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302 7 Health, Safety and Environment Fig. 7.5 Critical exposure times for different preparation tools (solid line according to BGV B3 L¨arm; points from different sources) the material to be subjected and the preparation tool. The evaluation parameters of the vibration are its amplitude and velocity (frequency). No measurements are available from dry blast cleaning operations. However, there are some measure- ments available from concrete facades treated with other surface preparation tools. Amplitudes and vibration velocities generated by the tools are plotted in Fig. 7.6. The two mechanical methods generated rather high values for vibration velocities, whereas the application of water jets led to low vibration velocities. 7.4 Emission of Dust A mist of paint debris and broken abrasive particles is generated during dry blast cleaning in the immediate environment of the operator. An example is shown in Fig. 7.7. A simple model for the evaluation of dust during the blast cleaning of mould casings was introduced by Engelberg (1967). Unfortunately, the dust is difficult to control. The only way to prevent it is the use of shrouded tools. Another way to protect the operator is the application of mechanically guided tools or robotic machinery. Anyway, both methods fail as it comes to the cleaning of complex structures. A reduction in dust exposure is pos- sible by adding water to the air particle flow (wet blast cleaning and slurry blast 7.4 Emission of Dust 303 v eff in mm/s s in 0.001mm 1.6 1.2 0.8 0.4 0 12345 preparation method target parameter Fig. 7.6 Measurements of body sound emitted from different surface treatment tools (Werner and Kauw, 1991); v eff – effective vibration velocity; s – vibration amplitude. Preparation methods: 1 – water jetting, 2 – hammer and chisel, 3 – jack hammering, 4 – pneumatic hammer, 5 – angle grinder Fig. 7.7 Dust formation during dry blast cleaning (Photograph: Muehlhan AG, Hamburg) 304 7 Health, Safety and Environment Fig. 7.8 Additional working time in a shipyard due to dust formation (Navy cargo ship in a dry- dock) cleaning). Reviews on such methods are provided by Momber and Schulz (2006) and in SSPC (2006). Some problems associated with dust formation are illustrated in Fig. 7.8. A very high amount of working time is required to wrap and unwrap the object (in the certain case a marine vessel in a drydock) before and after blast cleaning, and to clean up the yard site after the blast cleaning job. Several hundreds of additional working hours were spent in the example shown in Fig. 7.9. For a ship hull of about 8,000 m 2 , 5 to 7 days for wrapping up the vessel using an eight-man crew would be required. Unwrapping would require another 4 to 5 days (Nelson, 1996). Brantley and Reist (1994) investigated the exposure to respirable dust at ten dif- ferent blast cleaning sites where quartz sand was used. Their results revealed that in general, downwind respirable silica concentration varied as distance raised. The concentration of respirable silica (mg/m 3 ) reduced with distance from the source (feet) according to the following relationship: m D ∝ x −1.17 (7.1) The geometry of the worksite and the position of the workers affected concen- trations observed by orders of magnitude. The values measured for respirable dust varied between 0.01 and 10 mg/m 3 . Randall et al. (1998) reported on measurements performed during the removal of lead-based paint from a steel bridge with blast 7.4 Emission of Dust 305 Fig. 7.9 Effects of abrasive type on the particle size distribution functions of dust (Kura, 2005) cleaning. The authors measured total dust, respirable dust, total lead exposure and the exposureof respirable lead. Results of these measurements are listed in Table 7.2. The values are all above the permissible limits. This situation required the implemen- tation of feasible engineering andwork practice controls and theprovision of personal protective equipment(PPE) andhygienefacilitiessupplementedbyuse ofrespirators. Particle size distributions of airborne particles from blast cleaning operations were analysed by Kura (2005) with different methods. Some results are plotted in Fig. 7.9, and it can be seen that the type of abrasive determined the size distribution functions. Steel grit formed rather large dust particles, whereas the dust particles were small for bar shot. Table 7.2 Air sampling analysis results from the removal of paint from a steel bridge (Randall et al., 1998) Sampling point Exposure in μg/m 3 Total dust a Respirable dust b Total lead Respirable lead c Background 300 100 5 * Blast cleaning area 20,600 6,300 200 100 Blaster 1 4,000 400 50 * Blaster 2 44,600 6,200 450 120 a OSHA PEL: 15,000 μg/m 3 b OSHA PEL: 5,000 μg/m 3 c OHSA PEL: 50 μg/m 3 ∗Not detectable 306 7 Health, Safety and Environment Kura et al. (2006) investigated the effects of nozzle pressure, abrasive feed rate (in terms of number of turns of metering valve) and abrasive mass flow rate on the emission of dust during blast cleaning.Dust emissionincreased asthe nozzle pressure increasedifpaintedpanels wereblastcleaned.Ifrusted panelswere blastcleaned,dust emission was almost independent of nozzle pressure. The influence of the abrasive feed rate on the emission of particulate matter was sensitive to the nozzle pressure. For low and moderatepressures (p=0.55–0.69 MPa), the emission increased withan increase in the number of valve turns. For higher pressures (p =0.83 MPa), however, the emission showed maximum values at a moderate number of turns. The emissions for rusted panels were almost independent of the abrasive