The present invention relates generally to gate valves. More particularly, the invention relates to small bore gate valves for use with subsea oil and gas well equipment.
The construction and operation of gate valves are well known in the art. In order to open a typical gate valve, several forces must usually be overcome. The first force is the gate-seat drag, which is related to the area of the flow bore and the working pressure of the gate valve. The second force is the stem-packing drag, which is related to the sealing diameter of the stem. The third force is caused by any unbalanced fluid forces acting on the stem.
In gate valves with relatively large bores (that is, those several inches or more in diameter), the required opening force is dominated by the gate-seat drag. This is due to the fact that the gate-seat drag is proportional to the square of the flow bore diameter, while the stem-packing drag is directly proportional to the stem diameter. The challenge in designing gate valves with relatively small flow bores, for example, less than about an inch in diameter, is that as the diameters of the flow bore and the stem are scaled down, the drop in the gate-seat drag is much greater than the drop in the stem-packing drag. Eventually, the stem-packing drag begins to dominate the required opening force. This sets a lower limit on the stem diameter if a desired opening force is to be maintained. Consequently, the overall dimensions for small bore gate valves are often disproportionately large when compared to large bore gate valves.