A troublesome phenomenon in severe services is the audible noise produced by turbulence as the fluid moves through a control valve. Noise output is worse for gas services experiencing sonic (critical) flow and for liquid services experiencing cavitation, although it is possible for a control valve to produce substantial noise even when avoiding these operating conditions. Noise produced by a control valve also translates into vibration imposed on the piping, which may cause problems such as loosening of threaded fasteners over time. One way to reduce noise output is to use special valve trim resembling the trim used to mitigate cavitation. A common cage-guided globe valve trim design for noise reduction uses a special cage designed with numerous, small holes for process gas to flow through.
One of the ways in which these small holes reduce audible noise is by shifting the frequency of that noise upward. This increase in frequency places the sound outside the range where the human ear is most sensitive to noise, and it also helps to reduce noise coupling to the piping, confining most of the noise “power” to the internal volume of the process fluid rather than radiating outward into the air.
Fisher manufactures a series of noise-abatement trim for process gas service called Whisper trim. A “Whisper” plug and cage set is shown in this next photograph:
In some versions, the holes are merely straight through the cage wall. In more sophisticated versions of Whisper trim (particularly the “WhisperFlo”), the small holes lead to a labyrinth of passages designed to dissipate energy by forcing the fluid to take several sharp turns as it passes through the wall of the cage. This allows relatively large pressure drops to develop without high fluid velocities, which is the primary causal factor for noise in control valves.
Also Read : Types of Control Valves Noise