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Definitions
Journal of Electrostatics, 27 (1992) 5-11 Elsevier
5
2. Definitions
In this Chapter we define a number of key expressions that are used in the Guide. The defined terms fall into three categories; those that are vital to the development of the subject (e.g. Electrical Conductivity, Flash Point), those that require a precise definition in order to clarify what is meant (e.g. Large Tank, Insulating Pipe) and those whose meaning would otherwise be unfamiliar to the non specialist reader (e.g. Water Slug, Streaming Current). The defined terms are identified throughout the Guide by capital letters at the start of each word.
Anti-static Additive A material that is added in small concentrations to a liquid in order to raise its Electrical Conductivity. Anti-static Additives may be referred to as static dissipator additives in other publications. Anti-static Hose A hose with a resistance that is low enough to dissipate electrostatic charges but too high to transmit large Stray Currents. Anti-static Hoses minimise the risk of electrostatic discharges and will not produce Inductive Sparks by the interruption of Stray Currents. Anti-static Hoses may be referred to as partly conductive or semi-conductive hoses in other publications. A more detailed description is given in Section 4.13. Apparatus Group Flammable Materials are assigned to Apparatus Groups IIa, IIb or IIc according to the ease with which they can be ignited. Materials in Group IIa are of normal sensitivity whilst those of Groups IIb and IIc are of increased sensitivity. The Apparatus Groups are assigned on the basis of either the minimum spark current needed for ignition or the minimum gap through which a flame can propagate. Apparatus Groups are defined for the analysis of hazards from electrical equipment but because there is a strong correlation between Apparatus Group and Minimum Ignition Energy, they are also useful for the evaluation of electrostatic hazards. More details on Apparatus Groups are given in Appendix A.
© ShellPetroleumMaatschappijB.V. 1991.Reproducedby permission.
Bonding The provision of a low resistance electrical connection between conductive objects without necessarily providing a connection to earth (see Appendix I for details).
Breakdown The sudden formation of a conducting discharge channel in an insulating material subjected to a strong electric field.
Charge Density The net electrical charge per unit volume.
Clean Liquid A single product or mixture of substantially similar products that contains less than 0.5% v/v of undissolved water and less than 10 mg !- 1 of suspended solids.
Conductive Hose A hose with a resistance that is low enough to dissipate electrostatic charges and also low enough to transmit large Stray Currents. Conductive Hoses may give Inductive Sparks when Stray Currents are interrupted. A more detailed description is given in Section 4.13.
Conductive Pipe A pipe that has a resistance to earth of less t h a n 10 8 ~ at all points when the ends are earthed.
Conductivity See Electrical Conductivity
Contaminated Liquid A mixture of substantially different liquids or a liquid that contains gross amounts of undissolved water a n d / o r dirt {i.e. more than 0.5 % v/v of undissolved water a n d / o r more than 10 mg 1- ~ of suspended solids ).
Dielectric Constant The electric field strength produced by a gi-en charge distribution is lower in a material than in a vacuum. The Dielectric Constant of a material is the factor by which the electric fields are reduced in that material.
Earthing The provision of a low resistance electrical connection between conductive objects and earth (see Appendix I for details).
Electrical Conductivity The Electrical Conductivity is the current density (the current per unit area) divided by the applied electric field. It describes the ability of a material to transmit electric charges and is the reciprocal of the Volume Resistivity. For historical reasons the Electrical Conductivity is generally used for liquids and the Volume Resistivity for solids.
Electromagnetic Induction See Induction (Electromagnetic)
Electrostatic Induction See Induction (Electrostatic)
Equivalent Energy The Equivalent Energy of an ignition source is the ignition energy ¢f the Flammable Atmosphere that the source is just capable of igniting.
Flammable Atmosphere An atmosphere that contains Flammable Materials and air or oxygen in proportions that will allow combustion. For this Guide, concentrations of Flammable Material that exceed 25 % of the Lower Flammable Limit are regarded as Flammable (see Section 3.2.2).
Flammable Material A material that will burn when mixed with air or oxygen in suitable proportions.
Flammable Range The range of concentrations of a Flammable Material that will burn. The Flammable Range is bounded by the Lower Flammable Limit (LFL) and the Upper Flammable Limit (UFL).
