Engineering gavimetric monitoring methods

In the first-phase of the project, Oberholzer et al., (1998) developed the rationale for adapting an addition of a cowl (shield) to the existing South African cyclone for measuring dust at high concentrations. In this report, background to the reasons for sampling, rationale for adapting the sampling head, the evidence of various problems with discrepancies between instruments, the same instruments of different manufacturers, and the difficulties in evaluating these samplers are discussed.
 
This report, a second-phase of COL 515 project titled "Engineering Gravimetric Sampling Methods for Monitoring the Conditions of Excessively High Dust Levels-2." The objective of the second phase of the work was to assess the results of the field trials on modified cyclones for dust measurements.
 
Field experience of dust measurement in South African coal mines has suggested that in very high dust concentrations, cyclones for sampling respirable dust can become overloaded (Oberholzer, 1998). In the light of studies conducted by Kenny, Baldwin and Maynand (1998) in UK, the use of a cowl (shield) attachment was favoured. The study reported to have shown that there was no significant difference in the concentration level measured between the cyclone with and without the shield. The general view of the several international opinions is that the use of the cowl will help to increase the accuracy of dust concentration measurements without increasing the bias of samples. It is further recommended that the CEN/ISO respirable curve with a d50 of 4 µm should be introduced as the standard.
 
In overall, following conclusions were made from the current study:
 
  •  The work carried out in the field demonstrates equivalent dust concentration results from the two sampler types in different positions within the mine. The extent of the agreement between the two sets of results seems to be excellent and much better than that for results obtained in the laboratory results or in non-mining workplaces.
  • From the statistical analysis (t-test and F-test), the mean dust concentration measured by pairs of cyclones is not significantly affected at 95 percent level of confidence. In other words, there is no difference between the measured mean dust concentration levels between various sampler pairs.
  • From the general size distribution plots from the sample dust, we notice that the mine- face respirable dust has two relative maximums, called modes and can be referred to as a bi-modal distribution.
  • From the size analysis on dust samples (with and without cowl samples) of the measured dust concentrations in the range 2,37 mg/m3 to 10,59 mg/m3, it is seen that the sampled respirable dust contained particles greater than 10 µm, except for the BGI sampler dust (3,89 mg/m3 concentration).
  • At very high measured dust concentrations, the cowl sampler lowered the loading of large particles. From the size distribution analysis data, we can conclude that the cowl samplers are the preferred samplers for taking engineering samples at high dust concentrations.
  • However, the results showed that at very high dust concentrations, there is a huge variation in both the measured dust concentration and the size distribution for a paired sample. For a pair of cowl and without-cowl samplers, for the same dust concentration levels, the evidence showed the presence of wide size distributions.
  • It is thus evident that, although the dust mass collection of the samplers conforms to the requirements, the size distribution of the dust is not concomitantly accurate.
  • The field study failed to conclusively determine whether the modifications made to the South African cyclone either improve or degrade its performance for the conditions tested, based on the results of the size analyses. • From the field observations, it can be stated that the cowl sampler can be used for engineering sampling to avoid the dependence of the cyclone inlet on orientation in the collection of particles.
  • The data analysis and field experience have shown that in comparison with other samplers, BGI samplers greatly reduce the amount of oversized particles deposited onto the filter from the inversion of cyclones. Also, due to the design of its cyclone inlet, the BGI sampler is not affected by orientation dependence.
  • The field observations did indicate that when collecting samples closer to the face, where high concentrations of dust are present, sample handling is extremely difficult. At high dust concentrations, the probability of non-respirable particles depositing onto the filter from the grit pot is very high when the pump is switched off. However, the BGI sampler, due to its design, alleviates this problem to some extent. • In both the laboratory and field tests, a sufficient mass of sample dust was required for analysis with the Fritsch analyser to determine the presence of non-respirable particles. This led to the conclusion that there may have been non-respirable particles in the concentrations below 5 mg/m3 (as found from this study)
  • The size analysis of the collected dust mass for sample concentrations greater than 5 mg/m3 did not follow the D50 of either BMRC or ISO/ACGIH/CEN respirable curves.
  • The cowl sampler could not be recommended for personal sampling because the results of the size analysis are inconclusive and wide-ranging.
  • Finally, question of cowl under-sampling (under-estimation of dust concentration) or over-sampling (over-estimation of dust concentration) could not be resolved with confidence from size distribution data at concentrations below 5 mg/m3 despite the insignificant differences obtained from comparison of mass concentrations alone.

 

From the conclusions drawn, the following recommendations for future research are proposed, although it is suggested that there should be a paradigm shift in the existing sampling procedure and instruments in the future project:
 
  • The results were inconclusive with regard to the recommendations on the usage of adding a cowl to the cyclone in the personal sampling method. However, the use of a cowl for taking engineering samples can be recommended on the grounds that it lowers sample-handling errors, the probable reason being that in practice it reduces factors such as the wind and orientation-dependence of the cyclone inlet. At high dust concentrations, it was difficult to determine which sampler measured the “true concentration.” Therefore, future field tests should incorporate the aspect of “impactor sampling” from which both the mass concentration and the size distribution of ambient particles can be accurately determined.
  • Despite some of the cyclones being accepted for sampling in the industry, concerns were expressed about the cyclone performance with respect to the BMRC and ISO/ACGIH/CEN respirable curves. Fears were also expressed about whether the cyclones do follow the BMRC respirable curve under concentrations below 5 mg/m3. In addition to the investigation, further work on recommendations for the use of the cowl sampler for personal sampling could be considered.
  • Similar studies should be carried out to compare these samplers with CIP10 samplers and to investigate their variances and size distribution. The different makes of samplers should also be investigated to determine the relationship between them, not only with regard to the mass of dust collected but also in terms of the size characteristics of the dust. • Tests should be carried out to determine the size characteristics of personal dust samples and engineering dust samples to compare with the existing DME requirements for engineering sampling.
  • Immediate attention needs to be given to analysing both the compliance and non- compliance samples from the mines, which have difficulty in maintaining the 1997 DME directive. On the basis of this outcome, there needs to be clarification on the matter of performance evaluation of all the cyclones currently used in the mines.
  • An extensive facility, including a chamber and a particle size analyser for analysing sample mass on compliance samples of less than 5 mg/m3, needs to be established for this serious issue of sampling. A programme needs to be established for dust sampling, measurement and data analysis for exposure assessment.
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