Dust Explosions in the Process Industries Second Edition phần 9 ppt

Assessment of ignitability 5 19

Figure 7.36

ignition energy of dust clouds

Three different electric spark discharge circuits used for determining the minimum

ing the discharge with the dust cloud, may be appreciable. Sophisticated elements such as

thyrathrons have been employed to solve this problem.

However, synchronization of spark and dust cloud can also be accomplished by

incorporating a third, auxiliary spark electrode in the spark gap configuration. By

discharging just a very small energy in the gap between one of the main electrodes and the

auxiliary electrode, the main discharge is initiated. This method was used with success by

Franke (1978).

Mechanical synchronization constitutes a further possibility. Prior to the experiment the

capacitor is then charged to the high voltage required with the spark gap sufficiently long

for breakdown to be impossible at that voltage. Pneumatically or spring-driven displace￾ment of one of the spark electrodes towards a shorter spark gap, allowing spark-over, is

synchronized with the occurrence of the transient dust cloud, for example via solenoids.

Boyle and Llewellyn (1950) were probably amongst the first to use the electrode

520 Dust Explosions in the Process Industries

displacement method. Its drawback is that the actual spark gap distance at the moment of

the discharge is not known.

One way of avoiding the synchronization problem is to work with a semi-stationary dust

cloud and charge the high-voltage capacitor slowly until breakdown occurs naturally at the

fixed spark gap distance chosen. Because of arbitrary variations, the actual voltage at

breakdown will differ from trial to trial, and must be recorded for each experiment for

obtaining the actual given spark energy 1/2 CV.

Figure 7.36 (b) illustrates two versions of the direct high-voltage discharge circuit, one

without and one with a significant series inductivity, of the order of 1 mH. This difference

can be significant with respect to the igniting power of sparks of similar energies. The

induction coil makes the spark more effective as an ignition source by increasing the

discharge duration of the spark. Such an induction coil is automatically integrated both in

the original US Bureau of Mines circuit, and also in the CMI circuit, as shown in Figure

7.36 (a) and (c). (See Chapter 5 for further details concerning the influence of the spark

discharge duration.)

If the test is to simulate a direct electrostatic discharge of an accidentally charged

non-earthed electrically conducting object, the use of a discharge circuit with low

inductance (left of Figure 7.36 (b)) seems most appropriate.

7.10.2.3

The CMI discharge circuit

The method for synchronization of dust cloud and spark discharge, which was developed

by CMI (see Eckhoff (1975a)), is illustrated in Figure 7.36 (c). The method is similar to the

3-electrode technique in the sense that an auxiliary spark discharge is employed for

breaking the spark gap down, but the use of a third electrode is avoided. The energy of the

auxiliary spark is about 1-2 mJ. The CMI method requires that the spark energy be

measured directly, in terms of the time integral of the electrical power dissipated in the

spark gap. Figure 7.37 shows the traces of voltage and current for a spark of net electrical

energy 13 mJ, produced by the CMI circuit. The spark discharge was completed after

about 280 ps.

The general apparatus used by CMI was as otherwise shown in Figure 7.34, i.e. similar

to that originally developed by US Bureau of Mines.

7.1 0.2.4

A new international standard method

As a part of its efforts to standardize safe design of electrical apparatus in explosible

atmospheres, the International Electrotechnical Commission (1989) is considering a new

test method for the minimum ignition energy of dust clouds. The draft is to a large extent

based on work conducted by an international European working group and summarized

by Berthold (1987).

The detailed design of the apparatus to be used in a possible IEC test method, in terms

of explosion vessel, dust dispersion system, synchronization method, etc. was not

specified, but some suitable apparatus were mentioned, including direct high-voltage

discharge circuits as well as the CMI circuit. However, no matter which apparatus is

chosen, the spark generating system must satisfy the following requirements:

Assessment of ignitability 52 1

0 Inductance of discharge circuit 2 1 mH.

0 Ohmic resistance of discharge circuit < 5 R.

Electrode material: stainless steel, brass, copper or tungsten.

0 Electrode diameter: 2.0 mm.

0 Electrode gap: 6 mm.

0 Capacitors: low-inductance type, resistant to surge currents.

0 Capacitance of electrode arrangement: as low as possible.

0 Insulation resistance between electrodes: sufficiently high to prevent significant leakage

currents.

Figure 7.37 Spark gap voltage and spark current

versus time during discharge of a 13 ml electric

spark from the CMI spark generator. Spark dischar￾ge duration 280 p. Energy of trigger spark (spike to

the far left) is about 1-2 m/

It will be necessary to take account of the possible influences of dust concentration, dust

cloud turbulence and degree of dust dispersion on the test result. Preliminary tests must be

carried out to adjust the dust dispersion conditions and the ignition delay such that

prescribed minimum ignition energies are actually measured for three specified reference

dusts.

522 Dust Explosions in the Process Industries

Starting with a value of spark energy that will reliably cause ignition of a given

concentration of the dust being tested, the dust concentration being itself a variable, the

test energy is successively halved until no ignition occurs in 10 successive tests. The

minimum ignition energy is defined to lie between the highest energy at which ignition

fails to occur in at least ten successive attempts to ignite the dustlair mixture, and the

lowest energy at which ignition occurs within ten successive attempts.

