Designs to Prevent Firesand Explosions

Chapter 7

The specific topics include

inerting,

static electricity,

controlling static electricity,

ventilation,

explosion-proof equipment and instruments,

sprinkler systems, and

miscellaneous design features for preventing fires and explosions.

lnerting

Inerting is the process of adding an inert gas to acombustible mixture to reduce the concentration of oxygenbelow the limiting oxygen concentration (LOC).

The inert gas is usually nitrogen or carbon dioxide, althoughsteam is sometimes used. For many gases the LOC isapproximately 10%, and for many dusts it is approximately8%.

Inerting begins with an initial purge of the vessel with inertgas to bring the oxygen concentration down to safeconcentrations. A commonly used control point is 4% belowthe LOC, that is, 6 % oxygen if the LOC is 10%.

After the empty vessel has been inerted, the flammablematerial is charged. An inerting system is required tomaintain an inert atmosphere in the vapor space above theliquid. Ideally this system should include an automatic inertgas addition feature to control the oxygen concentrationbelow the LOC.

This control system should have an analyzer tocontinuously monitor the oxygen concentration inrelationship to the LOC and a controlled inert gas feedsystem to add inert gas when the oxygen concentrationapproaches the LOC. More frequently, however, theinerting system consists only of a regulator designed tomaintain a fixed positive inert pressure in the vaporspace; this ensures that inert gas is always flowing outof the vessel rather than air flowing in.

There are several purging methods used to initiallyreduce the oxygen concentration to the low set point:vacuum purging, pressure purging, combined pressure-vacuum purging, vacuum and pressure purging withimpure nitrogen, sweep-through purging, and siphonpurging.

STATIC ELECTRICITY A common ignition source within chemical plants is sparks

resulting from static charge buildup and sudden discharge.

Static electricity is perhaps the most elusive of ignitionsources. Despite considerable efforts, serious explosions andfires caused by static ignition continue to plague the chemicalprocess industry.

The best design methods for preventing this type of ignitionsource are developed by understanding the fundamentalsrelevant to static charge and by using these fundamentals todesign specific features within a plant to prevent theaccumulation of static charge or to recognize situations wherethe buildup of static electricity is inevitable and unavoidable.

For unavoidable static buildup design features are added tocontinuously and reliably inert the atmosphere around theregions where static sparks are likely.

Fundamentals of Static Charge Static charge buildup is a result of physically separating a poor

conductor from a good conductor or another poor conductor. When different materials touch each other, the electrons move

across the interface from one surface to the other. Upon separation, more of the electrons remain on one surface than

on the other; one material becomes positively charged and theother negatively charged.

If both the materials are good conductors, the charge buildup as aresult of separation is small because the electrons are able to scurrybetween the surfaces.

If, however, one or both of the materials are insulators or poorconductors, electrons are not as mobile and are trapped on one ofthe surfaces, and the magnitude of the charge is much greater.

Household examples that result in a buildup of a static charge arewalking across a rug, placing different materials in a tumble dryer,removing a sweater, and combing hair. The clinging fabrics andsometimes audible sparks are the result of the buildup of staticcharge.

Static electricity is an imbalance of electriccharges within or on the surface of a material.The charge remains until it is able to moveaway by means of an electric currentor electrical discharge.

An electrostatic discharge occurs when twomaterials at different potentials or polaritiescome close enough together to generate a chargetransfer. In an explosive environment this suddentransfer of charges may be energetic enough tobe an ignition source. To prevent these ignitions,one must understand

(1) how charges accumulate on objects,

(2) how charges discharge by means of chargetransfer, and

(3) how to estimate the resulting energydischarged in relation to the minimum ignitionenergy (MIE) of the explosive environment.

