Finding the Source of All that Noise(2)

SCIENCE OF PREDICTING NOISE LEVEL

Because source reduction is the best way to avoid excessive noise, this article will consider that subject shortly. But first, the issue of how to know if a noise problem will occur in a system must be addressed. The place to start is experience: Professional plant operations personnel know that certain control valve and piping configurations are inherently noisy. Valves in these services need to be sized very carefully with noise attenuation in mind.

Beyond experience, however, is developing an understanding of conditions most likely to generate noise. At low-pressure ratios, for example, the cause of aerodynamic noise is turbulence. At high-pressure ratios, the turbulent interactions of the shock waves become the major source of noise. A very potent determinant of noise is flow velocity because dBA levels increase geometrically (to the fourth, sixth and even eighth power) as flow velocities increase.

While this terse summary provides a simplistic explanation of where noise comes from, predicting noise generation in air and gasses is actually a very complex challenge. For example, scientists study noise at subsonic and supersonic velocities, and for subsonic sources, the noises in processing could be monopole, dipole or quadrapole (measurements of how sound waves radiate), with each of these levels adding more complexity to the equation. Also, noise created by the interplay of shock waves and turbulence can be influenced by the presence and geometry of physical boundaries in the vicinity of these interactions.

Shock waves that are perpendicular to a free jet stream are called normal or direct shocks. A shock wave can also be oblique if the flow passes a wedge or sharp object or if supersonic flow is forced to change direction by a solid boundary. Shock waves can be reflected from solid boundaries and can penetrate one another. A control valve outlet flow stream can create an oscillating chain of compression shock waves with declining cycles and intensity—a very powerful noise source.

With computational fluid dynamics, shock waves can be simulated in different geometries. Normally, super computers with substantial memory and processing power are used because the calculations required are long and complex. The results that these state-of-the-art computer programs produce are used in the design of low-noise trims and attenuators.

EXPERT SIMULATION AND SIZING

The scientific study of what creates noise in process lines is ongoing. However, after decades of research, much has been learned that can translate into equations to predict, with high levels of accuracy, the noise-generating potential for most piping and control valve configurations. These equations are available to process engineers within the International Electrotechnical Commission and the German Engineering Federation (VDMA) standards.

Control valve vendors have specialists who understand these variables and can work with processors to predict and modify the noise-generating characteristics of process loops. Some even have expert software developed to simulate line conditions for making realistic predictions of flow and noise generation. This expert software allows an experienced user to swap valves, trim and other components until the most favorable combination of noise attenuation and process performance has been achieved.

If a control valve vendor can simulate control loops with expert sizing software, it is likely that vendor can detect potential noise generation problems in advance and minimize those noises using cost-effective source treatment approaches. In such cases, high levels of mechanical vibration, which are always associated with noise, can be minimized or eliminated at the outset to ensure greater control reliability and process uptime.

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