Safety & Environment From Industrial Gases

7/27/2019 Safety & Environment From Industrial Gases

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262 INDUSTRIAL GASES

To illustrate how gas ballast works, consider a pumping workloadconsisting entirely of water vapour. This cannot be exhausted to atmosphereunless the pump is run at over 100C. At high throughputs, the pump oilwould soon be reduced to an opaque emulsion with a high vapour pressure,reducing its ultimate pressure. Contrast the situation with nitrogen ballastadded at 100 times the inlet water vapour rate. The pump can now run at asafer 30C and exhaust to atmosphere a nitrogen gas with 10 mbar watervapour, since the saturated vapour pressure of water is only 42 mbar.

2.5 Safety and environmental issuesThere is a justif iably greater emphasis on safety today than existed even 10 or20 years ago. 37 The larger gas suppliers all have technical safety departmentswho will be pleased to help any customer with safety issues, often free of charge. If there is in any doubt whatsoever about safety in a gas installationthen professional advice of this sort should be sought.

In many cases, before the gas suppliers will connect up large supplies of gas, or dangerous or unusual gases, they will frequently insist on qualifying

the user and his equipment as suitable. Occasionally they will perhaps carrythis as far as a safety audit or HAZOP: two species of precise analysis of thefailure modes, their probabilities and consequences for a plant or piece of equipment.

2.5.1 Where does gas go in an accident?

The most frequent cause of a gas accident is gas escaping from itscontainment, either into another part of a process or to the atmosphere.

37 Who today would inhale a lungful of heating gas and proceed to blow it down a Bunsenburner with the burner lit up? Yet this sort of lunatic party trick, which could easily kill theperpetrator, was regularly tried by schoolboys in England earlier in the century. Even officialwork was not so conscious of safety issues in the past. A common lecture demonstration was thefilling of a heavy glass gas-jar with a stoichiometric mix of acetylene and oxygen. The audiencewould be rendered temporarily deaf by an ear-splitting detonation, whilst the gas-jar, inside alength of iron pipe, was reduced to a heap of powdered glass. A remote concrete bunkerequipped with a robot to ignite the gas would probably be de rigueur today.


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Where the gas goes in the event of a leak or equipment failure is often the

most fundamental question in a hazard analysis. Is the gas lighter or heavierthan air? Is there a pit in the floor or a trap in the ceiling where accumulationcould take place? Is there a source of ignition in the place of accumulation?If an operator were rendered unconscious by the gas, would he fall into azone of more danger or a zone of less danger?

That gas does sometimes get into very peculiar places is not in doubt.Much has now been written about the percolation of radon gas fromuranium in some granitic areas such as Cornwall in England. Huge increasesin lung cancer were related to radon inhaled by the occupants of houses withstone floors. Less well known (Hendrick and Sizor, 1992) are incidents dueto old mine workings, some hundreds of years old. Mine workings undergobreathing, i.e. exhaling the carbon dioxide- and oxygen-depleted aircontained in them, during excursions in atmospheric pressure. In some areasof northern England this gas, picturesquely known as stythe, is seen inhouses above the workings even years after closures, and can be dangerous.

Nitrogen gas from cryogenic leaks flows downward; although is lighterthan air at the same temperature, the low temperature of means that itwill be denser than air. This has led to fatal asphyxia accidents whenattempts have been made to fix cryogenic leaks occurring in pits or inenclosed buildings. Strong ventilat ion, i.e. opening doors and windows andif possible a powerful fan, is needed to clear such gas accumulations byblowing in fresh air if an enclosed or pit area must be entered.

Finally, along with elaborate precautions to ensure safety with gasesshould go a reasonable degree of prudence with respect to the more ordinaryaccidents it is possible to have with gas equipment. Probably the mostcommon accident in the gas industry needs no more than an empty cylinderand a little ordinary carelessness: the toppled gas cylinder. A gas cylinder issufficiently heavy (up to 100kg or more, depending on the exact size andcontents) to be capable of spraining ankles, breaking toes and causingunpleasant flesh wounds, irrespective of its contents. Gas cylinders shouldbe secured to prevent toppling when not actually being moved.

2.5.2 Toxic and asphyxiant gases

When asked about gases, heating gases usually come to mind first. Secondthoughts might revolve around the poisonous nature of some gases. 38 It is anunfortunate fact that very few gas mixtures are safe to breathe. Even gaseswhich are not actually toxic may have other undesirable consequences and

38 My grandfather saw chlorine gas used on the Western Front in Belgium in the First WorldWar and talk of gases would quickly remind him of this, whilst millions of not-so-old peopleremember the gas masks that were carried, but fortunately never used, in the Second WorldWar.


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264 INDUSTR IAL GASES

all gas mixtures containing insufficient oxygen will cause asphyxia. Someexamples of typical classifications of gases and their toxic effects, giving theUSA National Institute for Occupational Safety and Health TLV (thresholdlimit value) or IDLH (immediately dangerous to life and health value) aregiven in Table 2.14. The TLV is the maximum concentration permittedduring the working day and is similar to the OES (occupational exposurelimi t) for 8 h TWA (time weighted average). As can be seen, descriptions donot follow a totally consistent pattern. Butane is probably a more powerfulanaesthetic than for example, whilst highly toxic chlorine has a higherIDLH than merely toxic ozone. Some of these discrepancies reflectgeneral practice. For example, ozone is rarely used in concentrations higherthan a few per cent, so leaks in practice are less serious than leaks of 100%

There are large numbers of gases that are so dangerously reactive or toxicthat most countries insist on special licenses for businesses that use them.The military poison gases (actually, most of them are liquids with anappreciable vapour pressure at room temperature) are some of these. Aftera initial crude attack in 1914 with cylinders of liquid chlorine, the deadlybusiness of war moved on to expressly designed chemical poison moleculessuch as phosgene Lewisite and mustard gas. These fair ly simplecompounds do have some peaceful uses. Phosgene is a relatively commonchemical intermediate and other materials of this type are occasionallyrequired in small quantities for scientific research purposes. Mustard gas, forexample, can be used to induce mutations in cell genetic material.

