Hazard and Risk Asessment

As the name implies, the term originally referred to physical phenomena associated with charges at rest, such as on charged, isolated conductors. However, as used in this book, “static” charges may either be at rest or moving. The elementary unit of negative charge is the electron, which carries –1.6 × 10–19 Coulombs of charge. A positive charge is equivalent to the absence of electrons. In semiconductor theory a group of covalent bonds deficient of one electron is treated as a mobile positively charged entity, or “hole.” This concept is used to describe the properties of semiconductive crystals used in transistors. However in the context of this book it is best to think of the flow of positive charge as a flow of positively charged particles or ions. Static electricity hazards or nuisances arise when charge separation occurs leading to an accumulation of one sign of charge within some defined boundary, such as inside a container. The work performed in separating the charges results in differences of potential within or across the defined boundary and the accompanying generation of electric fields. If an electric field locally exceeds some threshold value, electrical breakdown of the intervening medium occurs in the form of a static discharge. This might come as a shock. The “tingling” effects of static are caused by mutual repulsion between strands of hair carrying the same sign of charge, which tends to make them stand up. The phenomena occur either as the result of polarization  or a net charge on the body. When the body is polarized by a strong electric field, the charged strands of hair are both repelled from one another and attracted in the direction of the electric field. This can be especially hair-raising.

 Charge Separation

This occurs in a variety of ways. When solid surfaces are placed in contact, an electronic rearrangement occurs to minimize the energy at the interface. Since this process is generally not reversible, charge separation occurs when contact between the surfaces is lost. If the interface is disrupted at a rate faster than equilibrium conditions can be established, additional charge separation occurs. However, the maximum surface charge density is limited by electrical breakdown in the gap between the separating surfaces. During separation of nonconductive plastic sheets, one sheet gains a net positive charge and the other gains an equal quantity of negative charge. If this process occurs in air, corona discharges in the gap formed between the sheets limit the maximum surface charge density to £2.65 × 10–5 C/m2. As a stream of water breaks up, ions associated with aligned water dipoles at the water–air interface separate into the fine mist created as the surface layer shears away, while ions of predominantly opposite sign separate into the coarser droplets formed from the body of the water stream. This results in a charged water mist after the coarser droplets settle out. Ionic charge-carrying species in liquids are adsorbed nonuniformly at the wall of a pipe such that one sign of charge predominates in a tightly held “fixed layer” while the countercharge is situated farther from the wall in a less tightly held “diffuse layer.” When the liquid is pumped through the pipe the diffuse layer shears away and is convected downstream. The flow of charge is equivalent to a charging current or “streaming” current .

Magnitude of Current and Potential

Static electricity hazards and nuisances are typified by the generation of large potentials (0.1–100 kV) by small charging currents (0.01–100 mA) flowing in high resistance circuits (108–1015 W). This in part differentiates static electricity from other electrical phenomena. For example, stray currents in low resistance circuits are typically of the order 1 A for potential differences of the order 1 volt. The electric field at any point in relation to a conductor is proportional to its potential, while magnetic field is proportional to the current flowing through the conductor. Since static electricity involves high potentials and very low currents, it can be differentiated from “current electricity” phenomena by its associated electric field but the absence of any significant magnetic field.