Flash Point An approximate indication of the temperature at which the equilibrium vapour concentration above a liquid reaches the Lower Flammable Limit. It is the temperature at which a propagating flame can be produced above the liquid in a standard type of apparatus. There are, however, different types of Flash Point apparatus and slightly different results are obtained with each (see Appendix A). The use of the Flash Point to indicate flammability gives rise to problems in the case of residual fuels which may slowly release dissolved flammable gases and hence produce Flammable Atmospheres at temperatures well below their Flash Points.
Hazardous Areas Areas in which a Flammable Atmosphere could be present in quantities such as to require precautions against ignition. Hazardous Areas are classified into Zones as follows: Zone 0. A Flammable Atmosphere is continuously present or present for long periods. Zone 1. A Flammable Atmosphere is likely to occur in normal operations. Zone 2. A Flammable Atmosphere is not likely to occur in normal operation and when one does occur it will exist only for a short period. Zone z. Areas in which a flammable dust is, or may be, present as a cloud during normal operations (e.g. processing, handling or cleaning) in sufficient quantity to produce a Flammable Atmosphere. Zone y. Areas not classified as Zone z in which accumulations or layers of flammable dust may be present that may, under abnormal conditions {e.g. if the layers are disturbed), give rise to a Flammable Atmosphere.
High Conductivity Liquid A liquid having an Electrical Conductivity of more than 1 000 pS m - 1.
Incendive Capable of igniting a given Flammable Atmosphere.
Induction (Electromagnetic) The generation of high potentials by the interruption of electric currents.
Induction (Electrostatic) The raising of conductive objects to high potentials by the influence of other electrostatically charged objects or materials (see Appendix D).
Inductive Sparks Sparks produced by Electromagr, ='~ticInduction.
Inerting Rendering a Flammable Atmosphere non flammable by the introduction of Inert Gas.
Inert Gas A gas that is non flammable and that contains too little oxygen to allow combustion. Note: Inert Gas doesnot support human life;the asphyxiationdangermust be carefullyconsidered.
Insulated Conductor A conductor whose Earthing or Bonding resistance is high enough to permit the accumulation of hazardous potentials. Conductors are normally considered to be insulated if their Earthing or Bonding resistance exceeds 106 or 10s depending on the MIE of the Flammable Atmosphere concerned and the available electrostatic charging current.
Insulating Flange A flange inserted into a pipeline or hose string in order to prevent Stray Currents. An Insulating Flange must maintain at least a minimum electrical resistance between the pipes or hoses to which it is connected. Minimum values in the range 103 Y~to 2.5 × 104 ~ are commonly specified.
Insulating Hose A hose with a resistance that could be too high to safely dissipate electrostatic charges. A more detailed description is given in Section 4.13.
Insulating Material A poor conductor of electricity on which charge may accumulate or which may provide a high Earthing or Bonding resistance for conductive objects. A material is considered to be insulating if it has a Volume Resistivity of more than 108 ~ m or a Surface Resistivity of more than 10l° ~ per sq. (see Appendix
E). Insulating Pipe A pipe that is made of Insulating Material and that at some point has a resistance to earth (measured as described in Appendix F) that exceeds 106 Yt even when both ends are earthed.
Large Tank A tank whose maximum diagonal dimension is greater than 5 m (the capacity of a Large Tank will generally exceed 50 m 3).
Leakage Current A current to earth (generally through an Insulating Matel~a~ or Low Conductivity Liquid) that acts to dissipate accumulated electrostatic charge.
LFL (LowerFlammable Limit) The minimum concentration of Flammable Material that will burn. Known, in some documents, as the LEL (Lower Explosive Limit).
Low Conductivity Liquid A !iquid whose Electrical Conductivity is less than 50 pS m - 1.
10
Medium Conductivity Liquid A liquid whose Electrical Conductivity is between 50 and 1 000 pS m -1.
Medium Sized tank A tank whose volume is greater than 250 1 and whose maximum diagonal dimension is less than 5 m (the capacity of a Medium Sized Tank will generally be less than 50 m3).
MIE (Minimum Ignition Energy) The MIE of a Flammable Material is the minimum spark energy needed to ignite an optimum concentration of the material using a capacitative spark under ideal conditions.
