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In-depth survey report: experimental and numerical research on the performance of exposure control measures for aircraft painting operations, part I.

Bennett JS; Marlow DA; Hammond DR; Dietrich WL; Vonderhaar KM
Cincinnati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, EPHB 329-12a, 2011 Dec; :1-100
Researchers from the Centers for Disease Control and Prevention/National Institute for Occupational Safety and Health (CDC/NIOSH) investigated the performance of the ventilation system in a Navy aircraft paint finishing hangar, in terms of the efficiency and effectiveness of contaminant removal and worker exposure control. The Naval Facilities Engineering Service Center (NAVFAC ESC) and the Navy Medical Center San Diego (NMCSD), Industrial Hygienists collaborated with NIOSH in the study. The Navy seeks to keep worker exposures to air contaminants, including hexavalent chromium (CrVI), hexamethylene diisocyanate (HDI), methyl isobutyl ketone (MIBK), and others, below levels required by regulatory health and safety standards, while limiting the environmental footprint, i.e. energy use, and operational costs of paint finishing hangar ventilation. The specific operation under study was refinishing F/A-18C/D strike fighter aircraft in Bay 6 of Building 465, at Fleet Readiness Center Southwest, Naval Base Coronado, San Diego, California. Approximately twenty F-18s were processed per year in Bay 6, with each aircraft requiring 5 to 6 days. In early 2008, a pilot study of the relationship between air velocity and exposure level was performed through computational fluid dynamic (CFD) simulations of a Navy aircraft painting facility. In those initial results, decreasing the ventilation rate by 50%, from 100 fpm to 50 fpm, increased the modeled gas concentration in a worker's breathing zone by 15%. However, during the current and more comprehensive study beginning in 2009, field observations indicated that the ventilation system was unbalanced, thus complicating the flow pattern and the relationship of velocity and concentration. The ventilation system's variable frequency drive (VFD) provided six different operational modes and controlled the system. In full painting mode, the VFD attempted to match the exhaust flow rate to the supply flow rate, so that air and contaminants would flow efficiently from the supply at one end of the bay to the exhaust filters at the other end. The bay was observed to be under positive pressure, meaning there was more supply than exhaust. This was found to be due to the inability of the exhaust to match the supply under higher filter bank pressure drops (as much as 2.5 in water gauge) that are encountered when the filter loading, with particles and paint, is at the moderate or high end of the filter maintenance cycle. To the extent the system is unbalanced toward supply, air inside the hangar (which contains contaminants) is likely emitted through the outside doors and other openings leading to a lower pressure area. In so doing, energy is wasted during moderate and high filter loading and, in some operational modes, heating, and environmental compliance is compromised. Therefore, one goal of this project was to correct the pressure imbalance under all operating conditions (thereby saving energy, improving ventilation efficiency, and reducing air emissions). The testing described in this report evaluated how changes, such as ventilation rates, might affect contaminant concentrations, worker exposures, and pressure levels within the hangar. Evaluations of the hangar ventilation system were based on a combination of field studies and CFD simulations. Initially, a walk-through survey was conducted June 16-19, 2009, encompassing range-finding air-sampling (for CrVI, HDI, and any other contaminants found on the material safety data sheets) and the gathering of hangar dimensions, geometric details, and ventilation boundary conditions that would be used to set-up the CFD simulations. Next, the current ventilation system performance in terms of contaminant control was evaluated through comprehensive air sampling of all solvent, primer, and topcoat constituents, on July 22 and August 3, 2009 and April 13, 2010. At the same time, CFD simulations of the existing scenario were built and validated using the measurements. CFD was then used to predict concentration vs. air velocity and illustrate the relationship between volumetric flow rate of air (which has a large effect on energy use) and contaminant removal, from both health (contaminant exposure) and safety (fire and explosion) perspectives. Subsequently, a tracer gas study, with no workers present, was conducted April 12 and 14, 2010 to document the change in contaminant concentration resulting from lowering the ventilation rate (from a supply/exhaust velocity of 136/99.0 fpm down to 102/68.9 and 73.4/49.0 fpm) under real-world conditions. These studies led to the following conclusions: 1. Balancing the air supply and exhaust can improve exposure control and air pollution permit compliance. This finding is based on CFD simulations, and is consistent with ventilation standard practice. 2. Tracer gas measurements conducted during unbalanced supply and exhaust settings indicated that 3/4 of the normal supply and exhaust rates provided the lowest concentrations, when compared to full flow (supply = 136 fpm; exhaust = 99.0 fpm) and half-flow (supply = 73.4 fpm; exhaust = 49.0). 