Evaluation and Prevention of Electrostatic Hazards in Chemical Plants

Transcription

1 Evaluation and Prevention of Electrostatic Hazards in Chemical Plants Sumitomo Chemical Co., Ltd. Technology Nowadays, electrostatic theories are usefully applied to various industries. On the other hand static electricity cause ESD (electrostatic discharge) problem in high technology industries or fire and explosion in chemical industries. In order to prevent incidents caused by electrostatic discharge, it is important that operators or staffs working in chemical plants find electrostatic potential hazards and have consultations with safety experts. Electrostatic charge and discharge phenomena, countermeasures against static electricity and some methods for evaluating electrostatic hazards are described in this paper. Ω Fig. 1 Volume resistivity [Ω m] Electrical conductor Sufficient effect of grounding Moderate Moderate Insulator No effect of grounding Volume resistivity and effectiveness of grounding 2004-

2 Material A B (a) Transfer of charge by contact (b) Formation of electric double layer (c) Electrification by separation ε ε Ω Fig. 2 Mechanism of electrostatic charge 2004-

3 roll film/paper/cloth pipe/hose metal (+) fiber (+) native material (+) synthetic resin (+) (a) Frictional charging (c) Spray electrification (e) Collisional charging oil sedimentation drips drip 1 balance of charge (before splitting) liquid (b) Streaming charging (d) Peeling electrification (f) Electrification during transferring liquid oil (h) Splitting charging outside (+) ( ) (+) surfacing bubble (g) Sedimentation / Surfacing electrification 2 imbalance of charge (after splitting) outside fragment (+) Pb Zn Al Cr Fe Cu Ni Au Pt ( ) wool nylon rayon silk cotton hemp glass fiber acetate fiber vinylon polyester acrylic fiber polyvinylidene chloride ( ) asbestos hair/fur glass mica raw cotton wood human skin paper rubber celluloid cellophane ( ) ebonite polystyrene polypropylene polyethylene vinyl chloride polytetrafluoroethylene ( ) water ice 1 transfer of charge during freezing Fig. 3 electric field inside ( ) 2 internal separation of charge by complete freezing electric conductor surface induced charge insulator Example of electrostatic charge inside fragment ( ) (i) Electrification when freezing / breaking (j) Induction charging 3 electrification of each fragment by complete freezing Fig. 4 Examples of series of frictional electrification 2004-

4 Table 1 Classifications of electrostatic discharge Types of discharge Characteristics Hazards of ignition Condition for discharge Corona discharge It can occur when an electrode with radius of curvature less than 5mm experiences a not sufficiently energetic to ignite almost materials except sevral materials potential : more than several kilovolt strong electric field. which has very small minimum ignition energy such as H2, CS2. Brush discharge It can occur between a conductor with radius of curvature in the range 5 to 50mm and either another conductor or a charged insulating surface. sufficiently energetic to ignite gases and vapors and some dust cloud which has low minimum ignition energy (less than several mj) potential : more than dozens of kilovolt average of electric filed : more than V/m Cone discharge It can occur along the conical surface of the powder heap during filling of a large sufficiently energetic to ignite gases and vapors and some dust cloud which has diameter of powder : around 1~10mm silo with powder which has low conductivity. low minimum ignition energy (less than around 10 mj) Lightning-like discharge It can occur in a large container and a long spark jumps from the cloud to the sufficiently energetic to ignite gases and vapors and dust cloud average of electric field : more than V/m grounded container wall. Spark discharge transitional discharge phenomenon which lead to arc or glow discharge. sufficiently energetic to ignite gases and vapors and dust cloud electric field : more than V/m Surface discharge It can occur along the surface of thin insulating layer backed by a conductor. sufficiently energetic to ignite gases and vapors and dust cloud thickness of insulator : less than 8mm surface charge : more than 250µC/m Test conditions room temperature atmospheric pressure Upper spread in vertical tube (diameter: 5cm) about 15% about 5% Fig. 5 Methane [vol.%] E Upper flammable limit Oxygen [vol.%] Flammable limits of methane B Lower flammable limit A C Nitrogen [vol.%] Limiting oxygen concentration D About 13% 2004-

5 1 1 2 Table 2 Safety margin of oxygen concentration in case of continually monitoring in case of non-continually of oxygen concentration monitoring of oxygen concentration LOC 5% : LOC 2% LOC 5% : LOC 0.6% LOC < 5% : LOC 0.6% LOC < 5% : LOC 0.4% LOC : Limiting oxygen concentration Lower flash point flash point by JIS method [ C] Fig. 6 Tag closed Cup : flammable solids Each data was evaluated by Tag closed cup method Pensky Martens closed Cup Flash point (JIS closed cup method) [ C] Relations between flash point determined by the JIS closed cup and lower flash point 2004-

6 µ µ Fig. 7 Ignition energy [J] MIE Dust concentration [g/m 3 ] Estimating method of minimum ignition energy Ω 2000mm liquid gas 800mm Fig mm Wire mesh Cylinder (clear vinyl chloride) Electrodes Mist receiver 2 Fluid nozzle (not measured) Balance Wire mesh Mist receiver Test temp. : about 20 C Sample : aqueous solution of organic solvent (flash point 58 C) Photo of mist explosion Example of mist explosion and test apparatus 2004-

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8 Poisson s equation 2 φ 2 φ 2 φ ρ + + = (1) x 2 y 2 z 2 ε Dryer Faraday Cup (aspiration type) Energy calculation U φ : Potential [V] ρ : Space charge density [C/m 3 ] ε : Dielectric constant [F/m] U : Energy [J] E : Electric field strength [V/m] Fig. 9 1 = ε E 2 dv 2 Basic formula of electric fields (2) Dry air Wire mesh Electric field meter Probe Recorder Insulator Fig. 10 Fig. 11 Fig mm Thick paper Coaxial cable Shield Purge Flow meter Pump Measurement of charged density of dust cloud Details of faraday cup (aspiration type) Shape of paddle Filter paper Wire mesh Insulator Coaxial cable Simulation results of electric field strength

9 Ω Ω Ω Table 3 Recommendations (potential of insulator) Minimum ignition energy [mj] Indices of potential [kv] < 0.1 < ~ 1 < 5 1 ~ 10 < 10 > 10 < 10 Table 4 Results (human charge) Items Case 1 Case 2 Case 3 Case 4 Case 5 Case 6 Floor polyurethane floor + conductive grounded plate Shoes antistatic shoes Clothes antistatic working wares Maximum potential [V] 70 Electrostatic capacity of 120 human body [pf] Energy accumulated to 2.9E 04 human body [mj] polyurethane floor antistatic shoes antistatic working wares E 03 polyurethane floor + conductive grounded plate antistatic shoes polypropylene working wares polyurethane floor + conductive grounded plate slippers polypropylene working wares polyurethane floor slippers polypropylene working wares polyurethane floor slippers antistatic working wares

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