Electrostatic Hazards in the Process Industries

0957–5820/04/$30.00+0.00 # 2004 Institution of Chemical Engineers Trans IChemE, Part B, March 2004 Process Safety and Environmental Protection, 82(B2): 132–141

www.ingentaselect.com=titles=09575820.htm

REVIEW PAPER

ELECTROSTATIC HAZARDS IN THE PROCESS INDUSTRIES I. D. PAVEY* Chilworth Technology Ltd, Southampton, UK

S

tatic electricity continues to be the cause of fires and explosions in the process industries. A major problem is the lack of understanding of the fundamentals of electrostatics, and even the common misconception that static electricity is unpredictable. This paper provides a short overview of the subject of electrostatic hazards. It includes a discussion of some of the basic concepts, and also covers some of the ways in which static hazards can be addressed. Some of the issues are illustrated, and further developed, with reference to a range of realistic processing scenarios. Keywords: electrostatic; static; hazard; risk; protection; fire; explosion; flammable; gas; vapour; dust; powder; ATEX; DSEAR.

INTRODUCTION


Solvent-wet powder was being added from FIBCs to water in a previously inerted 10,000 l vessel. As one FIBC was being removed an explosion occurred. Windows were blown out, doors were damaged and two operators required hospital treatment.
During the transfer of solvent-wet powder from a filter drier into skips, a flash fire and fire occurred. There were no injuries.
An explosion occurred during the manual addition of kegs of solvent-wet powder to a dryer. One man died and others were injured. Substantial damage was also caused.

Static electricity is not the most common cause of fires and explosions in the process industries. It is, however, a very important one. This is well illustrated in European Directive 1999=92=EC, commonly known as ATEX 137 and implemented as DSEAR 2002 in the UK. ATEX 137 lays down ‘minimum requirements for improving the safety and health protection of workers potentially at risk from explosive atmospheres’. If the formation of flammable atmospheres cannot be prevented, the next approach set out by the directive is to avoid ignition sources. In Article 4 of the Directive, ‘The Assessment of Explosion Risks’, of all the ignition sources that might have been referred to, the only one to be explicitly mentioned is electrostatic discharge. Consider also the following brief incident summaries:

These incidents have all occurred since the beginning of 2000. They represent a small fraction of all the incidents, in the UK and Ireland alone that were investigated by Chilworth Technology Ltd between the beginning of 2000 and early 2003. It was concluded that all the above were caused by static electricity. Clearly, static hazards are a very current issue, and this is in spite of efforts to deal with them. Every incident mentioned above occurred in locations where static hazards had been considered and control measures were in place. The problem is that static electricity is not widely understood. It is not always properly understood by those responsible for safety and who are setting procedures aimed at ensuring safety. Inevitably, therefore, some measures are inadequate. It is also not well understood by the operators who are required to implement the procedures. In this case lack of understanding can lead to shortcuts, or even just inadvertently missed steps, because their importance, and the implications of missing them, are not appreciated. This review paper provides broad background information regarding static electricity and how it impacts on process safety. Armed with this information it should be possible to better analyse a process operation, especially with reference to some of the published standards and guidelines


An explosion occurred while transferring a powder from a dryer into kegs. The two operators required hospital checks but were not seriously injured. Plant damage was minor.
During a liquid addition to toluene in a 4500 l vessel an explosion occurred. Some damage to plant was incurred. The two operators saw flames emerging from an open manway but were sufficiently remote to escape uninjured.
In a laboratory scale production unit a flash fire occurred during vacuum transfer of a solvent from a bench-top container. The fire was quickly extinguished. Damage was superficial only and there were no injuries. *Correspondence to: I. D. Pavey, Principal Electrostatics Specialist, Chilworth Technology Ltd, Beta House, Chilworth Science Park, Southampton SO16 7NS, UK. E-mail: [email protected]

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ELECTROSTATIC HAZARDS IN THE PROCESS INDUSTRIES that are available. It may also help with internal training by explaining exactly why some procedures are in place. A better understanding of the basics will not only help when addressing the issues surrounding an actual process operation but, and perhaps even more importantly, will help to make it clear when external help may be required. The next sections deal with some of the important fundamentals of static electricity, basic control measures and flammable atmospheres. These are then put into the context of some practical examples. Finally, a few concluding remarks pull the whole paper together.

Just as pressure differences and pressure gradients provide the driving force for the transfer of gases from one place to another, so potential differences and potential gradients are the driving forces for the movement of charge. CAPACITANCE Given the relationship expressed in equation (3), electrical capacitance is clearly a very important variable. For the simple geometry represented by two parallel metallic plates capacitance can be calculated as shown in equation (4): C ¼ e0 er

CHARGE AND VOLTAGE In electrostatics the observed effects are the result of electric fields. Electric current (or charge flow) is largely incidental. This is quite the opposite of the situation with most electrical equipment (what might be termed electromagnetics), where, by and large, the effects of electric current are all important. The same fundamental physical laws apply to electrostatics, but common simplifications that may work in electromagnetics do not necessarily carry through. This is one of the main reasons that electrostatics seems a mystery to many, and is even sometimes referred to as a kind of ‘black art’. One of the commonest misconceptions that is frequently heard, at least by implication, is that charge and voltage are synonymous. This is simply not true. Electrical neutrality implies that a substance comprises equal numbers of positive and negative sub-atomic particles. Charge, which is measured in Coulombs, can be related directly to the number of electrons too few or too many when compared with electrical neutrality. In fact: 1 Coulomb
6:24  1018 electrons

(1)

On the other hand voltage, or potential measured in Volts (V), is actually potential energy per unit charge. It is well known that like charges repel one another. Clearly, therefore, if like charges are forced together, the potential energy of each will rise. When explained in these terms, an analogy in terms of pressure and volume comes to mind. For a fixed temperature the relationship between the amount of gas, the volume in which it is contained, and its pressure, are so well known as to almost seem intuitive, and can be expressed as: n / vP

Q ¼ CV

(3)

where Q is the charge in Coulombs, C the electrical capacitance of the object on which the charge resides (in Farads), and V the electrical potential (in Volts).

