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210 SHALE SHAKERS AND DRILLING FLUID SYSTEMS
HAZARDOUS LOCATIONS to burn in air without suppling additional heat
from an outside source. Some substances (sand
Explosion Protection and clay for example) will not ignite at any tem-
perature because the elements they contain have
Construction and installation of all electrical already combined with as much oxygen as they
equipment placed in a flammable or potentially are capable. In this case further reaction with
explosive location must receive careful consider- oxygen is not possible. Some substances ignite at
ation. In some drilling and production sites, where very low temperatures such as white phosphorus,
the occurrence of explosive mixtures of flammable which ignites at 35°C (95°F). Gasoline ignites at a
materials and air cannot be prevented, special lower temperature than kerosene; kerosene ignites
construction measures for prevention and/or con- at a lower temperature than motor oil; and ether
tainment of ignition sources are warranted. Such ignites at a lower temperature than alcohols.
areas are classified by hazardous ratings, which The combustion of some substances is accom-
will be discussed later in this chapter. panied by the production of flames, which are
Hazardous locations are those where poten- burning gases. When wood or coal (especially soft
tially explosive atmospheres can occur due to local coal) is heated to its ignition point or below, com-
and/or operational conditions. Leaks inevitably bustible gases are released. These combustible
occur during manufacture or movement of vola- gases usually ignite at a lower temperature than
tile, or slightly volatile, liquids. Such leakage the residue of wood or coal, and their combustion
may combine with oxygen from the atmosphere produces the effect know as "flame."
to form mixtures of explosive concentrations. Hydrogen burns with an almost colorless flame,
Ignition of such mixtures by an electrical spark as opposed to flames produced by wood, which
or by contact with an excessively hot surface are generally yellow colored. Flames are usually
may result in an explosion. colorless if solid particles are not present in the
An explosive atmosphere occurs when a mix- burning gases and are produced by the decompo-
ture of air (or oxygen) and flammable substances sition of substances into the gases as they burn.
in the form of gas, vapor, or mist exists in such The hydrogen flame can be yellow if a small quan-
proportions that the mixture can explode through tity of sodium chloride is vaporized and mixed
excessive temperature, arcs, or sparks. with the burning hydrogen.
Different techniques are used to minimize the Kindling temperature is the temperature at
risk of explosion including explosion-proof con- which a substance bursts into flames and combus-
struction, purging, pressurization, encapsulation, tion proceeds without further application of heat.
enriching, oil immersion, and intrinsic safety. Kindling temperature varies considerably with the
state of division of the substance (for instance, the
"wood" of a match), its surface area, porosity, and
Combustion, ignition, Flashpoint so forth. Finely divided particles offer much more
surface area than the same weight of a substance
When oxygen reacts with other elements or in one large mass. Iron and lead can both be pro-
compounds, heat is usually liberated. Because of duced in small enough particles (large surface area
this, the temperature rises and causes the reaction per unit mass) so that they will ignite without
to proceed at a more rapid rate. Generally, the preliminary heating when poured from a container
term "combustion" refers to the vigorous and rapid into air.
reaction with oxygen attended by liberation of Flashpoint is the lowest temperature at which
energy in the form of heat and light. the vapors above a volatile, combustible substance
Reactions other than those involved with oxy- (such as any petroleum product) ignite momen-
gen can also liberate heat and light. For example, tarily in air due to a spark or small flame applied
a jet of acetylene burns brilliantly in chlorine: near the liquid surface. It has also been described
as the lowest temperature at which a liquid will
C 2H 2 + Cl 2 -> 2HC1 + 2C give off sufficient vapor to ignite momentarily on
application of a flame. The degree of flammability
of a substance is mainly expressed by its flashpoint.
Hydrogen also burns brilliantly in chlorine:
An ignitable mixture is one within the flam-
mable range (between upper and lower limits)
H 2 + Cl 2 -^ 2HC1 capable of flame propagation away from the source
of ignition when ignited. Some evaporation occurs
Various substances must be heated to different below the flashpoint, but not in quantities suf-
temperatures before they will ignite and continue ficient to form an ignitable mixture. This term
