Compressor Instability with Integral Methods Episode 2 Part 3 ppt

Chapter 7 Health, Safety and Environment 7.1 Safety Features of Blast Cleaning 7.1.1 General Safety Aspects General aspects of health, safety and environment (HSE) for blast cleaning appli- cations are summarised in Fig. 7.1. ISO 12944-4 states the following for surface preparation in general: “All relevant health and safety regulation shall be observed.” Blast cleaning owns an injury potential. General sources of danger to blast cleaning operators include the following: r reactive forces generated by the exiting air-abrasive mixture (see Sect. 6.6.4); r hose movements; r uncontrolled escape of pressurised air; r damaged parts being under pressure; r dust and aerosol formation; r sound emitted from equipment and blasting jet; r impact from rebounding abrasive material and debris from the impact point. It is generally recommended to carry out a risk assessment of the actual envi- ronment where a blast cleaning job will be done before starting the job. This risk assessment may include (French, 1998): r how access is to be gained? r is there a need for scaffolding? r is there confined space? r what is the surface like where the operators will have to stand? r the availability of day light or artificial light; r the presence of electrical supplies/equipment; r nature of contaminate: Is it toxic? Is it a pathogen? Is it asbestos based? Is it harmful or corrosive? r general layout that will allow visual contact between the blast cleaning team; r permit requirements; A. Momber, Blast Cleaning Technology 295 C  Springer 2008 296 7 Health, Safety and Environment uncontrolled TPM emissions (kg/yr) uncontrolled TPM emission factors (kg/kg) dispersion modeling toxicity ratings for Cr, Mn, Ni, Pb cancer (URE) non-cancer (RfC) PM 10 / TPM fraction efficiency of APCD (%) annual abrasive usage (kg/yr) ambient long-term (chronic) metal concentrations at receptor locations Cr, Mn, Ni, Pb (μg/m 3 ) uncontrolled respirable PM (PM 10 ) emissions (kg/yr) controlled PM 10 emissions (kg/yr) total inhalation-induced cancer = Σ URE i * Conc i & non-cancer risk = Σ Conc i /RfC i controlled metal emissions: Cr, Mn, Ni, Pb (kg/yr) metal fractions in TPM Cr, Mn, Ni, Pb (kg/kg) Fig. 7.1 HSE risk analysis for blast cleaning processes (Kura, 2005) r safety of access (e.g. working on motorways or hazardous areas such as refin- ery where flameproof equipment and earthing to avoid static electricity may be required); r who or what will be affected by flying debris? r is noise a problem? r will containment be necessary? r where will the effluent go? (for wet blast cleaning and slurry blast cleaning) In that context, ISO 12944-4 states the following: “Personnel carrying out sur- face preparation work shall have suitable equipment and sufficient technical knowl- edge of the processes involved.” 7.1.2 Risk of Explosion Some source of explosion during blast cleaning can be electric discharge sparks. Safety hazard analyses identified that static electric charges occur in the following three circumstances: r small particles flowing through piping; r small particles passing through fine filters or nozzles; r abrasive particles impinging fixed parts. 7.2 Emission of Air Sound 297 Table 7.1 Results of spark measurements during blast cleaning (Stuvex Belgium) Parameter Method Dry blast cleaning Wet blast cleaning Voltage at the blast cleaned surface in V 500 2–3 Voltage of static electricity at the nozzle in V 5,000–10,000 0 Results of spark measurements performed on oil containers with dry blast and wet blast cleaning techniques are listed in Table 7.1. Dry blast cleaning generated high levels of voltage at the blast cleaned surface as well as at the nozzle. The use of wet blasting equipment helped to keep these levels low. Elbing (2002) reported about measurements of the electrostatic charging of steel during the blast cleaning with carbon dioxide pellets. The author measured values as high as 3,000 V, and he found that the discharge current increased with an increase in air pressure and stand-off distance. The discharge current was rather high for shallow impact angles, but reached a lower saturation level for impact angles ϕ>50 ◦ . The electrostatic discharge in hose lines due to friction between hose wall and flowing abrasive particles can be managed through the use of blast cleaning hoses with low electric resistance. Values for the electrical resistance lower than of 10 3 ⍀/m are considered to allow a safe charge elimination (BGR 132, 2003). The effects of impinging abrasive particles on the ignition of explosive gas mix- tures are not well understood. Dittmar (1962) highlighted the fact that sparks must create a certain temperature field in order to ignite gas mixtures. Duration and in- tensity of the temperature field determine the danger of explosion. The author cited experimental results from machining operations, and he reported that the forma- tion and subsequent combustion of small metal chips generated temperatures up to 2,300 ◦ C. These high temperatures were much more critical than the temperatures reached at the tool–chip interface during the material removal process (see Fig. 5.17 for the situation during blast cleaning). Smaller chips generated higher temperatures than larger chips. Dittmar (1962) also reported that rusted steel substrates were much more sensitive to spark generation compared with clean steel substrates. 