A d

(4)

where C is the capacitance in Farads (F), A the area of the facing surfaces (m2) and d the distance between the two plates (m). e0 is the electric constant, and equal to approximately 8.85  1012 F m1, and er the relative permittivity of the material filling the gap between the two plates. For air, the relative permittivity is 1. In qualitative terms, equation (4) indicates that the size and location of one plate relative to the other is important. A single conducting object will have a measurable capacitance that depends on its size and location relative something electrically connected to the rest of the world. For example, a single metal plate will have a measurable capacitance. However, a metal plate leaning against a wall will have a higher capacitance than the same metal plate suspended, say 1.5 m below the ceiling in the centre of a room. Unfortunately, the mathematical complexity of even a simple room and plate means that the capacitance of most objects in practical situations cannot be calculated. They can, however, be measured, and Table 1, taken from PD CLC=TR 50404 (2003), gives some typical capacitances of common conducting objects that can be found on many plants. RESISTANCE When considering fluid flow, it is the pressure difference between two parts of a pipe that, together with characteristics of the pipe, govern the rate of material transfer. Similarly, when dealing with electricity, the rate of charge transfer from one part of an object to another depends on the potential difference between the two parts and a characteristic of the object known as resistance. The simple equation relating these variables is: dq V ¼ dt R

(2)

where n is the number of moles of gas, v the volume (or capacity) of the container in which it is held, and P the pressure. Clearly, equation (2) is simply the equation of state for an ideal gas, rewritten to suit the illustration. The constant of proportionality that would normally be applied therefore includes both the universal gas constant and the temperature, which was earlier given as fixed in this case. The analogous equation relating charge and electrical potential (or voltage) is given in equation (3).

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(5)

Table 1. Typical capacitances of common objects. Extracts from PD CLC TR 50404: 2003 are reproduced with the permission of BSI under license number 2004DH0005. British Standards can be obtained from BSI Customer Services, 389 Chiswick High Road, London W4 4AL. Tel: +44 (0) 20 8996 9001. E-mail: [email protected] Object

Capacitance (pFa)

Small metal items (scoop, hose nozzle) Small container (bucket, 50 l drum) Medium container (250–500 l) Major plant items (reaction vessels) Human body

10–20 10–100 50–300 100–1000 100–300

a

1 pF ¼ 1012 F.

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PAVEY

where q is the charge in Coulombs, t is time in seconds (s) and R the resistance in Ohms (O). Given that the rate of charge transfer in Coulombs per second is also known as electric current, equation (5) can be rearranged into the more familiar expression of Ohm’s Law: V ¼ IR

(6)

where I is the current in Ampe`res (A). As with capacitance, the resistance of an object is calculable provided the geometry is relatively simple. For an object of constant cross-sectional area between two plane parallel faces the resistance between the faces is given by: d (7) A where d is the distance between the faces and A the constant cross-sectional area. r is a property of the material itself, known as resistivity, with units of Ohm-metres (O m). Although static electricity almost by definition is not directly concerned with electric currents, the resistivity of a material (or its reciprocal, conductivity) is extremely important. This is because resistivity controls the way charge is lost by conduction. During charge acquisition, resistivity therefore sets the dynamic equilibrium charge level. Furthermore, once charging ceases, resistivity controls the rate at which charge may be lost by conduction, and hence its longevity. R¼r

ELECTRICAL DISCHARGES IN AIR There are always a few free electrons in air. If an electron is exposed to an electric field (voltage gradient) between two electrodes it will be accelerated towards the more positive electrode. Acceleration will be proportional to the electric field and will continue until the electron hits a gas molecule. The kinetic energy acquired by the electron will therefore depend on the electric field and on the distance between gas molecules—or pressure. If sufficient energy has been acquired the collision will result in another electron being knocked off and two electrons beginning to accelerate where previously there was only one. When the potential gradient is high enough and this process is repeated, the result is a sea of electrons and positive ions in the gap between the electrodes. The conducting nature of this plasma permits transfer of charge between the two electrodes, and the entire phenomenon is referred to as breakdown of the gas. From this description it can be seen that, as the pressure is reduced, and the distance between gas molecules increased, electrons will accelerate for longer, and acquire more energy between collisions, leading to breakdown at lower electric fields. This will continue until the distance between electrodes is comparable with the distance between molecules. At this point there will be few electron–molecule collisions and breakdown will be inhibited. This explains the fact that, while a modest reduction in pressure leads to a reduced breakdown strength, hard vacuum will protect against breakdown between electrodes. These phenomena are embodied in Paschen’s Law (Paschen, 1889), which states that the breakdown voltage between two electrodes depends on the product of gas pressure and electrode spacing, as illustrated for air in Figure 1. The above qualitative description of gaseous breakdown explains the basic mechanism. However, the precise nature