Concentration of Charged Species

The occurrence of static electricity is highly dependent on the presence of charged chemical species at extremely small concentrations. This is because only a minuscule fraction of an electrostatically charged substance carries a net charge. One Coulomb represents the same charge as 6.25 × 1018 electrons, or an equal number of ionic species each carrying one elementary unit of charge. A mole of substance contains the Avogadro number, 6.023 × 1023 molecules. Hence a charge density of one Coulomb per mole is equivalent to 1 molecule in 96,400 (~10 ppm) carrying an elementary charge. One Coulomb per mole is an extremely large charge density. The volumetric charge densities found in charged liquids typically range from 1 to 5000 mC/m3. For a typical maximum charge density of 1000 mC/m3, assuming a liquid with specific gravity 1.0 and molecular weight 100, the involvement of molecules in the net charge carrying process is one part per trillion. Similarly, 1 m3 of this substance contains 6.023 × 1027 molecules. The maximum surface charge density of £2.65 × 10–5 C/m2 corresponds to 1.6 × 1014 electrons. If the surface charge is assumed to reside in a slice of the substance 10 Angstroms thick, containing 6.023 × 1018 molecules, approximately 27 ppm of these molecules carry an elementary charge. A similar concentration (about 7 ppm) is found for the more realistic case of a plastic with molecular weight 20,000 and specific gravity 0.8, where the charge is trapped in a surface layer ~1 mm thick.

 Importance of Trace Contaminants

The electrostatic behavior of intrinsically nonconductive substances, such as most pure thermoplastics and saturated hydrocarbons, is generally governed by chemical species regarded as “trace contaminants.” These are components that are not deliberately added and which may be present at less than detectable concentrations. Since charge separation occurs at interfaces, both the magnitude and polarity of charge transfer can be determined by contaminants that are surface active. This is particularly important for nonconductive liquids, where the electrostatic behavior can be governed by contaminants present at much less than 1 ppm. An unpredictable charge density increase or polarity reversal caused by a “pro-static agent” may lead to static ignition after years of uneventful operation under ostensibly identical conditions. For example, a change from positive to negative charging of a liquid may lead to formation of an incendive “positive brush” discharge. Such situations may be exacerbated by a coincidental decrease of ignition energy. An example is a temperature change that affects both charging of the condensed phase and flammability of the surrounding space. Many surface active trace contaminants increase the magnitude of charging in liquids. However, at higher concentrations they can have a beneficial effect by increasing the liquid conductivity to the extent that significant charge no longer accumulates in grounded containers. Special formulations are known as “antistatic additives”. Trace contaminants are also significant at charged solid surfaces, affecting both the charging process and the surface conductivity. In ambient air atmospheres their effect is often determined by interaction with adsorbed water vapor, whose dominant concentration may be sufficiently large to form a monolayer. Topical antistatic agents for solids typically rely on interaction with adsorbed water and can lose effectiveness at low relative humidity.

Hazard Evaluation

The ignition hazard analysis in 2-5 begins with evaluating whether static electricity can accumulate, with the assumption that flammability has already been addressed. However, as reflected in Chapters 3, 5, and 6, a practical safety analysis should begin by evaluating whether a flammable mixture may be present, since this determines whether or not any ignition hazard exists in the first place. The usefulness of a hazard evaluation is determined largely by the evaluation of flammability, since this governs the ignition hazard with respect to any ignition source. Order-of-magnitude estimates may in some cases be sufficient to rule out ignition via static discharges. Conversely, if a large risk exists, or in the aftermath of an explosion, order-of-magnitude estimates may be inadequate. In such situations the coverage given in this book should be especially helpful.

Statistics

Static ignition statistics must be used with prudence not only because of possible misdiagnosis of the cause of ignition but also because of the way the losses are often grouped together. As discussed in Chapters 5 and 6 the likelihood of ignition is related most strongly to the ignition energy of the flammable atmosphere involved. In road tanker operations, although switch loading is sometimes considered separately, accident statistics usually fail to identify fires involving pure liquids which can represent the most hazardous case. The sheer bulk of some operations having a small likelihood of ignition, such as gasoline trucking, can lead to misinterpretation of incident rate by incorporating and hiding those products with a much greater likelihood of ignition, such as toluene. Other grouping errors may involve the use of antistatic additives in certain products. Powder ignition is similarly much more likely for certain products, especially where a substantial mass fraction comprises fine powder or the process by its nature tends to accumulate easily ignitable dust. The methods outlined in this book should help identify those products and operations most at risk.

Source: Avoiding Static Ignition Hazards in Chemical Operations by LAURENCE G. BRITTON (p 7-13)