The organophosphorus poison nerve gas compounds Sarin and Tabunare much more complex molecules that work by interrupting normal


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GAS TECHNOLOGY 265

biological cell function. They inhibit the enzyme cholineresterase used in

biological nerve systems and are extremely dangerous. They are related tothe organophosphorus insecticides developed in the 1930s and almostunused in war, save for some incidents in the Iran/Iraq war and a recentterrorist attack in Tokyo. Their ability to interrupt particular biologicalreaction sequences means that there may be occasions in which milligramamounts are needed in biochemical research.

There is general agreement worldwide about toxicity of gases. However,in each country there are separate and different regulations about TLVs andIDLHs, and levels vary somewhat, as do the descriptions applied to gases.

Double-pipe containment. Double-pipe containment is occasionally usedfor highly toxic gases. Longer runs of pipe, usually those in between thesource cylinders and the equipments using the gas, are fabricated in twoconcentric pipes. The normal gas path is down the inner pipe. The outer pipeis purged continously with nitrogen and vented to an exhaust stack or to a gasscrubber. Ideally it will be monitored for toxic content.

2.5.3 Cryogenics

Safety with cryogenics is mostly a matter of common sense. If cryogenicsurfaces are treated with the respect accorded to hot surfaces, then fewproblems will in general be encountered. Cool surfaces should not betouched except with gloved hands and contact with cryogenic liquids shouldbe avoided.

Some additional phenomena need to be allowed for, however, relative tohot liquid handling; one is the welding of human flesh to cold metal andanother is the brittleness of many materials at low temperatures. Mostpeople have touched the walls of a domestic freezer and are none the worstfor their experience. Similarly, most of the time, the metal components of acryogenic liquid system, even those at very low temperature, can be touchedbriefly without apparent harm. This is because cold surfaces in a typicalambient atmosphere quickly condense moisture, accumulating a protectivefilm of ice. However, just occasionally, the metal touched will be relativelyclear of the condensed ice that quickly forms on a cold surface, and this clearsurface is very dangerous. The welding of flesh to the cold metal is instantand often the cause of unpleasant wounds.

There is a temptation, when a moving component is rendered immobile,


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frozen, by low temperature, to hit it with a hammer. This must, however,

be strenuously resisted because normally robust steel or plastic parts mayshatter and the small problem of sticking caused by the low temperature maybecome a much bigger problem of broken components.

Leidenfrost phenomenon. There is an effect, the Leidenfrost phenom-enon, which can give a degree of safety in handling cryogenic liquids. Theoccurrence of the Leidenfrost phenomenon is not confined to cryogenics andin fact occurs in many places in physics. Leidenfrosting is seen on scales fromthe largest to the smallest; from the collision of galaxies made of antimatterand matter, to the hovering of droplets of water on a hot saucepan.Basically, a cryogen such as liquid nitrogen (LN) dropped onto a muchwarmer surface forms and maintains a thin film of boiling vapour whichisolates the LN from the surface. In this way, personnel are to some extentprotected from small accidental spills and droplets landing on them fromboiling LN because the droplets do not wet skin easily because of Leidenfrosting and tend to run off harmlessly, even more effectively thanwater from a waxy surface.

This can lead to a false sense of security, however. If the LN pools in thesurface that it lands on or otherwise remains there long enough, it will coolthat surface unti l the rate of vapour production slows. At some point theprotective film of vapour disappears, allowing a suddenly much increasedrate of cooling and resulting in painful and sometimes dangerous burns topersonnel. Finally, it is worth mentioning that surfaces cooled in dry air byliquid nitrogen can condense liquid oxygen, which is itself a relativelydangerous substance (section 2.5.9).

2.5.4 Flammability

Many gases are inflammable and all flammable gases have the potential tocause explosions. Some gases ignite or explode spontaneously, such assilane. Gases specifically designed to ignite on contact with air or oxygen aresometimes dubbed hypergolic. Compounds such as pentaborane or triethylaluminium reliably ignite and have been used in small rocket motors (silaneignition is not reliable, particularly for pure grades).

Flammabili ty is a very complex subject in general but has been intensivelystudied over the years. For combustion, certain conditions must be metwhich are characterised by more or less quanti tative parameters:

the fuel/oxidant ratio must be correct within limits: these are the upperand lower explosive limits (UEL and LEL);

the oxidant stream must contain a minimum amount of oxygen: thispercentage of oxygen is called the oxygen index;

a minimum amount of energy must be provided to the combustionmixture: this is the ignition energy;


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GAS TECHNOLOGY 267

a minimum temperature of ignition source must be applied: this is theignition temperature.Flammability of mixtures in air at atmospheric pressure, at least in safety

matters, is the most common situation so this is mostly what is consideredhere. Flammability can be considerably enhanced by oxygen enrichment(section 2.5.9). 39

Flames. It is possible to react together fuels and oxidants using aheterogeneous catalyst; 1% platinum on a ceramic fibre plate will allow thecombustion of common fuels from around 300 or 400C upwards. However,

a catalytic system is not necessary because of the phenomenon of stablecombustion flames.

