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Using dry filtration to capture plasma cutting fumes

Such a move helps to keep employees healthy and minimizes the possibility of dust explosions

Figure 1
These before and after photos show the difference a dry filtration system capable of high-efficiency dust collection can make in a fabricating operation that performs plasma cutting.

The plasma cutting processes used to cut mild steel, stainless steel, and other metals generate fine particulate dust and fumes that can be harmful to workers, machinery, and electronics if not properly controlled. One solution is the use of dry downdraft tables (see Figure 1) with integrated dust and fume collectors and built-in ducts that convey contaminants to the collector for capture and removal.

Worker Health Risks

Before beginning a discussion of dry filtration systems and the standards that influence their design and performance (see Figure 2), metal fabricators should be aware of the often-overlooked health risks associated with plasma dust and fumes.

Whether you are working with mild steel, stainless steel, aluminum, galvanized, or another material, the Material Safety Data Sheet (MSDS) is a good starting point for identifying health risks. The Occupational Safety and Health Administration (OSHA) has established permissible exposure limits (PELs) based on an 8-hour time-weighted average (TWA) for hundreds of dusts, including the numerous metal dusts generated in plasma cutting.

Following are the worst culprits:

Hexavalent chromium, or hex chrome, is a carcinogenic substance that is a byproduct of cutting stainless steel and other metals that contain chromium.

Hex chrome overexposure can result in short-term lung, eye, or skin irritations. The greatest long-term health danger associated with hex chrome exposure is lung cancer. Other major health effects include damage to the upper respiratory system and dermatitis. Respiratory tract problems can include inhalation damage to mucus membranes, perforation of septum tissue between the nostrils of the nose, and lung damage.

Once in the body, hex chrome typically targets some of the body’s organs. A worker exposed to hex chrome may also experience symptoms such as sinus irritation, nosebleeds, stomach and nose ulcers, skin rash, chest tightness, wheezing, and shortness of breath.

The current OSHA PEL for hex chrome is extremely stringent, at 5.0 micrograms per cubic meter (µg/m3). For cutting stainless steel, HEPA filtration is required to stay below this threshold limit.

As a result, cutting stainless steel on wet systems is often not recommended. If a water table must be used, some type of air filtering device (such as a capture hood with a HEPA filtration system) is needed to capture any fumes that rise up from the table.

Zinc oxide is a pollutant generated by hot work on galvanized steel. Exposure can result in a condition known as metal fume fever, a short-term illness in which severe flu-like symptoms occur after a break from work, such as over the weekend or during a vacation. Because of the delayed reaction, it is often confused with regular influenza and many cases go undiagnosed.

Manganese, which is present in some steel alloys, can cause workers to feel exhausted, apathetic, weak, or headachy. Chronic overexposure to fumes containing manganese leads to a condition known as manganism, which is characterized by neurological and neurobehavioral health problems. Manganese discharges are now specifically regulated by the Environmental Protection Agency in its National Emission Standard for Hazardous Air Pollutants (NESHAP), Rule 6X.

Metal dust particles generated during cutting are an eye irritant and can cause eye injuries in factories.

Figure 2
In this dry downdraft table design, dust and fumes are exhausted to an outdoor dry media dust collector that captures them and then recirculates the clean air back into the shop.

It is imperative to know and follow OSHA exposure guidelines for these and other metals, particularly where workers are at risk for long-term health effects. But sometimes people simply get sick and miss work, experiencing headaches, upper respiratory symptoms, or general discomfort even when a facility is in compliance with OSHA. When this happens, it may be necessary to set even lower exposure limits to eliminate employee complaints.

A well-designed plasma downdraft table with a cartridge fume collector (see Figure 3) properly filters hazardous contaminants. Using long-life cartridge filters with high-efficiency nanofiber media, a dry collector can achieve removal efficiencies as high as 99.995 percent (MERV 15) on very tiny particles of 0.5 microns and larger by weight. It controls respirable particulate at the point of generation, ensuring that it will not spread and be inhaled by workers in other areas of the plant.

Facility Safety Risks Associated With Explosive Dust

Combustible dust hazards may be present in all types of metal cutting. The only way to know for sure is to test the explosive nature of the dust and document a dust hazard analysis for the plasma cutting operation.

A dust hazard analysis (DHA), as defined in the new NFPA 652: Standard on the Fundamentals of Combustible Dust, is needed to identify the full range of combustible dust deflagration, fire, and explosion hazards specific to an application. This standard directs the manufacturer to other industry-specific standards that may require more stringent controls on the hazard.

For example, NFPA 484 covers combustible metals. If plasma dust falls under this standard, a metal fabricator is going to find very specific and limiting requirements for dust and fume collection. The fabricator needs to determine the hazards associated with the dust and which standards apply.

Most of the information in NFPA 652 is carried over from other standards. However, there is one new requirement with broad-reaching implications. For existing processes handling combustible dust, the “owner/operator shall schedule and complete DHAs of existing processes and facility compartments within a three-year period from the effective date of the standard,” according to the standard issued in October 2015. Currently OSHA cites facilities that do not have a dust hazard analysis, and this new standard will increase enforcement efforts.

The type of dust collector, explosion protection, and duct isolation required for each application varies, and a DHA should be conducted to determine system requirements.

An engineer knowledgeable of the process should perform the assessment with support from the dust collector and protection control suppliers.

Proper System Design and Maintenance

A well-designed dry filtration system can have a positive impact on worker health, plant safety, and product quality. It also can reduce maintenance, minimize downtime for greater productivity, and reduce cost per part when compared to wet systems. (See Plasma Cutting on a Water Table: Potential Pitfalls sidebar.)

