static electricity and charge accumulation

Static Electricity and Charge Accumulation

Static electricity & charge accumulationDefinitionsTypes of dischargesMechanisms of charge accumulationfluid systems - Streaming currentSolids handlingBalance of chargesBonding and groundingCase studies

What is static electricityA stationary electrical charge that is built up on the surface of a material

Two kinds of chargesAfter being rubbed, a plastic ruler can attract paper scraps. Ruler carries electric charge.It exerts electric force on paper.The interaction between static electric charges is called electrostatics.This chagrin method is called friction charging

Where do charges comes fromMatter is made up of atoms.atomnucleusProton (positive charge) neutron (neutral)electron (negative charge)

Where do charges comes fromIf electrons = protons neutral

If electrons > protons gaining electrons, negative chargeIf electrons < protons losing electrons, positive charge

Insulators and conductorsInsulators: materials that do NOT allow electrons to flow through them easily.Insulators can be easily charged by friction as the extra electrons gained CANNOT easily escape.

Insulators and conductorsConductors: materials that allow electrons to flow through them easily.Conductors CANNOT be easily charged by friction as the extra electrons gained can easily escape.

GroundingAn object is grounded when it is connected to the earth through a connecting wire. What is grounding?If a charged conductor is grounded, it will become neutral.

GroundingHow does grounding occur?+++++When we touch a metal ball of positive charge...electrons flow from the earth to the metal ball to neutralize the metal ball.Metal ball becomes neutral.

Similarly, if the metal ball is of negative charge... GroundingHow does grounding occur?extra electrons flow from the metal ball to the earth and the ball becomes neutral.

Why do gasoline tankers usually have metal chains at the back?When cars run, their tires and bodies are usually charged by _______. For gasoline tankers, if the accumulated charge is large enough, _______can be produced and _________ will occur if gasoline vapor is ignited. Those metal chains conduct the charge on the bodies of tankers to the _______ and avoid the danger.frictionsparksexplosionground

Definitions - Types of materialsConductiveA material incapable of retaining a significant electrostatic charge when in contact with earth and having a volume resistively equal or lower than 104mDissipativeA material incapable or retaining a significant amount of electrostatic charge when in contact with earth and having a volume resistivity higher than 104m but equal to or lower than 109m measured at ambient temperature and 50% relative humidity.Non-conductiveA material having a volume resistivity higher than 109m

Spark dischargesDischarging of static electricity between two conductors.

Spark DischargeGeneration of Spark Discharges.Charge accumulation at a conductive object.Field strength exceeds the electric strength of the ambient atmosphere.Ignitability--gases, vapors, dustsEnergy transfer--up to 10,000 mJ

Brush discharge

Brush DischargeGeneration of Brush DischargesConductive electrode moves towards a charged nonconductive object.Nonconductive lining or surface must have a breakdown voltage greater than 4 kV and a thickness greater than 2 mm.Nonconductive coating can be a layer of the powdered solid.Ignitability--gases, vaporsEnergy transfer--up to 4 mJ

Propagating Brush Discharge

Propagating Brush DischargeGeneration of Propagating Brush DischargeBipolar charging of the high resistivity material (non conducting) that is lining another conductor.Field strength exceeds the electric strength of the high resistivity material. Non conducting lining must have breakdown voltage greater than 4 kV Ignitability--gases, vapors, dustsEnergy transfer--up to 100,000 mJMajor contributor to static electricity ignitions.

Cone Discharge

Cone DischargeGeneration of Cone Discharge.Vessels larger than 1 m3.Relatively fast filling rate, greater than 0.5 kg/s.High resistivity (>1010m) bulk product, larger than 1 mm diameter.Charge accumulation in the bulk product.Field strength exceeds the electric strength of the ambient atmosphere.Ignitability--gases, vapors, dustsEnergy transfer--up to 1000 mJ

Ignitability of dischargesType of DischargeEnergy transfer IgnitabilitySpark< 10,000 mJgases, vapor, dustsBrush< 4 mJgases, vaporPropagating Brush< 100,000 mJgas, vapor, dustsCone< 1,000 mJgas, vapor, dusts Corona< 0.1 mJsome gases with low MIE