7.1 1

SENSITIVITY OF DUST LAYERS TO MECHANICAL IMPACT

AND FRICTION

7.1 1.1

THE INDUSTRIAL SITUATION

This hazard primarily applies to powders and dusts with explosive properties, Le. which

are able to react or decompose exothermally without oxygen supply from the air. Strong

oxothermal reactions may be initiated in layers of such materials if they are exposed to

high mechanical stresses and fast heating by impact or rubbing, either accidentally or as

part of an industrial process.

7.1 1.2

LABORATORY TESTS

7.1 1.2.1

Drop hammer tests

As summarized by Racke (1989), a number of impactlfnction sensitivity test methods have

been developed in several European countries, as well as in USA and Japan. The most

common design concept for the impact test is the drop hammer, as illustrated in Figure

7.38.

Verein deutscher Ingenieure (1988) also mentioned the very similar test by Lutolf

(1978) as a suitable standard method. In the Lutolf test the dust sample size is about 0.10 g

and the theoretical maximum drop hammer impact energy 39 J (5 kg, 0.8 m). Up to ten

trials are conducted and observations are made with respect to occurrence of explosion,

flame, smoke or sparks. If all ten tests are negative, a new test series is conducted with the

dust samples wrapped in thin aluminium foil (10 pm thickness), in case the aluminium

should have a sensitizing effect on a possible exothermal reaction. If the tests with

aluminium are positive, a new test series without aluminium is conducted.

The American Society for Testing and Materials (1988) adopted the US Bureau of

Mines drop hammer method as their standard. Using a fixed drop hammer weight (2.0 or

3.0 kg), the drop height H,, giving 50% probability of a positive reaction, is determined.

The lower H,,, the more sensitive the material is to impact ignition. In the test description

Assessment of ignitability 523

Figure 7.38

5 kg and height of fall 7 m (From Verein deutscher Ingenieure, 1988)

Drop hammer test for dust layers by Koenen, Ide and Swart (7 96 I). Drop hammer mass

it is emphasized that the observation of the reaction of the sample is one of the difficult

points in impact sensitivity testing. A positive test result is defined as an impact that

produces one or more of the following phenomena: (a) audible reaction, (b) flame or

visible light, (c) definite evidence of smoke (not to be confused with a dust cloud of

dispersed sample), and (d) definite evidence of discolouration of the sample due to

decomposition. The problem arises with reactions that yield no distinguishable audible

response, no flame, and little sample consumption. The decision concerning reactiodno

reaction in these cases must be based primarily upon the appearance of the sample after

the test. The impact in most cases will compress the sample into a thin disc, portions of

which may adhere to the striking tool surface, the anvil, or both. One should then inspect

the tool and anvil surfaces and look for voids in the powder disc and discolouration due to

decomposition in areas where voids occur. If there is discolouration from decomposition,

the test trial is to be considered as positive. If there are small voids but no discolouration,

the trial should be regarded as negative. In the case of doubt as to whether or not

discolouration is present, the trial is to be regarded as negative. If the only evidence is a

slight odour or a small amount of smoke, which may be a dust cloud from dispersed

sample, the trial should also be considered negative.

7.1 1.2.2

Friction tests

As pointed out by Racke (1989), several different friction tests have been devised,

including three described by Gibson and Harper (1981). One of these is illustrated in

Figure 7.39.

524 Dust Explosions in the Process Industries

Figure 7.39 Example of laboratory method for

testing the sensitivity of powders to mechanical

rubbing/friction (From Gibson and Harper,

1981)

7.1 2

SENSITIVITY OF DUST CLOUDS TO IGNITION BY METAL

SPARKS/HOT SPOTS OR THERMITE FLASHES FROM

ACCIDENTAL MECHANICAL IMPACT

7.1 2.1

THE INDUSTRIAL SITUATION

Dense clouds of metal sparks, and also hot surfaces, are easily generated in grinding and

cutting operations. Such operations are therefore generally to be considered as hot work,

which should not be permitted in the presence of ignitable dusts or powders.

However, the evaluation of the ignition hazard to be associated with accidental impacts

is less straight-forward. Such impacts can occur due to mis-alignment of moving parts in

powder processing equipment, for example in grinders and bucket elevators. Or foreign

bodies such as stones and tramp metal can get into the process line. Whether or not metal

sparks/hot spots or thermite flashes from single accidental impacts between solid bodies,

can in fact initiate dust explosions, has remained a controversial issue for a long time. It

now seems that in the past ‘friction sparks’ have been claimed to be the ignition sources of

dust explosions more often than one would consider as reasonable on the basis of more

recent evidence. However, as long as necessary conditions for such impacts to be capable

of initiating dust explosions have been unidentified, one has been forced to maintain the

hypothesis that such sparks may be hazardous in general. This in turn has forced industry

to take precautions that may have been superfluous, and caused fear that may have been

unnecessary.

Generation of metal sparkshot spots by accidental mechanical impacts is a complex

process, involving a number of variables such as:

0 Chemistry and structure of the material of the colliding bodies.

0 Physical and chemical surface properties of the colliding bodies.

Shapes of the colliding bodies.

0 Relative velocity of the colliding bodies just before impact.

0 Impact energy (kinetic energy transformed to heat in an impact).

Single or repeated impacts?