Charge Accumulation

There are four charge accumulation processesthat are relevant to dangerous electrostaticdischarges in a chemical plant

Contact and frictional charging

Double-layer charging

Induction charging

Charging by transport

Electrostatic DischargesA charged object can be discharged to a ground or toan oppositely charged object when the field intensityexceeds 3 MV/m (breakdown voltage of air) or whenthe surface reaches a maximum charge density of2.7 10-5 C/m2 by six methods:

(1) spark,

(2) propagating brush,

(3) conical pile (sometimes known as Maurerdischarge),

(4) brush,

(5) lightning-like, and

(6) corona discharges.

A spark discharge (Figure 7-9) is a dischargebetween two metallic objects. Because bothobjects are conductive, the electrons move toexit at a single point of the charged object,and they enter the second object at a singlepoint. This is therefore an energetic spark thatcan ignite a flammable dust or gas.

A propagating brush discharge (Figures 7-9and 7-10) is a discharge from a groundedconductor when it approaches a chargedinsulator that is backed by a conductor. Thesedischarges are energetic, and they can igniteflammable gases and dusts. Data show thatpropagating brush discharges are not possibleif the breakdown voltage of the insulator is 4kV or less4

A conical pile discharge (Figure 7-9) is a form of abrush-type discharge that occurs at the conicalsurface of a pile of powder.

The necessary conditions for this discharge are

(1) a powder with a high resistivity (>1010 ohm m),

(2) a powder with coarse particles (>1 mm indiameter),

(3) a powder with a high charge to mass ratio (forexample, charged by pneumatic transport), and

(4) filling rates above about 0.5 kg/s. These arerelatively intense discharges with energies up toseveral hundred milli joules; therefore they can igniteflammable gases and dusts.

A brush discharge (Figure 7-9) is a dischargebetween a relatively sharp-pointed conductor(radius of 0.1-100 mm) and either anotherconductor or a charged insulated surface.

This discharge radiates from the conductor ina brush-like configuration. This discharge isless intense compared with the point-to-pointspark discharge, and it is unlikely to ignitedusts.

However, brush discharges can igniteflammable gases.

Lightning-like discharges (Figure 7-9) aredischarges from a cloud in the air over thepowder. It is known from experiments thatlightning-like discharges do not occur invessels with volumes less than 60 m3 or insilos with diameters less than 3 m.6 There iscurrently no physical evidence that lightning-like discharges have resulted in industrialdeflagrations.

A corona discharge (Figure 7-11) is similar to abrush discharge. The electrode conductor hasa sharp point. The discharge from such anelectrode has sufficient energy to ignite onlythe most sensitive gases (for example,hydrogen).

Energy from Electrostatic Discharges

The energy generated in electrostatic dischargescompared with the minimum ignition energies ofgases and vapors and dusts is illustrated in Figure7-12.

In general, the results illustrate that flammablegases and vapors can be ignited by spark, brush,conical pile, and propagating brush dischargesand that flammable dusts can be ignited only bysparks, propagating brush, and conical piledischarges. The regions enclosed by the dottedlines in Figure 7-12 indicate regions ofuncertainty.

Controlling Static Electricity

Charge buildup, resulting sparks, and theignition of flammable materials is aninevitable event if control methods are notappropriately used. In practice, however,design engineers recognize this problem andinstall special features to prevent

(1) sparks by eliminating the buildup andaccumulation of static charge and

(2) ignition by inerting the surroundings.

General Design Methods To Prevent Electrostatic Ignitions

The design objective is to prevent the buildup ofcharges on a product (liquid or powder) as wellas on surrounding objects (equipment orpersonnel

Three methods are used to achieve this objective:

1. Prevent charges from accumulating to dangerouslevels by reducing the rate of charge generationand increasing the rate of charge relaxation. Thismethod is generally used when handling liquids.

2. Prevent charges from accumulating todangerous levels by designing the system toinclude charge reduction by means of low-energy discharges. This method is generallyused when handling powders.

