Dust off your foundry's silica control measures: By understanding how to manage silica dust emissions in your foundry, you can make your control systems work more efficiently.
In light of OSHA and other groups' increased emphasis on silica exposure limits for workers (see "OSHA Silica PEL Update" sidebar), this article will address different methods to reduce employee exposure in a foundry. An appropriate approach to silica dust control must be integrated to address the ways in which silica dust particles become dispersed and migrate within the foundry air environment.
As a potential breathing zone hazard, silica dust is amenable to control by the same measures that industrial hygienists recommend for all toxic air constituents. These keys to control are substitution, isolation and ventilation.
Substitution replaces materials, processes, equipment and procedures with alternatives that produce reduced amounts of silica dust. For example, in sand molding, olivine sand may be a suitable silica sand replacement material for aluminum casting. Recently introduced refractories containing reduced amounts of silica could replace current furnace and pouring ladle linings. In addition, non silica materials can be used in pattern release and parting compounds.
These substitutions will help reduce silica dust, but much of the hazard is associated with operations that occur after shakeout. The shakeout process typically does not remove all of the sand adhered to a casting--that sand accompanies the casting through the sprue removal and grinding. Even shot blasting does not remove deep burnon, a condition in which sand particles are imbedded inside the metal surface and must be removed as a fine dust during grinding. Adhered sand continues to shed as the casting is transported, especially when by vibration.
This loose sand that is shed is called free sand and can emit dust at any point in the transfer process. Potential dust hazards associated with adhered sand and free sand can be reduced by casting processing methods that remove adhered sand from the casting as soon as possible after shakeout. Tumbling, media drums and shot blast techniques can be incorporated along with vibrations to achieve this separation.
Work procedures also can have a direct impact on dust control. Air nozzles used to clean machinery and workstations provide a significant dust source, especially around molding and coremaking, but can be replaced with vacuum methods. Vacuum housekeeping methods are more challenging to implement, but they can have a substantial impact on airborne dust levels a they suck up the dust rather than spreading it. When vacuuming, use a HEPA filter on the vacuum cleaner.
Also, to keep the sand from being reentrained off the floor by forklift trucks, fine tune housekeeping techniques. Designating sand-free aisles for forklift travel, co fining sand strictly to the area it is being used, and employing powered sweepers to clean the floor several times each day can lower the amount of airborne sand in the foundry.
Isolation is a method of dust control that uses physical barriers to prevent dust from dispersing. The shot blast machine, sand mulling operation, sand coolers and return sand system components isolate the processes so workers do not enter these enclosures during operation.
Some enclosures for manual operations, however, must admit workers, including large finishing booths and ladle relining. Workers must be protected with ventilation systems and respirators while conducting these operations, and the booths prevent the dust from migrating away from the process area.
To successfully isolate dust sources, the barriers always must be replaced and doors on machine enclosures must be closed after temporary removal for maintenance operations. Because sand use in a foundry cannot be isolated completely, isolation is seldom a successful technique by itself. Many isolating methods also use ventilation.
Ventilation addresses dust already dispersed into the foundry environment that needs to be controlled. Efficient ventilation depends on control at the source through local suction hoods. Often these hoods can capture dust before it has a chance to migrate into the breathing zones of workers or into the general foundry environment. A wide variety of hood methods have been developed and demonstrated in foundries, including shakeout enclosures and grinder, furnace, mold and pouring hoods.
Although use of local ventilation controls is a well-developed field, the management of dust particles that escape these controls is not well developed. This is the field of general ventilation. General ventilation is a dilution method that controls in-plant concentrations of dust by introducing air with a low dust concentration. Some foundries rely chiefly on roof ventilators to achieve dilution ventilation, drawing in fresh air from the outdoors through doors and wall openings by the negative pressure created by these roof fans.
However, totally exhaust-driven approaches are inefficient, so foundries have refocused their attention on powered-supply air systems that provide efficient methods of creating a thorough air exchange. These systems mechanically blow in air, creating a positive supply. Positive powered supply, when strategically combined with local ventilation controls, can create balanced zones in the foundry that prevent silica from migrating from one foundry area to another and building up background dust concentration.
