1. Technical Field
The present disclosure relates to methods and valves for controlling the flow of fluid through a bore and more particularly, the disclosure relates in some embodiments to methods and ball valves for use in the oil and chemical process industry. More particularly, this disclosure relates to double piston, trunnion-mounted ball valves.
2. Background Art
Ball valves are commonly used in both the oil and chemical process industries. A type of ball valve used to control flow of a fluid is an apertured ball valve such as is disclosed in PCT Patent Application No. WO 93/03255 published on Feb. 18, 1993, incorporated by reference herein. In an apertured ball valve the valve operation or function may be broken down into two separate stages. Firstly, the ball moves between an open and a closed position by rotating through 90 degrees such that the ball aperture from an orientation coaxial with the flow direction, i.e. when the valve is open, to a position whereby the ball aperture is normal or perpendicular to the flow direction and the valve is closed. Secondly, the valve seals in the closed position to prevent flow through the bore across the ball valve. Therefore, the on-off control of flow through the valve is achieved by rotating the ball through 90° within the valve housing. Another ball valve is disclosed in U.S. Pat. No. 6,708,946, the teachings of which are incorporated herein by reference.
There are two basic types of ball valve mechanisms which currently exist. First, there is the trunnion mounted ball system in which the ball element is positionally constrained inside the valve, usually by radial bearings. The ball is rotated by the application of torque through a valve stem to the trunnion. Sealing occurs as a result of the valve seat on the upstream (or high pressure) side of the valve “floating” onto the ball element and causing engagement between a surface of the valve seat and the surface of the ball. The advantage of this system is that it provides highly reliable rotation between the valve open and the closed positions. The principal disadvantage of this system is that seal reliability is reduced because the sealing force only develops in proportion to the annular area of the valve seat. Further, in high pressure applications, the force exerted on the ball on the upstream side of the valve can result in deformation of the ball and leakage between the ball and another valve seat located on the downstream (or lower pressure) side of the valve. Thus, when trunnion mounted ball systems are used in high pressure wells and especially those in which the well fluid has a high proportion of particulate matter, being generally known as “aggressive” wells, the pressure is such that fluids and/or particulate matter may leak past seals between the ball and the valve seats. This often results in the valve not achieving integrity of sealing. In such cases, this type of ball valve is unable to operate properly in such conditions.
The second type of ball valve mechanism which effects the abovementioned function is known as the “floating ball system”. In this system the ball is not positionally constrained relative to the valve body. Rotation is caused by the application of force to a point which is offset from the ball centre which, in conjunction with the mating curvatures of the ball and seat, cause the ball to rotate. Sealing occurs as a result of the ball “floating” onto the valve seat. The advantage of this mechanism is that the reliability of the seal is increased, because the sealing force develops in proportion to the circular area of the ball to seat contact. The disadvantage of this type of mechanism is that the rotational reliability is reduced as the friction factor between the ball and seat are considerably larger than that of trunnion mounted devices. With high pressure and aggressive types of wells and particulate flows of the type described above, the reliability of this valve in those applications creates a problem in that the torque necessary to rotate the ball becomes excessively high, and thus, the valve can seize between the open and the closed position giving rise to serious problems in both operational and safety terms.
It would be desirable to provide a method and/or improved ball valve design which may obviate or mitigate at least one or more of the aspects associated with the aforementioned disadvantages.