mass flow rate, whereas the emissions for painted panels again showed a complex relationship to abrasive mass flow rate and nozzle pressure. Results reported by Kjernsmo et al. (2003) are presented in Fig. 7.10. The emission of respirable dust increased for higher nozzle pressures.It can also be seen that quartzsand generated more dust than coppersand at equal nozzle pressures (p =0.7 MPa). But this trend turned upside downif water was Fig. 7.10 Effects of abrasive type, nozzle pressure and water addition on the formation of res- pirable dust (Kjernsmo et al., 2003) 7.4 Emission of Dust 307 added to the nozzle flow; in that case, the dust emission was higherforthe copper slag compared with that of quartz sand at equal high nozzle pressure (p = 1.0 MPa) and equal waterflow rate(1.1l/min). The graphs also illustrate the effectof wateraddition. The version with the highest amount of added water (4.5 l/min) generated the lowest dust level among all tested configurations. Greenburg and Winslow (1932) performed an early thorough study into the ef- fects of location, abrasive type and fresh air supply on the concentration of dust during blast cleaning operations. Some results are listed in Table 7.3. It can be seen that the use of a mineral abrasive (sand at that time), even when mixed with a metal- lic abrasive material, created much higher dust concentrations compared to the use of a metallic abrasive. Kjernsmo et al. (2003) reported on the effects of abrasive type on respirable dust concentration. As shown in Fig. 7.11, quartz sand generated the highest amount of dust (which agreed with the results shown in Fig. 7.10), whereas cast iron generated very low dust levels. Mineral-based abrasive materials are usu- ally more critical to dust formation compared with metallic abrasive materials. Kura (2003) and Kura et al. (2006) provided the following statistical model for the assessment of parameter effects on dust emission during dry blast cleaning: E f = a 1 + a 2 · p + a 3 · ˙m p · a 4 · p 2 + a 5 · ˙m 2 P + a 6 · p · ˙m P (7.2) Here, E f is a specific dust emission factor, given in g/ft 2 . The pressure is given in psi and the abrasive mass flow rate is given in lbs/min. This relationship holds for coal slag and bar shot, and for air pressures between p = 0.55 and 0.83 MPa. The constants a 1 to a 6 are regression parameters whose values as listed in Table 7.4. Plitzko et al. (1998) investigated the effects of abrasive type and water addition on the concentration of respirable dust during the blast cleaning of metal substrates. Some of their results are plotted in Fig. 7.12. It is clear from this graph that even the use of a slurry system (method “5”) could not avoid the exposure of impermis- sibly high dust concentrations. For dry blast cleaning with quartz, the permissible workplace limit was exceeded by a factor of 940. The use of an alternative abrasive material and the addition of water allowed for the reduction of this value, but the permissible limit was still exceeded by a factor of 4. Katsikaris et al. (2002) noted an effect of the desired substrate surface cleanli- ness on the concentration of respirable dust. The respirable dust concentration was 399 μg/m 3 for a cleanliness degree of Sa 2 and 525 μg/m 3 for a cleanliness degree of Sa 2 1 / 2 . Table 7.3 Results of dust measurements for different abrasive materials (Greenburg and Winslow, 1932) Abrasive material Dust concentration in 10 6 particles per cubic metre Minimum Maximum Average Sand 6.5 86.9 27.1 Steel 1.4 9.2 4.3 Sand/steel mixture 1.4 66.9 27.8 308 7 Health, Safety and Environment Fig. 7.11 Effects of abrasive material on the formation of dust (Kjernsmo et al., 2003). 1 – cast iron, 2 – aluminium oxide, 3 – aluminium silicate, 4 – olivine, 5 – quartz sand Dust concentration, especially in confined spaces, can be reduced due to the utilisation of ventilation systems. As shown in Fig. 7.13, ventilation could drop dust concentration to very low values. Critical parameters were ventilation time and system size. The longer the ventilation time, the lower was the dust concentration. It was also shown that small ventilation systems can work very efficiently. Blast cleaning operators must usually wear respiratory equipment, combined with a separate fresh air supply. It was already shown in an early investigation by Greenburg and Winslow (1932) that the amount of air delivered is of fundamental importance in determining the degree of protection of respiratory devices. Results of their measurements are provided in Fig. 7.14. It can be seen that the dust con- centration under the helmet reduced with an increase in air supply. The graphs also illustrate the effects of screens in front of the blaster’s eyes. A glass screen notably contributed to a reduction in dust concentration under the helmet. Table 7.4 Regression coefficients for (7.2) Target parameter in g/ft 2 Coefficients a 1 a 2 a 3 a 4 a 5 a 6 E f for painted steel 263.73 2.58 −57.17 −0.03 −0.85 0.71 E f for rusted steel −206.40 4.13 8.99 −0.01 1.04 −0.24 . Average Sand 6.5 86.9 27 .1 Steel 1.4 9 .2 4.3 Sand/steel mixture 1.4 66.9 27 .8 308 7 Health, Safety and Environment Fig. 7.11 Effects of abrasive material on the formation of dust (Kjernsmo et al., 20 03) area 20 ,600 6,300 20 0 100 Blaster 1 4,000 400 50 * Blaster 2 44,600 6 ,20 0 450 120 a OSHA PEL: 15,000 μg/m 3 b OSHA PEL: 5,000 μg/m 3 c OHSA PEL: 50 μg/m 3 ∗Not detectable 306 7 Health, Safety and. cleanliness degree of Sa 2 and 525 μg/m 3 for a cleanliness degree of Sa 2 1 / 2 . Table 7.3 Results of dust measurements for different abrasive materials (Greenburg and Winslow, 19 32) Abrasive material