No. 251

1970

On the Study of the Explosibility of Atmospheres in Enclosed Volumes

V. F. Panin, V. A. Marasanov, V. B. Nemirovsky, D. A. Zakharov

(Presented by the scientific seminar of the Department of Radiation Chemistry)

One of the pressing issues concerning the operation of certain facilities of new and modern equipment is currently the question of the explosibility of atmospheres in enclosed volumes in aviation, submarine, and similar technology. The atmospheres of such facilities, explosion-safe in their initial state, may become explosive as a result of the entry into them of combustible components formed as a result of the functioning of the facilities’ systems, as a result of the action of various destructive factors on structural materials, and, finally, as a result of negligence by facility personnel.

The difficulty of assessing the explosibility of such atmospheres lies in the following:

1). As a rule, the atmospheres are complex in composition (multicomponent), including both combustible and noncombustible gases, among which there may be diluents, inhibitors, or combustion catalysts.

2). Such complex gas mixtures may be affected by many poorly studied and specific factors that change their explosion (fire) hazard (low or relatively high temperatures, light and ionizing radiation, weightlessness, and so forth).

3). Under operating conditions of the facility, the most varied types of ignition sources (pulses) may appear as potentially possible.

Let us consider appropriate lines of investigation and methods for assessing the explosibility of such atmospheres.

It is obvious, first, that it is impossible to evaluate the combustible properties of complex atmospheres by theoretical means alone; at present, the chemistry and combustion properties even of such a simple system as a hydrogen–oxygen mixture have been far from fully studied. It follows from this that the path of investigation must be, for the most part, experimental.

Second, it is necessary to establish experimentally the most dangerous mixture; to study the influence of external conditions (irradiation, temperature, etc.); to determine their most dangerous combination; and, from the total number of possible impulses, to find also the most effective one.

The explosibility of the most combustible mixture under these most severe conditions will determine the technically important greatest danger of the atmospheres of the facility.

In this case, explosibility itself must be characterized by two parameters: the concentration limits of ignition and the value of the minimum ignition energy.

The first, obviously, characterizes the absolute safety of the atmosphere, while the second gives an idea of the permissible conditions at the facility (permissible energies of normal switching operations, electrostatic discharges, etc.).

Considering the first and third complicating circumstances among those indicated above, and bearing in mind that a considerable number of works has been devoted to questions of the influence of factors on explosibility.

In light of the data currently available on the properties of complex gas systems, the procedure for evaluating the most explosive atmospheres appears to be as follows.

First, the concentration limits of ignition are studied for systems composed of the principal components, with respect to the principal combustible component, and, in parallel, the minimum ignition energies of atmospheres with the principal compositions are determined.

On the basis of such preliminary investigations, compositions are determined that are characterized by lower concentration limits of flammability and lower ignition energies.

of ignition. The limiting values thus established could indicate, as noted above, the direction and intensity of measures to ensure the explosion safety of atmospheres under normal conditions.

However, for the conditions in which closed volumes are operated, these values cannot be considered sufficient. There is a certain probability that substances formed as a result of the decomposition of structural materials under the action of various destructive factors, as well as products of human activity, may have a substantial effect—especially under conditions of high temperatures and exposure to ionizing radiation—either on the values of the concentration flammability limits of atmospheres with respect to the principal combustible, or on the level of the minimum ignition energies of atmospheres, or on both, and, moreover, in the direction of decreasing them.

Therefore, the next stage of research should be the study of the influence of additive components on the concentration flammability limits and on the values of minimum ignition energies.

One variant of a (preliminary) study of such influence is the following. The concentration of the total product of thermoradiolysis and of personnel activity in the atmosphere under investigation is increased until its appreciable effect on the limiting values is detected. From this, an estimate can immediately be made of the operating time of the object under specified conditions during which a “threshold” concentration of the total additive product is reached.

In a similar way, it is possible to estimate the operating times during which a given concentration level of the total product is reached and, consequently, one or another change in the limiting values (it is understood that qualitative and quantitative ... must be carried out in parallel).

qualitative studies of the process of formation of additive gaseous components, i.e., the kinetics of their accumulation).

The ultimate goal of the complex of works indicated is to establish the sources (causes) of a possible (and appreciable) reduction of the limiting values. The latter can be achieved by gradually removing from the total additive gaseous product introduced into the atmosphere under study individual components or groups of related components—until it is established the removal of which component, or which components, “removes” the influence of the impurities.

This will make it possible to determine, first, which of the structural materials of the object, decomposing under the action of individual factors, introduces dangerous components into the atmosphere and, ultimately, to develop measures for protecting or replacing such material. Second, it may be indicated which component increases the hazard; these data will make it possible, conversely, without changing the structural material (or a circuit element), to raise the question of removing (binding) the hazardous component.

One of the important aspects of the proposed complex of works is the selection of the most suitable source of ignition pulses for the atmospheres under study. Let us briefly consider the possible ignition sources.

All real ignition pulses may—with a certain degree of convention—be divided into four groups:

a) pulses arising in exothermic reactions;

b) pulses arising in the conversion of mechanical energy into heat (frictional heat, impact and frictional sparks, etc.);

c) pulses arising in the conversion of electrical energy into heat (overheating of elements of electrical installations during short circuits and overloads, etc.);

d) electrical discharges.

Of the listed ignition pulses, in the practice of testing atmospheres for explosibility, [[unclear: continuation cut off at bottom of page]].

flame torches, heated surfaces, and electrical discharges.

For carrying out the above-described set of studies, electric-spark ignition sources are the most suitable.