A flame is the reaction zone between a fuel and an oxidant gas (Figure2.65). The zone typically appears more or less stationary but is in fact adynamic phenomenon formed out of the fast flowing gas mixtures. Aconventional flame burns at a rate which equals the flow rate of the gases so itis in dynamic equilibrium. A flame is a self-sustaining chain reaction: oncestarted, the flame generates heat, which raises the temperature of more fueland oxidant so that it, too, combusts, generating more heat, which raises the

temperature of more fuel and oxidant, and so on.The most useful distinction that can be made is between diffusion flamesand premixed flames. In the former case, pure fuel flows into theatmosphere, and the flame is formed where fuel and air mix a large,typically luminous, flame. In the latter case, the gases are premixed andallowed to flow from their containing tube into the atmosphere, where theyare ignited. The fuel and oxidant are raised in temperature above theignition temperature for the mixture as they approach the flame and thereaction zone begins. A complex molecular dance then occurs, with

molecules losing and then gaining electrons and atoms, and the temperaturerises rapidly, limited only by reaction kinetics to reach typically 1800C ormore. Within the space of a millimetre or so the luminous part of the flamehas begun. Finally, reactions proceed to reach equilibrium and the finaltemperature (the adiabatic temperature if the flame is not cooled) isreached in the bulk of the flame.

A majority of relatively stable species, such as carbon dioxide and watervapour, are then formed as ambient gas entrainment and radiation cools thegases beyond the tip of the flame to below 1000C. Mixed in with stable

39 Similarly, flammability is affected by pressure; the Victorian fire piston was a demonstrationshowing just that. In the fire piston a piece of highly flammable solid material at the bottom of acylinder , which can be made of glass, is ignited simply by compressing the air above it suddenlywith a piston. This both raises the temperature adiabatically (see section 2.5.9 adiabatic ignitionof oxygen mixtures) and raises the oxygen partial pressure, which itself renders the mixturemore flammable. Fire pistons were one kind of portable convenient source of ignition beforegaseous or petro fuelled lighters li t by misch-metal fl int or piezoelectric spark becameavailable.


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Figure 2.65 Simplified flame structure.

species will be low levels of undesirable gases, NO and (collectivelycalled if air has been the oxidant, along with par tial ly combusted fuel ,sulphur dioxide from any sulphur in the fuel and, with hydrocarbon fuels,microscopic carbon particles. The heat released sustains the chain reactionand tends to make it propagate back down the supplying tube at a fairlywell-defined velocity known as the flame speed. If the flow speed is lowerthan the flame speed, the flame will travel back up the tube; this is calledblow-back. On the other hand, if the flow velocity is too large, the chainreaction cannot propagate back against the flow fast enough and blow-offor lift-off of the flame occurs. Stabilisation of the flame is achieved in small,simple burners by the cooling and flow slowing effects of the edge of thesupplying tube. In larger burners specific measures such as a quarl (a smallante-chamber into which a furnace burner fires), rapid rotation of the gasstream or a bluff object in the gas stream, are used. These cause flamerecirculation, giving stabilisation.

Diffusion flames are only rarely used. Diffusion flames are typically large,luminous, unstable and, if using a carbon-containing gas, deposit carbononto cool solid surfaces placed in them. Premixed flames are those normallyemployed in most applications. They are typically much less luminous thandiffusion flames, are stable and are much smaller than diffusion flames withthe same thermal output. Partially premixed flames are very common andthey have two often distinct flame zones: a small inner zone of reaction of thepremixture and a large, less luminous outer zone where oxidant diffusinginto the still combustible reaction products of the inner flame allows furthercombustion.


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GAS TECHNOLOGY 269

There is a further distinction amongst flame types depending on their flow

characteristic. While small burners (such as cooking burners or laboratoryBunsen burners) tend to have laminar flow, larger industrial burners showturbulent flow. In turbulent flames, the reaction zones described occur insmall transient combustion cells formed in the intensively mixed gas.Turbulent flames are described as premixed or jet flames. In the latter,although the fuel supply is separate from the oxidant, the turbulent mixingmeans that, in effect, they have characteristics very like premix flames.

Turbulent flames can achieve greater combustion intensity (throughput of reactants per unit volume of flame) than laminar flames and many industrial

designs rely on this to achieve high rates of heating and effective heating athigh temperature. High intensity flames are unstable and have a strongtendency to lift-off. One stabilisation system for intense flames is an oxygenpilot. A small side stream of oxygen is used and the oxygen locally increasesthe flame speed, allowing a more intense flame before lift-off occurs.

Ultrahigh intensity burners sometimes use a combination of laminar orlow-velocity flames with high-velocity highly turbulent flames. The lowervelocity flames stabilise the unstable high-velocity flames, relighting themcontinuously.

To achieve a premix flame from a liquefied gas source involves vaporisingthe liquid stream effectively. Simple burners rely on the burner parts heatingup to do this. Other burners use an ambient air-heated vaporiser. Hot-airballoons have special vaporiser coils near the lower part of the flame toensure that the liquid propane fuel used is thoroughly evaporated into thegas prior to combustion (see Figure 3.45).

Rocket motors have a similar arrangement whereby liquid oxygen, andliquid hydrogen if used, is fed into pipes welded onto the outside of the thinmetal propulsion nozzle, ensuring complete conversion into gas using the

formidable heat of the rocket flame. The latent heat of vaporisation of thecryogens cools the rocket motor walls, allowing them to survive the intenseconditions for longer.

