Electrostatics what safety folk should know..ppt

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					Electrostatics: what safety folk
         should know.
                     Graham Hearn
       Wolfson Electrostatics 1971-2006
Department of Electrical Engineering, University of Southampton



   Wolfson Electrostatics Limited 2006 –
           32 Church Lane, Highfield, Southampton




    Consultancy Research Development Instrumentation
                 1 day to 3 years duration
                Client base

Aerospace/avionics, agricultural, automotive,
chemicals, construction, electronics, energy
production, explosives & pyrotechnics, fine
chemicals, food production, fuel handling,
marine, military, mining, packaging, paint
manufacture and application, petrochemicals,
pharmaceuticals, printing and coatings, textiles.
Southampton
                 Fire Triangle

                                           Ignition source
                                           •Flame & glowing combustion
                                           •Hot surface
    Oxygen                                 •Welding/cutting/abrading
                                           •Friction & impact
                                           •Electrical equipment sparks/arcs
                                           •Chemical reaction
                                           •Static electricity


        Fuel (electrostatic ignition)
•Solvent above flashpoint eg. acetone FP=-20C
•Solvent below flashpoint as mist, droplets, froth etc. eg white spirit
 which can be ignited at 18C (20deg below its FP of 38C)
•Fine dispersed powder and dust
•Hybrids of the above
‘In only about 50% of accident investigations
involving fires or explosions is the source of
ignition determined with any degree of
certainty. It is because of this uncertainty in
many cases that static electricity is blamed.’
From the book ‘Sources of Ignition’ - John Bond
                 Electrostatic Hazard Analysis Flowchart (5 conditions)

                          No   Are flammable                   Flammable vapours, gases, aerosols & fine dusts.
                               atmospheres present?            Explosives & pyrotechnics.

                                                        Yes
                          No
                               Is static  electricity          Friction, contact, separation.
                               generated?                      High voltage supplies, etc.

                                                        Yes

No electrostatic fire &   No   Can electrostatic               Insulating materials, isolated conductors, the human
explosion hazard.              charge accumulate?              body.

                                                        Yes
                          No   Can electrostatic               Spark, brush & propagating brush discharges.
                               discharges occur?

                                                        Yes
                          No   Is the discharge                Compare discharge energy with minimum ignition
                               sufficient to cause ignition?   energy of flammable atmospheres.

                                                        Yes

                                Electrostatic fire & 
                                explosion hazard present.
           Analysis


Consider each condition in turn
Are flammable
atmospheres present?


              Are flammable atmospheres present?
            (sensitive to ignition by static electricity)

             material      Vapour      LEL/MEC    UEL      Minimum    Ignition   Kst/Kg


                          Flashpoint                       Ignition    Temp          

                                                           Energy                    


  Acetone                   -19C         2%       7%        1.2mJ      535C      80-100


  methanol                   11C        6.7%      36%      0.14mJ      455C        64



  Epoxy powder coating                 10g/m3    6kg/m3     15mJ       430C       200


  Instant coffee powder                150g/m3   no data    120mJ      490C        30


  kerosene                   38C        0.70%     5%       no data     210C      no data


  styrene                    30C        1.10%     8%        0.2mJ      490C        96
Ignition
    ESD
   energy
Energy


  10 mJ


10mJ
                                                                           IGNITION
                                                                             ZONE




  1.0mJ


1mJ


   MIE

  0.1mJ

0.1mJ
                       LEL                            UEL         %conc.
                                            LEL                               UEL            % Concentration
     Electrostatic ignition of solvent
               mist/aerosol




Aerosol of narrow-cut kerosene (FP=78C) igniting at room temperature
     Electrostatic ignition of surface froth




                   (a)                                 (b)

Frothy surface of narrow-cut kerosene (FP=78C) igniting at room temperature
     (a) Low energy spark causes ignition (b) fire spreads across surface
                    Dusts and powders

In general, dust clouds are less sensitive to ignition than flammable solvents
and gases.

