Brochure

Science of Noise
4 | The Silent Treatment
Understanding the sources of noise and the nature of noise-
generating mechanisms is the foundation for designing and
implementing effective measures to control noise.
Noise Definition
Noise is the unwanted or undesirable sound produced by
process control equipment, including control valves. Sound
is produced by fluctuating pressure waves that arise from the
flow of fluid through the control valve. The specific sources are
either mechanical or fluid structure in nature. Dominant fluid-
generated sources are specific to hydrodynamic (liquid) flow
and aerodynamic (gas or vapor) flow.
These pressure waves are characterized by both amplitude
(loudness) and frequency. Amplitude is measured in decibels
(dB), a logarithmic measure of relative sound power level.
Since the units are logarithmic in nature, a 3 dB increase
represents a doubling in sound power level. Frequency
is measured in Hertz (Hz) or pressure cycles per second.
Industrial noise levels are often expressed in a form that takes
into account human sensitivity to frequency. This is known
as “A-weighting” and the units of measure are A-weighted
decibels or dBA.
Mechanical Noise
Mechanical noise results from the physical vibration of
components in control valves such as worn plugs rattling
in cages. Sound levels generated by this type of noise are
typically in a low frequency range of less than 1,500 Hz.
Another source of mechanical noise is resonance of movable
parts within the control valve, which are characterized by a
single pitch, or tone, and may reach frequencies of 7,000 Hz.
Hydrodynamic Noise
Hydrodynamic noise occurs in liquid flows and is predominately
caused by cavitation. Cavitation consists of the formation
and collapse of vapor cavities in the flowstream. This occurs
when the pressure drops to the vapor pressure of the fluid.
The energy released in this process is converted into pressure
fluctuations that create sound waves. This noise occurs over a
wide frequency range and is often described as sounding like
gravel flowing through the pipe.
Other possible noise sources include fluid turbulence and
flashing. However, testing has shown that noise generated
from these mechanisms is generally not problematic.
For more information on hydrodynamic noise,
scan the QR code or go to Fisher.com/D351912
to download the Fisher Cavitation-Control
Technologies brochure.
Throttling across the control valve generates high turbulence.
Turbulence will eventually decrease but the acoustic pressure field
will persist.
Aeodynamic Noise Sources
The primary source of aerodynamic noise is fluid turbulence.
There are several locations where turbulence can be a problem.
Two locations of interest are within the control valve body.
First is the throttling region where the relative fluid pressure is
low and the fluid speed is high. High levels of fluid turbulence
can result from the jets formed in the control valve trim. The
second location is the region between the control valve trim
and the body wall where high speed fluid impingement can
result in significant turbulence. These two sources of noise are
called the trim or valve noise source.
Another location of interest is the region downstream of
the control valve trim. Here the velocity-related turbulence
in the control valve outlet and downstream pipe acts as
an independent noise source. In effect, there can be two
independent noise sources in each installation.
Emerson testing and IEC standards validate that significant
noise is generated by fluid expansion when control valve outlet
velocity exceeds a Mach number of 0.3.
As the fluid moves downstream through the pipe the
turbulence will decrease, however, the acoustic field persists.
Depending on the pipe diameter, material, and wall thickness,
noise at some frequencies will pass through the pipe wall (sound
transmission) and create sound waves that people can hear.
Sources of noise can be classified as either a point source or
a line source. Noise from a point source radiates in a spherical
shape from the center and sound pressure levels decrease by
6 dBA with every doubling of the distance from the source.
An example of a point source is an atmospheric vent or flare.
Noise from a line source radiates in a cylindrical shape from the
center and sound pressure levels decrease by 3 dBA for every
doubling of the distance from the source. An example of a line
source is a pipeline.