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process safety consulting

Crucial to safer material handling is a thorough understanding of maximum safe processing and storage temperatures.

Many solids (powders) are capable of undergoing an exothermic decomposition when exposed to elevated temperatures. Thermal instability hazards can be manifested in a range of ways including "black bits" in product after drying, yield decrease after prolonged reaction or drying times, propensity for smouldering (often leading to fire), etc.

The definition of a maximum safe temperature is complicated by the impact of a number of variables such as scale, geometry, particle size and air availability. Whilst some materials decompose ("fall apart") at the molecular level at elevated temperatures, others (such as milk powder, coal dust, citrus peel and dried sewage sludge) undergo exothermic reaction with oxygen in air (ie. burning). For very finely divided powders, the surface area exposed for reaction with air, can be huge and can lead, in extreme cases, to pyrophoric behaviour (ie. near-spontaneous combustion).

Determining whether exothermic behaviour at elevated temperature is due to pure chemical decomposition or oxidation is absolutely crucial in selecting an appropriate test to measure the initiation conditions. In many cases in industry, companies blindly use Differential Scanning Calorimetry (DSC) or Differential Thermal Analysis (DTA) methods for establishing thermal stability limits. However, these sealed test methods are normally inadequate to detect oxidation processes and massively overstate safe processing temperatures. Alternative test methods that are specific to powders, that take into consideration scale and also air availability are available to overcome these small scale test limitation and to provide sound and conservative data for any processing condition.

Chilworth Global has many years of experience in evaluating thermal instability properties for a wide range of materials as well as being able to provide expert consultancy advice for the selection of the most appropriate test and to provide assistance and support for companies wishing to prevent hazardous thermal decomposition in an industrial situation.

How Chilworth Can Help

  • Laboratory testing (in GLP compliant facilities) to establish thermal instability properties for powders and also liquids.
  • Test techniques for determination of thermal instability for materials which undergo chemical decomposition:
    • Differential Scanning Calorimetry (DSC)
    • Differential Thermal Analysis (Carius Tube)
    • Adiabatic calorimetry (using the Accelerating Rate Calorimeter (ARC) or low phi factor Adiabatic Calorimeter system)
    • CLP and UN standard tests for classification of self-reactive substances (UN Class 4, Division 4.1) including test methods H.2 (Adiabatic Storage Test) and H.4 (Heat Accumulation Storage Test)
  • Determination of thermal instability properties such as onset temperature for decomposition, safe storage temperature and maximum duration time for safe storage for powders which undergo thermal oxidation including tests for evaluating the thermal instability of powders in:
    • Bulk driers (spray, tray, flash driers) using the Bulk Powder (Diffusion Cell) test and also the Basket test series which can be used to extrapolate thermal limits for very large scale storage facilities). The basket test approach can also be used under CLP and transportation regulations for definition of self-heating substances.
    • Aerated environments such as fluid bed dryers or rotating drum driers, using the Aerated Cell Test.
    • Driers where thin layers or deposits are found such as smaller tray driers, some spray driers or flash driers using the Air Over Layer test approach

Chilworth can also assist by providing:

  • Independent Consultancy advice to carry out hazard and risk assessments and auditing of thermal instability hazards.
  • Incident investigations to establish the cause of thermal instability incidents and how to prevent recurrence.
  • Training courses to improve awareness of process hazards and how to prevent thermal runaways.

