Feasible and Effective
BY DENNIS P. DRISCOLL
OSHA’s proposed reinterpretation of
“feasible” noise controls to mean those
that are “capable of being done” would
require employers to do everything short
of going out of business to control noise
exposures. One noise source likely to be
in OSHA’s crosshairs is compressed air.
In my 30-plus years’ experience in noise
control, I’ve observed that compressed
air is typically responsible for at least 30
percent of noise problems in manufacturing plants.
Pneumatic systems, such as air valves,
cylinders and/or solenoids, use compressed air to power equipment. These
devices generate high noise due to air
discharge and/or excessive pressure settings. Compressed air nozzles are commonly used in production tasks, such as
ejecting product or parts, moving parts
along sorter bowl tracks, evaporative
cooling, drying, and closing flaps on
corrugated containers. Some hand-held
devices, such as air guns, wands, and
brooms, also use compressed air. These
pneumatic systems and devices can generate noise levels in excess of 100 dBA
and are generally the most significant
contributors to exposures above the permissible exposure limit (90 dBA).
Problem and Opportunity
Compressed air, in my experience, is the
easiest source of excessive noise to remedy and represents the greatest potential
for exposure reduction. In addition,
companies often see return on their investment in less than a year.
When reducing noise from com-
pressed air, consider the application of
the air. Noise generated by air exhaust
from air valves, cylinders, and solenoids
is caused by turbulence due to mixing of
gases with widely differing velocities. In
most cases, high-velocity air is ex-
hausted into the stationary air surround-
ing the equipment. The shearing action
that occurs in the mixing regions results
in noise radiation, where the intensity of
the sound is proportional to the velocity
of airflow raised to the eighth power.
Therefore, the first step is to reduce the
air velocity to the lowest setting needed
for the equipment to function properly.
This action alone saves money through
energy conservation and minimizes wear
and tear on the machinery.
If reducing air pressure does
not provide enough attenuation,
try relocating the discharge
point away from typical worker
If reducing air pressure is impractical
or does not provide enough attenuation,
try relocating the discharge point away
from typical worker locations—for example, deep inside a machine’s casing or
into a manifold where it can be piped
away and released in an unoccupied
area. Another option is to install a pneumatic silencer. If the air is used to perform a service, as is the case for air guns
or wands, quiet-design nozzles are available for retrofit to open-ended air lines
or for replacement of standard nozzles.
Other practical options are available.
Attenuation of noise from compressed
air has a relatively short-term payback.
Consider a manufacturer that uses a series of vibratory sorter bowls as part of
its product assembly process. Each bowl
uses five open-ended air lines (such as
those indicated in the photo above) to
move parts around the tracks within the
bowl. The total open area is equivalent to
a 10 mm diameter pipe. The air pressure
is set to 5 bar (72.5 psig), which results in
185 Nm3/hr passing through the open-ended lines. At an average cost of $0.015
per 1 Nm3/hr and an estimated use time
of 40 percent, this equates to 704 hours
of consumption per year. Thus, the annual cost for the open pipe is $1,953.60
(185 Nm3/hr × $0.015 × 704 hours).
A Silvent 705 quiet-design nozzle not
only reduces the noise level by 20 dBA,
it provides the same airflow service at
only 95 Nm3/hr for an annual cost of
$1,003.20 per sorter bowl—a savings of
$950 over the open pipe scenario. The
Silvent 705 nozzle costs approximately
$200, so the financial payback period for
retrofitting five nozzles per bowl is approximately one year.
The use of compressed air systems is by
far the primary source of noise in manufacturing plants. Fortunately, noise control for compressed air is relatively easy
to accomplish, often saves energy, and
yields short-term return on investment.
This is a no-brainer, folks.
Dennis P. Driscoll, PE, is principal consultant at
Associates in Acoustics, Inc. He can be reached
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