Dust Collection for Thermal Spray Processes

Common thermal spray processes include:

• Cold spray
• Electric arc spraying
• Electric arc wire (EAW) spraying
• Plasma spraying
• Vacuum plasma spray
• Flame spray powder or wire
• High-velocity oxy-fuel (HVOF)
• High-velocity air-fuel (HVAF)

Metallic thermal sprays generate some of the most hazardous dusts and fumes—similar to the fumes generated by welding metals. Metal alloys may contain a number of different toxic elements, each with their own health risks. These may include nickel, aluminum, copper, cadmium, cobalt, manganese, zinc and beryllium. Prolonged exposure to metallic fumes from thermal processes is linked to lung cancer, other respiratory problems such as chronic obstructive pulmonary disease (COPD) or acute pulmonary edema, kidney or liver disease, thyroid problems and nervous system disorders. Metallic thermal sprays may also cause contact dermatitis and eye irritation. Some metals, such as nickel, may cause severe allergic reactions in susceptible people.

Non-metallic sprays also have health risks. All thermal sprays can cause irritation of the lungs and mucus membranes. Some plastics and composites are also considered to be carcinogens (causing various cancers) or endocrine disrupters (linked to developmental, reproductive, neurological and immune system disorders). The specific health risks of many non-metallic thermal sprays have not been well studied.

These are PELs for some of the elements and compounds commonly found in dust from metallic thermal spray processes:

• Nickel: 1.0 mg/m3
• Aluminum: 15 mg/m3
• Hexavalent chromium: 0.005 mg/m3
• Cadmium: 0.005 mg/m3
• Beryllium: 0.0002 mg/m3
• Manganese: 5.0 mg/m3
• Cobalt: 0.1 mg/m3

Manufacturers using thermal spray processes also have to follow OSHA standards for nuisance dusts and both OSHA and NFPA standards for combustible dusts.

• Nuisance dusts: Thermal spray dusts that are not associated with a specific PEL fall under the OSHA definition of “nuisance dusts,” which are regulated under the general particulate matter concentration limits set by OSHA. The general limit for “Particulates Not Otherwise Regulated” (PNOLs) is 15 mg/m3 (8-hour TWA limit) for total particulate and 5 mg/m3 for respirable particulates. Employers must also follow general Housekeeping standards (OSHA 1910.22, Walking-Working Surfaces) to prevent accumulation of dust on surfaces.
• Combustible dusts: Combustible dusts are regulated under OSHA’s General Duty Clause (Section 5(a)(1)) with additional requirements under the Hazardous Locations (§1910.307), Hazard Communication (§1910.1200) and Housekeeping (§1910.22) standards. In addition, manufacturers dealing with combustible dusts must follow National Fire Protection Association (NFPA) standards for prevention of fires and explosions. Thermal spray dusts fall under NFPA 654, Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing and Handling of Combustible Particulate Solids. OSHA’s Combustible Dust National Emphasis Program (NEP) outlines policies and procedures for inspecting workplaces that create or handle combustible dusts.

There are several other considerations in designing an effective dust collection solution for thermal spray processes.

• Negative pressure: Maintaining negative pressure in the spray booth or enclosure will ensure that dust and fumes do not escape into the rest of the facility.
• Spark arrestance: A spark control system should be used to prevent sparks from the thermal spray process from entering the ductwork or dust collector.
• Deflagration system: The dust collector should be equipped with a deflagration system to mitigate the effects of a possible dust collector explosion. The system may include an explosion vent, an isolation valve to prevent a pressure wave from propagating back into the facility, and an airlock to isolate the dust collection hopper.
• Ductwork and airflow: The system must be designed to prevent fine thermal spray dusts from settling in the ductwork of your dust collection system. That means selecting duct material that will ensure good laminar flow and maintaining adequate airflow in the system. A typical spray booth will require a dust collector with a CFM rating between 8,000 and 16,000 CFM.
• Cooling: You also need to make sure that the airstream has sufficient time to cool down before hitting the filter media to prevent the risk of a dust collector fire or melting of the filter media. This can be accomplished by using longer duct runs and/or introducing cool air into the ductwork during collection.
• Filter media: It is important to choose a filter material appropriate for the type of dust that is created by your application. For most thermal spray applications, we recommend our A15 cellulose/polyester nano-blended filter media. For higher-temperature applications (over 180°F), we can cool the airstream or use a medium-heat spunbond filter to handle the elevated temperatures.
• Air-to-cloth ratio: You also need to make sure you have adequate filter media for your airflow and dust volume. Thermal spray processes typically require a ratio between 0.5:1 and 2:1, based on a technical analysis of the thermal spray process, deposition rate and feedstock material.