Airborne dusts and powders are of particular concern when considering
electrostatic hazard because:

•Most industrial powders and dusts are electrically insulating in nature
•A given mass of powder represents a large total surface area and therefore
 a high capacity for the retention of electrostatic charge
•Many industrial processes handling powders (eg pneumatic conveying)
 also generate high levels of electrostatic charge.
   Flammable dust concentrations

Industrial hygiene                                      explosive range                 dust deposit




    1-10 mg/m3                                          20-2000 g/m 3                        500kg/m3
    A cloud of 40g/m3 of coal dust in air is so dense
that a glowing 25W light bulb can hardly be seen at 2m




                         2m
Flammable dust concentrations
  Ignition
  Energy

                   Ignition
  100mJ             Zone
    

  100mJ

    MIE
   10mJ


     1mJ
             MEC                 UEC

                20-2000 g/m3
Illustration of the potential hazards of even thin dust layers. A 1mm layer of dust
of bulk density 500kg/m3 (a) will generate a cloud of average concentration 100 g/m3
if dispersed in a room of height 5m (b). Partial dispersion of up to only 1m (c)
gives 500g/m3.




                                   5m


                                                                            1m

             (a)                                      (b)                                       (c)
Is static  electricity
generated?


                         Is static electricity generated?


                         •The contact and separation of solid surfaces
                          such as moving webs over rollers.
                         •The movement of personnel.
                         •The flow/movement of liquids.
                         •The production of mist or aerosols.
                         •The flow or movement of powders.
                         •Charging by induction in an electric field.
          Charging of powders

•The nature of the material comprising the particulate.
•Flow velocity.
•Mass flow rate/density (kg/m3)
•Particle size
•Composition of duct walls.
•Turbulence due to bends, constrictions etc.
•Temperature and humidity.
                      Charging of liquids

Unlike industrial powders most liquids are in electrostatic terms relatively
Electrically conductive. For this reason it is non-polar liquids such as paraffin,
gasoline, purified aromatics (toluene, xylene, etc), diesel and light oils,
which exhibit significant charge generation.
 Electrostatic charge is generated due to flow, agitation and droplet generation.
 In pipelines the level of charge generated depends principally upon:

 •The conductivity of the liquid
 •The flow velocity
 •The nature of the pipe wall
 •The presence of filters, constrictions etc.
 •The presence of an immiscible phase eg. water in solvent

 Note: The flow of gas produces little or no electrostatic charging.
Can electrostatic
charge accumulate?



         Can electrostatic charge accumulate?

           n    Non-conductive surfaces eg. plastic
           n    Electrically isolated conductive media
           n    Unearthed metal components
           n    Electrically insulating (resistive) powders
           n    Droplets/mist/aerosol
           n    Low conductivity liquid in tanks
                       Resistivity table

  Metals                          Soil     Natural       Concrete                Plastics
                                           rubber        Wood
                                         Skin                   Glass

                      Charge decay time    10μs    10ms     10s        3h


10-9      10-6      10-3        1          103       106        109       1012      1015   ohm.metres

                            Tap water                              Petrol
                                                                   Toluene
                                   Dist. water
                                                                      Diesel
                                              Alcohol                 Kerosene
                                       Conductive   Dissipative   Insulating
      Charge on the human body
n   Capacitance is usually in range150-300pF
n   Operators may be unearthed due to
    footwear, flooring, chairs etc.
n   Skin is conductive and may produce sparks
n   People are mobile and may carry hazardous
    potentials into sensitive areas
n   It may be difficult to apply grounding
n   Maximum resistance 108 ohms if practical
Can electrostatic
discharges occur?



