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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.. A reduction in dust exposu

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

veff in mm/s

s in 0.001mm

1.6

1.2

0.8

0.4

0

preparation method

Fig 7.6 Measurements of body sound emitted from different surface treatment tools (Werner and

Kauw, 1991);veff – 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)

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 m2, 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/m3) reduced with distance from the source (feet) according to the following relationship:

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/m3 Randall et al (1998) reported on measurements performed during the removal of lead-based paint from a steel bridge with blast

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 exposure of 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 and work practice controls and the provision of personal protective equipment (PPE) and hygiene facilities supplemented by use of respirators 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 μ g/m 3

Total dust a Respirable dust b Total lead Respirable lead c

a OSHA PEL: 15,000 μ g/m 3

b OSHA PEL: 5,000 μ g/m 3

c OHSA PEL: 50 μ g/m 3

∗Not detectable

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 emission increased as the nozzle pressure increased if painted panels were blast cleaned If rusted panels were blast cleaned, 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 moderate pressures ( p= 0.55–0.69MPa), the emission increased with an

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 quartz sand generated more dust than copper sand at

equal nozzle pressures ( p= 0.7 MPa) But this trend turned upside down if 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)

added to the nozzle flow; in that case, the dust emission was higher for the copper slag

compared with that of quartz sand at equal high nozzle pressure ( p= 1.0 MPa) and equal water flow rate (1.1 l/min) The graphs also illustrate the effect of water addition 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:

Ef= a1+ a2· p + a3· ˙mp· a4· p2+ a5· ˙m2

P+ a6· p · ˙mP (7.2)

Here, Efis a specific dust emission factor, given in g/ft2 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 a1to a6are 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

399μg/m3for a cleanliness degree of Sa 2 and 525μg/m3for a cleanliness degree

of Sa 21/2

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/steel mixture 1.4 66.9 27.8

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

E f for painted steel 263.73 2.58 −57.17 −0.03 −0.85 0.71

E for rusted steel −206.40 4.13 8.99 −0.01 1.04 −0.24