Ask the Expert Question-and-Answer Archive

by Ron Joseph

January, 2003

Spray Booth Filter Change - NESHAP

Q. Am I correct in saying that if your paint booth is covered by the NESHAP standard then in order to determine when the filter must be changed, one must go by the limits the vender set for the pressure differential? This is determined by using a manometer where a base-line sample is taken and then the vendor's suggested value can be taken into account. On the other hand, if the NESHAP does not apply to your paint booth then you can simply determine when the filter needs to be changed by taking air flow readings when the filters are clean and periodically measuring the pressure drop and when the average airflow velocity drops to the minimum allowed by the law, then the value is marked on your manometer or magnahelic gauge. This also is a method that tells you at which point you need to change your filter. For these paint booths, the value's set by the vendor do not need to be followed because they are not under the NESHAP. Is this correct?

A. First, the spray booth filter requirement applies only to the Aerospace NESHAP. Moreover, the requirement applies only to those spray booths in which inorganic HAPs, such as chromates are used. If your facility is subject to the Aerospace NESHAP for inorganic HAPs your analysis is correct.

Suppose you are not subject to that NESHAP. In the past OSHA required a minimum air velocity though the spray booth of 100 fpm for non-electrostatic spraying operations. Guidelines for this come from the National Fire Protection Agency, NFPA, which no longer uses air velocity as a limiting factor. Instead, NFPA suggests that the concentration of flammable vapors in a spray booth should never exceed 25% of the LFL. If you were to perform a velocity analysis in your spray booths, then based on their sizes and volumetric airflow rates (CFM) you might well find that even at air velocities below 100 fpm the concentrations might fall below the 25% LFL limit. This situation arises in spray booths in which relatively small quantities of paint are sprayed in large booths. In other words, as the ratio of paint used/volume of spray booth decreases, the 100 fpm requirement becomes less important. On the other hand, where large quantities of paint are being applied in small booths, (paint used/volume of spray is high) the air velocity might need to exceed 100 fpm in order to keep the concentration of flammable vapors below the 25% LFL level.

Pressure differential across the filters is a simple method for monitoring air velocity, but quite honestly it is not a good substitute for a proper velocity profile, for reasons that go beyond the scope of this answer.