An important component of ventilation system design to understand is that close-capture ventilation methods can remove critically needed fines from casting lines that employ recycled sand. These needed fines are designated as AFS mesh sizes 80120 and are essential to maintain mold strength and produce a quality surface finish. Although these fines are much larger than the respirable particles that create a health hazard for workers, they are not so large that they can resist updraft into suction hoods.
Recycled sand is agitated during the process of controlling temperature, moisture content and grain size distribution. The agitation can consist of tumbling, plowing or vibrating the sand while blowing on it or spraying it with water. The agitation itself, as opposed to the suction level, often propels these fines into exhaust transitions.
Solutions to this problem can involve actions such as:
* modulating the volume of cooling air based on sand throughput and closely controlling the velocity of air streams blown at recycled sand (for pneumatic sand cooling conveyors);
* restoring some of the moisture content needed for mold creation before the sand is processed;
* using cyclones on exhaust takeoffs to remove and return the 80-120 AFS size particles.
Measuring Ventilation System Performance
Ventilation techniques can impact the amount of silica dust in the foundry, but only if the system is functioning properly. Taking periodic readings of exhaust system performance ensures that the system continues to work as it was intended.
One easy way to measure an exhaust system's performance without expensive manometers, velometers or calculations is to use the maintenance department's amp meter. Because there is a direct correlation between amps and horsepower, the amount of air being moved is function of amps.
The motor amps should be read and recorded when the system is operating at known acceptable level and then monthly thereafter. If the amps decrease, there is a problem with the exhausts stem such as a worn fan, plugged ducts or blinded bags, which cause a reduction in air volume. A low flow rate also can be caused by backwards installation of a fan motor. Most centrifugal fans will operate in reverse, but at a rate of 75% or less than their rated capacity. If the amps increase, the system is moving more air than intended due to leaks or broken bags.
A U-tube manometer can be made from clear plastic tubing (min. 0.25 in. inside diameter) purchased at a hardware store and used to measure static pressure in inches of water within ducts. The direct relationship between static pressure and duct velocity allows a U-tube manometer to measure duct velocity. This affects the close capture hood's performance and the transport velocity. It is important to maintain transport velocities throughout the exhaust ductwork. Otherwise, dust can settle out in the ducts, reducing cross-sectional area and therefore effective exhaust. In extreme cases, the dust can build up to the point where the ducts collapse because they are not designed to support the weight of the dust.
The pressure gauge, usually installed with the baghouse, records baghouse performance by measuring the pressure drop in inches of water across the bags and cartridges. This measurement should be made when the bags and cartridges are new, and the maximum pressure drop recommended by the manufacturer should also be marked. Checking the gauge daily to see that it falls between these two marks avoids problems such as bags or cartridges that require cleaning or have torn and need to be replaced.
A properly designed ventilation system will achieved dust control while not sacrificing casting quality. However, foundries must pay attention to such systems to ensure that they sustain their performance over time.
This article is adapted from "Silica...Information for Metalcasters," an AFS Safety & Health Committee 10-Q publication.
RELATED ARTICLE: OSHA Silica PEL Update
Currently, OSHA has set the permissible exposure level (PEL) for silica at 0.1 mg/cu m. This is based on the percent of crystalline silica in airborne dust (see sidebar, "Monitoring Silica Exposures). Foundries need to know if they are in compliance with the PEL, and, if not, how to reduce levels to below the OSHA standard.
OSHA's data indicates that 30% of samples collected during inspections in general industry and construction have been in excess of the PEL. Because of the large number of noncompliant businesses, OSHA placed silica on its priority list. In 1996, the agency launched a Special Emphasis Program on silica to reduce and eliminate the workplace incidence of silicosis from overexposure to crystalline silica. OSHA also is conducting additional inspections in industries (including the foundry industry) having a higher risk of silicosis development in the workplace.
OSHA is considering developing a comprehensive crystalline silica standard that may reduce the PEL by 50% or more to a level of 0.05 mg/cu m respirable silica or lower. It believes that the current PEL may not be adequate to protect workers from silicosis. A new comprehensive silica standard likely would include provisions for product substitution, engineering controls, training and education, respiratory protection, arid medical screening and surveillance.
