Dust Collection for Thermal Spray Processes

Introduction to Dust and Emission Control for Thermal Spray Processes

Thermal spray processes are widely used across numerous industries, including automotive, aerospace, marine, oil & gas, industrial equipment and consumer products. This flexible and adaptable technology is used to coat solid surfaces using metals, alloys, plastics, ceramics and composite materials. When spraying metals, it may also be called metalizing, flame spraying, metal spraying or hardfacing.

Thermal spraying is favored for many coating applications that require an even deposition over a large area at a high deposition rate. Compared to other processes such as electroplating or physical or chemical vapor deposition, thermal spray processes generally provide a more even application of thick coatings at a higher rate of deposition, leading to higher quality coatings and faster production. Thermal spray coatings can be designed to add desirable characteristics to the surface, such as resistance to corrosion or wear, chemical resistance, electrical conductivity or insulation, or different friction properties. Thermal spraying is also used to build up worn parts (hardfacing).

Thermal spray processes produce large volumes of dust and fumes made up of very fine particulates of the feedstock material along with trace gases from the heating process itself. The intense heat used in thermal spraying processes atomizes the feedstock material, generating very tiny particulates. A portion of these particulates become airborne, much like the fine airborne mist generated by spray painting or other aerosol sprays. This excess dust—known as “overspray”—causes health, safety and maintenance problems in a thermal spray operation. Controlling dust and emissions is necessary to protect the health of workers, prevent re-entrainment of particulates onto the coated surface, and prevent combustible dust explosions.

HVOF (lightsaber)

Types of Thermal Spray Processes

Thermal spraying refers to any process in which a material is heated or melted and then sprayed onto a surface. The feedstock (which may be a metal, alloy, plastic, ceramic or composite material) is heated or melted using electricity (plasma or arc) or combustion flame. 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)

Human Respiratory System

Health Risks of Thermal Spray Dust and Fumes

Fine particulates generated by thermal processes create special health risks because they can be inhaled deeply into the lungs and, in some cases, cross from the lungs into the bloodstream, where they are carried throughout the body. Some sprays may also have toxic elements that cause skin, eye, nose or throat irritation, or that can be absorbed through the skin or mucus membranes.

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.

combustion

Combustion Risks of Thermal Sprays

Most of the dusts created by thermal spray processes are considered to be combustible dusts. That means they can explode when allowed to concentrate in the right levels in the presence of oxygen and an ignition source. Both metallic and non-metallic thermal spray dusts can be considered combustible in the right concentrations.

The flame or arc used to heat materials for thermal spraying acts as an ignition source, making it imperative that dust in the thermal spray room or booth is not allowed to accumulate in the air or on surfaces. Thermal spray dusts may also create a combustion risk inside the dust collector. Care must be taken to eliminate the risk of sparks inside the dust collector.

Regulations

Regulations for Dust and Emission Control for Thermal Spray Processes

The Occupational Safety and Health Administration (OSHA) has issued specific regulations for many different metallic and non-metallic dusts. These are expressed as Permissible Exposure Limits (PELs), which are measured and averaged over a worker’s 8-hour shift. Manufacturers are responsible for ensuring that workers are not exposed to hazardous dusts in levels above the PELs. Failure to control dust in areas where workers may be exposed can result in serious fines and legal action.

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.

Considerations in Dust and Emission Control for Thermal Spray Processes

Most thermal spray processes will be conducted within a spray booth or enclosed area. This allows thermal spray dust and fumes to be contained for efficient source capture. In a source capture solution, air is collected as close to the source as possible and filtered to remove contaminants. Because air collected from thermal spray processes may contain trace gases (from the heating process) as well as solid particulates, it is normally not returned to the facility, even when filtered.

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.

Solutions for Thermal Spray Dust Collection

RoboVent offers robust dust and fume control solutions for manufacturers using thermal spray processes. RoboVent Fusion is a rugged, high-powered dust collector that minimizes maintenance and operating costs for thermal spray dust collection. Fusion can be easily equipped with a spark arrestance system and deflagration system to mitigate combustion risks for explosive thermal spray dusts.

A RoboVent solutions engineer can design an effective dust control system for your thermal spray applications. We can help you evaluate the health and combustion risks of your thermal spray dust, determine the best dust collector and filter media for your application, and design a comprehensive air quality solution including dust containment and collection, ductwork design, and ventilation and makeup air.

Solutions

Fusion™ Series

A dust collector on the cutting edge of air filtration. The Fusion Series delivers exceptional performance and cost savings in a powerful, simple package.

SparkOut Caption Alt Text

Delta3™ Inline Spark Arrestor

Using groundbreaking technology to kill sparks before they become a problem, the Delta3 protects your workers, your equipment and your productivity.

PleatLock Cartridge Filters

PleatLock Dust Collector Cartridge Filters

RoboVent's premium filter engineered to provide the best filtration protection for your plant and employees. A widened pleat and proprietary filter media help deliver top performance.

Configurations

Traditional Configuration

The traditional configuration maximizes performance with ductwork sized for optimal airflow for your specific applications.

Services

VentMapping® Engineering

VentMapping is a systematic engineering and design service. Our comprehensive approach helps us find the safest, most effective and most efficient way to meet your goals.

RoboVent Maintenance Program, Clean Air and Industrial Air Filtration Solutions

ClientCare Maintenance Program

ClientCare lets you leverage RoboVent's expertise so you can reduce the burden on your maintenance staff and stay focused on your core business.

Collectors

Fusion™ Series

A dust collector on the cutting edge of air filtration. The Fusion Series delivers exceptional performance and cost savings in a powerful, simple package.

Products

PleatLock Cartridge Filters

PleatLock Dust Collector Cartridge Filters

RoboVent's premium filter engineered to provide the best filtration protection for your plant and employees. A widened pleat and proprietary filter media help deliver top performance.

RoboVent Streamline Hood, Fume Hood for Welding Cells

Streamline Hood

A modular system for enclosing robotic welding cells, the Streamline Hood is easy to install, use and modify. The system is highly flexible and can accommodate a variety of welding cell configurations.