Principles of safety

Having reached this stage in the book the reader may well be wondering about the factors that need to be considered in overcoming problems arising from static electricity and the principles on which safety precautions are based. From the systematic investigation of the case histories described in the following chapters various requirements for safety and the methods used in their implementation have been dealt with. Although static charges are almost everywhere, they are, for the most part,...

89 Emptying a tumble dryer

On pouring a vulcanizing additive from a tumble dryer into a silo an explosion occurred when the dryer was almost empty. As the additive was a solvent-wetted product it was suspected that a brush discharge from the possibly charged product itself was the cause of the ignition. However, a thorough investigation showed that the product had been dried under a vacuum of 6 mbar (0.6 kPa) after purging the atmosphere with nitrogen. An explosible solvent vapour air mixture could not, therefore, have...

333 Super brush discharges

There is a kind of electrostatic discharge which does not fit into the category of the different 'one electrode' discharges already described in this chapter. It is a very intense type of discharge which takes place only in special circumstances. Up to now little interest has been shown in them as they very rarely occur in industry. An insight into these strange discharges can be seen by referring to Fig. 3.5. Five plastic tubes of the same material are electrostatically charged by rubbing them...

78 Plastic tank inside a metal mould

Large plastic vessels with complicated shapes, e.g. petrol tanks for cars, are manufactured by tumbling molten plastic inside a metal mould. The mould comprises two parts which are hinged together and which can be rotated about two axes (vertical and horizontal) simultaneously. A plastic monomer is poured into the mould and while the latter rotates the monomer becomes polymerized at the heated walls of the mould, thereby forming the plastic container. The mould can then be swung open and the...

76 Polymethylmethacrylate PMMA window pane in the silo of a granulating plant

In a polyethylene processing plant several production lines were installed and in order to even out any variations and maintain constant quality the entire product was fed into a mixing silo. As is shown in Fig. 7.2, the product was taken from the bottom of the silo and conveyed to the top for recycling. Because the product was clean PE granules no danger of fire or explosion from electrostatic discharges was expected and so no special precautions were taken. It was, therefore, almost...

59Pumping polluted toluene

Minor faults may sometimes cause devastating results. A new plant was built with the economical and ecological purposes of recycling polluted toluene. The plant had to be started before the pipeline to the liquid recycling station was finally installed. This meant that the polluted toluene, which was stored in a tank on the ground floor, had to be disposed of by pumping it into metal drums. Lorries then transported the full drums to the recycling station and returned them empty. A worker was...

13 Minimum ignition energy

The minimum ignition energy (MIE) of an optimum mixture of a combustible material with air (or oxygen) is defined as the least amount of energy needed to cause the ignition of the mixture when measured by a standard method. It is a means of classifying hazardous situations where fires and explosions might be initiated. The energy can be supplied in a number of ways (see 1.2.3) but is directly quantifiable only when delivered in the form of an electrostatic capacitative spark discharge. The...

335 Propagating brush discharges

Compared with the types of gas discharge already described, propagating brush discharges (Lichtenberg discharges) are, with the exception only of lightning flashes, the most powerful kind of electrostatic discharge. As the inductive reactance (inductivity) of a propagating brush discharge circuit is extremely low in comparison with, say, that of a spark discharge circuit, the current rise-time of a propagating brush discharge is correspondingly much faster, resulting in sharp pulses. The effect...

87 Explosion when emptying a metal drum

It was at the end of winter and the first shift had started work at the plant after the weekend. When a steel drum filled with an organic powder was emptied into a mixing unit an explosion occurred at its opening. The drum was held by a fork-lift truck with a special device for holding and turning the drums (Fig. 8.3). Eyewitnesses reported that the explosion happened when the drum was almost empty and turned at such an angle as to touch the opening of the mixing unit. The first drum of powder...

232 Use of the triboelectric series

As has already been noted, the separation of electrostatic charges depends, primarily, on the difference in the work functions of the materials involved. The greater the difference, the larger is the charge separated and vice versa. On this basis it should be expected that to reduce static charging materials should be selected which differ as little as possible in their work functions. Indeed, the contact electrification between identical materials should be zero. Coehn 3 put forward a rule,...

131 Determination of minimum ignition energy 1311 Gases and vapours

In measuring the MIE of a gas or a vapour there are a number of factors which need to be considered relating to the experimental procedure. From the discussion in 1.3 it follows that, usually, it will be necessary to adjust the concentration of the fuel in a fuel 0,2 0,4 0,6 0,8 1 1,2 1,4 1,6 1.8 2 2,2 2,4 2,6 2,8 3 0,2 0,4 0,6 0,8 1 1,2 1,4 1,6 1.8 2 2,2 2,4 2,6 2,8 3 Concentration ot fuel in a fuel air mixture as a traction of the stoichiometric value Figure 1.4 Dependence of ignition energy...