Encyclopedia of Environmental Science and Engineering, Volume I and II

1022

PREVENTION OF TOXIC CHEMICAL RELEASE

Purging of process vessels, tanks and piping before startup

and after shutdown is imperative for pilot plants and large-

scale plants in the process industries or wherever combusti-

ble gases and vapors are handled. Gases most often used for

purging are nitrogen, carbon dioxide, or gases derived from

the combustion of hydrocarbons. The reader is referred to

the sections on Vapor and Gaseous Pollutant Fundamentals

and to Fossil Fuel Cleaning for unit operations and incinera-

tion procedures involved in hazardous gas removal. To show

how the purging procedure can be “mapped” a typical purg-

ing chart will be constructed and the salient points explained

(see Appendix).

Inert gases such as nitrogen have the property of not only

depressing or narrowing the explosive range of a combus-

tible gas or vapor, but also of preventing the formation of

explosive mixtures with air when these inert gases are mixed

in suitable proportions with either air or with the combus-

tible gas or with an explosive mixture of both. By displacing

or mixing air contained in a vessel, tank or piping system to

be placed into gas service, with a suitable amount of an inert

gas such as nitrogen, a combustible gas may subsequently

be introduced without the formation of an explosive mix-

ture. Similarly, by displacing or mixing the combustible with

a suitable amount of nitrogen, air may later be introduced

without causing an explosive mixture to develop. During

the purging procedure constant sampling of contents must

be pursued using standard accepted methods of chemical

analysis.

GAS FLAMMABILITY LIMITS

A flammable mixture of a gas, such as acetylene and air, may

be diluted with one of the constituents (acetylene or air) until

it no longer is flammable. The limit of flammability due to

dilution is the borderline composition: a slight change in the

direction will support burning, while in the other direction

combustion cannot be supported and maintained.

At the ends of these two extremes there are well defined

limits within which self-propagation of flame will take place

on ignition. These are known as “upper” and “lower” limits

as defined in terms of the percentage by volume of com-

bustible gas present in a mixture of the gas and air. Table 1

below lists these limits for some of the more common gases

and vapors for conditions of atmospheric pressure and

temperature.

Within these limits, the combustible gas and air mix-

ture liberates sufficient energy to continue to propagate

flame from one mixture layer to the other. Mixtures above

the upper limit may burn on contact with external air, since

these layers are formed in the zone where gases mix. Certain

conditions effect a shift in the two limits, either increasing

or decreasing the spread between them as we will note later.

These conditions include: ignition source, ignition inten-

sity, direction of flame propagation (upward, downward, or

across), size and shape of container, vessel, or piping orien-

tation, temperature, pressure and humidity in the containing

vessel, oxygen content and turbulence.

EXPLOSIONS

When a chemical reaction is accompanied by the libera-

tion of heat, as the reaction progresses, it is followed by an

increase in the amount of heat, which in turn helps to accel-

erate the reaction. Thus the two advance together, both in

highly intimate connection and mutually helpful, until the

entire mass has been heated and chemically converted.

When a burning substance, a match for instance, is placed

in contact with the extreme outer limit of an explosive mix-

TABLE 1

Per cent gas or vapor in mixture—flammability limits

Gas or

vapor Lower Upper

Gas or

vapor Lower Upper

Acetylene 2.5 80.0

Carbon monoxide 12.5 74.2 Acetone 2.6 12.8

Hydrogen 4.0 74.2 Ethane 3.2 12.5

Ethyl ether 1.9 48.5 Propane 2.4 9.5

Carburetted 6.4 47.7 Butane 1.9 8.4

Hydrogen sulfide 4.3 45.5 Benzene 1.4 8.8

Methyl alcohol 6.7 36.5 Gasoline 1.5 6.2

Coal gas 3.9 29.9 Turpentine 0.8 —

Ethylene 2.8 28.6 Ammonia 16.0 27.0

Pennsylvania 4.9 14.1 Ethyl alcohol 3.3 19.0

Natural gas — — Methane 5.0 15.0

Reference: G. W. Jones, Chemical review Vol. 22, 1938. pp. 1–26.

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