2.5.5 Flammability limits

Most substances that are not fully oxidised and therefore inert will displaya positive heat of combustion, according to standard chemical enthalpytables. However, compounds such as perfluorinated alkanes have very lowheats of combustion (less than 8000 and will not normally ignite. Othercompounds may be so diluted with inert materials, as with the asbestos

/rubber composition gaskets popular in gas systems, that their net heat of combustion is low. With these exceptions, all compounds are flammable andsolids such as polymers PE or nylon (combustion heats 42 000 and34 000 approximately, respectively) will burn vigorously. Solids burnby reaction at their surface with the air or oxygen in the same way as a


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candle, the tendency to ignite being formalised in the oxygen indexmeasurement. Liquids and gases can also be ignited in this way and burn atthe surface, giving a diffusion flame. However, in the case of vaporisableliquids or gases they can mix with air or oxygen and combustion can takeplace within the bulk of the mixture, often explosively. The tendency to burnin bulk in this way can be given a quantitative shape in the form of flammability limits.

If a gas is mixed with air in stoichiometric proportions, i.e. the oxygen inthe mix is exactly that needed to oxidise the fuel, then, if the gas is at allflammable, it will ignite. For instance, a mixture of 29% hydrogen to 71% airignites explosively. However, if the gas is mixed in proportions far from thisideal mix, it may also ignite explosively. The limits of percentage in air whichignite are an important safety parameter. The lower explosive limit (weakestfuel mixture) is important in situations where fuel gas leaks from a pipelineor vessel into the air, where there may be a source of ignition. It is used insetting flammable gas detectors; these are frequently set to alarm at 20 or40% of the lower limit. The upper explosive limit is important in situationswhere there may be an ignition source within a fuel stream or vessel.Furnaces for steel heat treatment , for example, involve temperatures well inexcess of ignition point with a fuel gas atmosphere. In the event of incomplete purging of air from the system, for example during start-up, theproportion of fuel in the mix may sink below the upper explosive limit andbecome dangerous.

Some gas flammability limits are listed in Table 2.16. Outstandingamongst them are the extraordinarily wide limits of hydrogen and acetylene.The flammable limits of mixtures are not in general predictable and shouldbe measured in cases where they are important. However, early in theinvestigation of combustion processes in France, le Chatelier proposed anapproximate formula on the following lines. A mixture is flammable if

where are the mole fractions in the atmosphere of flammablefuels 1, 2 , . . . and so on. This common sense and simple formula can,however, be quite inaccurate in particular cases.


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2.5.6 Ignition energy

If a gas mixture is subjected to a high-temperature spark, it may not ignite.Anyone who has snuffed out a lighted match in a saucer of paraffin (anexperiment to be tried only outside with small quantities) will have an idea of what is going on here. That same saucer of paraffin will be easily set alight bya roaring gas burner, whose flame may well be cooler than the lighted matchbut whose energy is larger.

For most gas mixtures flames only occur if a certain minimum amount of energy is supplied: the ignition energy. For the hypergolic fuels, the ignitionenergy is effectively zero; a passing cosmic ray or a random conjunction of

molecules is sufficient to start the chain reaction. Examples of hypergolicfuel mixtures are impure silane and air or unsymmetrical dimethyl hydrazineand dinitrogen tetroxide. The latter mixture is frequently used in aerospaceapplications where cryogenic tanks are not wanted.

For some gases, such as hydrogen, the ignition energy is exceptionallylow, just a few microjoules. This range of ignition energy can easily begenerated in static electrical discharges which occur in everyday activity. 40

For other fuel gases, ignition energies are in the millijoule range but stillsmaller than the static voltages which can occur. Proper precautions to avoid

all high voltages are necessary to avoid accidental ignition of gases where it isnecessary to use apparatus in an inflammable atmosphere (section 2.5.8).

The minimum ignition energy is usually estimated by gradually increasingthe size of an electrical spark in a stoichiometric mixture of fuel and oxidant.The spark energy is related to the energy stored in a capacitor dischargedacross the spark gap.

2.5.7 Oxygen index

The oxygen index (OI) of a substance is the lowest percentage of oxygen inthe air which still allows self-sustaining combustion of that substance. It isdetermined by ign iting a sample strip in a steady stream of mixtureand seeing whether the sample stays lit like a candle or extinguishes. Likethe LEL and UEL, the OI is a fairly quantitative concept, although OI isdependent on pressure and is affected by a number of other considerationsas well as ignition energy and temperature. It is interesting to note that manypolymers, even those such as PTFE which are general ly reckoned to be inertand nonflammable, have an oxygen index, although only at elevated

pressure in the case of PTFE (Table 2.17).

40 A plastic biro pen rubbed on a shirt sleeve will typically generate enough electric charge todeflect a stream of water from a tap or pick up small pieces of paper electrostatically. Taking outa biro to note where a flammable gas is leaking out of a pipe could thus be disastrous.


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Fluoroelastomers and PTFE are regarded as satisfactory materials for

oxygen systems because of their high oxygen index. Copper, nickel and theiralloys are regarded as the most satisfactory materials for arduous duty inoxygen systems, such as pumps, valves and other moving parts, and where afinely divided material is needed, such as in filters.

Another means of measuring the sensitivity of materials to ignition inoxygen systems is the autoignition temperature. Finely divided samples areslowly (200C per minute) heated in a heavy-walled reaction vessel at astandard pressure, often 100 bar, until they ignite spontaneously. Mostpolymers ignite under these conditions at between about 150 and 350C,

although PTFE does not. This sort of test gives a similar ranking to theoxygen index in most cases.

2.5.8 Safe working in flammable atmospheres

It is possible, and often essential, to be able to work in areas where theremay be a flammable atmosphere continuously or intermittently present. If itis necessary to work in an atmosphere containing a flammable mixture of hydrogen, precautions against accidental ignition must be stringent. Thestatutory requirements for flammable atmosphere working (i.e. Zone Irequirements) were therefore drawn up with this need in mind.

