Only one of the standards (EPA’s 6H Rule) that we see applied to spray booths addresses booth pressure specifically. One of the purposes of a spray booth is to confine and capture the overspray. While the spray booth’s ability to do this may be directly related to booth pressure, the standards for spray booth typically describe performance requirements and not specific values of pressure or velocity (with the exception of the IFC). The type of booth will have an impact on the pressure needed in the spray booth to achieve the performance required by the industry standards. The following is a summary of the statements in the various codes that address this issue.
National Fire Protection Association:
NFPA 33 Standard for Spray Application Using Flammable or Combustible Materials is the principal standard for spray booths. The IFC and OSHA codes are based upon the requirements of NFPA 33. NFPA 33 requires that “Each spray area shall be provided with mechanical ventilation that is capable of confining and removing vapors and mists to a safe location and is capable of confining and controlling combustible residues, dust, and deposits. The concentration of the vapors and mists in the exhaust stream of the ventilation system shall not exceed 25 percent of the lower flammable limit.” It also states that “Spray areas equipped with overspray collection filters shall have an effective means to ensure that the performance requirements of Section 7.2 are met.” Acceptable means to comply with this requirement include visible gauges, audible alarms, approved interlocks, or an effective inspection program.
International Fire Code: Chapter 15
The International Fire Code (IFC) does not specifically state a booth pressure requirement. It does state that “Mechanical ventilation shall be kept in operation at all times while spraying operations are being conducted and for a sufficient time thereafter to allow vapors from drying coated articles and finishing material residue to be exhausted. Spraying equipment shall be interlocked with the ventilation of the flammable vapor areas such that spraying operations cannot be conducted unless the ventilation system is in operation.” The IFC also adds the requirement that “Ventilation systems shall be designed, installed and maintained such that the average air velocity over the open face of the booth, or booth cross section in the direction of airflow during spraying operations, shall not be less than 100 feet per minute (0.51 m/s).”
OHSA also does not state a booth pressure requirement, but like the IFC requires that “All spraying areas shall be provided with mechanical ventilation adequate to remove flammable vapors, mists, or powders to a safe location and to confine and control combustible residues so that life is not endangered. Mechanical ventilation shall be kept in operation at all times while spraying operations are being conducted and for a sufficient time thereafter to allow vapors from drying coated articles and drying finishing material residue to be exhausted.”
Environmental Protection Agency (EPA):
The EPA is primarily concerned with outdoor air quality and lets other industry standards and OSHA address life safety issues (fire hazards, personnel exposure, etc), but in their 6H rule they allow for fully enclosed booths (no openings, adequate seals round doors) with an automatic pressure balancing system to operate at positive pressures up to 0.05 inches water gauge.