This invention relates generally to ball valves and, more particularly, to top entry ball valves.
Ball valves are well known and generally consist of a valve body having a central valve chamber, a substantially spherical valve member positioned in the valve chamber, and one or two seat members between the valve member and the ends of the valve chamber. The valve member has an internal passage therethrough which forms a flow path from valve inlet to valve outlet when the valve is in the opened position. A stem projecting through the valve body provides means for rotating the ball 90.degree. between the opened and closed positions.
Originally, seat members for ball valves were formed of various flexible materials, e.g. rubber. This was especially suitable for top entry ball valves; i.e. where the valve chamber has an open top and the ball and valve seats are inserted through this opening, and the opening closed by a bonnet. Top entry construction is beneficial in that it permits removal and replacement of valve seats and the valve member without removal from the flow line of the entire valve structure. Further, the top entry ball valve lends itself to unitary body structure which is capable of enduring line strain much more efficiently than bodies having joints.
Since the flexible seats were easily deformable, no great problem was presented in compressing the seats and valve members together and inserting them as a unit into the valve chamber. However, with increased frequency, valve seats are being formed of more rigid material, e.g. Teflon, which are not easily deformed and therefore present problems of assembly and repair.
As stated previously, most seals have been of the compression type, i.e., each seat is compressed between the valve member and an end wall of the body causing the spherical surface of the ball to maintain intimate contact with the seat, thereby establishing a seal between the body and the seat member and between the valve member and the seat member. Obviously, after a period of time, sufficient wear would result in a decrease of the initial built-in compression, resulting in a leaky valve. It is for this reason that the more sophisticated seat materials are used. However, their greater rigidity makes top entry a problem.
Several techniques have been employed to accommodate rigid seating members while taking full advantage of top entry construction. For example, the valve chamber may be tapered inwardly from the top, thereby resulting in an upper opening sufficiently large for insertion and retraction of the ball and seat assembly. Clearly, precautions must be taken using this type of arrangement in order to insure proper sealing between the ball, seat and body.
A further solution, taught in U.S. Pat. No. 3,067,978, employs a ball having an axial length substantially less than its diameter. In this way, when the ball is in the opened position, it may freely enter the valve housing from a top opening. However, this arrangement provides no sealing between the ball and valve seat when in the opened position, thereby increasing the possibility of leakage.