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Silica Dust Control Improvement: Grinding of Iron Castings with Portable Tools at Kennedy Valve Foundry in Elmira, New York |
Background
The control of airborne silica dust is a constant challenge in a sand casting
foundry, where large volumes of sand are used in the molding processes. Kennedy
Valve, a sand cast iron foundry in Elmira, New York that manufactures fire
hydrants and valves for waterworks applications, developed an innovative
approach to the ventilation of its grinding operations that helped overcome some
of these challenges. This case study describes the approach and successes
achieved by Kennedy Valve.
Kennedy Valve, a division of McWane, Inc., has more than 430 employees. McWayne
has implemented a comprehensive environmental, health, and safety management
system based on International Organization for Standardization (ISO) 14000,
Occupational Health and Safety Assessment Series (OHSAS) 18001, and OSHA’s
safety management guidelines. One of the foundations of this management system
is the commitment to search for innovative engineering solutions that will
reduce hazards in the workplace.
The demanding foundry environment presents unique safety and environmental
challenges. To meet these challenges, Kennedy Valve involves all levels of
employees in safety and health teams and other facets of the overall safety
program. Health and Safety Department staff, along with external experts,
coordinate all of Kennedy Valve’s industrial hygiene and occupational health
programs.
The Problem: Search for an Exposure Reduction Approach
One challenge that frequently arises in a sand foundry is the control of
airborne silica resulting from the chipping and grinding of castings,
particularly when portable tools are used. While workers were protected in these
jobs through a combination of personal protective gear (primarily respirators)
and ventilation, Kennedy Valve searched for an engineering solution that would
provide a more consistent and higher degree of protection for workers against
overexposure to silica at these work stations. Finding an engineering solution
proved especially difficult because of the limitations of current ventilation
controls for this process. Ventilated tools have not yet proved feasible for
this work, while the protection offered by stationary exhaust hoods, such as
downdraft or backdraft benches, is limited if the stream of particles (grinding
swarf) emitted from the tools cannot be continuously directed at the exhaust
openings.
The Solution: New Approach for Ventilation Controls
Kennedy Valve determined that it needed to devise a new approach for ventilation
controls for portable grinding tools on sand castings nearly three feet wide.
Kennedy Valve augmented its technical team with a foundry ventilation consultant
to investigate the feasibility of silica exposure reduction from the grinding
process. While the foundry already used grinding benches with backdraft slots,
Kennedy Valve sought more effective controls.
After a broad search of information on available ventilation control methods for
grinding with portable tools, the team identified a ventilation approach which
had been demonstrated to be effective in controlling emissions from another
foundry process, called air carbon-arc gouging, conducted on work benches with
steel castings. This method had been identified and documented by the National
Institute for Occupational Safety and Health (NIOSH) as Case History #6 in their
"Evaluation of Occupational Health Hazard Control Technology for the Foundry
Industry" (NIOSH Publication No. 79-114).
The air carbon-arc gouging process is as least as difficult to control as
portable grinding. The tabletop booth presented in NIOSH Case History #6
incorporated a wrap-around design, a three-foot diameter turntable for casting
repositioning, and a unique way of introducing supply air so that it swept past
the worker on both sides of the body (Figure A-1). This design appeared to
incorporate the best features seen to date on a ventilated booth. The team
decided to determine whether the design could satisfy the ventilation
requirements for grinding castings at Kennedy Valve.
There was one design characteristic of the booth that NIOSH evaluated on
fume-producing processes that appeared as though it could create a rebounding
issue when applied to grinding. That characteristic was the use of spaced
exhaust openings along flat collecting surfaces. As shown in
Figure A-2, respirable-sized dust follows in the low pressure wake of the large (inertial)
particles in the grinding swarf. If the large particles rebound off of a solid
wall, the dust will rebound with them and head toward the worker's breathing
zone.