3/4-flow was a supply velocity of 102 fpm and an exhaust velocity of 68.9 fpm. However, the only statistically significant difference among ventilation settings was between 3/4-flow and half-flow, which had the lowest and highest concentrations, respectively. 3. CFD simulations showed a large increase in contaminant concentration at typical worker locations, when the supply rate exceeded the exhaust rate, compared to when the supply and exhaust rates were equal. "Balancing," as in item 1, means maintaining a very small negative pressure, perhaps approximately -0.1 in. water. 4. Personal sampling of workers during typical aircraft refinishing operations showed that MEK (range: <0.03 to 665 ppm, with a STEL of 300 ppm), MIBK (range: 0.02 to 918 ppm, with a STEL of 75 ppm), isocyanates (range: 6.29 to 34.7 µg/m3, with an ACGIH TLV of 35 µg/m3) and hexavalent chromium (range: 145 to 537 µg/m3, with an OSHA PEL of 5 µg/m3, an ACGIH TLV of 10 µg/m3, and a NIOSH REL of 1 µg/m3) were the only air contaminants that approached or exceeded occupational exposure limits (OELs). The reported ranges were for exposures lasting approximately one hour, whereas the PELs, RELs, and TLVs are for an 8-hour or 10-hour time-weighted average (TWA), and the STEL applies to any 15-minute period. The sprayers have the highest exposures, and they wear air-line respirators, on continuous flow mode, making their exposures approximately 1000 times lower than concentrations in workplace air. 5. The ventilation system does not adequately address worker exposure and requires supplementing with respiratory protection. Area air sampling measurements taken between the process and the exhaust filters indicated that concentrations of methyl isobutyl ketone (MIBK), methyl ethyl ketone (MEK), and all other materials measured in the aircraft refinishing process were less than 1% of any LEL. Thus, explosion from chemical concentrations is not an issue here. Based on these conclusions, the following recommendations can be made: 1. The supply and exhaust airflow rates should be balanced to reduce exposure risk to workers. The balanced system should maintain the bay under slightly negative pressure (perhaps -0.1 in. water), if prevention of fugitive emissions to the environment is desired. 2. Tracer gas measurements should be performed at balanced ventilation settings to validate the concentration reduction predicted by the CFD simulations. 3. The respiratory protection program should be continued, under existing or feasibly modified ventilation. 4. Correcting the pressure imbalance should include replacing appropriate exhaust filters, pre-filters, or pre-layers during moderate or high filter loading to reduce pressure drop and save energy. The filter pressure drop value at which filters will be replaced should be recommended by NAVFAC ESC and the filter manufacturer. Balancing the system and improving system maintenance will have a marked effect on operational efficiency. 5. After balancing or any other system modifications, follow-up concentration and velocity sampling should be done to verify ventilation improvements. 6. Measurements should be made directly in the exhaust stream to demonstrate compliance with NFPA 33: "Standard for Spray Application Using Flammable or Combustible Materials 2011," if any significant changes are made to the existing ventilation system or settings. The current study did not include this specific measurement, because area air sampling during this study clearly indicated that an explosion hazard was not present. 7. In addition to correcting existing aircraft painting facility ventilation systems, innovative design should be explored using CFD. Reducing the hangar cross-sectional area to maintain a desired velocity at a lower flow rate, directing supply air to the work zones more precisely, and bringing exhaust terminals closer to contaminant sources are examples of possible paths to consider that will reduce worker exposures, while also reducing associated energy costs.
Military-personnel; Aircraft; Paints; Painting; Paint-spraying; Hexavalent-chromium-compounds; Chromium-compounds; Ventilation; Ventilation-systems; Isocyanates; Methyl-compounds; Ethylenes; Industrial-hygienists; Air-contamination; Employee-exposure; Ketones; Exposure-assessment; Exposure-levels; Exposure-limits; Air-quality-monitoring; Air-sampling; Air-quality-measurement; Air-flow; Exhaust-ventilation; Air-filters; Air-pressure; Filters; Work-environment; Simulation-methods; Engineering-controls; Control-technology; Volumetric-analysis; Time-weighted-average-exposure; Permissible-concentration-limits; Personal-protective-equipment; Respiratory-protective-equipment; Respirators; Sampling; Standards; Equipment-design; Combustible-materials; Spray-painting; Spraying-equipment; Sprays; Flammable-liquids; Fluids; Region-9; Author Keywords: Engineering Controls; Ventilation; Computational Fluid Dynamics; Hexavalent Chromium; Hexamethylene Diisocyanate
18540-29-9; 822-06-0; 108-10-1; 78-93-3
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Field Studies; Control Technology
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Transportation, Warehousing and Utilities
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National Institute for Occupational Safety and Health
Page last reviewed: September 2, 2020
Content source: National Institute for Occupational Safety and Health Education and Information Division