Foundries may have difficulty with a more restrictive standard from both a technical and economical angle. There is concern that the technology does not exist to achieve or adequately measure a PEL at a 50% reduction of the current level. Most foundries do not have the financial resources available to research and implement new and still-to-be-invented technology. These concerns have impacted OSHA's plans.
Due to serious problems with the sampling and analysis of crystalline silica, OSHA has moved silica to a long-term action on its regulatory agenda.
Despite this move, silica activity continues. Several states are looking to regulate crystalline silica as a hazardous pollutant under the toxic air rules. In addition, silica was upgraded to a human carcinogen on the National Toxicology Program report issued by the U.S. Dept. of Health and Human Services.
--"Silica...Information for Metalcasters," APS Safety & Health Committee 10-Q
A Short Course in Ventilation Systems
Ventilation systems can greatly impact the removal of silica dust emissions from the foundry environment, but only if they're designed correctly and function properly.
One specific design criterion to look for is the presence and flow of the makeup air, which is the fresh air that replenishes the foundry's air supply. Controlling the quality and entry location of this air prevents it from bringing in ore pollutants (such as through the coke feeder opening) and keeps the pressurized system balanced. Ideally, makeup air should flow in towards employees rather than towards sand or other dust particles. A poorly designed system will disintegrate over time due to stresses--the air pressure will drop, fans will stop working, and the foundry will face frequent repairs or simply be forced to replace the entire system.
The system isn't always to blame--foundries need to examine individual pieces of equipment and look for simple problems. Inspect the collector, duct work and hooding designs to ensure that they are working correctly (check for plugging, corrosion and holes). Ensure that equipment hasn't been modified. Often, maintenance and other employees remove seals or cut off hoods to access internal parts or handle large castings. Sometimes when a new grinder or conveyor is added, the foundry taps into nearby ducts to use the ventilation system. Any of these modifications alter the balance of the system, forcing it to work less efficiently and allowing dust to escape.
Ventilation systems require vigilant maintenance to keep them performing at their best. Some maintenance recommendations include:
* calibrating the differential pressure gauge and checking the pressure differential from the dirty to the clean sides of the baghouse cloth;
* verifying correct bag installation and sealing;
* ensuring that the compressed air used is dry and free oil;
* checking that the dust handling system is being emptied rather than building up with dust;
* avoiding condensation in baghouses that might mud up the material;
* examining the duct system for holes or worn spots (especially at elbows);
* checking that hoods have access doors and seals and are still in their original shape.
If a new system needs to be installed, these tips will make the process go smoothly. First, know the through put--both the tons of sand/hr and the tons of castings/hr--so that the system can be designed as tightly as possible to improve efficiency.
Second, select a maintenance-friendly system that considers the necessary access for repairs. A system without convenient access doors will be hacked apart to reach bearings and motors that need repair. A duct system with easily replaceable elbows may be more expensive, but frequent replacements make it worth the money.
Third, go with what is familiar. If the foundry has had good luck with a particular brand of equipment, stick with it. Improve upon current ventilation designs hut choose the size and type used previously to increase employee familiarity. Simplicity and familiarity is always better.
Fourth, educate employees about the importance of ventilation systems. Involve them in selecting the system and making it fit around the workflow. Explain to them the ramifications, both monetary and health-related, of making adjustments to the system.
--Gerald Auler, Jim Z-Hanegraaf and Keith Bretl, Industrial Ventilation, Inc., Greenville, Wisconsin
Clearing the Air at Acme Foundry:
Integrated Engineering Design of a New casting cleaning and Finishing Facility
Acme Foundry, Inc., Coffeyville, Kansas, produces 19,000 tons/year of gray iron castings, primarily for the hydraulic valve industry. The 350-employee foundry recently planned and implemented an expansion of its cleaning and finishing area to meet increasing customer demands. One of the main focuses in the new design was silica dust control.
Acme Foundry wanted to maintain tight control over its silica dust emissions. All dust-producing cleaning and finishing operations use local exhaust systems at each individual station. The close-capture hoods are manifolded and ducted to a series of air cleaners, most of which are located on an elevated mezzanine within the department. Acme selected an indoor location for the air cleaners so that the cleaned exhaust could recirculate back into the foundry.
