem structure, such as an annular lip, to flex under the low pressure conditions found in the test, while providing additional support for this flexing structure, preventing overstressing of the flexing structure under full working pressure conditions. The flexing annular lip on the stem forms a metal-to-metal seal against the backseat on the housing. The shape and thickness of the annular lip are designed to allow the lip to flex under the low pressure conditions used in the test and form an effective seal, as the stem is urged against the bonnet by the pressure within the valve body. An annular shoulder is also provided on the valve stem, between the annular lip and the housing. The annular shoulder on the stem is positioned to contact the housing only after the annular lip flexes a sufficient amount to create an effective seal. At pressures higher than the test pressure, the annular shoulder prevents the stem from moving farther toward the housing, thereby preventing overflexing of the annular lip. Positioning of the annular shoulder between the annular lip and the housing allows the internal pressure within the valve to continue to seal the annular lip against the backseat, even after the annular shoulder contacts the housing. The backseat can have a single angled surface, with both the annular lip and the annular shoulder contacting the single surface, or it can have two angled surfaces, with the annular lip contacting one angled surface and the annular shoulder contacting the other angled surface.
The novel features of this invention, as well as the invention itself, will be best understood from the attached drawings, taken along with the following description, in which similar reference characters refer to similar parts, and in which:
As shown in
The backseat surface illustrated has two separate frusto-conical surfaces 28,30, but it can be seen that the backseat surface could also comprise a single frusto-conical surface, if combined with a lip and a shoulder having appropriate sizes and positions to ensure that the lip contacts the backseat surface before the shoulder contacts the backseat surface. That is, with a single frusto-conical backseat surface, the annular lip would simply have a sufficiently larger diameter than shown, to cause the lip to contact the backseat surface before the shoulder contacts the backseat surface.
After the edge 32 of the annular lip 24 contacts the backseat surface 28, internal valve pressure or other influences can cause additional upward movement of the stem 20 relative to the bonnet 12, causing the lip 24 to deflect. The shoulder 26 has a diameter sufficient to limit passage of the stem 20 through the bonnet bore 25, thereby providing an up stop to limit the travel of the stem 20 and the gate (not shown), preventing overstroking or damage to the gate. Specifically, the stem movement is stopped by abutment of the shoulder 26 against the backseat surface 30, as shown in
As the stem 20 strokes a valve, such as in moving a reverse-acting gate valve to the closed position, the flexible annular lip 24 engages the housing or bonnet 12 to act as a low pressure metal to metal seal. Increased pressure inside the valve body forces the stem 20 upward, deflecting the lip 24, until such time as the support shoulder 26 contacts the bonnet 12. At this point, any additional upward force acting on the stem will be translated into a bearing force against the bonnet seat 30 through the support shoulder 26, preventing additional deflection of the flexible sealing lip 24. Controlled deflection of the flexible lip 24 prevents overstressing the lip 24. By sealing on the edge 32 of the lip 24, the design is made more resistant to failure by contamination, as debris is less likely to be trapped between the surface 28 and the edge 32 than between two matching surfaces. A relatively large seat angle is also provided on the backseat surfaces 28,30 and on the shoulder 26, thereby helping to prevent wedging of the stem 20 in the seat 30. Further, as mentioned above, by virtue of the positioning of the up-stop shoulder 26 above the sealing lip 24, between the housing bonnet 12 and the flexible lip 24, unintentional sealing at the up-stop shoulder 26 does not interfere with pressure acting to assist the flex lip 25 in sealing.
While the particular invention as herein shown and disclosed in detail is fully capable of obtaining the objects and providing the advantages hereinbefore stated, it is to be understood that this disclosure is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended other than as described in the appended claims.