Charge AccumulationWhenever two dissimilar materials come in contact, electrons move from one surface to the other. As these materials are separated and more electrons remain on one surface than the other,one material takes on a positive charge and the other a negative charge.Mechanisms for Charge Accumulation:Contact and FrictionalDouble layerInductionTransport

Contact and Frictional ChargingDust transporte.g. pneumatic transport of powders/solidsPouring powderse.g. pouring solids down chutes or troughsGears and beltse.g. transporting charges from one surface to another

Double layer chargingCaused by friction and movement at interfaces on a microscopic scale.Liquid-liquidSolid-liquidSolid-solidGas-liquidGas-solid

Induction chargingWhen an isolated conductor is subject to a electric field a charge polarity develops on the object. If the object is grounded then the charges closest to the grounding source flows away leaving the body with a net charge of opposite sign.

Charging by TransportResults from a charged dust, liquid or solid particles settling onto a surface and transporting their charges to this new surface.The rate of charge accumulation is a function of the rate of transportation.

Fluid handling operationsMany fluid handling operations can generate static electricity. This becomes a problem when non conducting pipes (glass or Teflon lined) are used without adequate bonding.

Fluid flow into vesselsWhen fluid flows into a vessel it carries a charge with it which can build up in the tank if the tank is not properly grounded.Routine inspection of grounding minimizes the change for fire or explosion due to a spark discharge from the charged tank.

Splash FillingWhen non conducting fluids (or solids) free fall through air they pick up a significant static charge.When there is spraying or splashing static electricity can build up.This can be a source of sparks

Spraying of LiquidsWhen fluids are spayed in air a static charge can built up fairly rapidly in some fluids. Non-conducting fluids typically build up static charge more rapidly.

Streaming currentWhen a liquid or solid is flowing, there is a transfer of electrons from one surface to another as they flow past each other.

Streaming currentFor fluids the streaming current, Is, is calculated using Eq. 7-12 for laminar flows.

Streaming currentFor turbulent flow, use Eq. 7-14.

Electrostatic Voltage DropsFor flow through a non conducting pipe (glass, Teflon lined) a voltage drop can develop from flowing liquid.

Charge Accumulation from IsCharges can accumulate as a result of streaming current:

Accumulated charge from solid handlingSolid geometries are almost always ill defined, so need to be based on empirical calculations. Solid processing operations have different empirically determined charge capacities. Q=Charge Capacity X Charge Rate X time

Charge capacities solids handling Table 7-5ProcessCharge Capacity (coulombs/kg)Sieving 10-9 to 10-11Pouring10-7 to 10-9Grinding10-6 to 10-7Sliding down incline10-5 to 10-7Pneumatic transport10-5 to 10-7

Capacitance

Capacitance of Various Objects Table 7-6Object Capacitance (farad)Small scoop 5 x 10-12Bucket 10 x 10-12Barrel 100 x 10-12Person 200 x 10-12Automobile 500 x 10-12Tank Truck1000 x 10-12

Static Energy Stored

CalculationsDetermine the capacitance, C, of the object or container contents, expressed in farads or coulombs per volt.Determine the accumulated charge, Q, expressed in coulombsCompute accumulated energy, E, expressed in J or mJ.Compare to the MIE of the dust or vapor.

Example Solids handlingDetermine the potential hazard of pneumatically transporting a dry powder (dry powder with a particle size greater than 1 mm) at a rate of 30,000 kg/hr into a metal vessel which has a volume of 70 m3.Given: The powder has a bulk density of 600 kg/m3; the vessel has a spherical geometry; 70 m3 of powder is charged into the vessel. The powder is flammable with a MIE of 20 mJ.

Example solids handling - solutionDetermine radius of sphere:

Calculate capacitance:

Example - solids handling - solution (cont.)Determine mass fed:

Calculate charge accumulated (Table 7-5)

Example - solids handling - solution (cont.)Calculate energy:

This is much greater than the MIE of the powder. If there is sufficient air this would be very hazardous.This is the total charge that could go into vessel while filling. Multiple discharges would occur, certainly there would be conical pile discharges (unless grounded).

