Finding the Source of All that Noise(3)

SOURCE TREATMENT

A variety of methods exist that can minimize the potential for noise generation. The most common are acoustic control, velocity control, location control and diffusion. Recently, trim has been defined specifically to address these types of aerodynamic noise.

Acoustic control: Two common methods of minimizing acoustics of control valves are dividing flow into multiple streams and modifying the acoustic field. Flow division into multiple streams is effective because the intensity of noise generated by a single orifice decreases dramatically when hole diameter is decreased. Thus, many small holes will attenuate noise more effectively than one large hole. A rule of thumb is that each doubling of the number of holes reduces noise by 3 dBA.

Increasing the number of flow passages by making them smaller also affects noise frequency distribution—the smaller the passage, the higher the noise frequency. The pipe wall easily attenuates high-frequency noise. High frequencies are also less of a problem because they extend beyond the capabilities of the human ear.

Modifying the acoustic field directs the flow path in such a way that the peak noise field is broken up into a more diffused space.

This figure shows that sound intensity increases faster as velocity of the stream increases. When the velocity passes into the supersonic range, shock waves increase and noise levels spike. If the increase in noise remained proportional it would have followed the green dotted line.Velocity control: Controlling the maximum fluid velocity inside a control valve trim is an effective way of controlling noise at subsonic flow velocities because the acoustic intensity of a jet is proportional to the sixth power of the flow velocity in a system with solid boundaries, such as valve trim or a pipe. The control valve trim velocity can be controlled most effectively by using a multistage pressure drop and by increasing the valve trim outlet area in a manner in which the flow velocity and pressure at the valve outlet are minimized and gas volume is maximized. The successive stages are spaced so that gas pressure is allowed to recover to an intermediate level and velocity before the next throttling stage. This intermediate recovery of pressure and the resulting velocity reduction prevents the fluid from reaching the velocity of a single-stage pressure drop system.

Location control: Two major sources of noise are turbulence that forms in the mixing region between where the jet exits from an orifice and the gas flows at the outlet region and attachment, and the interaction of shock waves. The piping system should be designed following documented best piping practices. Such practices specify factors such as the distance from transition pieces to valves for the purpose of delivering flow in as laminar a condition as possible (thereby limiting turbulence). Location control is employed to make sure the jet streams within and departing from the trim are located and shaped to minimize the potential for noise generation. One way to do this is to smooth the velocity profile of the jet by introducing a lower velocity gas stream alongside the jet.

Diffusers: Diffusers have fixed area flow resistances and are custom-made for a particular flow condition. Dividing the pressure drop between a control valve and a downstream diffuser provides an effective way to further increase the noise attenuation in cases where the control valve has a constant, high-pressure drop, and the flow rate is relatively constant.

Aerodynamically specific trim: Typically, source treatment approaches applied within and around the control valve can solve most noise generation issues. Valve manufacturers have been studying and refining approaches to noise and anti-cavitation trim for decades now. As a result, combinations of all of the commonly used source treatment approaches can be applied to noise attenuation within a single device.

igure 1. Trim designed to reduce aerodynamic noiseFigure 1 shows a rotary control ball valve with a trim design that incorporates pressure staging, flow division, peak frequency shifting and velocity control within one rotary control device, in this case a ball valve. This design concept, which was introduced more than two decades ago, has been refined recently to provide greater precision in attenuating high aerodynamic noise levels in gas and steam applications. The result is noise attenuation that is increased from 15 dBA to 30 dBA. Users with noise control issues within gas and steam control loops should be aware that new trim technology addresses aerodynamic noise specifically.

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