Finding the Source of All that Noise(4)

PATH TREATMENT

The path treatment for noise, i.e. suppressing the transmission of excessive noise levels along the path, can be performed by using at least three different tools: silencers, insulation and heavier pipe schedule. While less than ideal, path treatment is an approach that can be considered when vibration levels are not high enough to damage or inhibit the performance of the control loop or if source treatment alone could not bring noise generation down to acceptable levels.

Silencers: Silencers are mufflers used inline or at an outlet leading to the atmosphere to dampen the noise produced. Reactive silencers create frequency interactions to dampen noise; whereas dissipative silencers use sound-absorbing materials such as glass-fiber to dampen the noise. The main use for silencers is on outlets for gas or steam. Typically, a combination of reactive and dissipative methods is used in atmospheric vent silencers.

Figure 2. Jet velocity profile modification: The formation of turbulence in the mixing region between where the jet exits from an orifice and the gas flow at the outlet region as well as attachment and interaction of shock waves (generated during throttling if the flow reaches sonic velocity in the valve) are major sources of noise that can be controlled to an extent by intelligent valve trim design. One way to do this is to smooth the velocity profile of the jet by introducing a lower velocity gas stream alongside the jet, as shown here.Insulation: Pipe insulation may be used to dampen noise in gas and steam lines, particularly in steam lines, where there is already thermal lagging. Thermal insulation can dampen noise 1-2 dBA per 10 millimeters (mm) of insulation thickness (3 to 5 dBA per inch). The maximum is about 12 dBA because of leaks and acoustic bridges. Special acoustic insulation reduces noise up to 4 dBA per 10 mm of insulation thickness (10 dBA per inch). The maximum attenuation for acoustic insulation is 20-25 dBA.

Heavy downstream pipe schedule: A heavier downstream pipe schedule can be used to dampen noise because vibrating pipe wall generates part of that noise. The heavier the pipe wall (the bigger the schedule), the less that wall will vibrate. Note that the noise level inside the pipe does not decrease with a heavier wall pipe. Because of this, the heavier wall pipe should be used for the whole downstream piping to avoid recurrence of the high noise level further down the line, which can be very costly in the case of long pipe runs.

CONCLUSION

Effective aerodynamic noise reduction in pipelines and control loops has become an important consideration for many industries. This is because control of such noise ensures a safe working environment, protects process and plant surroundings, enables stable process control, extends maintenance intervals and ensures legal obligations are met.

While noise can be suppressed after it is generated through path treatment, a generally more effective and less expensive alternative is source treatment. Much progress has been made recently in developing expert software technology for predicting noise generation under actual line conditions and for designing specific valve trim attenuation for aerodynamic noise. Processors who deal with air and gas control applications that have a tendency to generate noise should seek the assistance of control valve vendors that understand and can suggest these technologies.

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