Relaxation Time The Relaxation Time indicates the time required to dissipate electrostatic charge from a liquid. It is the time required for the charge to fall to 1/e of its original value. Here, e is the base of natural logarithms and has a numerical value of approximately 2.7. The Relaxation Time (~) is inversely proportional to the Electrical Conductivity (a) of the liquid (see Appendix G) and for oil products is given approximately by ~= 18/awhere a is in pS m -1.
Residence Time The average time spent by a liquid in travelling from a charge generating component (e.g. a filter) to a location (e.g. a tank) where charge could accumulate.
Saturated Vapour A vapour in contact with its liquid and at equilibrium with it.
Small Container A container with a capacity of less than 250 1.
Stray Current A current flowing in a path other than one specifically provided for it.
Streaming Current A current produced electrostatically by the continuous motion of a charged Insulating Material or Low Conductivity Liquid.
Surface Resistivity The resistance between the opposite edges of a square on the surface of the material (see Appendix E). The surface resistivity represents the ability of a surface to transmit charge.
11
Switc]z Loading The practice of loading a low volatility liquid into a tank that previously contained a high volatility liquid. The residues of the volatile liquid can produce a Flammable Atmosphere even when the atmosphere produced by the low volatility liquid alone is non flammable. Many accidents have been related to Switch Loading.
UFL (Upper Flammable Limit) The maximium concentration of Flammable Material that will burn. Known, in some documents, as the UEL (Upper Explosive Limit).
Unsaturated Vapour A vapour with a lower concentration than the Saturated Vapour of the material at the same temperature.
Vapour The form of gas that occurs when the liquid and gaseous phases of a material are present together. Compressing a normal gas produces a rise in concentration and pressure. Compressing a Vapour merely converts some of the gaseous phase into liquid with no increase in either pressure or Vapour concentration.
Volume Resistivity The Volume Resistivity of a material is the electric field divided by the current density (the current per unit area). It is equal to the resistance between opposite faces of a cube of the material. It describes the ability of a material to transmit electric charges and is the reciprocal of the electrical conductivity. For historical reasons the Volume Resistivity is generally used for solids and the Electrical Conductivity for liquids.
Water Slug An isolated, free-falling mass of water produced, for example, during the water washing of ship's tanks.
5
2. Definitions
In this Chapter we define a number of key expressions that are used in the Guide. The defined terms fall into three categories; those that are vital to the development of the subject (e.g. Electrical Conductivity, Flash Point), those that require a precise definition in order to clarify what is meant (e.g. Large Tank, Insulating Pipe) and those whose meaning would otherwise be unfamiliar to the non specialist reader (e.g. Water Slug, Streaming Current). The defined terms are identified throughout the Guide by capital letters at the start of each word.
Anti-static Additive A material that is added in small concentrations to a liquid in order to raise its Electrical Conductivity. Anti-static Additives may be referred to as static dissipator additives in other publications. Anti-static Hose A hose with a resistance that is low enough to dissipate electrostatic charges but too high to transmit large Stray Currents. Anti-static Hoses minimise the risk of electrostatic discharges and will not produce Inductive Sparks by the interruption of Stray Currents. Anti-static Hoses may be referred to as partly conductive or semi-conductive hoses in other publications. A more detailed description is given in Section 4.13. Apparatus Group Flammable Materials are assigned to Apparatus Groups IIa, IIb or IIc according to the ease with which they can be ignited. Materials in Group IIa are of normal sensitivity whilst those of Groups IIb and IIc are of increased sensitivity. The Apparatus Groups are assigned on the basis of either the minimum spark current needed for ignition or the minimum gap through which a flame can propagate. Apparatus Groups are defined for the analysis of hazards from electrical equipment but because there is a strong correlation between Apparatus Group and Minimum Ignition Energy, they are also useful for the evaluation of electrostatic hazards. More details on Apparatus Groups are given in Appendix A.
© ShellPetroleumMaatschappijB.V. 1991.Reproducedby permission.
Bonding The provision of a low resistance electrical connection between conductive objects without necessarily providing a connection to earth (see Appendix I for details).
Breakdown The sudden formation of a conducting discharge channel in an insulating material subjected to a strong electric field.
Charge Density The net electrical charge per unit volume.
Clean Liquid A single product or mixture of substantially similar products that contains less than 0.5% v/v of undissolved water and less than 10 mg !- 1 of suspended solids.