3. When dangerous discharges cannot beeliminated, then prevent the possibility of anignition by maintaining oxidant levels below thecombustible levels (inerting) or by maintainingfuel levels below the LFL or above the UFL.Measures to mitigate the consequences of anexplosion are also options for consideration (forexample, deflagration venting and explosionsuppression).

Sparks are prevented by grounding andbonding. This procedure prevents two metallicobjects (close to each other) from havingdifferent potentials. Grounding and bondingare used especially to prevent the existence ofisolated metal parts or objects. Isolatedobjects are notorious for building up largepotentials and energetic sparks when they areapproached by another conductor at a lowerpotential.

Propagating brush discharges are preventedby keeping the nonconductive surfaces orcoatings thin enough or conductive enough tohave a breakdown voltage below 4 kV. Thesedischarges are also prevented by keeping themetallic backings grounded, to eliminate theaccumulation of a high-density charge on themetallic interface and a countercharge on thenonconductor surface.

Conical pile discharges are prevented byincreasing the conductivity (additives), bydecreasing the charge rate below 0.5 kg/s, orby using containers with a volume less than 1m3. The most effective way of preventingignitions from conical pile discharges isinerting.

Brush discharges are prevented by keeping thenonconductive surfaces thin enough orconductive enough to have a breakdownvoltage (Ud) of 4 kV. Nonconductive coatingswith athickness greater than 2 mm, however,are capable of brush discharges even with a U,less than 4 kV.

Lightning-like discharges are prevented bykeeping the vessel volume to less than 60 m3

or the vessel diameter to less than 3 m. If thiscondition is not met, then the system needs tobe inerted.

Bonding and Grounding

Dip Pipes

Handling Solids without Flammable Vapors

Charging solids with a nongrounded andconductive chute can result in a buildup of acharge on the chute. This charge canaccumulate and finally produce a spark thatmay ignite a dispersed and flammable dust.

Solids are transferred safely by bonding andgrounding all conductive parts and/or by usingnonconductive parts (drum and chute). SeeFigure 7-22.

Handling Solids with Flammable Vapors

A safe design for this operation includesclosed handling of the solids and liquids in aninert atmosphere (see Figure 7-23).

For solvent-free solids the use ofnonconductive containers is permitted. Forsolids containing flammable solvents, onlyconductive and grounded containers arerecommended.

Explosion-Proof Equipment and lnstruments

Explosion-Proof Housings

Area and Material Classification

The classes are related to the nature of theflammable material:

Class I: Locations where flammable gases orvapors are present.

Class II: Same for combustible dusts.

Class III: Hazard locations where combustiblefibers or dusts are present but not likely to bein suspension.

The groups designate the presence of specificchemical types. Chemicals that are groupedhave equivalent hazards:

Group A: acetylene

Group B: hydrogen, ethylene

Group C: carbon monoxide, hydrogen sulfide

Group D: butane, ethane, ethyl alcohol

Group E: aluminum dust

Group F: carbon black

Group G: flour

Division designations are categorized inrelationship to the probability of the materialbeing within the flammable or explosiveregions:

Division 1: Probability of ignition is high; that is,flammable concentrations are normallypresent.

Division 2: Hazardous only under abnormalconditions. Flammable materials are normallycontained in closed containers or systems.

Sprinkler Systems

Sprinkler systems are an effective way tocontain fires. The system consists of an arrayof sprinkler heads connected to a watersupply. The heads are mounted in a highlocation (usually near ceilings) and disperse afine spray of water over an area whenactivated.

Sprinkler system types Antifreeze sprinkler system: a wet pipe system that

contains an antifreeze solution and that is connected to awater supply.

Deluge sprinkler system: open sprinklers and an emptyline that is connected to a water supply line through avalve that is opened upon detection of heat or aflammable material.

Dry pipe sprinkler system: a system filled with nitrogen orair under pressure. When the sprinkler is opened byheat, the system is depressurized, allowing water to flowinto the system and out the open sprinklers.

Wet pipe sprinkler system: a system containing waterthat discharges through the opened sprinklers via heat.