Effect of ventilation velocity on hexavalent chromium and isocyanate exposures in aircraft paint spraying

Abstract

Exposure control system performance was evaluated during aircraft paint spraying at a military facility. Computational fluid dynamics (CFD) modeling, tracer gas testing, and exposure monitoring examined contaminant exposure vs. crossflow ventilation velocity. CFD modeling using the RNG k-ϵ turbulence model showed exposures to simulated methyl isobutyl ketone of 294 and 83.6 ppm, as a spatial average of five worker locations, for velocities of 0.508 and 0.381 m/s (100 and 75 fpm), respectively. In tracer gas experiments, observed supply/exhaust velocities of 0.706/0.503 m/s (136/99 fpm) were termed full-flow, and reduced velocities were termed 3/4-flow and half-flow. Half-flow showed higher tracer gas concentrations than 3/4-flow, which had the lowest time-averaged concentration, with difference in log means significant at the 95% confidence level. Half-flow compared to full-flow and 3/4-flow compared to full-flow showed no statistically significant difference. CFD modeling using these ventilation conditions agreed closely with the tracer results for the full-flow and 3/4-flow comparison, yet not for the 3/4-flow and half-flow comparison. Full-flow conditions at the painting facility produced a velocity of 0.528 m/s (104 fpm) midway between supply and exhaust locations, with the supply rate of 94.4 m³/s (200,000 cfm) exceeding the exhaust rate of 68.7 m³/s (146,000 cfm). Ventilation modifications to correct this imbalance created a midhangar velocity of 0.406 m/s (80.0 fpm). Personal exposure monitoring for two worker groups-sprayers and sprayer helpers ("hosemen")-compared process duration means for the two velocities. Hexavalent chromium (Cr[VI]) exposures were 500 vs. 360 µg/m³ for sprayers and 120 vs. 170 µg/m³ for hosemen, for 0.528 m/s (104 fpm) and 0.406 m/s (80.0 fpm), respectively. Hexamethylene diisocyanate (HDI) monomer means were 32.2 vs. 13.3 µg/m³ for sprayers and 3.99 vs. 8.42 µg/m³ for hosemen. Crossflow velocities affected exposures inconsistently, and local work zone velocities were much lower. Aircraft painting contaminant control is accomplished better with the unidirectional crossflow ventilation presented here than with other observed configurations. Exposure limit exceedances for this ideal condition reinforce continued use of personal protective equipment.

Keywords: Aircraft paint spraying; computational fluid dynamics; exposure monitoring; hexavalent chromium; isocyanates; ventilation.

Figure 1.

Drawing of the aircraft painting bay showing filter areas.

FIGURE 2.

Navy artisans (sprayers and hosemen) applying primer during F-18 strike fighter aircraft paint finishing operations.

FIGURE 3.

Geometry of workers, exhaust wall filter, and F/A-18C/D Aircraft.

FIGURE 4.

Detail of modeled worker geometry.

Figure 5.

Modeled MIBK concentration contours (ppm) along the centerplane of the painting bay. Seen are the sprayer on the scaffold, the hoseman standing on the ground, and the plume created by the sprayer underneath the far wing.

Figure 6.

Solution contours in the hangar centerplane; (A) Velocity, showing a complex field influenced by ceiling, floor, and aircraft; (B) TI, being mainly determined by internal flow conditions; (C) Mass imbalance residual, where nearly all values are within the interval [−2.44 × 10⁻⁸, 4.70 × 10⁻⁸], i.e. very close to zero.

Figure 6.

Solution contours in the hangar centerplane; (A) Velocity, showing a complex field influenced by ceiling, floor, and aircraft; (B) TI, being mainly determined by internal flow conditions; (C) Mass imbalance residual, where nearly all values are within the interval [−2.44 × 10⁻⁸, 4.70 × 10⁻⁸], i.e. very close to zero.

FIGURE 7.

Concentrations of a simulated gas with the properties of MIBK calculated using CFD, for various air velocities and observed worker locations. 0.549/0.330 m/s (108/65 fpm) indicates the unbalanced condition of 0.549 m/s (108 fpm) of supply and 0.330 m/s (65 fpm) of exhaust. “BZ Height” refers to the entire hangar, at a height of 1.50 m from the floor. The heights of “Under Plane,” “Hoseman Port,” “Hoseman Starboard,” “Sprayer Port (scaffold),” and “Sprayer Starboard (wing),” were 0.305 m, 1.50 m, 1.50 m, 3.00 m, and 1.50 m, repectively.

FIGURE 8.

CFD results at 0.381 m/s (75 fpm) and 0.508 m/s (100 fpm) using the RNG k-epsilon turbulence model and a convergence criterion of 10⁻⁴ for the normalized residuals. The lower flow rate yields greater protection on average. “BZ Height” refers to the entire hangar, at a height of 1.50 m from the floor. The heights of “Under Plane,” “Hoseman Port,” “Hoseman Starboard,” “Sprayer Port (scaffold),” and “Sprayer Starboard (wing),” were 0.305 m, 1.50 m, 1.50 m, 3.00 m, and 1.50 m, repectively.

FIGURE 9.

CFD simulation results for velocities and turbulent kinetic energy in the work zones of sprayers and hosemen, as a function of crossflow velocity through the bay. Transverse velocity was calculated as velocity magnitude minus velocity toward exhaust.

FIGURE 10.

Five-location-mean concentrations for CFD simulations and tracer gas experiment means, as a function of velocity.

FIGURE 11.

Flow rate comparison by CFD and tracer gas methods.