Page 229 - Shale Shakers Drilling Fluid Systems

ELECTRIC MOTORS 211

applies mostly to flammable and combustible liquids, position of ignition chambers, method and rate of
although there are certain solids (such as camphor heating, residence time, and method of flame detec-
or naphthalene) that slowly evaporate or volatil- tion. Reported ignition temperatures are affected
ize at ordinary room temperatures. Also, liquids, by the test methods employed.
such as benzene, freeze at relatively high tempera- Since ignition temperature is the temperature at
tures and, therefore, have flashpoints while in the which ignition may occur due to contact with a
solid state. hot surface, it follows that motor selection must
The term "propagation of flame" is used to be based on the maximum surface temperature
describe the spread of flame from the ignition that will never exceed the auto-ignition tempera-
source through a flammable mixture. A gas or ture of any potentially explosive mixture likely to
vapor mixed with air in proportions below the exist. The National Electrical Codes (Table 12-6)
lower limit of flammability, may burn at the source indicates the maximum external surface temperature
of ignition. In other words, they may burn in the for motors in service with combustible materials.
zone immediately surrounding the source of igni- Ignition temperature depends on the type and
tion, without propagating (spreading away) from the concentrations of gasses and vapors present. Table
source of ignition. However, if the mixture is within 12-7 compares the flashpoint and ignition (or auto-
the flammable range, the flame will spread through- ignition) temperatures for some common materials.
out when a source of ignition is supplied. The use
of the term "flame propagation," therefore, can be
used to distinguish between combustion that takes
place only at the source of ignition and that which
travels (propagates) through the mixture.
Ignition temperature (or auto-ignition tem- TABLE 12-6. NEC Maximum External Surface
perature) of a substance, whether solid, liquid, or Temperatures
gaseous, is the minimum temperature required to Ignition
initiate or cause self-sustained combustion in the Maximum Temperature
absence of any ignition source, such as a spark or Temperature Surface of Combustible
flame. To avoid the risk of explosion, the tempera- Class Temperature Material
ture of any part or surface must always be below Tl 450°C/842°F > 450°C/842°F
the ignition temperature.
Ignition temperatures observed under one set of T2 300°C/572°F > 300°C/572°F
conditions may alter significantly with changing T2A 280°C/536°F > 280°C/536°F
conditions, such as: T2B 260°C/500°F > 260°C/500°F
T2C 230°C/446°F > 230°C/446°F
• Percentage composition of the vapor or T2D 215°C/419°F > 215°C/419°F
gas-air mixture
• Shape or size of the space in which T3 200°C/392°F > 200°C/392°F
ignition occurs T3A 180°C/356°F > 180°C/356°F
• Rate and duration of heating T3B 165°C/329°F > 165°C/329°F
• Reactivity of any other materials present T3C 160°C/320°F > 160°C/320°F
T4 135°C/275°F > 135°C/275°F
Thus, ignition temperatures should be viewed T4A 120°C/248°F > 120°C/248°F
as approximations. T5 100°C/212°F > 100°C/212°F
There are many differences in ignition tempera-
ture test methods including size, shape and com- T6 85°C/185°F > 85°C/185°F

TABLE 12-7. Flashpoint and Auto-ignition Temperature for Some Common Materials
Auto-ignition Explosion
Gas/Vapor/Liquid Flashpoint Temperature Class Group
Acetone -20°C/ -4°F 465°C/869°F Tl IIA
Benzyl Alcohol 93°C/200°F 436°C/817°F T2 IIA
Benzene -11°C/ 12°F 498°C/928°F Tl IIA
Gasoline (petrol) -43°C/-45°F 280°C/536°F T2A IIA
No. 2 Diesel Fuel IIA
Hydrogen Sulfide gas 260°C/500°F T2B IIB