      Can electrostatic discharge (ESD) occur?
            n   Spark discharge from ungrounded conductor
                - energy of discharge given by ½CV2
                - plant, product/material being handled, personnel, tools

            n   Brush discharge from plastic/insulator
                - may be produced by surface voltages in excess of -20kV
                - very high voltages (>50kV) may give incendive ESD

            n   Propagating brush discharge
                - plastic sheet, plastic lined conductor, pipe wall

            n   Cone discharge (bulking brush)
                -hoppers, silos, bins
        Spark and brush discharges
         +                           +
        ++
     +++   + +
    + +        +               +             +
+                   +
        Unearthed                                +
+                          +       plastic
         metal
                    +                        +
    +                          +
                                     +

          Spark                    Brush
          energy =1/2CV2           energy = 4mJ max.
       Propagating brush discharge




Thin plastic sheet with earthed            Plastic pipeline
        Metal backing

     Note: Surface must become highly charged for PBD to occur
      Cone discharge




Looking down onto top of powder heap in silo
Note: Equivalent ignition energy up to 10mJ
Is the discharge 
sufficient to cause ignition?


        Is ESD energy sufficient to cause ignition?
          n    Brush discharge <4 mJ
          n    Cone discharge 10mJ
          n    Spark discharge 1mJ (single bolt) – 100 mJ (large
               plant item)
          n    Propagating brush discharge >100 mJ

          n    Solvent vapours and gases 0.2mJ
          n    Foam/froth ignition energy 1mJ
          n    Mist ignition energy <10mJ
          n    Dust/powder ignition typically 5-500mJ
           Minimum ignition energy

The minimum ignition energy of a flammable atmosphere can be defined as
the minimum electrical spark energy required to ignite and propagate flame
either partially or totally throughout the volume of that atmosphere



     •Coarse powders                    :        20-200mJ
     •Fine powders (<50μm)              :            3-30mJ
     •Metal dusts & fine aerosols       :           0.5-5mJ
     •Flammable hydrocarbons            :           0.1-2mJ
     •Acetylene                         :          0.017mJ
     •Hydrogen                          :          0.011mJ
     •Oxygen-rich vapours               :          0.005mJ
Table of MIE values for common
  industrial solvents and gases
    Typical dust data chart
       illustrating effect of particle size




Note MIE values against particle size distribution for fructose
Examples of Hazardous Situations
Electrostatic potential developed on plastic pipe during and
                      after solvent flow




                                    Flammable range
                               (Solvent with sufficient O2)


   Insufficient oxygen                                              Insufficient fuel




                         UEL                                  LEL
         Are flammable atmospheres
                  present?
vapour
2-13%           ESD
                          Dust cloud 20-2000g/m3 in air


          air

                          Vapour 2-13% in air
Is static charge generated?

        -           -
        -           -
        -           -
        -           -
        -           -
        -           -




               ++
              +++
             ++++
            +++++
   Can electrostatic charge accumulate?
                          Type A FIBC
                                                     -
               +
                                                 -
                 +
Mobile plant                                     -        Containers
                 +
   Trolleys                                      -        Drums
                 + 
                                                 -
                          -                 -
                          -                 -
                          -                 -
                          -                 -
                  -       -                 -         +
                          -
                          -                 -
                                            -
                      -   --     FIBC       -     +       Tools
  Operators
                      -      - surface- -         +
                               -
                    -                                + 
                  -
Electrostatic discharges (ESD)?

          spark                          spark



                                         brush
                   -                -
                   -                -
           brush   -                -
        spark      -                -            spark
                   -                -
                   -
                   -                -
                                    -
                   --               -
                      -         -
                        -   -
                brush                   brush
Is the discharge energy sufficient to
           cause ignition?
                 10mJ                        5mJ



                                             4mJ
                        -                -
                        -                -
                        -                -
              15mJ      -                -         0.5mJ
                        -                -
                        -
                        -                -
                                         -
                        --               -
                           -         -
                             -   -




 Minimum ignition energy vapour = 0.2mJ, dust cloud >5mJ

				
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posted:3/3/2014
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