Dust Explosions in the Process Industries Second Edition phần 2 pdf

Dust explosions: an overview 57

1.4

MEANS FOR PREVENTING AND MITIGATING DUST

EXPLOSIONS

1.4.1

THE MEANS AVAILABLE: AN OVERVIEW

The literature on the subject is substantial. Many authors have published short, general

surveys on means of preventing and mitigating dust explosions in the process industry. A

few fairly recent examples are Gibson (1978), Scholl, Fischer and Donat (1979), Kiihnen

and Zehr (1980), Field (1982a), Woodcock and Reed (1983), Siwek (1986, 1987), Field

(1987), Swift (1987, 1987a) and Bartknecht (1988). The books mentioned in Section

1.1.1.5 also contain valuable information.

Table 1.9 gives an overview of the various means that are presently known and in use.

They can be divided in two main groups, namely means for preventing explosions and

means for their mitigation. The preventive means can again be split in the two categories

prevention of ignition sources and prevention of explosible/combustible cloud. One

central issue is whether only preventing ignition sources can give sufficient safety, or

whether it is also necessary in general to employ additional means of prevention and/or

mitigation. In the following sections the means listed in Table 1. 9 will be discussed

separately.

Table 1.9 Means of preventing and mitigating dust explosions: a schematic overview

PRNEliTlON

MITIGATION

1.4.2

PREVENTING IGNITION SOURCES

1.4.2.1

Introduction

The characteristics of various ignition sources are discussed in 1.1.4, and some special

aspects are elucidated more extensively in Chapter 5. Test methods used for assessing the

ignitability of dust clouds and layers, when exposed to various ignition sources are

discussed in Chapter 7.

58 Dust Explosions in the Process Industries

Several authors have published survey papers on the prevention of ignition sources in

process plant. Kiihnen (1978) discussed the important question of whether preventing

ignition sources can be relied upon as the only means of protection against dust

explosions. His conclusion was that this may be possible in certain cases, but not in

general. Adequate knowledge about the ignition sensitivity of the dust, both in cloud and

layer form, under the actual process conditions, and proper understanding of the process,

are definite pre-conditions. Schafer (1978) concluded that relying on preventing ignition

sources is impossible if the minimum electric spark ignition energy of the dust is in the

region of vapours and gases (< 10 mJ). However, for dusts of higher MIE he specified

several types of process plants that he considered could be satisfactorily protected against

dust explosions solely by eliminating ignition sources.

In a more recent survey, Scholl (1989) concluded that the increased knowledge about

ignition of dust layers and clouds permits the use of prevention of ignition sources as the

sole means of protection against dust explosions, provided adequate ignition sensitivity

tests have shown that the required ignition potential, as identified in standardized ignition

sensitivity tests, is unlikely to occur in the process of concern. Scholl distinguished

between organizational and operational ignition sources. The first group, which can

largely be prevented by enforcing adequate working routines, includes:

0 Smoking.

0 Openflames.

Open light (bulbs).

0 Welding (gadelectric).

0 Cutting (gashotating disc).

0 Grinding.

The second group arises within the process itself and includes:

0 Open flames.

Hot surfaces.

Self-heating and smouldering nests.

0 Exothermic decomposition.

0 Heat from mechanical impact between solid bodies (metal sparks/hot-spots).

Exothermic decomposition of dust via mechanical impact.

0 Electric sparkdarcs, electrostatic discharges.

1.4.2.2

Self-heating, smouldering and burning of large dust deposits

The tendency to self-heating in powdeddust deposits is dependent on the properties of the

material. Therefore, the potential of self-heating should be known or assessed for any

material before admitting it to storage silos or other part of the plant where conditions are

favourable for self-heating and subsequent further temperature rise up to smouldering and

burning.

0 Control of temperature, moisture content and other important powder/ dust properties

Possible means of preventing self-heating include:

before admitting powder/dust to e.g. storage silos.

Dust explosions: an overview 59

Adjustment of powdeddust properties to acceptable levels by cooling, drying etc. ,

whenever required.

0 Ensuring that heated solid bodies (e.g. a steel bolt heated and loosened by repeated

impacts) do not become embedded in the powdeddust mass.

Continuous monitoring of temperature in powder mass at several points by thermo￾meter chains.

