A Numerical and Experimental Investigation of Crystalline Silica Exposure Control During Tuck Pointing

Abstract

National Institute for Occupational Safety and Health researchers investigated control measures for the removal of mortar between bricks, using a grinder. This task, "tuck pointing," is associated with crystalline silica exposures many times greater than the permissible exposure limit enforced by the Occupational Safety and Health Administration. Previous studies showed that local exhaust ventilation (LEV) of the grinding wheel through a shroud was often ineffective. Tuck pointing occurs on a scaffold. For practical purposes, this limits the size and power of the LEV system. Thus, the goal of this study was to develop a recommended flow rate for exposure control. Flow induced by the rotating grinding wheel, flow induced by the mortar particle stream, and particle momentum are potential control challenges. Computational fluid dynamic (CFD) simulation of the grinder, supported by some experimental measurements, showed the relative importance of these factors through varying parameters and tracking particles. In a simulation of the shroud and grinding wheel, with the wheel inserted to a cutting depth of 0.750 inch flush into the brick wall, -0.461 cubic feet per meter (0.461 into the exhaust takeoff) was induced by the rotating wheel. The more realistic situation of the wheel in a cut in the wall 1.25 inches deep (forming a trench circumferentially 0.500 inch below the wheel edge) induced an airflow of 8.24 cfm out of the shroud exhaust. Experimental measurements taken for validation were 7.3% lower than the CFD value. The trench effect disappeared when a stream of 10-mu m particles was launched from the grinding wheel edge, as the simulations with and without the trench had nearly identical induced flow rates, 10.8 cfm and 10.9 cfm. We thus interpreted the particle stream as more important than the wheel in inducing flow. This insight was possible because of the power of CFD, compared to intuition and classical boundary layer analysis. In this situation of no forced exhaust, all particles escaped through the gap between the shroud edge and the brick wall into the worker's environment. Experiments and simulations indicated that approximately 85 cfm was required for good control of silica exposure, clearly demonstrating that the exhaust rate must accomplish much more than balancing the induced flow. The simulations showed that the exhaust must create a vacuum in the shroud sufficient to bend the particle paths into the shroud. In the simulations, stopping the particle stream through collision (effectively removing or reducing the "daylight" between the wall and shroud) greatly lessened the required flow rate. This is difficult in practice because the gaps between the shroud and the brick and between bricks create escape paths.

Comprehensive report: evaluation of the dust generation and engineering control for cutting fiber-cement siding.

“…Figure 1 illustrates a diagram of the laboratory testing system used in this study. The overall dimension and components of the system were similar to those used by Beamer et al [2005], Heitbrink and Bennett [2006], and Carlo et al [2010], and they were consistent with European Standard EN 1093-3 [CEN, 2006]. A dust collection air handling unit (PSKB-1440, ProVent LLC, Harbor Springs, MI) was used as an air mover for the system.…”

“…The delta tube was mounted on the duct about 2.4 m downstream (8 times of the duct diameter) of the funnel section (not shown in Figure 1 for clarity). The accuracy of the flow rate measured by the Delta tube was verified by comparing the flow rate obtained from its manufacturer's calibration equation [Mid-West Instrument, 2004] and that measured by Heitbrink and Bennett [2006] using a 10-point pitot tube traverse of the duct performed in the horizontal and vertical planes (about 0.8% difference). Two aerosol sampling ports were open on the duct for mounting the sampling probes of all the sampling instruments used in this study.…”

The Effect of Debris Accumulation On and Filter Resistance to Airflow for Four Commercially Available Vacuum Cleaners

“…(5,6) The use of right-angle grinders to remove deteriorated mortar from buildings causes excessive exposure to respirable crystalline silica that, reportedly, can be as high as 5 mg/m 3 . (7) Other construction and renovation tasks such as concrete grinding, concrete drilling, brick cutting, and cutting roofing tile are also reported to cause excessive exposure to crystalline silica. (1) Thus, effective control measures are needed to reduce worker exposure to crystalline silica.…”

“…To capture the dust, a vacuum cleaner can be used to exhaust a minimum of 2.3 m 3 /min (80 cfm) from a hood that is mounted on the grinder as shown in Figure 1B. (7,9) During field trials, respirable crystalline silica exposures for 22 samples had a geometric mean of 0.06 mg/m 3 and a range of less than 0.01 to 0.86 mg/m 3 . (8) In other studies, geometric mean respirable crystalline silica exposures during mortar removal were between 0.35 and 1.1 mg/m 3 .…”

“…(8) In other studies, geometric mean respirable crystalline silica exposures during mortar removal were between 0.35 and 1.1 mg/m 3 . (1,7,10) The use of local exhaust ventilation (LEV) during tuck point grinding dramatically decreases personal dust exposure levels about FIGURE 1. Mortar removal with and without dust control.…”