Static Electricity 2000 Edition

Chapter 7 Flammable and Combustible Liquids 
and Their Vapors
7.1 Scope.
This chapter discusses the assessment and control
of static electricity hazards involved with the storage, handling,
and use of flammable and combustible liquids and their
vapors and mists. While focused on flammable and combusti-
ble liquids, the principles of this chapter also apply to non-
combustible liquids and vapors (e.g., wet steam) where their
storage, use, and handling can cause a static electricity igni-
tion hazard. The chapter begins with a discussion of the com-
bustion characteristics of liquids and their vapors and mists,
followed by a discussion of charge generation and dissipation
in liquids. Emphasis is then given to processes involving the
following:
(1) Flow in pipe, hose, and tubing
(2) Storage tanks
(3) Process vessels
(4) Gauging and sampling
(5) Tank cleaning
(6) Portable tanks and containers
(7) Vacuum cleaning
7.2 Combustion Characteristics of Liquids, Vapors, and
Mists.
The following combustion properties of liquids need
to be understood in order to properly assess the static electric-
ity ignition hazard:
(1) Flash point
(2) Flammable limit and vapor pressure
(3) Ignition energy
(4) Oxidant concentration
7.2.1* Flash Point.
Flash point is the minimum tempera-
ture at which a liquid gives off sufficient vapor to form an
ignitible mixture with air near the surface of the liquid.
Flash point is determined using a variety of test procedures
and apparatus, the selection of which sometimes depends
on other physical characteristics of the liquid. (See A.7.2.1 for
more detailed information.)
If the flash point of a liquid is at or below typical ambient
temperatures, it is likely to evolve an ignitible vapor. The lower
the flash point, the higher the vapor pressure and the more
likely that a vapor will be present to ignite. Because of the vari-
ability in flash point test methods, the published flash point of
a particular liquid only approximates the lowest temperature
at which ignition is possible for that liquid. Thus, an allowance
of 4
°C to 9°C below the published flash point should be made
when evaluating ignition hazard. The following effects also
can generate an ignitible vapor:
(1) Off-gassing of flammable vapors from solids or low-vola-
tility liquids
(2) Processing at pressures below atmospheric pressure
(3) Nonhomogeneity of the vapors above the liquid
(4) Mist, droplets, or foam on the surface of a liquid