of a discharge depends also on the type of material on which the charge resides. This has led to a number of types of discharge being defined. Each occurs from a particular material or electrode type, dissipates a characteristic range of energies and therefore carries with it a characteristic ability to ignite flammable atmospheres. Clearly knowledge of these discharge types is crucial when considering electrostatic hazards, as is the basic mechanism described above, and which controls all discharge types. SPARK DISCHARGES Spark discharges occur between conducting items at different potentials. The discharge is initiated as described above, and charge begins to be transferred between the two objects. As charge leaves the point of initiation, charge on the rest of the object will move to maintain a constant potential across the whole of the object. Since the object is conducting, this movement of charge occurs very quickly, effectively supplying charge to, and maintaining charge transfer at, the point of discharge initiation. Even though the potential difference between the objects will drop as charge is transferred [see equation (3)], the plasma will be maintained by the current flow through it, until virtually all of the charge has been transferred. The characteristic of a spark discharge between conducting objects is therefore that virtually all the charge is transferred in a single discharge event. In transferring the charge energy is dissipated, resulting in the characteristic blue (in air) flash, and audible crack due to the local temperature (and hence pressure) excursion. From the earlier definition of potential it can be seen that an incremental charge, dQ, transferring across a potential difference V, will dissipate energy, dE, as given below: dE ¼ V dQ

(8)

Using equations (8) and (3), an equation for the total energy dissipated by a spark discharge can be defined: ðV (9) E ¼ C V dV 0

or 1 E ¼ CV 2 2

(10)

Figure 1. Paschen’s curve for air.

Trans IChemE, Part B, Process Safety and Environmental Protection, 2004, 82(B2): 132–141

ELECTROSTATIC HAZARDS IN THE PROCESS INDUSTRIES where C is the capacitance of the charged object and V its initial voltage. Now, using equation (10), and knowing that a typical voltage readily attained in electrostatics is 104 V, Table 1 may be redrawn with typical spark discharge energies possible from common objects. This is shown in Table 2. On its own, of course, Table 2 means little. However, it becomes of more practical significance when compared with Table 3, which gives the effect of different discharge energies on the human body. When considering ignition hazards, even more important than the effect on the human body is the igniting power of such discharges, which will be discussed shortly.

BRUSH DISCHARGES Brush discharges occur when a charged insulator is involved. The initiating mechanism is exactly the same as described previously. However, charge cannot be transferred through an insulator to the point of discharge initiation, so only the charge on a very localized area of the surface is involved. In practice, several small discharges from nearby parts of the surface usually occur almost simultaneously. The multiple, essentially parallel, discharge channels then usually join to form one larger channel a short distance above the insulator surface, and it is this brush-like appearance that gives the discharge its name. Because only a small part of the total charge is involved many brush discharges can be drawn from the same surface without it having to be charged again. Because the amount of charge involved, and even the local potential, are not easily determined, it is not possible to calculate the energy of a brush discharge. However, many experiments have shown that brush discharges can ignite flammable gases and vapours in air if the minimum ignition energy (see ‘Flammable atmospheres’, below) is less than about 3–4 mJ. There is no positive evidence that a brush discharge has ever ignited a dust cloud. Nevertheless, it is usually considered prudent to assume that a brush discharge could also

Table 2. Typical spark discharge energies for common objects. Object

Typical energy (mJ)a

Small metal items (scoop, hose nozzle) Small container (bucket, 50 l drum) Medium container (250–500 l) Major plant items (reaction vessels) Human body

0.5–1 0.5–5 2.5–15 5–50 5–15

a

Assuming an initial voltage of 10,000 V.

Table 3. Effects on human body of different discharge energies. Discharge energy (mJ)

Reaction

1 10 100 1000 10,000

Just perceptible Definite sensation Unpleasant shock Severe muscular spasm Possibly lethal shock

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ignite dust clouds with minimum ignition energies of less than 4 mJ, of which there are many. CONE DISCHARGES When a highly charged insulating powder falls to form a heap of powder, whether inside a vessel or not, the effect is to use the force of gravity to compress that charge against its own force of mutual repulsion. The result is an increase in potential energy, which manifests itself as a relatively high voltage at the centre of the heap compared with that at the sides. If the electric field across the surface of the heap is high enough, the air over the heap will be ionized as described earlier and discharges across the surface will occur. Since the surface of a powder heap being fed from a single stream is broadly conical in shape, these discharges are often referred to as cone discharges. Cone discharges occur radially across the heap, and are often a significant fraction of the heap radius in length. In addition to occurring repeatedly during powder transfer to a heap, they may also occur if a heap is disturbed. It is generally accepted that cone discharges can be more energetic than brush discharges in terms of their ability to ignite flammable materials. However, there is some controversy surrounding the actual discharge energy. Glor et al. (1995) have carried out experiments in which the charge involved in actual cone discharges was transferred via a cable to a spark gap, and the energy of the resulting spark discharges assessed by observing their effect on flammable atmospheres of known ignition energy. The result is an empirical equation relating the maximum cone discharge energy to the particle size of the powder and the diameter of the vessel containing the powder. This equation, which is included in the guidelines of PD CLC=TR 50404 (2003), is shown below: Emax ¼ 5:22  D3:36  d 1:46