7.2 Emission of Air Sound There are four major sources of air sound generated during blast cleaning operations: r sound emitted from the pressure generating unit (compressor, engine and power transmission); r sound emitted from the abrasive air jet travelling through the air; r sound emitted from the erosion site; r sound emitted from accompanying trades. Two other items of concern are noise generated by air supply in the helmet and sound attenuation of the helmet. 298 7 Health, Safety and Environment State-of-the-art air compressors are regularly equipped with sound insolating hoods or even placed in containers. Thus, the air sound emission is limited up to 70–75dB(A). More critical is the air sound emitted by the jet. This noise is gener- ated due to friction between the high-speed jet and the surrounding air as well as due to turbulences. Thus, the sound level depends on the relative velocity between jet and air, and on the surface exposed to friction. Consequently, air sound level increases as compressor pressure, nozzle diameter and stand-off distance increase. This is verified in Fig. 7.2 where the effect of the nozzle pressure on the noise level is shown. The noise level increased almost linearly with increasing air pressure. Equal trends have been reported for the noise emitted during dry blast cleaning with carbon dioxide pellets (Elbing, 2002). Fig. 7.3 illustrates results of measurements performed at different blasting sites, where dry blast cleaning, shot blast cleaning and wet blast cleaning were applied. Figure 7.3a includes results from measurements at a dry blast cleaning site. The actual blast cleaning application generated the highest noise levels. Figure 7.3b and d shows results from measurements at wet blast cleaning sites. The noise gener- ated during the actual wet blast cleaning application was lower than the noise level measured for the dry blast cleaning in Fig. 7.3a. Figure 7.3c contains results from measurements at a shot blast cleaning site. The noise level was again lower than the noise level for the dry blast cleaning process mentioned in Fig. 7.3a, which was due to the facts that no air was involved in the mechanically driven shot blast clean- ing process, and that the blast cleaning head was sealed. It can be recognised that Fig. 7.2 Effect of compressor pressure on noise level (Schaffner, 1997) 7.2 Emission of Air Sound 299 jobs: 1 - dry blast cleaning 2 - scaffolding 3 - maintenance air supply system 4 - picking up solid waste (grit,paint) 5 - transportation of solid waste 140 120 100 80 1 60(a) 3.07 1.6 1.77 working time in h 0.550.55 1.03 1.52 equivalent sound level in dB 2 1 2345 120 100 80 60 179 jobs: 1 - wet blast cleaning 2 - masking windows working time in min equivalent sound level in dB 34 12 (b) Fig. 7.3 Results from noise-level measurements during steel surface preparation jobs (Knipfer and Funke, 1997). (a) Dry blast cleaning; (b) Wet blast cleaning; (c) Shot blast cleaning; (d) Wet blast cleaning 300 7 Health, Safety and Environment 140 100 120 80 1 60 jobs: 1 - shot blasting 2 - disposal of removed coating working time in h equivalent sound level in dB 3.2 1.72 2 (c) (d) Fig. 7.3 Continued 7.3 Emission of Body Sound 301 Fig. 7.4 Noise levels at different locations (Ognedal and Harbak, 1998) the actual blasting operations (dry blast cleaning, shot blast cleaning and wet blast cleaning) generated the highest noise levels among all trades. Shot blast cleaning (which works with shrouded blasting tools) and wet blast cleaning are comparatively silent. Noise emission can notably be reduced if shrouded or sealed tools are used. Figure 7.4 shows results of air noise measurements performed inside the helmet of blasters. Ognedal and Harbak (1998) concluded from these measurements that blast cleaning may create loss of hearing to the workers, if no additional hearing protection is provided. The permissible air noise level depends on the exposure time. This is illustrated in Fig. 7.5 based on regularity limits stated in the German standard ‘BGV B3 L¨arm’. It can be concluded from the graph that ear protection equipment must be worn by any personally involved blasting cleaning operator (see Sect. 7.8). 7.3 Emission of Body Sound Body sound characterises waves, which carry noise and travel through solid materi- als. Therefore, even if windows, doors, etc. are properly closed to lock out airborne noise, persons may any way experience certain noise levels. This noise is generated due to vibrations; they occur during the tool impact and depend on the acoustic properties, especially on the sound velocity and the acoustic impedance, of both 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 [...]... Arithmetic mean Arithmetic standard deviation 0.05 . 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 26 3. 73 2. 58 −57.17 −0. 03 −0.85 0.71 E f for rusted steel 20 6.40 4. 13 8.99 −0.01 1.04 −0 .24 7.4. lead concentration in μg/m 3 Range Median Blasters and sweepers 12 4,401 36 6 Equipment operators 14–1,400 21 9 Foremen 26 3, 4 23 160 7.5 Emission of Airborne Metals 31 3 Table 7.6 Measured airborne. of solid waste 140 120 100 80 1 60(a) 3. 07 1.6 1.77 working time in h 0.550.55 1. 03 1. 52 equivalent sound level in dB 2 1 23 45 120 100 80 60 179 jobs: 1 - wet blast cleaning 2 - masking windows working