In a Zone I area all high-power electrical apparatus must be completelyisolated from the gaseous atmosphere. It should be either hermeticallysealed or well-sealed and then pressurised continuously from within by aninert gas such as nitrogen. Low-power electrical apparatus may be eitherisolated or intrinsically safe. The latter is achieved by ensuring that themaximum voltage in the circuit is only a few volts and that the maximuminstantaneous current possible is in the milliamp region or below. Smallelectronic devices, for example, may be used if they contain no largecapacitors and have batteries of a high internal resistance. (Caution must besounded over electric torches and flashlights, however; on these the bulbmust be isolated, as the 2500C+ tungsten fi lament in it would be a splendidsource of ignition.) Barrier circuits are available, using Zener diodes andresistors, that limit the voltage and current that can be transmitted into thezoned area along wiring.


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All metal surfaces should be electrically bonded together so that ignitioneffects from static discharges or electrochemical currents can be avoided.Plastic materials should be of the slightly electrically conductive anti-statictype. Finally, the use of ferrous tools may need to be controlled. Striking of ferrous alloys on other metals, stones and other hard materials can lead topyrophoric particles. These are microscopic particles of iron which ignitespontaneously in air or oxygen and can easily lead to igni tion of a flammablemixture.

2.5.9 Gas release accidents

Burning gas leaks. Gas suppliers should always be consulted as to fireprecautions, along with local fire service officials if appropriate. Differentgases will require different approaches, as will different sizes of installation.However, some general principles are perhaps useful to know.

It is not infrequently the case in large and extensive pipeline systems thatsmall leaks of flammable gas occur. A small burning leak, especially if it isoutside or in an accessible place inside a build ing , is often not part icularlydangerous and if prompt action is taken it can be easily extinguished beforereal problems occur. First, the gas supply must be shut-off , and , if possible,the pipeline depressurised via a suitable vent stack. The fire can then beextinguished in the way appropriate to the circumstances of the leak. Itshould be pointed out, however, that extinguishing a gas leak fire, if the gassupply is still on, is dangerous. If the fire is successfully extinguished, gas willcarry on accumulating and will mix with air, presenting the risk of anexplosion much more dangerous than the original gas leak fire.

Hydrogen installations are particularly subject to leaks, because leaksvery readily, and installations such as thin membrane purifiers and arrays of many hydrogen cylinders offer many opportunities for leaks. Hydrogeninstallations are often placed outside where the impact of a small leak isnormal ly very small. One of the dif ficult ies of leaks is that they areexceptionally difficult to see. Outside in bright sunshine the flames arealmost invisible. Old fashioned manuals advise the use of a piece of paper ora rag on a stick as a flame detector. With glowing carbon particles and atomsadded in this way, a hydrogen flame is indeed very much more visible.

Detonation, deflagration and BLEVEs. The effect of an ignited gas clouddepends on the speed at which the flame front travels. Gas/air mixtures canburn rapidly emitting copious heat but causing little immediate mechanicaldamage. BLEVEs (burning liquid expanding vapour explosions) are aspecial case of this and are typically seen where a container of flammableliquefied gas such as propane is ruptured and ignited. The mechanicaldamage seen is relatively slight, extending only a li ttl e beyond the zone of


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the visible fireball. In these cases the flame front in the mixture of gases

travels slower than the speed of sound so heat released by the ignition isdissipated smoothly in the expansion of the mixture. This situation isreferred to as a deflagration explosion. As the burning mixture approaches astoichiometric composition, the speed of the flame front will increase and adeflagration explosion becomes more dangerous. If a sealed vessel full of such a mixture is ignited then the vessel may well be ruptured by theadiabatic expansion. 41

Much rarer are those accidents where a large volume of near stoichiome-tric mixture is accumulated before ignition occurs. In these cases, the flame

front travels faster than the speed of sound and, with or without acontainment vessel, there will be detonation explosion. Large volumes of gas can be burnt and high temperatures and pressures accumulated beforethis energy can be dissipated by expansion of the surrounding air. As aresult, the energy of the burning gas is efficiently converted into anexpanding shock wave of immense destructive power. Military highexplosives are designed to produce this kind of detonation explosion.Military devices (fuel-air bombs) have been devised to utilise the power of gas-air explosions. Gas burner and boiler manufacturers, and fuel gassuppliers are of ten equipped with special explosion test chambers. These areused to investigate the effects of small detonation and deflagrationexplosions as part of their development efforts towards safer burner andboiler design and use.

Oxidant accidents. Two of the common oxidants in industry are relativelyharmless gases: oxygen and nitrous oxide. Unlike reactive oxidants, such ashydrogen peroxide or fuming nitric acid, both the gases are stable whenmixed with many oxidisable materials. This makes them in some ways morehazardous than the obviously perilous nitric acid because they can leak outin large quantit ies before ignition occurs.

Liquid oxygen (LO) has more potential than other oxidants for bigaccidents because it is used in such large quantities on a routine basis. LO iseasily soaked up by organic material, forming a low explosive. 42 A typicalLO/organic mixture will release more energy per unit mass than anexplosive, e.g. combustion of

nitroglycerine =octane + mixture =

41 If the vessel is vented a great rushing sound, like a jet engine, will be heard as a very rapidexpansion tak es place, followed by the expulsi on of a sudden spout of hot gases.42 For example, one of the party tricks performed by schoolmasters used to be the soaking of acigarette in liquid o xygen. An ord ina ry Benson and Hedges King Size, normall y smoulderingfor 10 minutes or so, is gone in two seconds, showering sparks everywhere like a firework.Given this behaviour from a few grams of material, it is clear that the contents of a 50 tonneliquid oxygen tank soaked in an organic substance is a major hazard.


