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STATIC ELECTRICITY AND CLEANING OF EQUIPMENT

Presented by:Dhairya MehtaShamel MerchantShashank MaindarkarManish Medar

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OVERVIEWWhat is Static ElectricityMajor Sources in IndustrySome Calculations for Spark IgnitionMinimum Ignition EnergyHazard AssessmentPrecautions to be takenCase Study

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What is Static Electricity?Electrical Imbalance on the surface of a

materialTransfer of Electrons Causes Spark ignition, which under

favorable (?) conditions can lead to explosion

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Major Sources of Static in Industry

Use of Power / Conveyor beltsPulverized materials / dusts

pneumatically transportedThe flow of fluids through pipes or

conduits, or from orifices into tanks or containers

The flow of gases from orificesThe use of rubber-tyred vehicles

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Typical NumbersCharge production in typical operations for non-accumulators[C/kg]:

Non-Accumulators: Conductivity < 50 pico - mhos/cm

Sieving: 10-11 - 10-9

Pouring: 10-9 - 10-7

Micronising: 10-7 - 10-4

Pneumatic Transport: 10-6 - 10-4

Calculation example:Pouring operation of 100 kg Product: (10-8 C/kg)Charge on product: 10-6 CSpark energy: E = 0.5 x C x VC = 10-6 C; V = 10 kV (typical value for spark discharge) Then discharge energy E = 5 mJ

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Minimum Ignition Energy

Ene

rgy

for

Igni

tion

C o ncen tra tion (v.p ; tem p )

M in (ox ygen)

M in (a ir)

MIE the minimum energy that can ignite a mixture of a specified flammable material with air or oxygen, measured by a standard procedureTypical minimum ignition energy values for Combustible Vapors:Soot: > 4000 mJ Natural products: > 10 mJ Organic chemicals: 1-10 mJ Aluminium, Sulphur: < 1mJ Methane: 0.3 mJ Carbon disulphide: 0.068 mJ Hydrogen: 0.012 mJ

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Electrostatic Hazard Assessment - logic

◦ Is there a flammable atmosphere?◦Will charge be generated?◦Can charge accumulate?◦ Is the field strength high enough to

breakdown the surrounding air?◦ Is there sufficient energy to ignite the

flammable atmosphere – discharge type?If the answer is YES, then there is a risk of

ignition!

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General Means of Control Bonding and earthing of stationary

conductive equipment.

Increasing the conductance of floors, footwear, wheels and tyres

Increasing the conductivity of non-conductors

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Fig. Filling a Tanker with a Flammable Liquid

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PrecautionsMINIMISE CHARGINGFluids

◦ Keep Flow Velocity Low◦ Avoid 2nd Phase entrainment◦ Avoid Pumps, Filters etc. specially near vessels

AVOID CHARGE ACCUMULATIONEarth All Conducting Parts - e.g. plant & itemso Generally <10 ohmo Special cases <106 ohm

Earth Personnel

MAXIMISE CHARGE DISSIPATIONLiquidsIncrease Conductivity (e.g. ‘improver’ Stadis 450)

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Barton Solvents Wichita facility Case - Study

Flammable Liquid: VM & P NaphthaIncident: While Transferring VM & P

Naphtha to a storage tank, an explosion occurred. (July, 2007)

Investigated by: U.S. Chemical Safety and Hazard Investigation Board

Main Cause: Static Electricity Spark Ignition

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Key Findings

Tank Contained ignitable vapor-air mixture in the head space Stop-start filling, trapping air in the transfer piping The tank had a liquid level gauging system float with a loose linkage The MSDS for the VM&P naphtha involved in this incident did not adequately communicate the explosive hazard.

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RecommendationsAdd a Nonflammable, Nonreactive (inert)

gas to tank head SpaceModify or Replace Loose Linkage tank

level floats Use Anti-Static Additives Reduced flow (Pumping) Velocity

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CLEANING OF EQUIPMENT IN PROCESS INDUSTRIES

Main methods:

1. Chemical

2. Mechanical

3. Combination of chemical and mechanical

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Chemical cleaning to removeDeposits build up due to:

1. Carbonaceous or organic structure molecules

2. Algae and slime organisms

3. Degradation deposits

4. Preoperational deposits

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CHEMICAL CLEANING SOLVENTS• Alkaline Cleaners - Degreasing of metal surface - Caustic Soda-Surfactant - Caustic Soda plus potassium permanganate (for sulfide

deposits)• Organic Acids - Remove oxides, mill scale and other impurities - Monoammoniated Citric Acid Citric Ion – Chelating agent for iron (pH 3.5)• Inorganic Acids - Remove water side deposits, iron oxides and calcium

scales - Inhibited Muriatic Acid (HCl), Inhibited Sulphuric Acid

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CHEMICAL CLEANING SOLVENTS• Organic Solvents - Removal of grease and oil spots - Spent solution might be recyclable - M-Pyrol for PVC Reactors• Complexing, chelating or sequestering agents - React with hardness ions, forming water soluble

complexes - Expensive but selective complexation - Ease and safety - EDTA, gluconates and polyphosphonates

Solvent Cleaning Methods• Circulation - Circulation of solvent • Cascade Method - Used for towers - Chemical pumped through reflux line

and cascade down over trays and interior tower

- Soils at bottom of tray –unremoved - High pumping capacity of solvents

Solvent Cleaning Methods• Fill and Soak - Vessel filled with solvent and let to soak - 15 min – 1 hour - Proper flushing to remove loose soil - Vent – to remove gases produced during reaction between soil and

deposit• On stream Cleaning - Steam Vapor phase cleaning – solvents introduced at high

pressure, soils carried with vapor - Foam Cleaning – Foamed solvent solution to increase contact time - Foam also has characteristic property of reducing static electricity - Cost effective compared to fill and soak - Aeration reduces total weight – important when structural integrity

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Solvent Cleaning Methods• Gel Cleaning - Similar to foam cleaning - Gel type cleaning agent sprayed or brushed

on surface - Remove iron oxide prior to painting - Eg. naval gel• Pickling and passivating - Agents act as corrosion inhibitors and

passivators - CS use HCl, Alloy use ammoniated citric acids

Mechanical MethodsWater jetting - Hyperblasting water used at 1000-10000 psi - Sheer force remove deposits - Consists of lances and specially designed nozzle - Extremely dangerous

Mechanical Methods

Hydrodrilling, Plugs, Crawlers - Special drills used with water to cut

through heavy deposits in tube walls - Water –Lubricant and flushing

Case StudiesXerox670000 pounds of emissions Replacement of chlorinated solvents with

citric acidReduction of emissions by 90%Saves USD 40000/yr in hazardous waste

disposal

Case StudiesDuPont-MerckInstalled integrated ultrasonic transducer

and rod-shaped tubular ultrasonic resonator

Eliminates the need to buy cleaning solvents and to dispose of solvent waste

Vapor emissions on site were reduced by 80%

8 cleaning cycles to pay for the system

Case StudiesParr PaintsInstalled high pressure systemReduction in latex adhering to wallsCost of high pressure unit, $800. Savings in waste disposal, $3000/yr. Payback, 1.7 months

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