(11)

where Emax is the predicted maximum energy in milliJoules, D the vessel diameter in metres, and d the particle diameter in millimetres. Equation (11) is said to be valid for vessel diameters up to 3 m and particle diameters up to 3 mm, which equates to a value for Emax of 1041 mJ. It is well accepted that the energy of cone discharges increases with both vessel diameter and particle diameter. Nevertheless, many consider that the energy calculated using equation (11) is unrealistically high when compared with the igniting ability of the actual discharges across the powder heap. By comparison, the US guideline document NFPA 77 (2000) states that these discharges ‘have a maximum effective energy of 10 mJ to 25 mJ’. Whenever a charged insulating powder is being loaded into a container, the possible effect of cone discharges on the powder, or any gases present, must be considered. However, with such widely differing guidelines it is clear that more work on this subject is required. PROPAGATING BRUSH DISCHARGES Propagating brush discharges occur when a thin insulating sheet is highly charged with opposite polarity charge on each side. The most common situation where this occurs is when a thin insulating film is in close contact with an

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PAVEY hydrogen) can be considered to be a benign means of transferring charge across a gap. CHARGE ACQUISITION

Figure 2. Propagating brush discharge.

earthed conductor. Viewing the opposite sides of the film as a capacitor, and referring to equations (3) and (4), it is clear that, in this situation, charge can accumulate on the free side of the insulator with an unusually low increase in potential. Conversely, in high charging situations by the time the surface potential of the insulating film has reached a typical value in electrostatics of a few tens of kilovolts, the amount of charge accumulated will be very high indeed. Now if a discharge is drawn from the surface of the insulator, extensions to the initial discharge channel propagate radially across the surface from the initial discharge, as shown in the photograph of Figure 2. In this way a great deal of charge may be transferred in a single discharge event that is characterized by a loud crack and visible, lightning-like, channels radiating across the surface. The energy dissipated in a propagating brush discharge can exceed 1000 mJ. From Table 3 it will be apparent that this type of discharge can even cause direct injury to personnel. Furthermore, since it comes from an insulating surface, not all of the charge is transferred and several propagating brush discharges can be drawn without re-charging. CORONA Corona occurs at an electrode in the form of a sharp point. In the immediate vicinity of a sharp point the local electric field will be much higher than it is further away. Consequently, even with modest potential differences, ionization will often occur around a sharp point held at high voltage, or around an earthed sharp point near another object at high potential. Because the field is not uniform, the ionization region does not extend across the entire gap. However, positive ions formed in the ionization region, or negative ions formed by combination of electrons and gas molecules outside the ionization region, will be repelled from the sharp point and may traverse the gap. Corona is characterized by a faint blue haze in the vicinity of the point and sometimes an audible hiss. It is a low energy form of discharge and apart from when in the presence of exceptionally sensitive materials (such as

Electrostatic charge can be acquired by three methods— corona charging, induction charging and tribocharging. Limited space means they cannot all be discussed in detail. Corona charging occurs by capture of ions arising from a nearby corona discharge, and induction charging by exposure of an object of finite conductivity to an electric field. It would therefore be correct to say that, although both these mechanisms are quite different, they both occur in the vicinity of an object that is already charged. The consequences of that statement are two-fold. First, if there is anything on plant that is known to be charged (such as a plastic IBC), effects beyond those discussed in this article may occur and help must be sought elsewhere. Secondly, tribocharging must be considered to be a more fundamental source of charge and will be discussed in a little more detail. Tribocharging occurs as a result of contact between dissimilar materials, and can be explained in terms of electron transfer across the interface. A metal, A, has a well-defined work function, FA, which is the energy required to remove an electron from the metal in electron-Volts (eV). If metal A is brought into contact with metal B (with work function FB), electrons will move across the interface from the higher energy state (or the metal with the lower work function) to the lower energy state. The result will be a potential difference between the two metals, DV, as given below: fB  fA (12) e where e is the charge on an electron (electronic charge ¼ 1.602  1019 C). The potential difference in this case is the well-known contact potential for the metals concerned. Since metals are conducting, charge will move through the metals such that they are each at a constant electrical potential throughout, which means that the total charge transferred will depend on capacitance [see equation (3)]. Also, as two metals are separated, the transferred charge will be conducted back through the last point of contact and there will be no lasting evidence of the transfer that took place. Qualitatively, a similar mechanism for electron transfer can be considered when one or both contacting surfaces is an insulator (a full discussion of the models that have been proposed is not appropriate here). However, in this case, charge will not be conducted through the insulating material and the transferred charge will only be that required to set the local potential difference at a point of contact (on a microscopic scale) to the equivalent of a contact potential. Furthermore, when the materials are separated the transferred charge will not be conducted back through the last point of contact but will be carried way leaving one material positively charged (having lost electrons) and one negatively charged. From this description it can readily be appreciated that repeated contacts where one or both materials is an insulator will lead to a greater total charge having been transferred. Increased intimacy of contact, brought about by higher pressures or sliding will also lead to higher charge. In the DV ¼

Trans IChemE, Part B, Process Safety and Environmental Protection, 2004, 82(B2): 132–141

ELECTROSTATIC HAZARDS IN THE PROCESS INDUSTRIES Table 4. Typical charge–mass ratios for common processes. Extracts from PD CLC TR 50404: 2003 are reproduced with the permission of BSI under license number 2004DH0005. British Standards can be obtained from BSI Customer Services, 389 Chiswick High Road, London W4 4AL. Tel: +44 (0) 20 8996 9001. E-mail: cservices@ bsi-global.com Operation

Typical charge–mass ratio (C kg1)