Conductive Hose A hose with a resistance that is low enough to dissipate electrostatic charges and also low enough to transmit large Stray Currents. Conductive Hoses may give Inductive Sparks when Stray Currents are interrupted. A more detailed description is given in Section 4.13.
Conductive Pipe A pipe that has a resistance to earth of less t h a n 10 8 ~ at all points when the ends are earthed.
Conductivity See Electrical Conductivity
Contaminated Liquid A mixture of substantially different liquids or a liquid that contains gross amounts of undissolved water a n d / o r dirt {i.e. more than 0.5 % v/v of undissolved water a n d / o r more than 10 mg 1- ~ of suspended solids ).
Dielectric Constant The electric field strength produced by a gi-en charge distribution is lower in a material than in a vacuum. The Dielectric Constant of a material is the factor by which the electric fields are reduced in that material.
Earthing The provision of a low resistance electrical connection between conductive objects and earth (see Appendix I for details).
Electrical Conductivity The Electrical Conductivity is the current density (the current per unit area) divided by the applied electric field. It describes the ability of a material to transmit electric charges and is the reciprocal of the Volume Resistivity. For historical reasons the Electrical Conductivity is generally used for liquids and the Volume Resistivity for solids.
Electromagnetic Induction See Induction (Electromagnetic)
Electrostatic Induction See Induction (Electrostatic)
Equivalent Energy The Equivalent Energy of an ignition source is the ignition energy ¢f the Flammable Atmosphere that the source is just capable of igniting.
Flammable Atmosphere An atmosphere that contains Flammable Materials and air or oxygen in proportions that will allow combustion. For this Guide, concentrations of Flammable Material that exceed 25 % of the Lower Flammable Limit are regarded as Flammable (see Section 3.2.2).
Flammable Material A material that will burn when mixed with air or oxygen in suitable proportions.
Flammable Range The range of concentrations of a Flammable Material that will burn. The Flammable Range is bounded by the Lower Flammable Limit (LFL) and the Upper Flammable Limit (UFL).
Flash Point An approximate indication of the temperature at which the equilibrium vapour concentration above a liquid reaches the Lower Flammable Limit. It is the temperature at which a propagating flame can be produced above the liquid in a standard type of apparatus. There are, however, different types of Flash Point apparatus and slightly different results are obtained with each (see Appendix A). The use of the Flash Point to indicate flammability gives rise to problems in the case of residual fuels which may slowly release dissolved flammable gases and hence produce Flammable Atmospheres at temperatures well below their Flash Points.
Hazardous Areas Areas in which a Flammable Atmosphere could be present in quantities such as to require precautions against ignition. Hazardous Areas are classified into Zones as follows: Zone 0. A Flammable Atmosphere is continuously present or present for long periods. Zone 1. A Flammable Atmosphere is likely to occur in normal operations. Zone 2. A Flammable Atmosphere is not likely to occur in normal operation and when one does occur it will exist only for a short period. Zone z. Areas in which a flammable dust is, or may be, present as a cloud during normal operations (e.g. processing, handling or cleaning) in sufficient quantity to produce a Flammable Atmosphere. Zone y. Areas not classified as Zone z in which accumulations or layers of flammable dust may be present that may, under abnormal conditions {e.g. if the layers are disturbed), give rise to a Flammable Atmosphere.
High Conductivity Liquid A liquid having an Electrical Conductivity of more than 1 000 pS m - 1.
Incendive Capable of igniting a given Flammable Atmosphere.
Induction (Electromagnetic) The generation of high potentials by the interruption of electric currents.
Induction (Electrostatic) The raising of conductive objects to high potentials by the influence of other electrostatically charged objects or materials (see Appendix D).
Inductive Sparks Sparks produced by Electromagr, ='~ticInduction.
Inerting Rendering a Flammable Atmosphere non flammable by the introduction of Inert Gas.
Inert Gas A gas that is non flammable and that contains too little oxygen to allow combustion. Note: Inert Gas doesnot support human life;the asphyxiationdangermust be carefullyconsidered.
Insulated Conductor A conductor whose Earthing or Bonding resistance is high enough to permit the accumulation of hazardous potentials. Conductors are normally considered to be insulated if their Earthing or Bonding resistance exceeds 106 or 10s depending on the MIE of the Flammable Atmosphere concerned and the available electrostatic charging current.
