Static ElectricityA shamelessly stolen introductionAlbert NessSeptember 15, 2008

Sensory EffectsSlide * Static Electricity -David Wechsler October, 2002

Visual EffectsSlide * Static Electricity -David Wechsler October, 2002

Workplace Effects?Slide * Static Electricity -David Wechsler October, 2002

Static Electricity3 StepsGenerationAccumulationDischargeControl & Mitigation

1) Whenever two materials are placed in contact, an exchange of charge occurs between them.

2) Unless the materials are conductive and electrically bonded together, when the surfaces are separated each acquires an equal and opposite net charge. The net charges drain away to ground at a rate that depends on the conductivity of the materials.

3) Electric charges may build up on ungrounded conductive materials (such as metals) non-conductive materials (most plastics) until The electrical breakdown field of the surroundings (usually air) is exceeded and an electrostatic discharge (such as a spark) occurs. The static discharge may be of sufficient energy to ignite vapors or dust, depending on the Minimum Ignition Energy (MIE) of the ignitable mixture. A General Static Condition - Slide * Static Electricity -David Wechsler October, 2002

Generation - Streaming Current

Splash Charge+

Dumping Powder

Contact and Frictional Charging

Induction

Accumulation of Charge

Charge Out (grounded vessels)Conductive liquid Nearly instantaneousNon-conductive liquid - eventually dissipates in grounded vessels.

5 min1 min Relaxation Time30 min

Extra kicker - Examples of some nonconductive liquids that will accumulate charge and are flammable:gasolinepropanetoluenebutadienebenzenemineral spiritsstyrene monomerSlide * Static Electricity -David Wechsler October, 2002

Accumulation of Charge on EquipmentCharge on fluid places opposite charge on equipment.Conductive equipment (metal), if grounded, safely brings charge to ground.Charge on non-conductive (or non-grounded) equipment will linger on equipment.

++++Accumulated charges remain in non conductive and in non-grounded vessels.

Conductive Sections in non-conductive Line

Discharge

Flammable LimitsSlide * Static Electricity -David Wechsler October, 2002

Minimum Ignition EnergySlide * Static Electricity -David Wechsler October, 2002

Spark

Spark

Discharge

Static Electricity ControlGround all isolated conductorsReduce flow velocityInstall dip pipesMove filters further upstreamEliminate top electrodesAntistatic additives, materialsIncrease HumidityIonize the air

Grounding & BondingGrounding. The process of bonding one or more conductive objects to the ground, so that all objects are at zero (0) electrical potential; also referred to as earthing. Bonding. The process of connecting two or more conductive objects together by means of a conductor so that they are at the same electrical potential, but not necessarily at the same potential as the earth.

ExampleBondingGrounding

Slide * Static Electricity -David Wechsler October, 2002Example of a tank car dip-pipe liquid transfer system

Safe Charge Discharge from Bags+++-------Conductive BagConductive GlovesConductive ShoesConductive Floor

Prevent Static DischargeReduce charge velocityGround all isolated conductorsMove filter further upstreamInstall dip pipeEliminate top electrodes- - - - - - -- - - - - - -- - - - - - - - - - - - - - - - - - - - - - - - - - - -- - - - - - - - - - - - - - - - - - - - -- - - - - - -xyzxmicro filter

Inert EnvironmentVent N2 to a safe place!

Important Summary Points1- Ground and Bond prior to any transfer2- Non conductive fluids are more likely for static generation3- Bottom filling or top fill using dip tube less static generation4- Slow filling will generate less static than rapid filling. Magic number slow start velocity of 3 feet per second until loading lance is submerged.5- Spray filling- keep nozzles from extending into the container to avoid pulling in air and creating flammable vapor mixture.6- Inerting eliminates the potential for combustion, but does noteliminate the static. Padding with nitrogen may not achieve thesame full potential as inerting (5% or less oxygen).Slide * Static Electricity -David Wechsler October, 2002

With respect to Static electricity before you transfer a flammable liquid material from a 55 gal drum to an open container, what should you do?A. Make sure it is raining and you have rubber boots on, so static will not be generatedB. Make sure that the 55 gal. Drum and the open container are grounded and bonded C. Make sure you fill the container very slowly with a thin stream and place the fillingnozzle close to the bottom of the container.

You have just finished unwrapping a skid of material with several layers of saran wrap and are going to remove and add a combustible dust material into a reactor containing a flammable solvent. What steps should you take.A. Open the reactor, start the agitator, and dump the contents into the reactor B. Open the reactor, ground the reactor, bond the agitator, start the agitator, dump the contents and close the reactor. C. Ground the reactor, bond the agitator, open the reactor, dump the contents, close thereactor, start the agitator.

Slide * Static Electricity -David Wechsler October, 2002Quiz

With respect to Static electricity before you transfer a flammable liquid material from a 55 gal drum to an open container, what should you do?A. Make sure it is raining and you have rubber boots on, so static will not be generatedB. Make sure that the 55 gal. Drum and the open container are grounded and bonded C. Make sure you fill the container very slowly with a thin stream and place the fillingnozzle close to the bottom of the container.

