Field Dust Explosions

1. Auflage 2012
ISBN: 978-0-444-59644-4
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, Band Volume 4, 262 Seiten

Reihe: Handbook of Powder Technology

Handbook of Powder Technology, Volume 4: Dust Explosions presents the dust explosion problem in general terms and describes how and why dusts explode. This book discusses the various approaches used to deal with the dust explosion hazards. Organized into five chapters, this volume begins with an overview of the methods used to assess, remove, or minimize the hazard of dust explosions. This text then examines the factors that influence the initiation and severity of an explosion. Other chapters consider the explosion prevention and explosion protection techniques. This book discusses as well the characteristics of a dust explosion and the historical development of the problem. The final chapter deals with the significant concept of explosion protection to reduce the worst effects of an explosion to an acceptable level. This book is a valuable resource for managers, engineers, scientists, safety personnel, and others involved in the handling ad processing of materials in solid particulate form.

Autoren/Hrsg.

Field, P.

Leseproben

CHAPTER 3

DETERMINATION OF DUST EXPLOSION PARAMETERS AND THEIR INTERPRETATION

Publisher Summary

This chapter discusses that a wide variety of industrial plant and buildings used in the processing and handling of combustible materials are only capable of withstanding internal pressures between 0.1 and 0.2 bar without damage. The pressures produced by the majority of dusts when they explode in confined situations are greater than this and consequently, measures are taken to protect the plant concerned. Explosibility tests are based on laboratory-scale equipment because results are obtained rapidly and the amount of dust required for testing purposes is relatively small. A number of dusts handled in industry present a health hazard, owing to either a toxic or an allergic effect, occurring through ingestion, inhalation, or skin contact. The chapter also explains the classification test, which offers a qualitative assessment of whether or not a suspended dust is capable of initiating and sustaining an explosion in the presence of a small source of ignition. The classification is made purely on a visual observation of flame propagation, an explosible dust being one which causes flames to move away from the ignition source.

3.1 GENERAL BACKGROUND

A wide variety of industrial plant and buildings used in the processing and handling of combustible materials are generally only capable of withstanding internal pressures between 0.1 and 0.2 bar without damage.

The pressures produced by the majority of dusts when they explode in confined situations will be greater than this and consequently measures need to be taken to protect the plant concerned. A question often raised by those involved in industries handling powdered material reflects the uncertainty concerning firstly, whether a hazard exists, and secondly, if it does, how serious the hazard is. The question is clearly posed in order to satisfy plant protection needs and in many cases the requirement to comply with national and/or local standards and codes of practice.

In the absence of a sound theoretical approach capable of predicting the explosion hazards of dusts, a range of explosibility tests has been developed in a number of countries. Explosibility tests which have the common object of assessing both the existence and severity of an explosion hazard are carried out throughout Europe, the USA, and else where. Although a number of similarities are present in the tests, a current lack of international standardisation has led to variations in the test procedures that have been adopted. In terms of explosibility, the tests under consideration deal with the dust in a suspended form. Tests do exist which assess the ignition characteristics of dust layers but since dust layers do not generally explode these tests are beyond the scope of this book. However, consideration is given to dust layers in respect of ignition hazards; this problem is discussed in Chapter 4. In view of the variations, the tests and procedures adopted by the countries of interest will be discussed separately. Explosibility tests are generally based on laboratory-scale equipment, since results tend to be obtained rapidly and the amount of dust required for testing purposes is relatively small. This is particularly important in cases where the material is either very expensive or when only small amounts are available from pilot-scale plants. The tests, although on a laboratory scale, attempt to recreate typical industrial conditions in terms of dust concentrations, turbulence, and probable sources of ignition, the main aim being to simulate the worst possible conditions so that the most serious hazard is recognised.

The seemingly insurmountable problem of producing a consistently homogeneous suspension of dust at the required concentration, under reproducible aerodynamic conditions, in a given apparatus means that very accurate reproducibility of data is unlikely. However, this difficulty is largely overcome by making a number of repetitive tests from which the data representing the most serious hazard are chosen as those required to be protected against.