Indeed, when working with a flame torch, certain difficulties arise in determining the minimum ignition energies, in working at low pressures and, perhaps most importantly, difficulties in studying the effect of additive products on limiting values (the introduction of a flame torch into the mixture under study is tantamount to introducing additive products).

When heated surfaces are used, difficulties also arise in determining the minimum ignition energies, associated in particular with the strong dependence of the ignition temperature on the experimental conditions: the size of the vessel, the properties of its inner surface, etc.

Let us dwell somewhat more fully on electric-spark pulses.

An electric spark is accompanied by a short-duration pulse of energy. The energy of the pulse is transferred to the volume of gases occupied by the spark and heats it. The heated volume of gas, being essentially an element of flame, transfers the energy of the electric-spark source to the surrounding molecules of the mixture, and the oxidation reaction of the fuel begins.

It has become customary to regard this as the initial formation of a certain initial nucleus of hot gas in the form of a sphere [1].

The development of this sphere is possible only if the temperature of its surface is sufficiently high, the effect of which on the surrounding mixture is apparently analogous to the action of a heated body. The pulse of energy introduced by the spark, together with the energy of the gas that has reacted in the sphere, constitutes a kind of initial ignition asset, which immediately begins to be spent on heating the surrounding medium. In this process, heat transfer from the sphere is proportional to the square of its radius, while heat input (due to reaction in the volume of the sphere) is proportional to the cube of the radius. In this connection,

In the initial period of development of the sphere (at its small diameters), heat removal can substantially exceed the heat input, and the development of the sphere at this stage can occur only at the expense of the initial electro-spark energy pulse.

The minimum energy of the spark pulse that is necessary for the development of the initial nucleus to critical dimensions, [1, 2] et al., at which the required temperature on its surface can be maintained solely by internal heat input, is the minimum ignition energy of the given mixture.

Let us note that, according to the data available in the literature, the optimal time for development of the initial source to critical dimensions when energies comparable with the minimum are introduced into the mixture is \(10^{-4}\) sec, [3, 4]. Therefore, ignition by longer or shorter sparks requires an increase in their energy content, [5].

Thus, in spark ignition, from the very beginning of initiation there is formed—without mediation in the form of any heated bodies—an element of flame, the heat transfer from which (with appropriate electrode dimensions) is practically completely utilized for the development of the initial source. It is obvious that with no other pulse is it possible to achieve such complete utilization of energy, and in this respect the minimum spark-ignition energy approaches an absolute, i.e., physical, characteristic of an explosive gas mixture.

Despite the successes in the development of phenomenological concepts of electro-spark ignition—the models of Zeldovich, [1], Most, [6], Lewis and Elbe, [8], confirmed by a number of experimental works, in particular by the work of Lintin and Budwig, [3]—up to the present time these concepts have not been brought to more or less rigorous quantitative forms. In particular, the dimensions of the “critical” diameter of the ignition source remain sufficiently conventional. Another conventionality is the proposition that the condition for the development of the initial flame nucleus to critical dimensions is the maintenance on its surface

temperature equal to the temperature of stationary combustion of the given mixture.

In this connection, at present it is impossible to make an accurate quantitative estimate of the minimum energies of even the simplest combustible mixtures. However, such an estimate can be made experimentally: by creating in the mixture under test an electric spark with a strictly controlled current in the spark and potential difference between the electrodes of the discharge gap. Such an approach is now technically quite feasible.

Having the possibility of such an estimate of the energy released in the spark, it is possible, first, for any fuel content in the mixture, to specify the minimum ignition energy (including the lowest ignition energy), and, second, by passing to the concentration limits of ignition, to determine strictly and with the required accuracy the compositions of mixtures corresponding to the concentration limits of ignition.

Thus, the electric-spark method of testing an atmosphere for explosion hazard gives both the concentration boundaries of ignition and the energetic boundary of mixture ignitability.

Moreover—and this, perhaps, is the main advantage of the method for the conditions considered—the use of a spark as a research tool makes it possible to study with great accuracy the effect of admixtures on limiting values.

The electric-spark method also has a number of other advantages: the possibility of creating an electric-spark source that permits regulation, over very wide ranges, of both the discharge energy and its duration, which makes it possible, among other things, to study the effect on the ignitability of atmospheres of the time of energy input.

The electric-spark method is especially preferable for studying the behavior of atmospheres in enclosed volumes under the conditions described, i.e., in complex electronic systems, electric discharges are, apparently, the most probable sources of ignition.

Finally, sparks are present as a necessary element in virtually every electronic or electrical circuit operating in normal mode, whereas the appearance of other sources of ignition presupposes, as a rule, the occurrence of an emergency situation.

REFERENCES

1. Ya. B. Zel’dovich, N. N. Semenov, ZhFKh, 23, No. 11, 1949.

2. D. U. Uidginotov, “Ignition of methane–air mixtures under the action of electric discharges,” report at the 3rd International Congress, Salzburg, 1964.

3. D. R. Lintin, E. R. Wooding, “Investigation of the ignition of a gas by an electric spark,” British Journal of Appl. Phys., 10, Apr., 1959.

4. D. Arnold, R. Sherwood, “Investigation of ignition and the initial stage of development of a flame kernel in hydrocarbon–air mixtures,” Problems of Combustion, IV International Symposium on Combustion and Detonation Waves, 1958.

5. E. S. Shchetinkov, Physics of Gas Combustion, Nauka, Moscow, 1965.

6. W. Iost, Zeit. Phys. Chem., 196, 4, 1950.

7. B. Lewis, G. Elbe, Combustion, Flames and Explosions of Gases, p. 4, 1951.

8. B. Lewis, G. Elbe, “Basic Principles of Ignition and Inflammation,” in Problems of Combustion of Reactive Fuels, Moscow, 1959.