You have just finished unwrapping a skid of material with several layers of saran wrap and are going to remove add a combustible dust material into the reactor containing a flammable solvent. What steps should you take.A. Open the reactor, start the agitator, and dump the contents into the reactor B. Open the reactor, ground the reactor, bond the agitator, start the agitator, dump the contents and close the reactor. C. Ground the reactor, bond the agitator, open the reactor, dump the contents, close the reactor, start the agitator.

Slide * Static Electricity -David Wechsler October, 2002Quiz - Answers

You are preparing to load a non conductive flammable solvent into a tank truck via the top manway. From a static's perspective, what two things should be done?A. Ground the tank truck and get the filling done as fast as possibleB. Ground the tank truck and fill slowly through a grounded down pipeC. Stand way back and fill the tank quickly

You are asked to load a tank car and in checking the grounding clamp, you notice that it is heavily corroded. What should your action be?A. Continue on with the loading operation, but dont use the grounding clamp.B. Continue on with the loading operation by repairing the ground clamp yourself.C. Get the ground clamp repair and then continue with the loading operation.

Slide * Static Electricity -David Wechsler October, 2002Quiz

You are preparing to load a non conductive flammable solvent into a tank truck via the top manway. From a static's perspective, what two things should be done?A. Ground the tank truck and get the filling done as fast as possibleB. Ground the tank truck and fill slowly through a grounded down pipeC. Stand way back and fill the tank quickly

You are asked to load a tank car and in checking the grounding clamp, you notice that it is heavily corroded. What should your action be?A. Continue on with the loading operation, but dont use the grounding clamp.B. Continue on with the loading operation by repairing the ground clamp yourself.C. Get the ground clamp repair and then continue with the loading operation.

Slide * Static Electricity -David Wechsler October, 2002Quiz - Answers

More InfoNFPA 77IChemE GuidesKnovel Avoiding Static Ignition Hazards in Chemical Operations Electrostatic HazardsElectrostatic Ignition of Fires and Explosions

Rub wool over plastic or rubber. The wool gives up electrons to the plastic, resulting in charge to the plastic.The ability of material to surrender its electrons or absorb excess electrons is purely a function of the conductivity of the material with which you are working. For example, a pure conductor, such as copper, has a rigid molecular construction that will not permit its electrons to be moved about freely. However, as you approach the semi-conductor range, such as some bond papers, the ability of this material to surrender its electrons is relatively easy and can be accomplished by friction, heat or pressure. As you approach the purely non-conductive materials, such as plastics, it is extremely easy to disrupt the molecular construction and cause the material to charge with the slightest friction, heat or pressure. If the conductivity of your processed material can be controlled, then, preventing static electricity becomes relatively easy. For example, adding surface conductivity to plastics will move them up into the higher conductivity range and prevent the build up of static electricity that is caused by friction. This is normally accomplished by use of additives such as moisture and anti-static sprays. The average anti-static spray is made up from a soap based material that's been diluted in a solvent, such as mild alcohol. A fire retardant is added to combat flammability of the solvent, then pressure and Freon are added, and you have your aerosol can of anti-static spray. As this spray leaves the nozzle of the aerosol can, the two Freons normally used evaporate immediately. A short time after con- tact with your material, the fire retardant and solvents evaporate leaving you with a conductive coating on the surface of the material. The plastic has now become conductive and as long as this coating is not disturbed, it will be difficult to generate static electricity in this material.

Static electricity is important enough to spend some time on. There are 3 steps to an electrostatic discharge. The charge must be generated, it must stay on an object and then a sudden discharge must occur.

Charge is generated whenever 2 dissimilar materials are brought into contact and the separated. The amount is a function of the where the two materials are relative to each other on the tribo-electric series and the strength of the contact.

Fluid flowing through pipes is one way static can be generated in a chemical plant. The faster the flow, the higher the charge.

Charge develops from splashing at surface, separating charges that are aligned there (double-layer).Charge develops for both non-conductive AND conductive liquids. Charge from splash charging can be large.Dumping powder is another common source of static charging.Requires non-conductive contact. If conductive, then charges would not stay separated, but would readily flow back to source.This slide, as well as being amusing, illustrates some principles about static charging. Lower humidity (which typically happens in the winter months, favors SE charging). Notice the men are wearing rubber boots and are walking on a carpet. The materials are very far apart on the tribo-electric series.A large charged object can INDUCE a charge on something nearby.

Next step would be to ground back half of person, removing positive charges, leaving him now charged with a negative charge. He is now a walking ungrounded conductor (will discuss more of that later).If the charge can drain out rapidly, there usually is no problem.How quickly s the charge drains is a function of the amount of charge and how conductive, or non-conductive the materials involved are. Here you see the drum is on a piece of plastic (non-conductive) which prevents the charge on the drum from draining. The wood pallet by itselfwould be semi conductive.If the charge build up enough it can break down suddenly. This chart is from NFPA 77. We will not go into the types of discharges in detail, but note that metal to metal sparks can be very powerful and ignite almost anything, dusts, vapors, gases.

The middle set of discharges are types of discharges that involve non-conductors. They range they can all ignite vapors and the more powerful can ignite dusts.

A discharge form a person, remember the Siberian static torture, is abou20-30 mJ. So you must consider operators as potential ignition sources for vapors and sensitive dusts. Here are some practical thing s you can do to control static electricity.Grounding and bonding are required to eliminate isolated conductors.Heres an example of grounding and bonding in pratice.