Here are some rule-of-thumb recommendations for designing and maintaining a dry system.

Sizing. Three of the most important factors in collector sizing are the expected amount of smoke/fume particulate being collected, the size of the downdraft table, and the material being cut.

Figure 3
This dry plasma cutting table is served by a nearby cartridge dust and fume collector to maintain air cleanliness.

Several factors affect the amount of particulate being collected, referred to as the “loading.” First is the amperage of the plasma cutter itself. A 400-amp plasma torch cuts much faster and has much higher particulate loading than a 130-amp torch. Some tables may have multiple heads operating at the same time, which generates more particulate than a single-head system. Second is the size of the downdraft table. This affects the volume of air needed to create the appropriate capture velocity at the face of the table. Third is material type. Different materials produce varying amounts of particulate. For instance, galvanized metals are known to produce higher levels of particulate.

An air filtration specialist can calculate the right collector size for the application and the best filter media for the job. Extended-life flame retardant filters with open-pleat nanofiber media are recommended for these applications.

Cartridge Mounting Configuration. The placement of filters can have a major impact on filter performance and service life. Some fume collectors are designed with horizontally mounted cartridges that allow dust to become embedded on the top of the filters. This condition can shorten filter life and provide a dusty surface for sparks to ignite. Vertical mounting, by contrast, reduces the load on the filters and helps improve service life while reducing fire and explosion risks.

Filter Service. Cartridge collectors use automatic cleaning systems that allow units to run for extended periods of time between filter change-outs. The only operator inputs are occasional changing of the pulse-cleaning on-demand pressure settings as the filters wear and eventual replacement of filters when they have reached the end of their life. Collected dust must also be cleaned out periodically from the storage drum.

When selecting a fume removal system for plasma cutting, be sure to evaluate health risks, safety and explosion concerns, product quality, productivity, maintenance, and operational issues. Dry filtration systems offer potential advantages in all these areas, bringing long-term cost and performance benefits while providing optimum protection of workers.

References

Department of Health and Human Services, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, “Criteria for a Recommended Standard: Occupational Exposure to Hexavalent Chromium,” DHHS (NIOSH) Publication No. 2013–128, 2013, www.cdc.gov/niosh.

National Fire Protection Association (NFPA), NFPA 484: Standard for Combustible Metals (2015) and NFPA 652: Standard on the Fundamentals of Combustible Dust (2016), www.nfpa.org.

Occupational Safety & Health Administration (OSHA), OSHA Standard on Hexavalent Chromium (OSHA Standard 3373-10), (2009), and OSHA Permissible Exposure Limits – Annotated Tables (2013), www.osha.gov.

Plasma Cutting on a Water Table: Potential Pitfalls

Plasma cutting on a water table may be viewed as a cost-effective way to cut sheet metal and plate, but metal fabricators need to keep in mind some of the concerns associated with employing such a cutting arrangement.

A few of the potential problems with water tables are revealed in these close-up views. Metal has rusted (left) from the wet environment, even in the chemically treated water. A small part coated in film (middle) has fallen into the water. Sludge and small parts that remain (right) have to be removed after the table is drained.

For example, fabricators need to make themselves aware of some of the health and safety issues. In some plasma cutting applications in which the part sits inches above the water, a percentage of fumes escape into the work area, exposing the shop floor employees to potentially harmful contaminants. Also, water tables are prone to leakage, and even routine handling of parts in and out of the table creates spillage, both of which can cause slip and fall hazards.

There is also the risk of an explosion when aluminum and water react to produce hydrogen gas. When a machine is shut down, hydrogen pockets can form underneath the plate being cut, and when the plasma arc is reactivated, it can trigger a dangerous explosion.

A fabricator also must concern itself with quality and productivity issues. For instance, water decreases cut accuracy by grounding the plasma arc. As a result, high-definition and small-hole plasma cutting have to be done above water, increasing fume exposure. Plasma cutting in a dry system maintains a clean column and can extend the life of consumable parts. It also may be 10 to 15 percent faster than cutting underwater.

Water may compromise cut-edge quality by inducing ripples known as “striation.” Turbulence in water agitation almost always affects bottom edge quality, perpendicularity of cut, and concentricity of holes.

Excess dross is created if the space between the plate and the water is not held to exact specifications. If the dross is quenched too quickly by water, it sticks to the back of the part and is difficult to clean off.

The quenching effect of water on cut parts can affect the hardness of mild steel material, making it difficult to perform additional machining operations. It also can cause product to be out of compliance with customer specifications. If a part is completely submerged, the water actually reduces the heat-affected zone around the area being cut, but it will still be hard to do additional machining.

Finally, steam and moisture from plasma cutting on the water table can clog the torch head, causing it to overheat.

A fabricating operation with a plasma cutting machine and a water table also has to contend with maintenance issues unique to this kind of cutting application:

  • Chemical stabilizers are needed to keep the water as clean as possible, and rust inhibitors have to be added to prevent corroding of parts.

  • A water table must be drained periodically for cleaning. It can take as long as 10 to 20 hours to clean an average-size table.

  • Water must be added regularly to compensate for evaporation, and dirty/oily water must be disposed of properly. (Many recyclers will not take wet material. Some shops lay out the metal dust to dry, while others purchase a drying oven, adding equipment and energy cost as well as processing time.)

Plasma cutting with a water table creates a very challenging environment for the fabricator looking to keep the equipment looking new. Those rust-inhibiting additives in the water reduce but do not eliminate all steel corrosion. Mild steel components, including the gantry, rails, and gear rack, are subject to rusting and premature failure. Manufacturers of wet tables recommend wiping these components dry every day.