Sieving Pouring Scroll feed transfer Grinding Micronising Pneumatic conveying

1011–109 109–107 108–106 107–106 107–104 107–103

process industries this means that the more energetic a process, or the higher the interfacial velocities, the greater will be the charge acquired. This is shown in Table 4, which gives typical charge to mass ratios acquired by powders undergoing different processes, and is taken from PD CLC=TR 50404 (2003). Of course, there is no fixed natural boundary that differentiates between conducting and insulating materials. Rather there is a continuum of possible resistivities (or conductivities). It was said earlier that on separating contacting metals all exchanged charge would be conducted back through the last point of contact. There will, of course, be a tendency for the same thing to occur when less conducting materials are involved. Now, though, the degree to which it occurs depends on both the resistivity of the material(s) and the rate at which separation proceeds. This leads to the general observation that for a given charging mechanism the level of charge acquired will increase as the material’s resistivity increases. If one of the materials is a metal, or at least a reasonably good conductor, its potential can be held at zero by electrically connecting it to earth. Careful consideration of the charging mechanisms discussed will show that this will not stop charge exchange with a contacting insulator, nor will it stop a contacting insulator retaining transferred charge. This explains why, for example, earthing a metal pipe, important though this is, will have no effect on the charge acquired by a powder being pneumatically transferred inside it.

LIQUID CHARGING The double layer of charge in the liquid at a solid-liquid interface (and other phase interfaces) is a familiar concept that has been modeled by various workers since it was first postulated by Helmholtz in 1879. One layer of charge, derived from the liquid, is strongly bound to the surface, leaving a diffuse layer of the opposite charge decreasing in concentration as the distance from the surface increases. If the solid–liquid interface is the inside wall of a pipe, and the liquid is flowing, the diffuse charge layer will be carried along with the liquid. This convective transfer of charge is referred to as a streaming current, and the result can be a charged liquid emerging from the pipe. Unless the pipe is conducting and earthed, it, too, will be left with a net charge.

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Clearly as the two layers of charge are separated there will be a tendency for charge recombination by conduction through the bulk of the liquid. Consequently high-conductivity liquids are virtually immune from charging in this way, while low conductivity liquids can become charged even in the simple pipe flow scenario. Even medium conductivity liquids (between 50 and 1000 pS m1) can acquire significant charge where there is a high interfacial area and=or velocities are high, such as in filters. (A conductivity of 1 pS m1 is equivalent to a resistivity of 1012 O m.) A number of equations for streaming current have been proposed. One commonly used relationship is the Scho¨n equation: is ¼ 4u2 d 2

(13)

where is is the maximum streaming current after a long pipe (mA), u the velocity of the liquid (m s1), and d the diameter of the pipe (m). A double layer is generated at all liquid surfaces, including those between a continuous liquid phase and a solid or liquid dispersion. Because of the high interfacial surface area in this situation, even if the continuous phase is of medium conductivity, a dispersed phase can lead to significant charging in processes as gentle as stirring in a vessel.

STATIC ELECTRICITY AND FIRE AND EXPLOSION HAZARDS Having set the background we can now begin to see where electrostatic hazards can arise. Charge will be generated wherever there is contact, and especially relative movement, between materials, at least one of which is not a good conductor. Charge can accumulate as a result of charge generation on either or both of the materials involved. The only exception is conducting materials that have been properly and effectively connected to earth. These will not acquire charge but, as indicated earlier, this does not prevent the other half of the material pair becoming charged. Once charge has accumulated somewhere, whether it is on a person, a plastic bag, an isolated metal pipe section, a heap of powder, or any other material or object, there is the possibility of an electrical discharge. The energy dissipated in that discharge will depend on the degree of charging that has occurred and the type, geometry and location of the material on which the charge has accumulated. It should be noted here that it is never a practical argument to suggest that a discharge can be avoided once charge has accumulated. If charge is there, it must be assumed that sooner or later a discharge will occur. The only exception is if the voltage is less than the minimum as determined from a Paschen curve. From the Paschen curve shown in Figure 1, it can be seen that no discharge is possible in air for voltage differences of less than about 300 V. If the discharge that could occur is more energetic than the energy required to ignite a flammable atmosphere that might be present, there is a fire or explosion hazard. It is therefore necessary to give some consideration to possible flammable atmospheres.

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PAVEY FLAMMABLE ATMOSPHERES

As has been seen, with the exception of a propagating brush discharge, electrostatic discharges in themselves are not usually dangerous. However, if they occur in a flammable atmosphere and are capable of igniting it, the combination presents a very real hazard. The issues surrounding flammable atmospheres are complex and will not be discussed in detail here, although some brief comments are essential. In order to assess ignition hazards arising from unwanted static electricity, it is necessary to compare the possible discharge energy with the energy required to ignite the flammable atmosphere. The minimum ignition energy (MIE) of a material capable of forming a flammable atmosphere is defined as the energy dissipated by a spark discharge that will just ignite the most sensitive mixture of the test material with the test oxidant—usually air. Any deviation from this worst-case mixture and the energy required to ignite it will increase until either a lower or upper explosible limit (LEL or UEL, respectively) is reached, at which point the mixture will not ignite, whatever the discharge energy. Clearly unless the mixture can be guaranteed to be outside its flammable limits, avoidance of an ignition or explosion hazard will require ensuring that no discharge with an energy greater than the material’s MIE could occur. For example, if the MIE is 15 mJ, brush discharges from insulating surfaces would not constitute a hazard. However, spark discharges and propagating brush discharges must certainly be avoided. Also, the possibility and likely energy of cone discharges arising from powder handling operations would need a process and plant-specific assessment. Many powders dispersed in air also constitute a flammable atmosphere, for which an MIE can be determined. The problem with most powders is that the combustion process is much more complex than for the gases and vapours of relatively simple molecules. Now, particle size, particle shape, water content, and other factors, as well as the material itself, all influence the MIE. For this reason, ignition sensitivity data for powders will not usually be found in the literature and, even if they are, such data cannot be used reliably for other samples of nominally the same powder. It is essential to test a sample of the actual material of interest in order to undertake a reliable electrostatic hazard assessment of a powder. Also, unlike gases and vapours, powders do not disperse. Instead they settle as layers on surfaces, which can later be lifted to produce an extensive dust cloud. Hence, although varying the concentration of dispersed powder will alter the determined ignition energy, in practice, if a flammable powder is being handled, it must usually be assumed that sometimes, somewhere, there will be a worst-case flammable atmosphere. Table 5 gives some examples of measured MIEs for different gases, vapours and powders. Comparing these values with those mentioned for the various types of discharge it can be seen that discharges from small conducting objects or insulating surfaces, which are barely perceptible to people, can readily ignite many flammable gases and vapours. Even many powders could be ignited by discharges from moderately sized pieces of equipment or personnel, which, if they were received by a person, would probably hardly be given a second thought in an industrial environment.

