Static Electricity 2000 Edition

Vapor Pressure. The vapor pressure can be used together
with a measured LFL to estimate the flash point. Usually, the
calculated flash point is less than the measured value due to
limitations in the flash point test technique. Conversely, only
an approximate estimate of the LFL can be made from the
flash point. The vapor pressure can be used to replace the
“concentration” axis in Figure 7.2.3 with the corresponding
temperatures required to generate the concentrations shown
in the figure. This method allows one to determine the equi-
librium liquid temperature at which vapor ignition is most
probable, corresponding to generation of the vapor–air mix-
ture having the lowest ignition energy. For many liquids, this
point is approximately halfway between the LFLs and UFLs.
For example, benzene generates its lowest MIE vapor–air mix-
ture at about 7
°C (4.8 percent benzene vapor in air) and tolu-
ene at about 2.6
°C (4.1 percent toluene vapor in air).
Therefore, for operations conducted at room temperatures,
toluene is more prone to ignition from a low energy static elec-
tric discharge than is benzene. In closed containers at equilib-
rium, benzene becomes too rich to burn (the concentration of
vapor exceeds its UFL of 8 percent) at temperatures above
about 16
°C. Conversely, at about 7°C, benzene is more easily
ignited than toluene, because the latter will generate a vapor
composition not far above its LFL. Some lowest MIE composi-
tions are given in Appendix B.
High Vapor Pressure Liquids. High vapor pressure liquids are
defined in API RP 2003, Protection Against Ignitions Arising Out
of Static, Lightning, and Stray Currents, as having a Reid vapor
pressure greater than 4.5 psia (31 kPa absolute). At normal
handling temperatures, rapid evaporation of these liquids
minimizes the duration of a flammable atmosphere above the
liquid during loading and the UFL is soon exceeded. How-
ever, if there is no initial heel in the tank and the tank is not
inerted, the flammable range will be traversed prior to attain-
ing vapor equilibrium. The duration of the ignitible atmo-
sphere is minimal for liquefied gases such as propylene, but
could be considerable for certain petroleum distillate fuels.
Inerting might be considered when loading high vapor pres-
sure nonconductive liquids to tanks containing air with no liq-
uid heel.
Intermediate Vapor Pressure Liquids. Intermediate vapor pres-
sure liquids are defined in API RP 2003 as having a Reid vapor
pressure less than 4.5 psia (31 kPa absolute) and a closed-cup
flash point below 100
°F (37.8°C). They are most likely to gen-
erate ignitible mixtures in vessels at ordinary temperatures.
Although graphical methods have been proposed to estimate
whether liquids are likely to generate ignitible atmospheres at
various temperatures, based on their Reid vapor pressures,
such graphs were originally derived for petroleum fuel mix-
tures and do not always apply to other flammable liquids.
Low Vapor Pressure Liquids. Low vapor pressure liquids are
Class II and Class III combustible liquids [i.e., those with
closed-cup flash points above 100
°F (37.8°C)] and will gener-
ate ignitible atmospheres only if handled at elevated tempera-
ture, suspended as a mist, or subject to slow vapor evolution.
However, static electricity generated during handling could
ignite vapors present from previous operations.
A.7.2.4
Preventing an ignitible atmosphere can be accomplished
using any of the methods described in NFPA 69, Standard on
FP
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0.25 101.3