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For grinding processes and for paddle dryers with bars etc., the sensitivity of a product to shock has to be determined. A product is impact-sensitive and therefore explosive if it disintegrates with a bang upon its exposure to impact energy under given test conditions.
For a single test, the result is regarded as positive if the test sample burns or decomposes with detonation, flame (sparks) or development of heavy smoke.
Substances which detonate under the drop hammer are classified as explosives, and may not be milled in the plant.
MP-1 - Burning Test
This test is designed to determine the ignitability and the combustibility of a dried product according to the following criteria:
- Can the product be ignited at all?
- Is the ignition propagated?
- Is there a fire or a flameless reaction?
- What is the deflagration, propagation velocity ?
The tests are carried out at room temperature as well as at an elevated temperature (e.g. 100°C). An electrically heated platinum wire is shortly dipped into the test substance and the response of the compound is observed.
This test is designed to determine the lowest temperature at which a substance shows an exothermic reaction under defined oxygen-lean conditions. It is done to products which are subjected to an elevated temperature over long periods of time, closed off from fresh air (e.g. in vacuum ovens, fluid-bed drier etc.). With this test, the maximum permissible temperature of the heating medium can be derived. The substance to be tested is heated up at a linear rate of 2.5°C/min (dynamic test) along with graphite powder as reference.
The temperatures of the samples are recorded. An exothermic reaction is considered to have occurred if the temperature curve of the test sample rises above the reference sample temperature. Alternatively, a series of isothermal tests is performed at temperature intervals of 10°C. Regarding the test result, the highest temperature at which no overheating of the sample is observed for 8 hours is reported.
Elevated temperatures may subject a product to chemical transformations which will not require oxygen. The reaction may be endothermic or exothermic. An exothermic reaction is classified as an exothermic decomposition. It is, from a safety point of view, of great importance, contrary to endothermic decompositions. Such reactions may liberate gases of decomposition (smolder gases), which will result in a pressure rise in a closed vessel, which may subsequently tear or burst. In addition, the gases of decomposition may be flammable and present an explosion risk. In case of an exothermic change, the heat may be trapped, resulting in self-heating, with a thermal explosion as the consequence. As with auto ignition, the exothermic decomposition depends upon the volume and size of the sample. With increasing volume, the danger of a heat accumulation increases. The decomposition temperature measured is a relative value. It depends on the test method, and a conversion to actual conditions is problematic. In any case, the test method has to be stated together with the temperatures.
Determination under temperature-programmed conditions. Approximately 2 grams of the test sample and 2 grams of graphite as a reference substance are each heated in a test tube at 2.5°C/min up to 350°C oven temperature. The sample temperature is recorded. An exothermic reaction is indicated when the temperature curve of the product is above the one of the reference substance.
If the temperature-programmed test results in an exothermic reaction, the sample is subjected to an isoperibolic test (at constant oven temperature). Once again 2 g of product are used, and the starting temperature will be the one where the product recording in the programmed test crosses the reference curve. The isoperibolic test is repeated with fresh samples in decreasing 10°C steps until the substance does not show an exothermic reaction for 8 h (in certain cases even longer).
Proceed as above. 100 mg of the material of construction are added to the sample. For instance, iron powder or stainless steel turnings. The results obtained are compared with and without the additional material.
While heating up, check the flammability of the gases or vapors produced at a certain temperature interval (e.g., 50°C) with an ignition source (glowing platinum wire).
A longer test tube (160 mm) containing 1 g product is sealed with a rubber stopper equipped with a glass pipe with latex tube. The test tube is put into a heating block at 350°C. The gases are passed through an empty wash bottle to a gas meter. In general, gas production stops after a few minutes. The gas quantity is stated in l/kg. In special cases, the gas evaluation is also measured under different parameters such as isothermally at 250°C over 8 hours.
Exothermic reaction in an air streamThis test is designed to determine the relative auto ignition temperature of a product in a hot air-stream. It is applied to products which are subjected to elevated temperatures in an air-stream for a short period of time (e.g. spray drier, circulating air tray drier etc.). With this test, the permissible inlet temperature of the drier can be derived.
The auto ignition of the product tested occurs by reaction with the oxygen in the air and/or by exothermic decomposition. The same test may be done in a nitrogen-stream instead of an air-stream. If both results are compared, the influence of the oxygen in the air can be clearly distinguished.
The milled and dried test substance and a same quantity of graphite powder as reference substance are heated up in a fine wire mesh container at a linear rate of 1°C/min, while a hot air stream is blown through them. The ©Kuhner temperatures of the samples are recorded.
The test set consists of:Test Equipment, as per Grewer: Electrically heated stainless steel block with 6 holes to receive 1 reference and 5 test samples. The lower hollow section contains a packing of small cuttings of copper tube through which an air stream is passed. The heated air passes through the holes in the upper part of the block, and flows around and through the test substance contained in a fine wire mesh container.