3.2 ACTION REQUIRED BEFORE SUBMITTING A SAMPLE FOR EXPLOSIBILITY TESTING

Before deciding whether or not it is necessary to submit a dust for explosibility assessment, a number of steps should be taken.

If the composition of the dust is known a study should be made of the list of dusts that are known to be explosible, as given in the Appendix. If the dust is listed then it need not necessarily be submitted for an explosibility assessment unless the particle size or moisture content are such that the dust in question may not be explosible. If the dust is novel or a mixture of materials (some of which may possibly be inert or non-explosible) then the dust may not be listed.

Although a general guide is given in the Appendix to some dusts that are known to be non-explosible, it is not advisable to regard this as fool-proof since changes in particle size, moisture content, and chemical composition can result in a dust becoming explosible.

A consideration of the chemical composition can often help in positively establishing a dust that will present an explosion hazard. It is known, for example, that organic dusts containing chemical groups such as COOH, OH, NH2, NO2, C=N, and N=N are generally explosible.

If after these preliminary steps have been taken the dust is thought to present a possible explosion hazard, or if it cannot be positively identified as a non-explosible dust, it should be submitted for an appropriate test.

If the material is known to be explosible, reference can be made to the explosibility data of a wide range of dusts given in the Appendix. These data offer a general guide to the various hazards of a particular dust, but explosion data for a specific dust should generally be obtained since there is no guarantee that the dust in question has a chemical purity, chemical composition, particle size distribution, and moisture content identical to that reported. In cases where any doubt exists a dust should be submitted for a test

3.2.1 Sample selection

If the tests are to be valid and the data obtained applied to the industrial situation with confidence, the sample selected for any test must be representative of the process and plant at risk. In general, the sample should be as dry as the driest material in the plant, and the particle size of the dust should be as small as that likely to occur in any part of the process. In cases where difficulty is experienced, the driest sample having a particle size less than 75 µm should be selected, since submission of samples having a higher than normal moisture content or a larger than normal particle size, will lead to an underestimation of the explosion hazard, with the likelihood that explosion protection measures fall short of those required to ensure complete safety in the plant.

If the dust is a mixture of compounds an added complication arises when attempting to select a representative sample; a satisfactory sample can often be obtained by the use of a spinning riffler (Plate 18). Some dusts are subject to an ageing process which can result in a reduced explosion hazard, this being due typically to oxidation or a tendency to absorb moisture after production. Samples of these dusts should therefore be supplied freshly produced and packaged in an appropriate manner, e.g. stored under nitrogen in an air tight container. Difficulty is sometimes experienced in selecting a plant location from which the dust should be collected, particularly if the material alters at different parts of the process, since its explosion characteristics are also likely to change. Under these circumstances it is normally advisable to select a number of samples from different parts of the plant; the explosion hazard can then be evaluated with possibly varying levels of explosion protection being required throughout the plant. This approach, although expensive in terms of explosibility testing fees, may well prove to be economic in terms of explosion protection, since reduced protection can often be incorporated in parts of the plant in which a comparatively low explosion hazard has been determined. If, after consideration of the factors described above, problems of sample selection are still experienced, advice can normally be obtained from the laboratory which carries out the tests.

Plate 18 Spinning riffler (reproduced by permission of FRS, Crown Copyright)

A number of dusts handled in industry can present a health hazard, owing to either a toxic or an allergic effect, occurring through ingestion, inhalation, or skin contact. A data sheet listing the known characteristics of the dust to be tested should therefore always be submitted. In cases where the dust is known or suspected to be highly toxic, e.g. radioactive or toxic at a few parts per million, standard laboratory tests cannot normally be carried out, and a special test rig may be required to cope with the problem.

3.3 EXPLOSIBILITY TESTS IN THE UK

A number of commercial organisations in the UK carry out explosibility tests for their own purposes; this may be due to the large number of powdered materials handled, making in-house testing economical, or because the company manufactures explosion protection equipment, e.g. automatic suppression devices, and carries out tests for...