Table 5. Minimum ignition energies for some common materials. Gas, vapour or dust

Minimum ignition energy (mJ)

Hydrogen Methane Propane Methanol MEK Acetone Sulfur Aluminium Sugar Wheat Flour

0.016 0.21 0.25 0.14 0.53 1.15 <1 10 30 40

AVOIDANCE OF ELECTROSTATIC HAZARDS If an electrostatic hazard has been identified and the flammable atmosphere cannot be avoided, the first approach for avoiding the hazard is almost always to prevent charge accumulation. This is achieved by the effective earthing of all conducting and semiconducting (usually referred to as static dissipative) materials. Wherever possible, insulating materials, which cannot be earthed, should be exchanged for dissipative or conducting ones, which can. If that is not possible the level of charge generation might be reduced by altering the charging materials in some way. This is commonly done with liquids in which very low levels of additive can increase the conductivity significantly and prevent charge generation. However, if purity is critical, and for most other process materials, this approach may not be practical. Another approach to reducing charging might be to alter the process in some way. This is also an approach commonly used for liquid transfers into vessels. Here it is recommended that low conductivity single phase liquids should not be pumped at velocities greater than 7 m s1. Multiphase liquids in which the continuous phase is of low conductivity should not be pumped faster than 1 m s1. However, for many other operations (for example pneumatic conveying or micronising) changes to the process may not be practical. Occasionally, it may be possible to reduce charge levels in some types of process by the use of hardware known as static eliminators. Most static eliminators reduce charge by corona—directing a stream of ions of opposite polarity at the charged object, thus neutralizing it. This is commonly used where charge must be reduced to deal with problems such as material adhesion. However, where the objective is hazard avoidance a good deal of care must be exercised, and this approach, if practical at all, is probably best left to an expert in the area. If the static hazard cannot be avoided by the methods discussed above the only approach is to enclose the area in question and remove the flammable atmosphere by inert gas blanketing. The above represents the generalised approach to the avoidance of electrostatic hazards. There now follows a few specific examples of operations and situations that could be hazardous in respect of static electricity, but in which appropriate measures can be implemented. SIEVING INTO A BIN Consider the situation in which a powder product from a sieve is running down a chute and being collected in a stain-

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ELECTROSTATIC HAZARDS IN THE PROCESS INDUSTRIES less steel bin on a plastic pallet. The powder has a minimum ignition energy of 15 mJ, and is being collected at the rate of 10 kg min1. No other flammable materials are present. We know that brush discharges from insulating plastics have a maximum discharge energy of about 4 mJ. The plastic pallet, even if it were becoming charged, would therefore be no problem, in this case. Consider also the bin itself. It will be isolated from earth by the plastic pallet. Furthermore, it will be collecting powder that, using the figure for pouring (as it is running down a chute) from Table 4, could have a charge–mass ratio of 108 C kg1. At the powder transfer rate of 10 kg min1 this means charge being transferred to the bin at the rate of 107 C min1. From Table 1 the bin could have a capacitance of 200 pF. Using equation (3) this would mean the bin potential would be rising at the rate of 500 V min1. After collecting 300 kg powder in 30 min the bin potential will therefore be 15 kV. Using equation (10), a discharge from the bin at that stage, perhaps because somebody touches it as they have a look inside, would dissipate 23 mJ—more than enough to ignite the powder. The problem could be completely avoided by connecting the bin to earth using a flying lead before beginning the operation. The question then arises—how good should that connection to earth be? From the Paschen curve of Figure 1 we have seen that no discharge will occur in air if the voltage is less than 300 V. For simplicity, and to include a safety margin, it is common to round this down to 100 V. Using a rearranged equation (6) we can calculate the resistance to earth that will allow all the charge acquired to be conducted away whilst ensuring the bin voltage does not exceed 100 V, and thus guaranteeing that no discharge could occur, irrespective of energy: R¼

100
6  1010 O 1:7  109

(14)

