1. Static Electricity and Charge Accumulation

2. Static electricity & charge accumulation Definitions Types of discharges Mechanisms of charge accumulation –fluid systems - Streaming current –Solids handling Balance of charges Bonding and grounding Case studies

3. Definitions - Types of materials Conductive –A material incapable of retaining a significant electrostatic charge when in contact with earth and having a volume resistively equal or lower than 10 4 Ωm Dissipative –A material incapable or retaining a significant amount of electrostatic charge when in contact with earth and having a volume resistivity higher than 10 4 Ωm but equal to or lower than 10 9 Ωm measured at ambient temperature and 50% relative humidity. Non-conductive –A material having a volume resistivity higher than 10 9 Ωm

4. Spark discharges Discharging of static electricity between two conductors.

5. Spark Discharge Generation of Spark Discharges. –Charge accumulation at a conductive object. –Field strength exceeds the electric strength of the ambient atmosphere. Ignitability--gases, vapors, dusts Energy transfer--up to 10,000 mJ

6. Brush discharge

7. Brush Discharge Generation of Brush Discharges –Conductive 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, vapors Energy transfer--up to 4 mJ

8. Propagating Brush Discharge

9. Generation of Propagating Brush Discharge –Bipolar 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, dusts Energy transfer--up to 100,000 mJ Major contributor to static electricity ignitions.

10. Cone Discharge

11. Generation of Cone Discharge. –Vessels larger than 1 m 3. –Relatively fast filling rate, greater than 0.5 kg/s. –High resistivity (>10 10 Ωm) 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, dusts Energy transfer--up to 1000 mJ

12. Ignitability of discharges Type of DischargeEnergy transfer Ignitability Spark< 10,000 mJ gases, vapor, dusts Brush< 4 mJ gases, vapor Propagating Brush< 100,000 mJ gas, vapor, dusts Cone< 1,000 mJ gas, vapor, dusts Corona< 0.1 mJ some gases with low MIE

13. Static electricity & charge accumulation Definitions Types of discharges Mechanisms of charge accumulation –fluid systems - Streaming current –Solids handling Balance of charges Bonding and grounding Case studies

14. Charge Accumulation Whenever 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 Frictional –Double layer –Induction –Transport

15. Contact and Frictional Charging Dust transport –e.g. pneumatic transport of powders/solids Pouring powders –e.g. pouring solids down chutes or troughs Gears and belts –e.g. transporting charges from one surface to another

16. Double layer charging Caused by friction and movement at interfaces on a microscopic scale. –Liquid-liquid –Solid-liquid –Solid-solid –Gas-liquid –Gas-solid

17. Induction charging When 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.

18. Charging by Transport Results 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.

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

20. Fluid flow into vessels When 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.

21. Splash Filling When 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

22. Spraying of Liquids When 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.

23. Static electricity & charge accumulation Definitions Types of discharges Mechanisms of charge accumulation –fluid systems - Streaming current –Solids handling Balance of charges Bonding and grounding Case studies

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

25. Streaming current For fluids the streaming current, I s, is calculated using Eq. 7-12 for laminar flows.

26. Streaming current For turbulent flow, use Eq. 7-14.

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

28. Charge Accumulation from I s Charges can accumulate as a result of streaming current:

29. Static electricity & charge accumulation Definitions Types of discharges Mechanisms of charge accumulation –fluid systems - Streaming current –Solids handling Balance of charges Bonding and grounding Case studies

30. Accumulated charge from solid handling Solid 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

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

32. Capacitance

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

34. Static Energy Stored

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

36. Example – Solids handling Determine 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 m 3. Given: The powder has a bulk density of 600 kg/m 3 ; the vessel has a spherical geometry; 70 m 3 of powder is charged into the vessel. The powder is flammable with a MIE of 20 mJ.

37. Example – solids handling - solution Determine radius of sphere: Calculate capacitance:

38. Example - solids handling - solution (cont.) Determine mass fed: Calculate charge accumulated (Table 7-5)

39. 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).

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

41. Example – Fluid Handling (cont.) Additional information: –Non conducting hose length 20 ft –Hose diameter 2 in. –Liquid conductivity 10 -8 mho/cm –Liquid diffusivity 2.2x10 -5 cm 2 sec -1 –Dielectric constant 25.7 –Density 0.88 g/cm 3 –Viscosity 0.60 centipoise –MIE0.10 mJ

42. Example – fluid handling - solution Procedure Calculate voltage drop using V=I s R (Eq. 7-17) Calculate R using Eq. 7-18 Calculate I s using Eq. 7-12 or 7-14 Calculate Q using Q=I s t Calculate E=(QV/2) Compare to MIE

43. Example – fluid handling – solution (cont.) Calculate the Resistance

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

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

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

47. Static electricity & charge accumulation Definitions Types of discharges Mechanisms of charge accumulation –fluid systems - Streaming current –Solids handling Balance of charges Bonding and grounding Case studies

48. Balance of Charges When 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.

49. Charge Balance

52. This relationship is used to determine the charge developing in the tank as a function of time relative to an initial charge of Q 0. 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=Q 2 /2C. Examples 7-9 and 7-10 demonstrate using this relationship.

53. Static electricity & charge accumulation Definitions Types of discharges Mechanisms of charge accumulation –fluid systems - Streaming current –Solids handling Balance of charges Bonding and grounding Case studies

54. Bonding and Grounding Charge 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

55. Bounding and Grounding Historically 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.

56. Bonding and Grounding Bonding –Is 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 object Grounding –Is the connection of a conducting wire between a charged object and the ground. –Any charge accumulated in the system is drained off to ground.

57. Bounding and Grounding Figure 7-7 and 7-8 should say “non” conductive hose.

58. Bounding and Grounding

61. Bonding and Grounding

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

63. Dip Legs to Reduce Splash Filling To 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.

64. Static electricity & charge accumulation Definitions Types of discharges Mechanisms of charge accumulation –fluid systems - Streaming current –Solids handling Balance of charges Bonding and grounding Case studies

65. Case Studies from a production plant Following 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.

66. Solids Filling Operation Situation –A 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. Cause –All 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. Precaution –Either fill into a closed, inerted vessel or avoid charge generation.

67. Operator Situation –An 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. Cause –The 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. Precaution –Always guarantee ground connection.

68. Valve Situation –A ball-valve is installed in a waste gas collecting system. During usual production an explosion occurred; the pipe system was destroyed. Cause –A 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. Precaution –Guarantee ground connection of conductive equipment.

69. Lined metal drum filling Situation –A 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. Cause –Electrostatic 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. Precautions –Guarantee ground connection of all conductive equipment.

70. PE-drum filling Situation –A 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. Cause –Splash 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. Precaution –Install a level indicator so that an unintended release of hydrocarbons does not occur.

71. Liquid Agitation Situation –After 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. Cause –Intense 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. Precaution –Need 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.

72. Super sack filling operation Situation –A reactor vessel was purged with N 2 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. Cause –Charge 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 N 2 purging, but the fast release of the bulk product ejected toluene/dust/N 2 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. Precaution –Only packaging with sufficient antistatic treatment should be used.

73. Filter basket Situation –A 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. Cause –The 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. Precaution –In 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.

74. Maintenance of a level indicator Situation –A 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. Cause –The 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. Precautions –For 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.

75. Case Studies Those 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.