THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

396 14. Machinery Noise ControlGaseous Flows in Pipes or DuctsVelocities in pipes or ducts usually occur in the subsonic range, but where there arevalves or vents present to control the flow, extremely high noise levels can occur.Noise levels have been measured as high as 140 dB downstream of reduction valvesin large steam pipes. In many cases, pressure drops across a control or regulatorvalve are sufficiently large to choke the flow at the discharge, with the result thatthe flow of the gas jet is sonic or almost sonic with corresponding generation ofhigh intensity aerodynamic noise. This noise can propagate through the pipe wallsinto the immediate surroundings, and what is even worse, it can propagate almostunabated downstream with very little attenuation.Because of the complexity of the noise source mechanism and the degree ofuncertainty in transmission loss of the pipe and ducts, it becomes quite difficultto predict the magnitude of the aerodynamic noise. But some guidance can bederived from empirical data, which can be used to establish first-order estimates.The turbulent mixing areas downstream of the valve constitute the principal regionof noise generation. But if valves are encased in thick housings, the noise levels aretypically 6 to 10 dB lower. The spectral character of the noise resembles that forhigh-velocity gas jets, i.e., there are peak levels present in the range of 2–8 kHz. Inshort, it can be expected wherever high-pressure steam and gas flows are regulatedor discharged through valves, noise levels exceeding 100 dB will most likely occur.The valves will have relatively thick walls, so the piping system itself, downstreamof the valve, is the primary source of externally radiated noise.Three basic approaches can be considered in reducing the noise from the controlvalve regions, namely, (a) revising the dynamics of the flow, (b) introducing anin-line silencer to absorb acoustic energy, and (c) increasing the transmission lossin the pipe walls.Of the three approaches just mentioned, altering the dynamics of the flow, whichmeans actually reducing noise reduction at the source, is probably the most preferredmethod. Changing the dynamics entails reducing the flow velocity via multiplestages of pressure reductions or diffusion of the primary jet. Figure 14.16shows multiple stages of pressure reduction with the use of expansion plates. Theflow velocity is lessened sequentially in each expansion chamber. These plates alsoact as a diffuser, reducing turbulent mixing. As much as 20 dB noise reduction haveFigure 14.16. Multiple stages of pressure reduction in a throttling system with use ofexpansion plates.