Note that in the above calculation the charging rate of 107 C min1 has been converted to 1.7  109 A (Coulombs per second). Although the maximum safe resistance to earth calculated in equation (14) would be adequate for this specific example, it would never be used in practice. The reasoning for this, and why certain rules in respect of earthing are adopted by published guidelines, is explained below. Suppose the bin is to be earthed with a copper wire. Knowing that the resistivity of copper is about 2  108 O m, equation (7) can be used to show that the diameter of a wire to achieve the calculated maximum resistance of 6  1010 O is around 109 m (1 nm). This illustrates the fact that, in electrostatics, earth wires that are just capable of providing an appropriate connection to earth are always impracticably small. For practical purposes only, a flying earth lead in an industrial environment must be substantial and robust to ensure that it does not become damaged. If a substantial metal cable is being used to earth a metal plant item, there is also no reason why the resistance to earth should not be very low. Consequently, the practical target resistance to earth that is set for metal plant items, irrespective of the resistance actually required for the avoidance of static charge accumulation, is 10 O. Even for the highest

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imaginable charging rate, this is many orders of magnitude less than necessary. However, it can be easily measured and a higher resistance suggests there is something wrong with the metal–metal contacts that should be in place. PERSONNEL, HAND-HELD TOOLS AND MOVEABLE EQUIPMENT Common experience tells us that people acquire charge simply by moving around. If the floor, footwear or both are insulating, every step results in an exchange of charge. In a very few steps a potential of a few kV can easily be attained. Table 2 gives the energy of a spark discharge from a person as being typically 10 mJ (assuming a capacitance of 200 pF and potential of 10 kV). This is far in excess of what is required to ignite most common solvent vapours, and could even ignite many dust clouds. Small to medium sized pieces of moveable equipment probably have a similar capacitance, and if their wheels, the floor, or both are insulating, they too will become charged and could produce similar sized discharges. In both cases the problem could be avoided by the use of continuously connected earth leads, though that would be inconvenient. It is much better to ensure earthing of personnel and moveable equipment through the floor. This requires both a suitably conductive floor that is connected to earth, and suitably conductive footwear in one case, or suitably conductive wheels in the other. As before, the question arises: what is suitably conductive? Neither personnel nor equipment being moved are subjected to high charging regimes. In practice this means that a charging current in excess of 1 mA would be virtually inconceivable. It is therefore usual to calculate the maximum resistance using this current, and the static-safe voltage of 100 V, to give a value of 108 O. This is therefore the generally accepted maximum resistance through the sole of static dissipative footwear, through the wheels of moveable equipment when not in use, and from the floor surface to earth. When equipment is in use, such as a bin collecting powder, it is possible that the charge acquisition rate could be higher and a maximum current of 104 A (100 mA) is assumed. Using the same calculation, but this new higher current, it can be shown that any conductive items of plant must be earthed via a resistance that is not greater than 106 O. As already discussed, if a flying earth lead is used a resistance to earth of less than 10 O must be the target. Also, if the equipment is mains powered it should be connected to the power supply’s protective earth. This should be checked, but if it is, no additional earthing in respect of static hazards will be necessary. If, however, the equipment is to be earthed through the floor when in use, the resistance across the wheels must be less than 106 O, and the floor resistance to earth, at least locally at the point of use, must also be less than 106 O. Finally consider hand-held tools that personnel may be using, such as scoops. Even these, if isolated from earth, could produce spark discharges capable of igniting flammable solvent vapours in air, and even some dust clouds (see Table 2). This hazard could be avoided by the use of flying earth leads, but this would not always be convenient. Much more practical is to ensure that they are earthed through the

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PAVEY

person using them. Obviously if the personnel earthing requirements have been taken care of by appropriate specification of footwear and floors, holding the scoop in the bare hand would automatically ensure that it, too, would be suitably earthed. However, this has to also be true when wearing gloves. Gloves for use in hazardous areas should therefore be specified as static dissipative with a resistance between the wearer and the item being held of not more than 108 O. ADDING POWDER TO A VESSEL CONTAINING FLAMMABLE SOLVENT Many flammable solvent vapours have a minimum ignition energy of well under 1 mJ and can therefore be ignited by a brush discharge from an insulator. In this situation a powder addition via an open manway and from a standard plastic-lined keg must be considered highly dangerous. If the addition must be via an open manway the keg must be dissipative or conducting (fibre board is good enough) and positively earthed using a flying lead. Any liner must also be static dissipative and effectively earthed. As a conductor and a potential source of spark discharges, the operator must be properly earthed, preferably by standing on an earthed dissipative or conductive surface while wearing static dissipative footwear. Of course, until it is dissolved the powder constitutes a dispersed second phase. If the solvent is non-conducting, such as toluene, the result could be that the vessel contents become highly charged even when only being gently stirred. This could lead to incendive brush discharges from the liquid itself. In this latter situation the electrostatic hazard is inherent to the process and an enclosed powder addition station, such that the powder can be added to an inert gas blanketed vessel, is appropriate. LIQUID TRANSFER TO VESSELS Electrically isolated charged conductive liquids in a vessel could lead to spark discharges. To avoid this, any vessel that is lined with glass, or any other insulating material, must have an earth contact passing through the lining near the bottom of the vessel. This ensures that as soon as liquid starts to accumulate it is connected to earth. A particularly effective way of charging liquids, even those that are not insulating, is spraying or splashing. Splash filling is therefore always to be avoided by the use of dip pipes or bottom entries. The accumulation of charged insulating liquids can lead to brush discharges. Equation (13) indicates that liquid charging can be controlled by limiting pumping velocities. As already stated, the guidelines given in PD CLC=TR 50404 (2003) recommend that the velocity of single phase insulating liquids is limited to 7 m s1 when delivered to a vessel. For the high-charging situation of an insulating liquid with a dispersed second phase (solid or liquid) the maximum recommended velocity is reduced to 1 m s1. However, units such as fine filters can lead to unusually high charging because of the large interfacial area or high local velocity. In these cases it is recommended that charge is allowed to dissipate in a relaxation chamber, or suitable