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The flammability limits of inflammable substances with oxygen are wide,

ignition energy and temperature is low, flame temperature is high , and flamespeeds are high. These effects all conspire to make a fire or explosion in thepresence of oxygen both likely and serious.

Lubricating grease or oil should not be applied to compressed oxygen gassystems. The density of oxygen at 230 bar is only about a quarter of thedensity of liquid oxygen and high-pressure gaseous oxygen tends to dissolveand form an intimate mixture with organic materials. A typical compressedoxygen failure occurs when an oxygen system becomes contaminated withan organic compound such as lubricat ing grease. The grease may have been

applied with good intentions, for example, to a sticking valve. However,high-pressure oxygen dissolved in the organic consitutes an explosive,possibly even a detonating explosive. A small but very dangerous explosionthen occurs, which will rupture the pipework system and perhaps fire outsmall parts, such as valve components, like bullets.

It is essential not only not to use lubricating greases and oils in oxygensystems but also to eliminate any adventitious organic compounds by arigorous cleaning procedure for all components used in oxygen service.There are standard degreasing/cleaning procedures for oxygen components

defined in the national and international codes, and these should befollowed. It is worth noting that cleaning of a complete system, afterassembly, may well be impossible. There may well be dead-legs, parts of cylinders and loops of pipe which cannot be degreased in situ. In the eventthat a system is contaminated with oil or grease, it may well be necessary todismantle and degrease each component, followed by reassembly.

As noted in section 2.4.2, the composition of seals in oxygen systems isalso problematical, with perfluorinated polymers most suitable, at least forless extreme service. Asbestos and related composition seals are oftenspecified for oxygen service as these are essential ly non-combust ible, at leastif they are not contaminated.

A much rarer fa ilure occurs when the gas velocity of oxygen down a pipe isvery high, perhaps because of a serious leak somewhere else. In this case,iron or steel pipes can be ignited, and once ignited will burn and melt in thestream of oxygen. The pipe then behaves exactly like a thermic lance and cancause great damage. To avoid this possibility, oxygen pipes should be madefrom brazed copper or high-alloy stainless steel, such as 316L, or restrictedin pressure and flow rate. Even stainless steel is not recommended above70 bar. An accepted rule of th um b is tha t for steel pipes at lower pressures,below 40 barg, flow velocity should be kept below whilst at higherpressures flow velocity should be kept even lower, less than Alloxygen valves and fittings for service at higher pressures should ideally bemade from brass, copper or 316L. However, note that local constructioncodes should in al l cases be obtained and followed in the design,construction and operation of oxygen systems.


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Sources of ignition in oxidant accidents. It is usually not possible to exclude

all ignitable material from an oxygen system. Polymer seals and flexiblehoses, for example, will always have some degree of flammability inhigh-pressure oxygen; even Teflon will ignite at high enough pressures withsufficient ignition effort.

It is surprising, when a survey of investigated oxidant accidents is made,how many ways there are of generating sufficient heat to give ignition. Themost unlikely mechanisms have been implicated; for instance, heat gener-ated by particles of rust or dust in the pipe hitting an obstruction has beenshown to be sufficient under adverse circumstances in a high-pressure

oxygen pipe. The frict ional heat generated by turning a valve in a tight stemseal has also been shown to yield enough heat; these sorts of accidents areparticularly unpleasant because the operator turning the valve may well beinjured. It should be remembered that many powdered materials arepyrophoric in pure oxygen. Just as tiny particles of misch-metal from alighter flint ignite in air as they fall through it, so minute steel particles, forexample generated by abrasion, can ignite in oxygen. Electrical orelectrostatic sources of ignition are commonly blamed for igniting volatileorganic vapours in air and although such sources should have beeneliminated from contact with oxygen systems by design they can occasionallycause accidents.

Adiabatic compression ignition in oxygen systems. The most commonsource of ignition in oxygen systems at high pressure, however, is probablyadiabatic compression heating following rapid opening of a valve onto arestricted low-pressure system. As discussed in section 2.4.7, this effectoccurs because a small volume of low-pressure gas is sufficiently confined(perhaps it is open to the atmosphere but only through a relatively smallorifice) that when high-pressure gas is suddenly admitted, the low-pressuregas is compressed, with a flow pattern that in narrow pipes does not allowmuch mixing, unlike the case in cylinder filling. Whereas in cylinder fillingtransient temperature rises of a few hundred degrees or less are typical, inpipes the full 1000C or more calculated above might be seen.

In any oxygen system with a high-pressure source, valves opening flowfrom that source should ideally be slow-opening screw-jack operated seatvalves; cylinder valves are obviously suitable in this regard. Valves withparticularly slow-opening characteristics (typically by an extended plug,somewhat like a needle-valve type) are available which will guaranteeparticularly slow opening.

The mode of operating of a manual system (or, equivalently, the softwarein an automated valve system) can also provide protection against adiabaticcompression ignit ion problems. For instance, procedures can ensure that al lvalves which may have high pressure behind them are opened slowly or partsof the system which include flexible hoses are always maintained at low


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Figure 2.66 Design considerations in a small oxygen installation.

pressure. However, it is most unwise to rely on procedural protection; it isfar better to design a system which cannot easily be put into danger. Anexample will show the sort of design considerations that apply (gas suppliersas well as national guidelines and regulations such as ASTM or CGA shouldbe consulted before designing or modifying oxygen systems which involveany high-pressure parts).