Temperature controller and data acquisition system: Sipcon
Metal-sheathed iron-constantan thermocouples, inserted in thin glass tubes to protect against corrosion.
Test for Exothermic Reaction and Pressure IncreaseThis test is designed to determine the lowest temperature at which a substance or reaction mixture shows an exothermic reaction under defined isochoric conditions, while the pressure is measured.
The test is applied to products or reaction mixtures with low boiling points to suppress evaporation effects in the temperature range of interest.
The substance to be tested and graphite powder as reference are heated up at a linear rate of 2.5°C/min. (dynamic test). The temperatures of the samples are recorded. An exothermic reaction is considered to have occurred if the temperature curve of the test sample rises above the reference sample temperature.
Alternatively, a series of isothermal tests is performed at temperature intervals of 10°C. As the test result, the highest temperature is reported at which no overheating of the sample is observed ©Kuhner for 8 hours.
Weight 2 g of the test substance into a test tube (15,5 mm dia, 45 mm long). Place test tube into the mini-autoclave. Slide glass sheath over thermocouple. Screw cover on, check whether rupture disk is inserted. Connect capillary tube. In another test tube 2 g of fine graphite powder are inserted in a autoclave as a reference sample for comparative temperature measurement. This reference-autoclave is not closed tightly, i.e. it will always be under atmospheric pressure.
Raise the temperature of the heating block continuously at a rate of 2.5°C/min up to at least 50°C above the desired operating temperature in the plant. Maximum test temperature 400°C.
An exothermic reaction is considered to have occurred if the temperature curve of the test sample on the recorder rises above that of the reference sample. The test report gives the starting temperature of the first exothermic reaction found by the dynamic test described above.
If required an isoperibolic test is carried out. To this end, a fresh sample is placed into the heating block at a temperature chosen by the operator (e.g. temperature of first exothermic reaction determined as above) and held at this temperature for 8 hours. If an exothermic reaction occurs during heating up or holding the temperature, a fresh sample is tested at a temperature 10°C lower. This is repeated, the test temperature being reduced by 10°C each time, until a temperature is reached at which a fresh sample shows no exothermic reaction during a period of 8 hours. The highest temperature at which no exothermic reaction occurs is reported.
A pressure increase is considered to have occurred if the pressure curve on the recorder shows a slope. The test report describes the course of pressure in relation to temperature (dynamic test) or time (isoperibolic test). The pressures that are measured are not necessarily representative for those to be ©Kuhner expected ones in production equipment (different volumes of apparatus).
Test for heat accumulation and hot storageThis test is designed to determine the lowest temperature at which, under insulated conditions, a substance shows an exothermic reaction. The test set consists of:
- Electrically heated, thermostatically controlled oven
- Dewar flask (sensitivity of test increases with increasing volume)
- Cylindrical containers of stainless steel wire mesh
- Temperature control and recording system Sipcon
- Thermocouples, protected by thin glass tubes
The testing for thermal stability is carried out under almost adiabatic conditions (the energy generated by the exothermic decomposition is not removed). Therefore self-heating of the substance occurs, which accelerates the decomposition. The decomposition reaction is excited to the maximum heat generation. In case of a sufficiently high temperature increase, which is independent of the total energy of decomposition, there may be a thermal explosion.
A so-called hot storage test can be used to assess the dangers which may exist in handling larger product quantities. The products to be tested are filled into cylindrical wire mesh baskets which are stored in an air-purged oven (2 l/min) at constant temperature. The test in the wire mesh container must be carried out on products which are to be dried in a fluid bed. It can also be applied to products which are to be dried in a recirculating air oven.
Example: temperature course during the hot storage testing
This test is designed as qualitative pre-test of the explosion behavior of a dust/air mixture.
The product, which is milled down to a particle size of max 63 µm (main part), is whirled up in a vertical pyrex glass tube by an air blow. A test is considered to be positive if the indication instrument shows a deflection of the hinged cover (indication "1" or "2"), or if a dust fire occurs (even if the hinged cover is not moved, indication "0"). The test has to be carried out at different dust concentrations.
Only dust explosions leading to an indication "1" in the Modified Hartmann-Apparatus are true St 1 dusts. These data sets are therefore suited as a base for designing explosion protection measures.
Otherwise, for the indication "0" and "2", additional tests in the 20L-Apparatus are necessary.
This test is designed to determine the ignition temperature of airborne dust on a hot surface (e.g. spray drier). First the oven is heated up to 600°C. With the heater switched off and the oven temperature falling, the dust samples are blown into the oven by means of a rubber bulb, e.g. every 50°C.
Ignition is considered to have taken place if the injected dust ignites or decomposes producing flames and/or explosion.
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