in the Chemical laboratory Edited by N O R M A N V. STEERE, 1 4 0 Melbourne Ave., S.E. Minneapolis, Minn. 5 5 4 1 4
LXV. Handling and Use of Cryogenics* R O Y REIDER, Safety Director, Los Alamos Scientific Laboratory, Lor Alomos, N e w Mexico 87544
The science of cryogenics is perhaps close to one hundred years old, but it is only in the last generation that it has become a matter that has significantly left the labarstory and became a commercial product. Cryogenic risks are actually few in number. Many of the safety problems are due to the nature of the gas itself which is condensed to a cryogen. Pressure Buildup a n d Relief
Since a11 cryogenic fluids exist as liquids only at temperatures considerably below ambient, normal storage must account for rtn unavoidable heat input from the environment which is an inexhaustible heat source. Therefore, cold systems in general must have suitable vent systems containing provisions far pressure relief in order to prevent excessive pressure buildup. A cryagen has a volume change from gas at standard condit,ions of 700 to 800 fold. So that, if a cryogen is in a sealed container, i t theoretically wants to return to the normal volume, and to exert great forces. For example: The pressure to maintain helium a t liquid density a t room temperature is 18,000 psi. For hydrogen it is 28,000 lb per square inch, and for nitrogen 43,000 lb per square inch. Therefore, we have to look a t the spaces in which we contain cryogenic fluids and keep them vented. First, there is the experimental volume; this could be any space with the experimental device immersed i n the cryogen within which cryogenic fluid could leak and later cause excess pressure when the system is warmed up. There are many cases where experimental volumes have blown up bemuse no relief was provided. Then we h w e the bath space, that is, the space above and including the cryogenic fluid. This too must be vented. Another volume which is no less important is the vacuum space; this, too, must be inde~endently relieved. The problem with the vacuum space is as follows. Let's assume we have cryogenic fluid and i t leaks into the vacuum space from the fluid. I t is all right as long as the cold fluid is there, but once we remove the cryogenic fluid then the vacuum space will warm up, and this can blow up. From the other
'This paper was prepared for pmsentation a t a regional safety workshop presented by the Iteseareh and Development Section of the National Safety Council. Reprinted with Special Permission from the Research and Development Section.
direction you might have a leak into the vacuum space, say of air. If the cryogen is an air condensing fluid, the leaking air can condense to a solid exerting almost no vapor pressure which will give no warning. Even if i t is not air condensing fluid, of course, we can get air in a t reduced volume and pressure. Then, when the cryogenic fluid is withdrawn, the air or gas that has leaked in can expand, and we can blow up the vacuum space hy imploding the inner vessel. There are cases where glass dewars cracked and the air that leaked in eondensed. When the dewar warmed up, whst had been a crack a t a cryogenic temperature was not a crack at the warmer temperature, and the dewm just blew up. So, one has to worry about the experimental space, the bath space, and the vacuum space (See Fig. 1).
One typical problem is given as an example: argon was being purified in a small glass system by immersion in a liquefied nitrogen trap. The argon condensed in the system which subsequently was sealed off. When the nitrogen trap was removed the argon evaporated causing enough pressure to rupture the glass system. The experimenter who had not realized that areon as well as im~urities condensed a t liquefied nitrogen temperaturessuffered an eye injury. Cold D a m a g e l o Tissue
All cryogenic fluids exist a t temperatures low enough to damage tissue. Surfaces
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cooled by such fluids, or t.heir evolved gases, may $so be cold enough to bring tissue to a paint below 32°F where damage occurs. Such coaling does not take place immediately as the blood supply to tissue act,s as a heat source; the contact of a. cryogen with warm tissue creates a gas film which is not a. good conductor of heat. This time delay usually permits successful measures to be taken if the exposed part of the body is splashed by a cryogenic fluid. Two measures make significant contributions to safety: 1)the ability to elude the continuous contact of the cryogen by shutting off the flow or escaping the area (or both). 2) Immediately flooding the tissue or clothing with water. The enormous heat capscity of water, its harmlessness and its ready availability all combine to make water an important safety contribution to cryogenic operations. So universal is the regard for this safety measure that cryogenic operations except for rautine, well-proven procedures should not proceed without its ready presence. An internationally known major producer and handler of cryogenics for over 30 years states, "our personnel injury experience from 'cold' bums has been practically nil." This authority compares liquid nitrogen a t -320 t,a 160DFhot water in its body-contact effect. Momentary or very brief contact with a. small amount of crvo-
when working with laboratory and industrial cryogenic and pressurized, or potentially pressurized, systems. Where splashing is a real possibility face shields may be indicated. Personnel may require special clothing, gloves and footwear. Gloves should be clean and dry; loose fitting gloves are preferred so they may he cast off quickly if splashed or spilled upon. While tissue damage may he virtually unknown with such cryogens as helium and hydrogen, they are more frequently seen among those cryogenic fluids with higher heat capacity. Liquefied oxygen spills have produced belated accidents from fire burns because of smoking and stray sparks a n clothing even after the individual has left the environment of the spill. The treatment of cryogenic burns is simply to restore tissue to normal body temperature (98.6'F) as rapidly as possible fallowed by protection of the injured tissue from further damage and infection. Rapid warming of the affected part is best achieved hy using water a t 108'F. Under no circumstances should the water he over 112°F; nor should the frozen part be rubbed either before or after rewarming. A patient should not smoke, nor drink alcohol.