Example Fluid HandlingDetermine the voltage developed between a charging nozzle and a grounded tank and the charge accumulated during the filling process at 150 gpm.

Example Fluid Handling (cont.)Additional information:Non conducting hose length 20 ftHose diameter 2 in.Liquid conductivity 10-8 mho/cmLiquid diffusivity 2.2x10-5 cm2sec-1Dielectric constant 25.7Density 0.88 g/cm3Viscosity 0.60 centipoiseMIE0.10 mJ

Example fluid handling - solutionProcedureCalculate voltage drop using V=IsR (Eq. 7-17)Calculate R using Eq. 7-18Calculate Is using Eq. 7-12 or 7-14Calculate Q using Q=IstCalculate E=(QV/2)Compare to MIE

Example fluid handling solution (cont.) Calculate the Resistance

Example fluid handling solution (cont.)Determine type of flow (laminar or turbulent)

Example fluid handling solution (cont.)Calculate the streaming current:

Example fluid handling solution (cont.)Calculate the voltage drop, accumulated charge and energy:

Balance of ChargesWhen you have a vessel that has multiple inputs and outputs, you can determine the charge accumulation by a charge balance.Consider streaming currents in, charges carried away by flows going out, and charge loss due to relaxation.

Charge Balance

Charge BalanceThis relationship is used to determine the charge developing in the tank as a function of time relative to an initial charge of Q0.The capacitance of the vessel is calculated as before (typically assume equivalent spherical vessel).The static energy stored in the vessel is then calculated from E=Q2/2C.Examples 7-9 and 7-10 demonstrate using this relationship.

Bonding and GroundingCharge buildup is always possible when you have moving fluids or solids. The potential for discharge is always present.We can eliminate sparks if we ensure that all parts of the system are connected with a conductor

Bounding and GroundingHistorically there was little problem when piping was all copper, stainless steel or iron. The problem comes when pipes or vessels are glass or Teflon lined or made from polymers or connected with non-conducting gaskets.There has always been a problem when you are pouring either liquid or a solid through an open space i.e., a filling operation.

Bonding and GroundingBondingIs the connection of a conducting wire between two or more objects.The voltage difference between the two objects is reduced to zero, however they may have a voltage difference relative to ground or another non connected objectGroundingIs the connection of a conducting wire between a charged object and the ground.Any charge accumulated in the system is drained off to ground.

Bounding and GroundingFigure 7-7 and 7-8 should say non conductive hose.

Bounding and Grounding

Bonding and Grounding

Grounding Glass-lined VesselsGlass and plastic lined vessels are grounded using tantalum inserts or a metal probe.This is less effective if fluid has low conductivity.

Dip Legs to Reduce Splash FillingTo eliminate the static charge that builds up from a fluid free falling through air, a dip leg is used. Note hole to prevent back siphoning.An angle iron can also be used so fluid runs down the angle iron instead of free falling.

Case Studies froma production plantFollowing are a series of case studies of accidents that actually happen at BASF and Dow and shared with the SACHE Chemical Process Safety Workshop participants.

Solids Filling OperationSituationA non-conductive bulk product is fed out of 25 kg PE-bags in a vessel, in which a flammable liquid is being stirred. During shaking of the the just empty bag an ignition occurred.CauseAll handling of non-conductive solids or bulk products may generate static electricity. Due to contact charging of the sliding bulk product, both the bulk product and non conducting package materials became charged. Brush discharges form the surface of the bad ignited the vapor/air mixture.PrecautionEither fill into a closed, inerted vessel or avoid charge generation.

OperatorSituationAn operator filled a non-conductive bulk product out of 25 kg PE-bags in a solvent free mixer. Exhaust system operated. All equipment grounded, the floor was dissipative, the operator wore dissipative footwear. During pouring the product in the reaction vessel explode.CauseThe plastic wrap that held the sacks on the pallet was on the floor and the operator was standing on it. This allowed a static charge to build up in him.PrecautionAlways guarantee ground connection.

