In typical control valves, a trim assembly, such as a valve cage, may be used to condition the flow of fluid through the control valve, reduce noise, cavitation, and turbulence through the control valve, and in some applications can also be used to provide guidance for a fluid control member or valve plug as the valve plug moves from a closed position in which the valve plug sealingly engages a valve seat to an open position in which the valve plug is disposed away from the valve seat. When the control valve is in the open position, fluid flows from a valve inlet, passes through a passage between the valve seat and the valve plug, passes through the trim assembly where the fluid flow is conditioned, and exits through a valve outlet.
In some applications, the trim assembly/cage will have a plurality of passages formed through a circumferential wall, which are used to reduce the noise produced as the gas passes through the cage. The passages are spaced specifically such that the jets of gas that are produced as the gas exits the passages do not converge and produce aerodynamic noise. Cages used in these types of applications are typically used in a “flow up” orientation (e.g., fluid enters the center of the cage and passes from an inside surface to an outside surface) and the spacing of the passages that is crucial to reduce the aerodynamic noise is on the outer surface of the cage. The spacing of the passages on the inner surface of the cage is also important, as this spacing is used to keep sufficient space between the passages to not allow flow to pass through more passages than desired for accurate flow characteristics throughout the travel of the valve plug.
For solid cages used in applications where the process conditions produce aerodynamic noise as the fluid flows through the control valve, drilled holes through the circumferential wall of the cage are typically used to form the passages. However, drilled hole cages are very cumbersome, time consuming, and costly to produce. Some drilled hole cages may contain thousands of holes and the only real feasible way to produce the passages was to drill them. Acceptance criteria exists that allows a percentage of drill bits to break and be left in the cage and this process requires the use of special drilling machines that have a high degree of accuracy.
In addition to the spacing of the passages on the outer surface of the cage, aerodynamic noise can also be reduced by providing a tortured, or non-linear, flow path for the passages or by varying the cross-sectional diameter of the passages as they pass through the wall of the cage. However, with drilled holes through a solid cage, creating passages having a non-linear flow path or having a variable cross-sectional area is not possible.