Page 21 - OHS, January/February 2021

in the process under normal and abnormal conditions, then the DHA team can formu- late corrective measures.
The best way to approach a What If Analysis for a DHA is to use the Explosion Pentagon depicted in Figure 1. For each unit operation, process line, room and building the team creates scenarios where the five components for an explosion, or three components for a fire, can come to- gether to cause an accident. The team can then develop hazard prevention and pro- tection plans.
For this What If Analysis and subse- quent hazard prevention and protection plan to be effective, the combustion and explosion potential of the material must be ascertained. The schema present in Fig- ure 2 is a method to follow to effectively characterize the combustion hazard of the material. Combustion characteristics of dusts, unlike gases and vapors, are specific to the sample.
Varying factors like particle size distri- bution, particle morphology, surface char- acteristics and moisture content can greatly affect the physical combustion parameters. This prevents the usage of historical data from previous studies of the material un- less the sample of the material in question is exactly the same as the sample used in the historical study. Do not trust data from Safety Data Sheets unless there is informa- tion provided that the data applies to the specific sample in hand.
Sample Testing
Once it has been decided which materials to consider for the DHA, unique samples can be taken for study from various loca- tions in the plant. For example, the sample from the rafters and overhead lights can be different than the material on the floor, dust collector or cyclone. As a result, mul- tiple samples of the materials at the facility may need to be studied.
Once the samples have been taken and uniquely identified, the analysis process moves to Stage 3 where the explosion po- tential of the sample is qualitatively de- termined in Stage 3a. The combustion potential of the sample as a pile of layers is qualitatively determined in Stage 3b. For all the samples in Stage 3a and 3b that demonstrate reactivity, further quantitative combustion and explosion qualities are de-
termined in Stage 4.
From Stage 4a, the Explosion Severity,
MEC and MIE are the three characteristics that are used in a DHA with the other Stage 4a tests used for unique process situations. In Stage 4b; the Burn Rate Test, LIT, Hot Storage Screening and Exothermic De- composition Screening tests are more often used with the other tests being performed when identified as warranted in the DHA.
Ultimately, conducting quantitative testing to determine combustion and ex- plosion characteristics of the dusts present at a facility in concert with performing the
required DHA from NFPA 652 will be cru- cial to the reader in assessing the dangers of a dust explosion at a facility.
Dr. Ashok Ghose Dastidar, PhD MBA is a Fellow Engineer at Westinghouse Electric Company and the Vice President of Dust & Flammability Testing and Consulting Ser- vices at Fauske & Associates, LLC.
REFERENCES
1. http://dustsafetyscience.com
www.ohsonline.com
Circle 27 on card. 17
Combustible Dust Testing
OSHA Directive CPL 03-00-008
• Initial Sample Characterization
• Explosion Severity
• Minimum Explosive Concentration • Minimum Ignition Energy
• Minimum Ignition Temperature
• Full Class II Analysis
West Coast
866.798.1089
East Coast
800.220.3675
www.emsl.com