Static electricity is a common phenomenon that may result in electric discharges, causing fires and explosions in hazardous locations. The question when an electrostatic spark occurs in a flammable atmosphere is whether or not the energy released will bring about ignition.
Figure 1. Hazardous locations, such as petrochemical plants, are susceptible to static electricity ignition and special attention must be paid.
The MIE-Based Approach to Evaluate the Hazard of Charge Accumulation
The matter of electrostatic ignition is complicated, and there is no entirely accepted theory. An approach compares the available capacitive energy to the minimum ignition energy (MIE) of the flammable mixture. The MIE is the smallest energy capable of igniting a flammable atmosphere at its most easily ignitable concentration.
Figure 2. The steps leading to the ignition of a potentially explosive atmosphere for electrostatic charges and discharges.
The potential developed on a body is the ratio of its charge to its capacitance:
where:
V = potential, in volts
Q = charge deposited on the body, in coulombs
C = capacitance between the body and the surroundings, in farads
The body increases in energy when its charge rises from zero to the value Q — equal to the work done during the process — represented as:
If:
Ignition is then likely.
NFPA 497, “Recommended Practice for the Classification of Flammable Liquids, Gases, or Vapors and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas,” lists selected combustible materials with relevant physical properties – such as lower flammability limit (LFL), Upper flammability limit (UFL), and MIE.
Table 1. MIE for common chemicals.
For flammable vapors, the MIE “rule of thumb” is 0.025 mJ. The energy released by a small coin – about 2.3 g weight – dropped from a height of 25 mm is 0.25 mJ.
A static discharge from a human being may reach 60 mJ or more. A safety precaution is to consider the human body capable of igniting flammable vapors with an MIE equal to 100 mJ or lower. A spark perceivable to the touch has around 20 mJ.
Flammable liquids are very volatile, and the resulting flammable vapor can diffuse throughout an entire facility. If the concentration reaches the flammability range in the presence of an ignition source, a fire or explosion may occur. But if the vapor and air are not adequately mixed or the mixture is either too lean or too rich, there will be no ignition.
Accumulation and Discharge
The process of charge separation, accumulation, and discharge gives the risk associated with static charges on flammable and combustible liquids.
Good conductor products are the best electrostatic generators, but this property allows them to discharge quickly.
Low conductivity liquids cannot discharge swiftly and they accumulate electrostatic charges, making them (aptly named) static accumulators. The low conductivity products are the most dangerous.
Ungrounded conductive objects – like truck tanks on rubber tires receiving charged products – and liquids of highly refined products are typically static accumulators.
An increase in static potential accompanies the accumulation of charges, which gives rise to an electric field. The greatest danger exists when the electric field’s intensity is high in the vapor phase since it may reach 3 kV/mm values, which is about the dielectric strength of air, and sparks may occur due to the dielectric breakdown. Fortunately, charge dissipation occurs at relatively high resistances to the ground, reducing the likelihood of a discharge that may ignite a given flammable atmosphere.
Short relaxation times do not allow the accumulation of enormous static potentials. Low generators – typically the cleaner products – can generate substantial static potentials owing to their long relaxation times.
In conclusion, the generated and accumulated electrostatic charge may ignite a flammable mixture if the amount of energy is enough to produce an incendiary spark, and there is a spark gap. Taking actions to prevent these conditions from coexisting is a must.
About Static Electricity as an Ignition Source in Hazardous Locations
Electrostatic discharges are capable of igniting the flammable atmospheres handled in hazardous locations.
Spark discharges occur when releasing the accumulated static electricity through a spark gap in a flammable atmosphere.
The ability of a spark to ignite a flammable atmosphere depends on its energy and duration. If the spark’s energy exceeds the minimum ignition energy (MIE) of a flammable mixture, the result will likely be a fire or explosion.