In Figure 2.66 a single oxygen cylinder or bank of cylinders is used toprovide relatively high pressure oxygen at two pressures to two user stations.High flow ball-valves are specified for V1 and V2 so that large flow rates can

be catered for. System 1 will be safe in most circumstances; opening Vc willnot cause adiabatic heating in the diaphragms of the regulator because it is aslow-opening type. Opening V1 and V2 does not cause a problem becausethe pressure (< 30 bar) behind them is un li ke ly to be high enough to cause aproblem at the user station. In the event of either of regulators Rl or R2leaking into the downstream (low-pressure) side when V1 and/or V2 are off,the pressure relief valves PR1 or PR2 will lift, thus assuring low pressurebehind V1 and V2. V1 and V2 could be paralleled by screw-operated plugvalves which have to be opened (slowly by their nature) before opening V1and V2 for high flow rate, providing protection against adiabatic heating inthe user device.

System 2, although using the same components, has the potential for anoxygen ignition accident. If Vc has been opened and V2 is then openedsuddenly, high-pressure oxygen from the cylinders and the pipeline to V2will compress the oxygen just before R2, and R2 may ignite or even explode.

Other rules have evolved from the gas companies long experience with


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making and using oxygen systems based on the science explained above, for

instance:

to limit oxygen gas velocities pipes should be generously sized and placeswhere velocity is art ificia lly increased over the mean, such as at thrott lingvalves, must be made of inert materials;

high-speed rotating equipment should be designed with positive clearanceunder all service conditions to avoid frictional heating;

pressures used at all stages in a system should be minimised.

Nitrous oxide and other oxidants. In general, nitrous oxide is a very safegas to handle, with fewer problems than the more ubiquitous oxygen. It doesshow all the oxidant accident potential of oxygen, however, with a lesserdegree of reactivity giving a greater safety margin. (However, it should benoted that some rubber materials are decomposed by nitrous oxide, socaution must be exercised with seals.)

In addition to its oxidant properties, nitrous oxide has been known to bethe cause of accidents due to thermal decomposition. Occasional failures of this type happen as follows. A section of pipe carrying nitrous oxide is seento be hot, perhaps for no apparent external reason. This section of pipequickly becomes red hot and then fails, releasing a large volume of nitrousoxide at very high flow rate, together with a shower of molten or burningmetal. This sort of incident can occur because nitrous oxide, like acetylene,is an endothermic gas, i.e. it has a large positive heat of formation (seeKaye and Laby, 1995). When it is decomposed at a few hundred C into itsconstituent nitrogen and oxygen, it releases heat, which causes furtherdecomposition. The released oxygen may result in combustion of pipematerial, further exacerbating the situation.

Finally, it should not be forgotten that other reactive gases have thepotential to cause oxidant accidents. Fluorine is such a powerful oxidisingagent that most ordinary substances spontaneously ignite or explode. Onlyspecially passivated stainless steel systems are normally suitable and gassuppliers specific recommendations for handling this exceptionally hazard-ous gas should be adhered to precisely. Nitric oxide and nitrogen dioxide areboth relatively unstable compounds, easily yielding their oxygen atoms incombustion reactions. The other commonly found oxidant gas, chlorine,also has the power to support vigorous combustion and, furthermore,

combustion in chlorine generally leads to the formation of copiousquant it ies of hazardous hydrogen chloride gas.

Reactive gas accidents. Purging of all reactive gas pipelines is essential,with effective venting during purging. Ideally, purging is by a pressure orvacuum cycle (section 2.5.10), with care taken in venting of the purged gas.


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Failing this, purging for a much longer time by passing an inert gas flow will

usually be acceptable from a safety point of view.43

Silane plants and other dangerously pyrophoric or explosive gas plants arecommonly constructed in friable buildings. Operators observe from behindsolid walls with heavy armoured glass windows. The outside walls and roof of the building are light, however, to allow any pressure waves to escapeoutwards and reduce damage to the operators room. Ultrapure silane, afterits distillation step, is particularly dangerous because it ignites spon-taneously only occasionally. It can therefore build up to a large volumebefore ignition, often explosive, takes place. Frequently, the area around

potential leak sources is exhausted into a duct where there is a continuoussource of ignition, e.g. an electric spark or flame. This burn box thenguarantees that ignition will occur early during the development of a leak, sothat any explosion which may occur will be very small. If a large flame isused, the burn box also acts as a scrubber, oxidising any suitable reactive gaspresent to less harmful oxides.

Helpfully, leaks of many reactive gases can be easily located because of the reactivity of the gas. Many reactive gases react with moisture in the air toproduce a mist of particles or droplets (the boron trichloride mentioned

earlier is an example). Small leaks of acid gases, such as hydrogen chlorideor sulphur dioxide, can be easily seen if a sponge soaked in ammoniasolution is brought near to the leak; a tell-tale white smoke will appear.

2.5.10 Safe equipment design

Materials selection for gas pipelines and vessels was dealt with in sections2.4.1 and 2.4.7 but it may be worth saying something in addition aboutmaterials for cryogenic service with respect to safety. Many properties of

materials are better at low temperatures but some are not. The problems of materials selection for cryogenic use boils down to two prime consider-ations:

will the material become undesirably brittle at the operational tempera-ture?

will thermal contraction during cool-down from room temperature matchother components or, if it will not, has the component been designed toallow for this?

Thermal expansion is more serious in cryogenics because of the simul-taneously increased stiffness and brittleness of materials; greater strains areinduced and these strains are more likely to lead to failure. Whereas even

43 For example, boron trichloride hydrolyses with moist air to form a dense white cloud of boricoxide and hydrochloric acid droplets. On purging with nitrogen a very large cloud of unpleasantly acrid white smoke will appear before the nitrogen purge finally exits the pipe.


