spray finishing with flammable liquids: fire hazard
Spraying with flammable liquids (paints) is a topic that needs to be revisited frequently. Although best practice standards- of which National Fire Protection Association (NFPA) 33 is considered the “bible”- have been around for decades now, still many people do not understand or implement proper controls.
- The term “flammable liquid” is defined by NFPA as a liquid that emits vapors less than 100°F and that sustain combustion (the term that defines this process is “flash point”). Several factors make flammable vapors dangerous:
- The low flash point means that unlike liquids with higher flashpoints, they do not need to be exposed to a heat source in order to be ignited.
- In most common flammable liquids the vapors are heavier than air and therefore sink to floor level and tend to disperse less readily.
- The vapors burn very quickly in what is called a “deflagration”- essentially an explosion. They also have a very high heat release.
- Fires are difficult to extinguish. Water can be used to extinguish, but non water-miscible liquids will present more hazards (such as floating puddles of flammable liquids ready to spread the fire elsewhere).
How to tell
Flammable liquid containers must display a distinct red label in order to comply with Department of Transportation (DOT) regulations. Don’t count on just reading the label, however, as the label might not be on the containers (e.g. it may have fallen off). Obtain a “material safety data sheet (MSDS) from the vendor/supplier of the liquid. This will detail the liquids along with the safety precautions.
Common flammable liquids include petroleum or alcohol-based paints and coatings, lacquers, mineral spirits, thinner, turpentine, kerosene, methyl ethyl ketone, and toluene.
For the purpose of this blog, we can’t go into great detail (we’ll give the Reader’s Digest version), and we’ll not cover storage in detail (this is another standard- NFPA 30)- but as an overview, below are some key ways to control the exposure (again- NFPA 33 outlines these in detail). Note that any spraying should be done in a UL listed or Factory Mutual Approved spray booth if possible since this helps with the below controls and the design is such that lends itself to easier cleaning, etc. (e.g. equipped with seamless/smooth metal interior walls).
Ignition Source Control
All electrical systems near the spray booth should be designed to prevent inadvertent ignition of flammable spray vapors. All electrical and lighting fixtures within a twenty feet horizontal or ten feet vertical distance from the booth need to be protected in accordance with NFPA 70 NFPA 70 'National Electric Code'. NFPA 70 'National Electric Code' specifies that Class I, Division 1 & 2 electrical should be installed. This diagram illustrates the general requirement:
The purpose of ventilation is to prevent accumulation of vapors. The best way to do this is with a spray booth. The booth should be mechanically exhausted to have a cross-sectional airflow of 100 feet per minute (CFM) in order to remove the flammable vapors. It is also imperative to maintain the concentration of vapor in the exhaust air stream below 25 percent of the lower flammable limit. This requires a sufficient flow of air moving through the booth at a sufficiently high velocity.
Note that the equipment (fans) that moves the vapors must not themselves be an ignition source, therefore, they should be constructed of non-ferrous metals or other non-spark generating materials.
Equipment (spray guns) should be interlocked with ventilation of the booth in order to assure that the ventilation is on (and take possible human error out of the process).
This may go without saying, but the air must be exhausted safely outside the building (make sure all “clean-air” / EPA regulations are adhered to by filtering/scrubbing the air).
The area/booth should have fire suppression- either automatic sprinkler protection or automatic fire suppression (such as an FM-200 chemical extinguishing system). Despite good dilution, ignition source control, and human element factors, a fire can still develop. Make sure that nozzles are protected from overspray of paint (approved cloth bags can be used, however, make sure they are clean). It is also recommended that sprinkler heads or fire suppression nozzles be located within ductwork (e.g. leading outside).
Make sure that all flammable liquids (paints) are stored in accordance with NFPA 30 Storage of Flammable Liquids. These can be in an approved storage room or in Underwriters Lab (UL) listed containers (there is a limit though). A small supply (generally less than 25g) can be in use outside the storage, but it should be placed in safety cans (UL/FM approved).
Last but not least, the controls that rely on the operator are also very important. These include:
- Equipment and storage containers should be bonded and grounded to prevent static electricity buildup (ignition source).
- There should be a regular documented cleaning schedule to prevent excessive accumulations of overspray residue. Use of non-spark generating materials.
- Filters should be changed regularly. There are various means of filtering the air, but the most common is non-woven cloth filters. Check airflow gauges to make sure the filters are not blocking airflow (usually due to clogged filters).
- Previously used rags should be placed in self-closing cans during the day, and removed to a safe location outside the building at the end of the working shift.
- Any portable fans or radios that are not in accordance with NFPA 70 (class I electrical) should not be used within the distances given above (this is a common violation).
- Though this article concentrated on the fire/property hazards, the health hazards are also critical. Appropriate respirators and personal protective equipment (clothing, eyewear, etc.) should be used by employees. If lead or other metals are involved, there may need to be a further OSHA- required controls.