An industrial ventilation designer working on the team who is also a firearms
instructor offered a way to address this issue. He cited the method of stopping
air rifle pellets using an energy-absorbing hanging curtain. In this case, if
the grinding swarf impacted a hanging curtain, the large particles would be
stopped "in their tracks" and be unable to rebound (Figure A-3). The fine dust
particles at that point would be pressed up against the curtain. If vertical
dividers were employed to restrict sideways air motion, the fine dust could be
readily directed through suction into exhaust plenums both above and below the
impact zone for the grinding swarf and be removed from the bench (Figure A-4).
Demonstration of Dust Control Effectiveness.
Figure A-5 shows the
control method selected for the demonstration phase at Kennedy Valve. A
prototype grinding booth was constructed for the demonstration and tested in an
isolated part of the facility to eliminate the potential for cross-contamination
from any other silica-producing process. Figure A-6 shows the grinding booth
following the prototype demonstration, in its current form for production
grinding. Castings are moved on and off of the work surface by overhead hoist.
After the castings are set down on the workbench, they may be rotated via a
turntable for better access to surfaces to be ground, to assist the ergonomic
aspects of the work, and to allow the grinding swarf to be directed as far as
possible into the capture zones. The worker can initiate the turntable using a
"bump" switch which does not require hand use and thus does not slow down the
grinding operation.
In the process of evaluating the ventilated grinding booth, respirable dust was
measured in the breathing zone of the grinding operator and in the general
background air next to the grinding booth. For this purpose, two real-time
particle sensors were used. Each sampling inlet was fitted with a cyclone to
remove the non-respirable portion of dust. These instruments simultaneously
logged respirable dust during grinding operations that were also video recorded.
The particle sensor data was downloaded to a computer and graphed.
Since the particle sensors did not provide a real-time measure of silica in the
dust, the data was used to evaluate general dust control from the process.
Follow-up testing was therefore conducted to determine the time-weighted-average
(TWA) of respirable silica dust throughout a workshift in which both respirable
dust levels and silica content of that dust were measured and averaged.
The first testing was done on a single type of casting (i.e., large fitting with
four-inch flanges) selected to eliminate variability of the work in the initial
assessment. From a visual standpoint, the only obvious exposure was associated
with the manner in which dust was emitted from the inside of the casting in a
"chimney effect" during internal grinding. This dust was discharged very close
to the breathing zone before it was withdrawn by the push of the supply air and
the pull of the hood exhaust. The dust produced by one particular task was
directed in such a way that the exhaust hood could not directly capture it.
The task in question involved grinding a portion of the casting which was
overhanging the front edge of the bench. The grinding swarf was directed
downward toward the floor. This type of grinding did not appear to affect the
dust exposure to the same extent that the chimney effect did, but the dust
induced into the grinding swarf was definitely fugitive to the ventilated
grinding bench. For this reason and also because of the anticipated housekeeping
issue associated with directing grinding swarf at the floor, the decision was
made to alter the basic prototype bench to add a small hood in front of the
grinding bench to capture the grinding swarf and the dust induced with it when
grinding was conducted on the overhanging portions of castings.
The tests also included measurements made without the supply air operating. The
absence of supply air almost doubled the average dust exposure level. It is
postulated that supply air improved the speed of dust removal by pushing the air
which was just in front of the operator into the capture hood, thus reducing the
residence time of this potentially dust-laden air in the breathing zone.
After it was confirmed that the supply air was important to the protection
offered by the ventilated bench, it was decided to try to optimize the
interworking of the exhaust and supply airstreams to enhance the benefit
received. It had already been established that testing would be conducted over a
range of exhausts from roughly 2,000 to 6,000 cubic feet per minute (CFM).
Dampers had been installed to adjust airflow rates and a flow meter was used to
measure these rates. Three exhaust rates were tested and at each of these
exhaust rates baby powder was used to set supply air at what appeared to be the
most effective, non-turbulent pattern.
The time-weighted-average dust concentrations measured during grinding of the
test casting at three different ventilation rates are presented in
Figure A-7.
The lowest personal dust exposure occurred at the middle ventilation condition.