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Figure 2.67 Elasticity of polystyrene showing glass transition from glass to tough polymer torubber and liquid states.

serious mismatches would be unimportant in most applications wheretemperatures are cycled 200C upwards, this is not necessarily the case for200C downward cycles.

Metals with body-centred cubic (BCC) structures are in general unsuit-able for cryogenic service, as they become as brittle as glass long before LN

temperatures. BCC metals include, unfortunately, most grades of steel.Carbon and many alloy steels are serviceable in cryogenics only if verylightly loaded because of these problems and are only used if economicfactors are overwhelming. Consequential failure due to cryogenic embritt-lement of steel or other parts because of a cryogenic spill should always beborne in mind in confined layouts. A small LN spill or leak may lead toexcessive cooling of the outside of nearby parts under tensile stress (pipesunder pressure, steel beams or cables, motor or compressor castings, forexample) leading to their failure in service.

Similarly, most plastic polymers become very britt le at a li ttle below roomtemperature. The movement of one polymer chain past another, whichaccounts for the flexibili ty and toughness of polymers, becomes increasinglydifficult as temperature is lowered, until the polymer becomes a rigid solidwith a random arrangement of atoms, i.e. a glass. The so-called glasstransition of most useful plastics is below room temperature but only a littlebelow for most. PTFE is useful to the lowest temperatures (Figure 2.67).

Polymers also have the problem that they have large coefficients of thermal expansion and poor thermal conductivity. This means that rapidcooling, for example dunking them in liquid nitrogen, can result in themundergoing differential thermal expansion leading to distortion andcracking. Polymer adhesives frequently peel off adherent surfaces aftercycling between room temperature and LN.

Safety in pipework. A set of guidelines for safe pipework is a standard partof the stock-in-trade of industrial gas firms. These guidelines will cover:


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labelling: labels should be attached every few metres of pipeline and givena colour code if possible; end-fittings: ideally these should be selected to match unique gas cylinderor tank fittings so that the wrong gas cannot be connected;

location: location of pipework in the ceiling of a factory, for example,generally leads to warming of the gas, which may be useful when the gashas come from a cryogenic source; accessibility for fixing leaks should alsobe a consideration for joints in the pipework;

material suitability for gas; material suitability for pressure; suitability for contaminants in gas: e.g. particulates in silane leads to theuse of tied diaphragm regulators so that blockage and leakage cannot take

place with particles underneath control valve so easily; bending, minimum radius, type of bending machine to be used, anneal

state of material needed; joining: welding, compression joints, flanges, soldering, grades of solder; flow rates: e.g. flow rates of high-pressure oxygen through steel pipes

allowed are limited.

It should not be thought that all of these guidelines are completely hardand fast but any deviations from these mostly well-thought out andtime-honoured rules should be carefully just ified and agreed with everyoneconcerned.

Purging. Purging of a gas system means the replacement of the initial gasmixture (often air) in a container or pipeline network with the correctprocess gas at the highest possible purity. The purging of flammable or toxicgas systems is necessary for safety reasons. There are three basic ways of purging:

flow purging: simply passing the process gas through the network; pressure purging: the system is repeatedly pressurised and then let down

to a lower pressure (typically atmospheric); vacuum purging: the system is evacuated then backfilled with gas,

sometimes repeatedly.

Flow purging is the least efficient because gases do not flow as perfectplugs; there is always a slow-moving boundary layer . Also, any dead-legs inthe system wi ll act as long-term reservoirs of impuri ty . Pressure purging andvacuum purging deal with these problems, with vacuum purging being by farthe most efficient (Figure 2.68).

Purging theory follows a simple harmonic progression. If the maximumpressure is P+ , and the lowest pressure is P, then the concentration of impurity C will reduce from an in it ia l in the following way, for n purgecycles


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Thus a vacuum purge of 1/1000 bar is equivalent in efficiency to pressurepurging to an impracticable 1000 bar. With 1/1000 bar vacuum and 10 barprocess gas pressure, the concentration of wil l decrease by a factor of 10 000 for each cycle in this simple theory. With a simple 1 bar vent eachtime instead of vacuum, wil l go down only by a factor of 10 for each cycle.

The harmonic progression of purging depends on perfect mixing of theadded gas to the lower pressure gas in the pipeline. Clearly this will not bethe case if pressure cycling is carried out from one end of a long pipeline; thedead-leg end of the pipeline will behave partially as a plug flow and simplyexpand and contract as the single-ended purge proceeds. If a line can bepressurised from one end, and vented from the other, purging will be muchmore effective and better than a perfect mixing assumption.

Naturally, practice is more complex than this theory indicates. Forexample, if the impurity is water , this tends, at ordinary temperatures, to liein substantial amounts in a thin film over the inside of a pipeline or cylinderand will only slowly evolve under the influence of heat and vacuum. Heatand vacuum must therefore be used on cylinders for ultrapure semicon-ductor gases where u ltralow moisture content is needed.

Bursting discs and pressure relief valves. No gas system can withstandinfinite pressure and in fact most systems cannot withstand the approxi-mately 1000 bar pressures which liquid nitrogen can exert if trapped in adead-leg and evaporated, so relief valves or bursting discs are essential.

For high-pressure systems, bursting discs (thin diaphragms of a cor-rosion-resistant metal) can be fi tted into special mount ing jigs. These act as aweak link in the system, bursting before any major component can bedamaged and releasing the gas pressure. Only a small amount of debris isejected when a disc blows; an import ant safety feature, alt hough the event isspectacularly noisy and not a little frightening for nearby spectators.

Figure 2.68 Purging a gas line of air by vacuum and pressure.

Safety & Environment From Industrial Gases

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