length of pipe, before the liquid is discharged into a vessel. The next few paragraphs explain the choice of residence time for this relaxation zone. Consider a cuboid sample of a material with electrodes covering two opposing faces. The assembly can be viewed as a capacitance and resistance in parallel. The electrodes will have capacitance, C, as defined by equation (4), and the resistance between them will be defined by equation (7). The relaxation of charge from a parallel capacitance and resistance is well known:
t  (15) Qt ¼ Q0 exp RC where Qt is the residual charge (Coulombs) at time t (s), and Q0 the charge at time t ¼ 0. Equations (4) and (7) can be used to evaluate the time constant, RC, in terms of material properties and sample geometry. When this is done the terms relating to sample geometry cancel and equation (15) can be rewritten as:   t (16) Qt ¼ Q0 exp e0 er r where e0 is the electric constant (8.85  1012 F m1), er the relative permittivity of the material and r the resistivity of the material (O m). This charge relaxation equation is correct for all solid materials that are Ohmic (resistivity independent of voltage gradient), and often a useful approximation for others too. It can also be used for liquids that are not highly insulating. The product e0err is often referred to as the charge relaxation time for the material and may be calculated as indicated, or determined directly by a charge relaxation experiment. For liquids, where conductivity is more frequently used than resistivity, the charge relaxation time, t, is often given as: e e (17) t¼ 0 r g where g is the liquid conductivity (S m1). After three time constants, the value of the time dependent subject of an exponential decay has been reduced by 95%. For moderately conducting liquids the recommended residence time of a relaxation chamber is therefore three times the charge relaxation time for the liquid in question. Unlike solids, low viscosity liquids can transfer charge by convection as well as conduction. For highly insulating, low viscosity, liquids this leads to a much faster charge relaxation than would be predicted by equation (16). Instead, the hyperbolic relaxation of equation (18) describes the decay (Bustin and Dukek, 1983): Qvt ¼

Qv0 1 þ (mQv0 t=e0 er )

(18)

where Qvt is the charge density (C m3) at time t (s), Qv0 the charge density at time t ¼ 0, and m an apparent charge carrier mobility (m2 V1 s1). The effect of equation (18) is that, no matter how insulating the liquid, provided it is of low viscosity (about 106 m2 s1), the residence time of a relaxation zone need not exceed 100 s.

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ELECTROSTATIC HAZARDS IN THE PROCESS INDUSTRIES For insulating liquids with viscosities in the range 3  105 to 104 m2 s1, or even higher, the 100 s maximum residence time no longer applies, as these can retain charge for an hour or more (Britton, 1999). With these liquids charge relaxation may not be practical and inert gas blanketing may be the only option, if flammable solvent vapours or gases could also be present. CONCLUSION Contrary to a frequently cited misconception, static electricity is predictable. Effective measures can usually be taken to control static hazards. In those situations where effective control measures cannot be taken, a static hazard assessment should make this very clear, enabling alternative strategies to be adopted. The fundamentals of static electricity are not complicated and can be readily understood by engineers of all disciplines. As always, some help may initially be required in putting the theory into practice for undertaking static hazard assessments and setting appropriate safety measures. This is readily available. It is to be hoped that the examples given in this review effectively serve to illustrate a range of typical hazards and, more importantly, practical ways for dealing with them. Inevitably, however, more of the available advice has been omitted than included. References already cited [especially PD CLC=TR 50404 (2003), NFPA 77 (2000), and Britton (1999)] contain much more advice for particular operations. Another frequently cited source of helpful information is BS 5958 (1991a,b).

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Static electricity is a real and important cause of fires and explosions. It must be given proper consideration and effectively guarded against if operations in the presence of flammable atmospheres are to be safely carried out.

REFERENCES ATEX 137, 1999, Directive 1999=92=EC of the European Parliament and of the Council of 16 December 1999 on minimum requirements for improving the safety and health protection of workers potentially at risk from explosive atmospheres [15th individual Directive within the meaning of Article 16(1) of Directive 89=391=EEC]. Britton, L.G., 1999, Avoiding Static Ignition Hazards in Chemical Operations (American Institute of Chemical Engineers, USA). BS 5958, 1991a, Part 1: Control of undesirable static electricity—general considerations (British Standards Institution, London, UK). BS 5958, 1991b, Part 2: Control of undesirable static electricity— recommendations for particular industrial situations (British Standards Institution, London, UK). Bustin, W.M. and Dukek, W.G., 1983, Electrostatic Hazards in the Petroleum Industry (Research Studies Press, Letchworth, UK). DSEAR, 2002, The Dangerous Substances and Explosive Atmospheres Regulations 2002, UK Statutory Instrument 2002 no. 2776. Glor, M., Maurer, B. and Rogers, R., 1995, Recent developments in the assessment of electrostatic hazards associated with powder handling, in Proceedings of Loss Prevention and Safety Promotion in the Process Industry (Elsevier Science, Oxford), Vol 1, p 219. NFPA 77, 2000, Recommended Practice on Static Electricity. Paschen, F., 1889, Wied Ann, 37: 69. PD CLC=TR 50404, 2003, Electrostatics—Code of Practice for the Avoidance of Hazards due to Static Electricity (British Standards Institution, London, UK). The manuscript was received 28 May 2003 and accepted for publication after revision 11 December 2003.

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