Monitoring of possible development of gaseous decomposition/oxidation products for

early detection of self-heating.

0 Rolling of bulk material from one silo to another, whenever onset of self-heating is

detected, or as a routine after certain periods of storage, depending on the dust type.

0 Inerting of bulk material in silo by suitable inert gas, e. g. nitrogen.

Thermometer chains in large silos can be unreliable because self-heating and smould￾ering may occur outside the limited regions covered by the thermometers.

Inerting by adding nitrogen or other inert gas may offer an effective solution to the

self-heating problem. However, it introduces a risk of personnel being suffocated when

entering areas that have been made inert. In the case of nitrogen inerting. negative effects

of lack of oxygen in the breathing atmosphere become significant in humans when the

oxygen content drops to 15 vol% (air 21 vol%).

If inerting is adopted, it is important to take into account that the maximum permissible

oxygen concentration for ensuring inert conditions in the dust deposit may be considerably

lower than the maximum concentration for preventing explosions in clouds of the same

dust. Walther (1989) conducted a comparative study with three different dusts. using a

20 litre closed spherical bomb for the dust cloud experiments and the Grewer furnace (see

Chapter 7) for the experiments with dust deposits. In the case of the dust clouds,

oxidizability was quantified in terms of the maximum explosion pressure at constant

volume, whereas for the dust deposits it was expressed in terms of the maximum

temperature difference between the test sample and a reference sample of inert dust,

exposed to the same heating procedure. The results are shown in Figure 1. 67. In the case

of the pea flour it is seen that self-heating took place in the dust deposit right down to 5

vol% oxygen or even less, whereas propagation of flames in dust clouds was practically

impossible below 15 vol% oxygen. Also for the coals there were appreciable differences.

Extinction of smouldering combustion inside large dust deposits e.g. in silos is a dual

problem. The first part is to stop the exothermic reaction. The second, and perhaps most

difficult part, is to cool down the dust mass. In general the use of water should be avoided

in large volumes. Limited amounts of water may enhance the self-heating process rather

than quench it. Excessive quantities may increase the stress exerted by the powdeddust

mass on the walls of the structure in which it is contained, and failure may result.

Generally, addition of water to a powder mass will, up to the point of saturation, reduce

the flowability of the powder and make discharge more difficult (see Chapter 3).

Particular care must be taken in the case of metal dust fires where the use of water

should be definitely excluded. Possible development of toxic combustion products must

also be taken into account.

The use of inert gases such as nitrogen and carbon dioxide has proven to be successful

both for quenching of the oxidation reaction and the subsequent cooling of smouldering

combustion in silos. However, large quantities of inert gas are required, of the order of

60 Dust Explosions in the Process Industries

Figure 1.67 Comparison of the influence of oxygen content in the gas on the oxidizability of dust

clouds and dust deposits (From Walther, 1989)

10 tonnes or more for a fair size silo. In the case of fine-grained products as wheat flour or

maize starch, the permeability of the inert gas may be too low for efficient inerting of large

bulk volumes.

Further details concerning extinction of powder and dust fires are given by Palmer

(1973) and Verein deutscher Ingenieure (1986). The use of inert gas for extinction of

smouldering fires in silos was specifically discussed by Dinglinger (1981) and Zockoll and

Nobis (1981). Chapter 2 gives some examples of extinction of smouldering fires in

practice.

Some synthetic organic chemicals, in particular cyclic compounds, can decompose

exothermally and become ignited by a hot surface, a smouldering nest, frictional heat or

other ignition source. Such decomposition does not require oxygen, and therefore inerting

has no effect. Zwahlen (1989) gave an excellent account of this special problem. He

pointed out that this type of exothermic decomposition can only be avoided by eliminating

all potential ignition sources. However, by taking other processing routes one can

eliminate or reduce the problem. Zwahlen suggested the following possibilities:

0 The hazardous powder is processed in the wet state, as a slurry or suspension.

0 If wet processing is impossible, one should avoid processes involving internal moving

mechanical parts that can give rise to ignition.

If this is not possible, strict control to prevent foreign bodies from entering the process

must be exercised. Furthermore, detectors for observing early temperature and

Dust explosions: an overview 6 1

pressure rise, and sprinkler systems must be provided. Adiabatic exothermal decompo￾sition of bulk powder at constant volume can, due to the very high powder concentra￾tion, generate much higher pressures than a dust explosion in air.