This finding was consistent with expectations for, and observations of, the
capture efficiency of the grinding bench. At the lowest ventilation rate,
evacuation of dust appeared to be complete but not rapid. The dust seemed to
dwell above the bench briefly before being extracted. At the other end of the
spectrum, the highest ventilation rate produced a near-turbulent condition which
tended to spread the dust. Area samples taken simultaneously with the personal
samples showed consistent readings for the lower two ventilation rates, with a
sharp increase at the highest ventilation rate. This finding is consistent with
the observations summarized above, which suggested that turbulence could lead to
dust loss from the capture zone of the bench.
Full Scale Operations. After successful completion of the
prototype test program, 15 production booths were constructed and installed in
the renovated finishing area. These benches have consistently controlled silica
exposures during grinding to below OSHA’s Permissible Exposure Level (PEL) for
Kennedy Valve’s grinding needs when operating at exhaust rates down to 3,000 CFM
and supply airflow rates at half of that flow rate. The supply air is provided
by a tempered (i.e., heated in winter) makeup air unit with proportional heater
control for steady temperature conditions.
The particles which are "stilled" through contact with the enclosure walls
readily fall out into collection areas in the workbench. This feature has
reduced the loading of abrasive particles on the filter media in the baghouse.
Foundries considering this type of approach should perform prototype
demonstrations before proceeding with full scale operations.
The expected effectiveness of this exposure reduction approach depends on a
number of variables, including:
-
Extent of sand burn-in/burn-on on the castings.
-
Effectiveness of shot blasting in removing sand material
from surfaces.
-
Amount of grinding performed on a casting.
-
Proportion of grinding internal to the casting.
-
Ability to perform all grinding on top of the workbench.
The Impact
Kennedy Valve workers performing this grinding have long been protected by
ventilation controls, augmented as needed by respiratory protection. The
improvement program described here has resulted in consistent levels of silica
exposure below the OSHA PEL.
Although the engineering controls have been effective, the worker in
Figure A-6
voluntarily continues to wear the air-supplied helmet. This worker and other
workers appreciate the eye and face protection and the stream of air circulating
around the head. In addition, spikes in exposure are not impossible with any
local ventilation method applied to manual processes; the air-supplied helmet
provides a safety factor against these spikes. In sum, Kennedy Valve is closer
to its goal of state-of-the-art processes, including worker protection, for
cleaning a variety of castings.
Source: Arne Feyling, Assistant General Manager; Mike Maziur, Plant Manager; Tom
Shaw, Health and Safety Manager, Kennedy Valve, Elmira, New York
________________________________________
The views expressed herein do not necessarily represent the official position or
policy of the U.S. Department of Labor (DOL).
-- As of June 2009.
 
Figure 1. Grinding hood with supply air plenums. NIOSH foundry ventilation assessment.
Figure 2. Effect of inertial particle rebound from a rigid surface causing respirator particles to escape from the exhaust plenum.
Figure 3. Effect of inertial particle not rebounding from an energy absorbing surface causing respirable particles to remain in the exhaust plenum.
Figure 4. Exhaust patterns in the grinding hood equipped with impact absorption curtains to trap respirable particles in the exhaust plenums.
Figure 5. Effect of supply air on worker protection from dust exposure.
Figure 6. Production module of grinding booth during grinding of a casting.
Figure 7. Measurements of respirable particulate matter for three different
castings and at different grinding bench ventilation rates.
TEXT VERSION OF FIGURE 7:
Chart Title: Measurements of Respirable Particulate Matter
Chart Type: Vertical Bar
Chart Elements: 6 elements - Two bars each measured by respirable particulate
matter (mg/m3) for three different castings at different grinding
bench ventilation rates.
Values:
- Exhaust (CFM) = 2,016
- Supply Rate (CFM) = 1,554
- Casting = 4" flange
- Personal Air Sample = 0.754
- Area Air Sample = 0.598
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- Exhaust (CFM) = 4,032
- Supply Rate (CFM) = 2,190
- Casting = 4" flange
- Personal Air Sample = 0.551
- Area Air Sample = 0.596
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- Exhaust (CFM) = 6,250
- Supply Rate (CFM) = 2,684
- Casting = 4" flange
- Personal Air Sample = 0.813
- Area Air Sample = 1.229
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