Transportation Testing (including UN)

  • Burning Rate Test - DOT/UN Division 4.1
    The Burning Rate or Fire Train Test is used to classify materials as "flammable solids" in accordance with U.S. Department of Transportation (DOT) and United Nations (UN) requirements. Chilworth performs the Burning Rate Test in accordance with the method described in 49 CFR 173 Appendix E and UN Recommendations on the Transport of Dangerous Goods. The test involves igniting a 250 mm x 20 mm x 10 mm powder train using a suitable ignition source and measuring the burning rate (time).
  • Burning Rate Test - DOT/UN Division 4.1
    The Burning Rate or Fire Train Test is used to classify materials as "flammable solids" in accordance with U.S. Department of Transportation (DOT) and United Nations (UN) requirements. Chilworth performs the Burning Rate Test in accordance with the method described in 49 CFR 173 Appendix E and UN Recommendations on the Transport of Dangerous Goods. The test involves igniting a 250 mm x 20 mm x 10 mm powder train using a suitable ignition source and measuring the burning rate (time).
  • Pyrophoric Solids Test - DOT/UN Division 4.2
    The Pyrophoric Solids Test is used to classify solids materials as "spontaneously combustible" in accordance with U.S. Department of Transportation (DOT) and United Nations (UN) requirements. Chilworth performs the Pyrophoric Solids Test in accordance with the method described in 49 CFR 173 Appendix E and UN Recommendations on the Transport of Dangerous Goods. The test involves pouring a powder sample from a height of one meter onto a non-flammable surface and observing whether the sample ignites during its descent or after settling.
  • Self-Heating Substances Test - DOT/UN Division 4.2
    The Self-Heating Substances Test is the second of two tests used to classify materials as "spontaneously combustible" in accordance with U.S. Department of Transportation (DOT) and United Nations (UN) requirements. Chilworth performs the Self-Heating Substances Test in accordance with the method described in 49 CFR 173 Appendix E and UN Recommendations on the Transport of Dangerous Goods.
    The Self-Heating Substances Test involves placing a powder sample in a 100 mm x 100 mm x 100 mm stainless steel cubic mesh basket and placing the basket in an oven where it is exposed to a constant temperature of 140 °C for 24 hours. If necessary, a second trial is performed using a 25 mm x 25 mm x 25 mm mesh basket. The sample and oven temperatures are continuously monitored at several locations for the duration of the test.
  • Dangerous When Wet Test - DOT/UN Division 4.3
    The Dangerous When Wet Test is used to identify materials that -- upon contact with water or moisture -- are liable to become spontaneously flammable or evolve a flammable or toxic gas, as per U.S. Department of Transportation (DOT) and United Nations (UN) requirements. Chilworth performs the Dangerous When Wet Test in accordance with the method described in 49 CFR 173 Appendix E and UN Recommendations on the Transport of Dangerous Goods.
    The Dangerous When Wet Test is comprised of four (4) parts, each of which involves exposing a small amount of solid sample to a limited amount of water. The samples are observed to determine whether gas is evolved and whether such gas is flammable. The volume of gas evolved is measured to determine the gas evolution rate.
  • Solid Oxidizing Substances Test - DOT/UN Division 5.1
    The Solid Oxidizing Substances Test is used to classify materials as such in accordance with U.S. Department of Transportation (DOT) and United Nations (UN) requirements. Chilworth Global performs the Solid Oxidizing Substances Test in accordance with the method described in 49 CFR 173 Appendix F and UN Recommendations on the Transport of Dangerous Goods. The test involves mixing powder samples with sawdust in various proportions and observing the combustion characteristics when a conical pile of the sample/sawdust mixture is ignited by a hot loop ignition source.
  • Oxidizing Liquids Test - UN/DOT Division 5.1
    The Oxidizing Liquids Test is used to classify materials in accordance with U.S. Department of Transportation (DOT) and United Nations (UN) requirements. Chilworth Global performs the Oxidizing Liquids Test in accordance with the method described in 49 CFR 173 Appendix F and the UN Recommendations on the Transport of Dangerous Goods using the Time/Pressure Test Apparatus.
    The test involves mixing a 2.5 gram sample of the suspected liquid oxidizer with an equivalent sample of dried fibrous cellulose. The mixture is then loaded into the test cell and ignited using a fuse wire. A pressure transducer fitted to the test cell is used to measure the maximum deflagration pressure and maximum rate of pressure rise. The results for the sample and mixture are then compared to results for testing of mixtures of cellulose and known oxidizers to determine the classification and packing group.

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