0 Generally the processed batches of the powder should be kept as small as feasible.

Use of additives that suppress the decomposition tendency may be helpful in some

cases.

1.4.2.3

Open flamedhot gases

Most potential ignition sources of the open flame type can be avoided by enforcing

adequate organizational procedures and routines. This in particular applies to prohibition

of smoking and other use of lighters and matches, and to enforcement of strict rules for

performing hot work. Hot work must not be carried out unless the entire area that can

come in contact with the heat from the work, indirectly as well as directly, is free of dust,

and hazardous connections through which the explosion may transmit to other areas, have

been blocked.

Gas cutting torches are particularly hazardous because they work with excess oxygen.

This gives rise to ignition and primary explosion development where explosions in air

would be unlikely.

In certain situations in the process industry, hot gaseous reaction products may entrain

combustible dust and initiate dust explosions. Each such case has to be investigated

separately and the required set of precautions tailored to serve the purpose in question.

Factory inspectorates in most industrialized countries have issued detailed regulations

for hot work in factories containing combustible powders or dusts.

1.4.2.4

Hot surfaces

As pointed out by Verein deutscher Ingenieure (1986), hot surfaces may occur in

industrial plants both intentionally and unintentionally. The first category includes

external surfaces of hot process equipment, heaters, dryers, steam pipes and electrical

equipment. The equipment where hot surfaces may be generated unintentionally include

engines, blowers and fans, mechanical conveyors, mills, mixers, bearings and unprotected

light bulbs.

A further category of hot surfaces arises from hot work. One possibility is illustrated in

Figure 1 .lo. During grinding and disc-cutting, glowing hot surfaces are often generated,

which may be even more effective as initiators of dust explosions than the luminous spark

showers typical of these operations. This aspect has been discussed by Muller (1989).

A hot surface may ignite an explosible dust cloud directly, or via ignition of a dust layer

that subsequently ignites the dust cloud. Parts of glowing or burning dust layers may

loosen and be conveyed to other parts of the process where they may initiate explosions.

It is important to realize that the hot surface temperature in the presence of a dust layer

can, due to thermal insulation by the dust, be significantly higher than it would normally

be without dust. This both increases the ignition hazard and may cause failure of

equipment due to increased working temperature. The measures taken to prevent ignition

by hot surfaces must cover both modes of ignition. The measures include:

62 Dust Explosions in the Process Industries

Removal of all combustible dust before performing hot work.

Preventionhemoval of dust accumulations on hot surfaces.

0 Isolation or shielding of hot surfaces.

0 Use of electrical apparatus approved for use in the presence of combustible dust.

0 Use of equipment with minimal risk of overheating.

Inspection and maintenance procedures that minimize the risk of overheating.

1.4.2.5

Smouldering nests

Pinkwasser (1985, 1986) studied the possibility of dust explosions being initiated by

smouldering lumps (‘nests’) of powdered material that is conveyed through a process

system. The object of the first investigation (1985) was to disclose the conditions under

which smouldering material that had entered a pneumatic conveying line would be

extinguished, i.e. cooled to a temperature range in which the risk of ignition in the

downstream equipment was no longer present. In the case of > 1 kg/m3 pneumatic

transport of screenings, low-grade flour and C3 patent flour, it was impossible to transmit

a 10 g smouldering nest through the conveying line any significant distance. After only a

few metres, the temperature of the smouldering lump had dropped to a safe level. In the

case of lower dust concentrations, between 0.1 and 0.9 kg/m3, Le. within the most

explosible range, the smouldering nest could be conveyed for an appreciable distance as

shown in Figure 1.68, but no ignition was ever observed in the conveying line.

In the second investigation Pinkwasser (1986) allowed smouldering nests of 700°C to fall

freely through a 1 m tall column containing dust clouds of 100-1OOO g/m3 of wheat flour or

wheat starch in air. Ignition was never observed during free fall. However, in some tests

Figure 1.68 Distance travelled in pneumatic tran￾sport pipe by smouldering nest before becoming

extinguished, as a function of dust concentration in

the pipe. Air velocity in pipe 20 m/s (From Pink￾wasser, 1985)