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Noise
control (of anything) only becomes an operating concern when sound
generated by some plant process is considered “objectionable,”
whether inside or outside the plant.
This is obviously a subjective characteristic, as a “noisy” plant
process in some remote area attended by only a handful of
technicians will probably be dealt with, internally, on a
microscopic scale: The workers may be required to wear hearing
protection and work primarily within the confines of a noise
attenuating enclosure.
However, when noise is emitted outside the plant, often through a
stack, the noise
becomes a community problem.
All sound that we naturally experience is diminished by moving
through distance
in air. As long as the plant boundary or property line is large
enough, the nearest
non-trespassing observer will be sufficiently far away to not be
disturbed by the
noise of the plant. However, certain noise will not be attenuated
well by distance;
such noise is often “pure-tone” in nature, a noise at a particular
frequency, to be determined by the noise control consultant.
The escalating cost of real estate, the pressure (and profit) to
develop neighborhoods and the resulting urban sprawl has, over time,
brought residents further out into the hinterland. Not infrequently,
plants originally located far from communities and the public, are
suddenly encroached upon by families with homeowner’s associations
and neighborhood noise ordinances.
These requirements are often quite stringent, and the close
proximity of the residents combined with 24-hour plant operation,
can pose significant challenges for plant management.
Typically, noise is referred to in terms of Sound Pressure Level (SPL)
in the Aweighted,
decibel scale (dBA). The “Aweighting” refers to a series of standard
correction levels applied to the various octave bands considered in
a typical noise measurement, which serve to adjust the so-called
“linear” sound pressure levels (measured by the instrument) closer
to the way the human ear works. Generally, when “octave band
analysis” is employed, these bands comprise 32.5 Hz, 65 Hz, 125 Hz,
250 Hz, 500 Hz, 1000 Hz, 2000 Hz, 4000 Hz, and 8000 Hz.
When noise complaints arise, this confrontation usually begins with
candid reports of “noise” from residents. Often the noise emanates
from the plant stack as a pure tone, which means noise at a certain
frequency (125 Hz, for example) is at least five decibels higher
than the noise in adjacent frequencies. In this instance, plant
managers usually find themselves needing
to comply with noise ordinances, however unfair they may appear.
The “pure tone” is one of the largest contributors to offsite
complaints. Eliminating this pure tone usually silences these
complaints, even though overall noise level may not be reduced
greatly.
The noise solution usually has two parts. First, a noise survey
performed by a reputable noise consultant may be required to
characterize the actual noise problem. Having defined the
problem, the second part involves determining the silencing system
required to resolve the problem. In the case of an existing stack
this may mean a so-called “stack-insert”
type silencer. Where the offending noise is the outlet of some
blowdown operation,
a “vent silencer” may be called for.
While the installed result of the vent silencer and stack stuffer
are the same — greatly reduced noise levels, their construction and
design are entirely different. Although pressure drops for either
silencer are low, the stack stuffer is the champion of extremely low
back pressure, typically less than 1.0 inch, water gauge.
This is due to special design with very high open (flow) area,
mandated by installation
right inside an existing stack. Since the stack insert silencer fits
into the existing stack, it has no outer shell, which reduces
silencer cost. As such, external appearance (of the stack) is
unchanged and no additional mounting structure is required, and no
installation preparation is required — further reducing cost. See
photograph.
The vent silencer is usually used in bleeding excessive pressure
(rather than a continuous process). Consequently, back pressure may
be less of a consideration — although it’s typically below 3 psi in
most vent silencers. This design features an outer shell, an
internal diffuser
section, and mounting to a simple structure for this purpose. Often
an optional rain cap is provided, which provides greater longevity.
In either case, the correct application of the appropriate
technology, either the Vent Silencer or the Stack Insert Silencer
means the end of the noise problem with the community. From the
moment of installation, the neighbors breathe a sigh of relief, as
does the plant manager. From
coal tar operations in Illinois to platinum processing in
Pennsylvania, to power generation
and refineries in Canada, South American, Europe and the Middle
East, silencers provide an excellent return on investment, in
dollars as well as improved community relations for the plant.
FSM
Mark Behrens is a project engineer at Quietflo Noise
Control Division in Nanuet,
NY. For more, go to
www.quietflo.com. |
