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|Ventilation Upgrade Reduces Silica Exposure at Grede Foundries, Inc. Facility in Reedsburg, Wisconsin|
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.