Ask the Expert Question-and-Answer Archive

by Ron Joseph

March, 2004

Shelf Life of Paints

Q. In the last two years I have heard from many Engineers in the Automotive Industry (Toyota and GM) that we no longer have to abide by the antiquated booth flow restrictions. This usually has to do with air flow reduction as a means of energy conservation. The usual reference to
reduced flow is cited out of NFPA manuals. There are several sections cited.

I believe they are correct in the reduced expectations from NFPA, however, the OSHA requirements have not changed. A robotic booth can be reduced to a comparatively low air change rate. When humans are added to the equation the regulations from OSHA come into prominence.

1926.66(b)(5)(i)
The spraying operations except electrostatic spraying operations shall be so designed, installed and maintained that the average air velocity over the open face of the booth (or booth cross section during spraying operations) shall be not less than 100 linear feet per minute. Electrostatic spraying operations may be conducted with an air velocity over the open face of the booth of not less than 60 linear feet per minute, or more, depending on the volume of the finishing material being applied and its flammability and explosion characteristics. Visible gauges or audible alarm or pressure activated devices shall be installed to indicate or insure that the required air velocity is maintained. Filter rolls shall be inspected to insure proper replacement of filter media.

http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&p_id=10657

Please review the OSHA site above and let me know if there is something I am missing.

A. In recent years the NFPA-33 has suggested that the airflow or air velocity in a spray booth should be less than 25% of the Lower Flammable Limit (LFL), which is the same as the Lower Explosive Limit (LEL). In OSHA 1910.94(c)(6)(ii) the regulation incorporates the NFPA recommendation (See http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&p_id=9734), but is ALSO requires that the air velocity meet 100 fpm for air-operated guns, or 50 fpm for electrostatic and automatic airless operations contained in a booth without an operator. Of the two regulations, 1910.94 and 1926.66, the former one appears to be more thorough.

My interpretation is that the minimum air flow is 100 fpm for manual operations BUT, even if you meet that requirement, nevertheless you must insure that you never exceed 25% of the LFL. Said another way, if you use large quantities of coatings such that the 100 fpm requirement is not adequate to lower the LFL to 25%, then you need to increase the air velocity until you have met the LFL requirement.

Bear in mind that the airflow in a spray booth serves two functions; to remove the flammable solvents AND to remove the particulates. All to often I go into a spray booth in which most of the paint overspray settles on the floor or walls of the booth and never makes its way all the way to the filters at the other end. Therefore, this poses a health hazard to the painters when they remove their respirators.

I hope this helps.

BTW, I perform air flow analyses in spray booths and determine how well they are performing. Since I live so close to New United Motors you might be interested in taking advantage of this service.