ValveSituationA ball-valve is installed in a waste gas collecting system. During usual production an explosion occurred; the pipe system was destroyed.CauseA valve consists of conductive and non-conductive parts. Conveying of dust suspensions or droplets may generate charge accumulation on the ball and/or shaft if not bonded to the grounded housing. Spark discharge from charged ball to housing caused explosion.PrecautionGuarantee ground connection of conductive equipment.

Lined metal drum fillingSituationA pure liquid was filled in a steel drum with an inner plastic liner. To avoid splash filling a short funnel was inserted in the spout. The nozzle, the drum and the weighing machine were all grounded. Despite having an exhaust system there was an explosion during drum filling.CauseElectrostatic charge generation at the surface of the non-conductive coating cannot be transferred. The funnel had sufficient capacitance was insulated from the ground by the PE lined filler cap. Spark discharge from funnel caused explosion.PrecautionsGuarantee ground connection of all conductive equipment.

PE-drum fillingSituationA mixture of water and hydrocarbon was separated; the water phase was released from time to time into a PE-drum located below the separator. During such a release a fire occurred on top of the PE-drum.CauseSplash filling the PE-drum generated charge accumulation at the wall material. The unintended release of a small amount quantity of hydrocarbon generated a flammable atmosphere in the drum and an ignition by brush discharges occurred.PrecautionInstall a level indicator so that an unintended release of hydrocarbons does not occur.

Liquid AgitationSituationAfter intense mixing, a non-conductive flammable dispersion was poured from the mixing vessel into a PE-drum just positioned below. The exhaust system was in operation, and to avoid charge accumulation a grounded rod was inserted. During drum filling a fire occurred.CauseIntense stirring of non-conductive liquids or multiphase liquids leads to charge accumulation. Splash filling in the non-conductive drum led to high charge accumulation on the inner walls of the drum and brush discharges from wall to grounded rod.PrecautionNeed to have another exhaust system and filling method since an explosive atmosphere and static electricity are formed at the same time in the same location.

Super sack filling operationSituationA reactor vessel was purged with N2 and feeding toluene was started. During the feeding operation a resin was prepared for pouring from an antistatically treated super sack via the filling port. The exhaust system was operating. Just at the beginning of pouring the bulk product into the vessel, an explosion occurred.CauseCharge build up was generated both by splash filling the liquid and pouring the bulk product. Flammable atmosphere in the gas space of the vessel was avoided by N2 purging, but the fast release of the bulk product ejected toluene/dust/N2 mixture up into the air where ignition occurred from either a spark discharge from the charged-insufficiently treated-super sack or charged operator by brush discharge.PrecautionOnly packaging with sufficient antistatic treatment should be used.

Filter basketSituationA fine pigment was conveyed pneumatically from a jet mill to a filter. The product settled in the filterhousing was set on fire and transported through the rotary valve in a silo. All conductive parts were properly grounded.CauseThe pneumatic conveying and the collection of charged fine particles usually generates high charge accumulation in filters. Extremely high charging at the rubber coating of a metal flange generated a propagating brush discharge. Settling particles were ignited and fell into the powder heap.PrecautionIn systems where high charging rates are possible, the combination of conducting and non-conducting materials must be avoided. Replace rubber gasket with a conducting one.

Maintenance of a level indicatorSituationA level indicator at a pressurized vessel was blocked. Usual maintenance procedure is the fast release of product in a pail until the connection between indicator and vessel is cleared. During such a procedure a fire occurred and two persons were injured.CauseThe release of a pressurized liquid generates highly charged droplets thus generating both an explosive atmosphere in the surrounding and brush discharges between the opened valve and the surface of the non-conducting pail used.PrecautionsFor effective cleaning a fast release is required. To avoid ignition the procedure needs to be changed to discharge the pressure in a waste gas collecting system.

Case StudiesThose who ignore history are doomed to repeat it.Those who ignore case studies are likely to repeat the same operational behavior and are doomed to experience the near miss, the serious, or the fatal accident.

static electricity and charge accumulation

Documents