Airborne mists emitted during machining processes pose numerous risks to human health and safety. It is important to understand the regulations that limit these emissions as well as the various technologies available to achieve compliance with OSHA and other regulatory agencies. Camfil's John Dauber and Kevin Tucker explain what you can do about mist collection issues in your facility; how to evaluate which type of collection system is most suitable for your application; and how to establish an overall safety program that will best protect employees and equipment.
The metalworking fluids (MWFs) commonly used during machining processes generate airborne contaminant mists that must be carefully controlled: Otherwise, they pose a variety of health risks to workers, while also creating a dirty and unsafe work environment. It is important to understand the hazards associated with oil mists as well as exposure limits set by OSHA and other entities.
MWF Emissions -- Laws, Regulations and Guidelines
The emissions generated by machining processes are formed due to a combination of mechanical and thermal effects and fall into three general categories:
This machining center is equipped with high efficiency collection equipment designed for removal of emulsion coolant mist.
Occupational Safety & Health Administration (OSHA) presently uses two air contaminant permissible exposure limits (PELs) that apply to MWFs. They are:
A number of adverse health effects are associated with MWFs. Harmful effects due to emissions include the following:
Fiberglass V-bag style mist collector
The personal exposure limits (PELs) have been developed to protect workers against many of these adverse health effects. If your facility is meeting the PEL requirements but workers are still experiencing symptoms, it may be necessary to set lower goals.
Coolants/Emulsions vs. Straight Oil: Understanding the Differences
There are two general categories of MWFs used in machining processes:
Chlorine-containing oils are very hazardous. Other additives provide rust protection, the reduction of foaming and oil mist, and also reduction of the viscosity. Straight oil coolants are generally used for their excellent lubricating properties -- unlike the water-mixed emulsion compounds which provide some lubrication but are used primarily for cooling.
Applications for straight oils include turning, drilling, milling, roughing, grinding, broaching, honing, rolling, deep-drawing and pressing. There is a variety of equipment used to capture mists generated when these coolants and lubricants are used. The most common type are fiberglass V-bag mist collectors offered by a wide range of manufacturers. These collectors use a first-stage Chevron metal filter, a second-stage aluminum mesh filter, and a third-stage fiberglass V-bag with a 95 percent ASHRAE efficiency rating.
Fiberglass V-bag style collectors are usually designed for double duty, i.e., they can be used on both straight oil and emulsion coolants. The units lose efficiency as the primary filter bags become saturated with fluid, so filters must be changed regularly to prevent harmful emissions from escaping into the workplace. V-bag collectors work well on lighter-duty applications but are not as effective for heavy-duty use and long production runs.
Centrifugal-type mist collector
Centrifugal-type mist collectors use a rotating drum to spin out the oil. Typically, there is a pad inside the unit that functions as a final filter, but most contaminants are removed by the rotating action of the drum. If chips collect in too high a volume, the unit can go out of balance and malfunction, creating a health hazard as well as a maintenance headache.
Like V-bag collectors, centrifugal collectors may also be used for straight oil and emulsion coolants; and again, they are best suited for lighter-duty use such as machining centers that operate a few hours a day or change out parts only occasionally. Frequent opening of the door for parts change-out adds to the mist in the air if equipment is not properly vented. V-bag and centrifugal collectors offer relatively low initial cost but are limited in terms of run time, filter service life and filtration efficiencies.
Because of these limitations, many machining center operators recognize that one collector size and type does not fit all. Straight oils and emulsion coolants have very different properties and characteristics. As a result, a number of equipment manufacturers offer highly engineered collection systems that are specifically designed for use with one type or the other.
When designing equipment for mist removal, the best solution is a source capture collection system that will contain mist at the machine. Ceiling units that provide ambient ventilation are sometimes used: These will remove some of the mist but are not as effective as source capture. By contrast, ambient collection works well in welding shops, where the warm weld fumes rise to the ceiling and can be removed with a properly sized system and the correct number of air changes. But this approach does not translate well to mist removal, with mist-laden air and slippery floors inevitably resulting. Source capture requires less air, and is safer and more efficient with just about any type of wet or dry collection system.
High efficiency mist collector for emulsion/coolant oils
The design challenges will be greater when retrofitting equipment into an older machining center. Equipment built 20 or 25 years ago was often not designed for mist collection. Newer machines utilize better ductwork designs and better airflow patterns to pull air out of the machine at the recommended cfm airflow.
A good general rule is to maintain a slight negative pressure when doing machining. This will allow you to capture the fine mist without pulling chips, emulsion or oil into the collector.
One recommended option is to equip the oil or emulsion mist collector with a variable frequency drive (VFD). During machining, the VFD speed should be reduced to a level just adequate to contain the mist in the machine. In a newer machining center that is fairly airtight, you may be able to ramp down to 25 percent of full airflow while the doors are shut. Maximum airflow when doors are shut can pull mist and chips into the collector and shorten filter life.
Another strategy is to use a special hood or chip gate to separate out metal chips and preclean the oil. Basically, it acts like a horizontal cyclone designed for the airflow you need in the machining center. It works by spreading out the airflow to prevent creating a high velocity suction area that will pick up a lot of chips. Ductwork is generally about 6" and the area pulling the air is about six times that size. With that ratio, you will not pull excessive air into one place in the cabinet. Chip gates can sometimes be retrofitted onto older equipment.
Chip gate for pre-separation of metal chips
Any time you can reduce the load on a collector, it's a good thing. As noted, lots of metal chips can drastically shorten filter life. Oil or emulsion can run for a very long time, if pickup hoods and airflow in the machines are optimally designed.
It is also important to equip ductwork with proper fittings and seals that are specifically made for oil. There is a misconception in the field that mist collectors and ductwork always leak, and unsuitable fittings and seals can often be the cause.
Also, make sure the cabinet is leak-proof and won't seep oil: Otherwise, employees are at risk of slipping and falling on oily floors. Even very small cracks will allow oil to seep out. Using equipment that has been dye-tested and certified as leak-proof by the manufacturer is a good way to make sure such leaks and cracks don't occur.
Clogged ductwork conditions caused by inadequate filtration.
In addition to a guarantee that the collector and its components won't leak, a reputable manufacturer should also guarantee filter emissions efficiency, to ensure that you keep below required exposure limits. Some manufacturers will also provide written guarantees on filter life and run time to fit your operation.
9 Best Practices and Controls for Reducing Exposure
You can tell when you walk into a machine shop whether good work practices are being observed. When they are not, you can literally smell, feel and taste the oil in the air. There are many factors involved in keeping machine shops as safe and clean as possible. Your approach to safety should be multi-faceted and should incorporate the following items:
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