Insulating Flange A flange inserted into a pipeline or hose string in order to prevent Stray Currents. An Insulating Flange must maintain at least a minimum electrical resistance between the pipes or hoses to which it is connected. Minimum values in the range 103 Y~to 2.5 × 104 ~ are commonly specified.
Insulating Hose A hose with a resistance that could be too high to safely dissipate electrostatic charges. A more detailed description is given in Section 4.13.
Insulating Material A poor conductor of electricity on which charge may accumulate or which may provide a high Earthing or Bonding resistance for conductive objects. A material is considered to be insulating if it has a Volume Resistivity of more than 108 ~ m or a Surface Resistivity of more than 10l° ~ per sq. (see Appendix
E). Insulating Pipe A pipe that is made of Insulating Material and that at some point has a resistance to earth (measured as described in Appendix F) that exceeds 106 Yt even when both ends are earthed.
Large Tank A tank whose maximum diagonal dimension is greater than 5 m (the capacity of a Large Tank will generally exceed 50 m 3).
Leakage Current A current to earth (generally through an Insulating Matel~a~ or Low Conductivity Liquid) that acts to dissipate accumulated electrostatic charge.
LFL (LowerFlammable Limit) The minimum concentration of Flammable Material that will burn. Known, in some documents, as the LEL (Lower Explosive Limit).
Low Conductivity Liquid A !iquid whose Electrical Conductivity is less than 50 pS m - 1.
10
Medium Conductivity Liquid A liquid whose Electrical Conductivity is between 50 and 1 000 pS m -1.
Medium Sized tank A tank whose volume is greater than 250 1 and whose maximum diagonal dimension is less than 5 m (the capacity of a Medium Sized Tank will generally be less than 50 m3).
MIE (Minimum Ignition Energy) The MIE of a Flammable Material is the minimum spark energy needed to ignite an optimum concentration of the material using a capacitative spark under ideal conditions.
Relaxation Time The Relaxation Time indicates the time required to dissipate electrostatic charge from a liquid. It is the time required for the charge to fall to 1/e of its original value. Here, e is the base of natural logarithms and has a numerical value of approximately 2.7. The Relaxation Time (~) is inversely proportional to the Electrical Conductivity (a) of the liquid (see Appendix G) and for oil products is given approximately by ~= 18/awhere a is in pS m -1.
Residence Time The average time spent by a liquid in travelling from a charge generating component (e.g. a filter) to a location (e.g. a tank) where charge could accumulate.
Saturated Vapour A vapour in contact with its liquid and at equilibrium with it.
Small Container A container with a capacity of less than 250 1.
Stray Current A current flowing in a path other than one specifically provided for it.
Streaming Current A current produced electrostatically by the continuous motion of a charged Insulating Material or Low Conductivity Liquid.
Surface Resistivity The resistance between the opposite edges of a square on the surface of the material (see Appendix E). The surface resistivity represents the ability of a surface to transmit charge.
11
Switc]z Loading The practice of loading a low volatility liquid into a tank that previously contained a high volatility liquid. The residues of the volatile liquid can produce a Flammable Atmosphere even when the atmosphere produced by the low volatility liquid alone is non flammable. Many accidents have been related to Switch Loading.
UFL (Upper Flammable Limit) The maximium concentration of Flammable Material that will burn. Known, in some documents, as the UEL (Upper Explosive Limit).
Unsaturated Vapour A vapour with a lower concentration than the Saturated Vapour of the material at the same temperature.
Vapour The form of gas that occurs when the liquid and gaseous phases of a material are present together. Compressing a normal gas produces a rise in concentration and pressure. Compressing a Vapour merely converts some of the gaseous phase into liquid with no increase in either pressure or Vapour concentration.
Volume Resistivity The Volume Resistivity of a material is the electric field divided by the current density (the current per unit area). It is equal to the resistance between opposite faces of a cube of the material. It describes the ability of a material to transmit electric charges and is the reciprocal of the electrical conductivity. For historical reasons the Volume Resistivity is generally used for solids and the Electrical Conductivity for liquids.
Water Slug An isolated, free-falling mass of water produced, for example, during the water washing of ship's tanks.