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Ventilation Upgrade Reduces Silica Exposure at Grede Foundries, Inc. Facility in Reedsburg, Wisconsin |
Background
Grede Foundries, Inc. has eight foundries in the United States and a joint
venture in Mexico that produce gray iron, ductile iron, steel, and aluminum
castings. Grede’s Reedsburg, Wisconsin, plant produces ductile iron castings.
The 300,000 square-foot facility can generate up to 130,000 tons per year of
ductile iron castings, including calipers, crankshafts, flanges, yokes, bearing
caps, carriers, cases, knuckles, and exhaust manifold castings.
The safety and health culture established at Grede Foundries can be summed up
with the following statement:
"No job is so important and no service so urgent
that we cannot take time to perform our work in a safe and healthful manner."
Grede’s safety and health record has been continually improving for more than a
decade. As part of its efforts to improve is safety and health performance,
Grede adopted a safety manual customized for its facilities. The manual
incorporates OSHA and State regulations and many best management practices.
Other components of Grede’s safety and health program include employee training,
attitude seminars, and safety audits. These measures, along with hard work and
commitment, enabled Grede to reduce the incidence of OSHA recordable cases to
1.9 per 100 employees in 2006. The national average for iron and steel foundries
was 14.8.
Although the company’s safety and health record exceeds the industry average,
Grede believes safety and health is a managed function and that job-related
accidents and illness can be managed out of the workplace. Part of this effort
involves monitoring the air quality in the workplace. Grede Foundries has a very
aggressive industrial hygiene testing program and works diligently to minimize
employee exposures to hazardous substances using engineering controls, employee
rotation, and respiratory protection devices.
The Problem: Process Background
Over the last several years, the Grede facility in Reedsburg has been working to
reduce employee exposure to silica in the knockoff areas, with an emphasis on
eliminating the source of the dust. The knockoff process is conducted by one
operator working in a booth. The worker manually strikes the castings against
the edge of the vibratory conveyor or against the top of the metal sorting
barrier. The worker’s trunk is bent slightly forward over the conveyor during
the knockoff work. Dust is generated during the vibratory conveying of the
castings through the booth and by the knockoff process itself. As much of the
residual sand as possible is removed from the castings before they enter the
booth on a vibratory conveyor. Minimizing adhered sand is essential to limiting
dust exposure for workers whose breathing zones are close to the dust source.
At the workstation, a local exhaust hood behind the conveyor was intended to
pull dust back away from the worker and evacuate the dust (Figure 1). Supply air
was located above and behind the worker to support this airflow pattern and keep
the air environment inside the enclosure well diluted. The exhaust rate from the
booth was around 8,000 cubic feet per minute. In order to allow the operation of
an auto knockoff device on a monorail, the top of the side draft hood was angled
back from the base (the top of a side draft hood normally angles forward from
the base). The "opposite" angle of the side draft hood drew the emissions away
from the point of dust generation (vibratory conveyor) in an arced pattern which
raised the airborne dust toward the breathing zone before drawing the air into
the capture hood.
The forward leaning of the worker’s upper body caused the air to "roll" in front
of the worker and confine some of the dust generated by the process into the
vicinity of the breathing zone. Makeup air was pushed unrestricted into the
plenum over the worker’s head. Most of the makeup air exited the front of the
plenum and went directly to the collection exhaust hood, which was believed to
multiply the "rolling" action of the air in front of the worker. Much of the air
behind the worker was stagnant and became saturated with dust and silica. When
the worker stood up straight during process breaks, the worker’s breathing zone
was in the stagnant dust-saturated air.
The Solution: Process Changes Made to Reduce Exposure
The makeup air was redistributed through the air supply plenum over the worker’s
head by dividing the plenum in two sections, front and back (Figure 2). The
front section (closest to the worker) would have a controlled airflow velocity
of 125 to 175 feet per minute using 25 percent perforations and the back section
would have a lower airflow velocity with 50 percent open perforations.
A baffle plate was added in front of the worker near the back side of the
conveyor. The baffle plate extended from the ceiling of the booth downward to
the middle of the exhaust inlet opening. A rubber material was used for the
baffle plate to prevent injury if the worker accidentally struck the plate while
raising his or her hand/arm during the knockoff process.
The redistribution of the air in the plenum above the operator caused all the
air in the booth to move downward to about waist level and then over to the
collection exhaust hood. The baffle caused the dust emissions from the conveyor
to cut under the baffle and "roll" behind the baffle away from the worker’s
breathing zone.
The Impact: Benefits of the Process Changes
Silica exposures were reduced by more than 50 percent to a level well below the
OSHA permissible exposure limit. Workers are no longer required to wear a
respirator during this operation. Because there is air flow throughout the
entire booth, the workers indicate they feel that the booth is cooler during the
warmer seasons. In addition, because the makeup air no longer blows in one small
area, the employees do not turn off the makeup air unit during the cold season.
Figure 1. BEFORE
Figure 2: AFTER. Changes made to improve dust capture during knockoff
operations.
Source: Peter Mark; Senior Safety, Health, and Environmental Engineer; Grede
Foundries, Inc.; Reedsburg, Wisconsin
________________________________________
The views expressed herein do not necessarily represent the official position or
policy of the U.S. Department of Labor (DOL).
-- As of September 2008.
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