Cartridge-type collectors accommodate the mezzanine height. One air cleaner, a non-recirculating baghouse handling snag grinders, is located outdoors. A pneumatic transport system simplifies dust consolidation and disposal. The system transfers the collected dust from all of the air cleaners on the mezzanine to a single ground-level filtered receiver where the dust is automatically loaded into large sacks.
A three-stage filter system reliably recirculates cleaned exhaust air back into the cleaning and finishing area. Secondary dust collectors, referred to as safety monitoring filters (SMF), automatically refilter air passing through the primary dust collectors. If a cartridge in a primary filter unit develops a leak, the SMF will filter out any dust leakage through its automatically recleaned filters and provide a signal that the leak is occurring. HEPA filters back up t e recirculation system.
The new casting cleaning facility uses an air mass balance that creates a positive pressure in he foundry areas where it is attached. Positive pressure prevents other foundry emissions, especially gaseous emissions, from entering the cleaning room. Because the recirculation systems in the cleaning room don't remove gases, the cross-contamination of gases would prevent the use of recirculation in the cleaning room. The positive pressure is created by tempered (i.e., heated or evaporatively cooled) makeup air units that direct air to 98 individual adjustable air drops over the workstations. Workers prefer these air drops because of the close overhead location and individual adjustability. Their use completely eliminates the need for pedestal fans that blow in door air and disrupt capture hood performance. The roof fans and sidewall supply fans provide summer heat relief only.
The foundry uses a computer to continuously monitor the entire ventilation system. Airflow parameters are controlled within preset ranges, with alarms to indicate any out-of-range parameter.
Ventilation System Performance
Following construction in June 1999 and commencement of operation, the foundry conducted air sampling to assure that stack emissions met the limits set by permit and that workers' exposure to respirable silica met OSHA's PEL. Stack testing was undertaken on two dust collectors, and silica exposure results indicate control well within the OSHA PEL.
-Don Pusa, Acme Foundry, Inc., Coffeyville, Kansas and Robert C. Scholz, RMT, Inc., Brook field, Wisconsin Monitoring Silica Exposures
In order to evaluate employee exposure to respirable crystalline silica dust and determine compliance with the OSHA PEL, three key actions must be accomplished:
* separate respirable from non-respirable sized particles;
* trap and measure the weight of the respirable fraction of particles;
* distinguish the form and amount of crystalline silica present in the respirable fraction of the dust.
Monitoring for respirable crystalline silica dust requires a sampling train consisting of three primary components:
* a calibrated, portable air pump operating at a prescribed air flow rate;
* a particle size selection device, usually a cyclone, to separate the respirable particles;
* a pre-weighed filter to trap the respirable particles passing through the cyclone.
After collection, laboratory analysis will determine the amount of respirable dust present, and the amount and form of crystalline silica in each sample. Respirable dust will be reported as weight/volume of air (mg/cu m), and the quantity of crystalline silica either as percentage of the respirable dust or as a weight (mg or mg). If weight is reported, the percentage of respirable dust must be calculated to determine the PEL.
The PEL for a sample can be calculated using the formula from Table Z-3 of 29CFR1910.1000, OSHA regulations:
PEL (mg/cu m) = (10 mg/cu m)/( % Si[O.sub.2] + 2)
Insert the percentage quartz (Si[O.sub.2]) reported by the lab and compare the resulting PEL value against the amount of respirable dust in the sample to determine compliance. For example, with 15% quartz present, the PEL would be 0.59 mg/cu m. With 10% quartz present, the PEL would be 0.83 mg/cu m. If the amount of respirable dust is less than the calculated value, then the exposure is compliant.
Remember, the PEL must be calculated for each sample and is dependent on the percentage of crystalline silica found in the collected respirable dust.
The complex nature of crystalline silica exposure monitoring and analysis calls for a careful and methodical approach that considers all of the factors required for meaningful results. When data is used to determine compliance or to guide the design and implementation of expensive engineering controls, it is critical that good technique be employed and sources of possible error be considered.
Robert D. Safe, Safe Technology, Inc., Chicago
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|Comment:||Dust off your foundry's silica control measures: By understanding how to manage silica dust emissions in your foundry, you can make your control systems work more efficiently.|
|Author:||Euvrard, LeRoy Jr.|
|Article Type:||Brief Article|
|Date:||Nov 1, 2001|
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