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Asphyxiation and Toxicity Cryogenic fluids released or spilled in a confined area can rapidly alter the air composition, by displacement or contrtmination, and introduce an asphyxiating or tnxir risk.
cent COpair mixture. Thus, although carbon dioxide is a physiological necessity in the normal breathing cycle because i t stimulates breathing, the effect of an excessive amount on the breathing rate can affect the ability of the body to rid itself of the carbondioxide in the blood.
Air Condensation and Oxygen Enrichment
Liquefied helium and hydrogen are capableaf condensing and solidifying air that may came in contact with the fluids. The ties of contaminant gases if these gases are polidified air can plug vent paths and jeopnonreactive and only slightly soluble in the mdise vessels because of pressure buildblood. Thus, with physiologically inert up; solid air, probably oxygen enriched, in gases such as helium, methane, hydrogen, nitroeen. .. . areon .. and neon.. oxveen .. concen- liquefied hydrogen may present an explosion hazard. Liquefied helium end hydrorr:iriow a . low , I , 1 3 volume pw,w~tI:," 1111 rol+.rdlnl11). hwil~ltys~~hjrccs.~ I o w ~ w ~gen , must be stored, transferred, and otherwise handled only in closed systems presrhr rt.lr.imun> ~ot.ecn!mIiv~.!,I G Y W W ~ surized to greater than atmospheric presnecessary to support human? is also related sure to prevent bilekflow of air; relief deto tho activity and health of the individual. vices should be designed to prevent back With an increase in activity there is an leakage of air if i t is necessary to operate increased demand for oxygen. the system below atmospheric pressure. If the oxygen content of the air falls Air coming in contact with a surface slowly and the individual is a t rest, sympcooled below 82 K will condense; Zabetakis toms ensue gradually. If these sympgives this condensate a t approximately toms arise slowly enough an individual's SOY0 oxygen, 50% nitrogen. Such an difficulties may be masked by 8. state of oxygen enriched mixture will significanfly "euphoria" and s. lack of danger warning enhance burning rates of combusbibles and which is false and may preclude selfpermit some materials to become combusrescue. tible that might not qualify as such in When the diluent gas is carbon dioxide, normal air. oxygen concentrations below about. 19 volWhen vacuum insulation is not recluired, ume percent can be tolerated only for s f e w applied external imulation is frequently minutes; 10 volume percent oxygen eonused. These me generally poured-in-place centration is found in a nine volume per~
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foams with densities of 0.03 to 0.1 gm/cm3; polyurethanes and polyvinyl chloride f o a m are frequently used. Since these are cellular structures, it may be possible for oxygen enriched air to accumulate, although most foams are closed-cell which are less susceptible to penetration than open-cell foams. I t is usually desirable to provide a barrier agaillst the infusion of air as it might also accumulate behind the foam adjacent to t,he pipe and expand when the pipe warms up. A foam of proven noncamhustibility should be selected not only because of the enriched air possibility but also because i t is always desirable to reduce the quantity of cambustibles present in cryogenic facilibies; these facilities are usually so expensive that a fire of just a few pounds of extraneous combustibles can do a dis.proportionate amount of damage.
Special Considerations for Liquefied Oxygen The common classification of materials into combustible and noncombustible is based upon experience with ztmospheric air. The removal of inert Ns as well as the 700 fold concentration by liquefaction in making liquefied oxygen concentrates O1 4,000 fold over the atmosphere. Oxidation reactions, once initiated, proceed a t enormously enhanced rates in liquefied oxygen. Alaterials not normally combustible react a t rapid rates and combustible materials react a t explosive-like rates. These two reported examples typify the problems with liquefied oxygen:
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1) A maintenance man wanted some material to solidify some plastic. A millwright was sent to the sir separation plant where he contacted the operator. At that moment a technician was taking a. sample of liquid oxygen. The operator painted to the sample and the millwright took the container nf liquid oxygen back to the plastic plant where it was promptly used to freeze the molten plastic %round the equipment. The idea was to get the plastic hard so that they could chip i t off. After the plastic way hard the millwright hit it with s. hammer and chisel. I t exploded in his face; although he was badly eat his vision was saved by safety glasses. 2) A substantial spill of liquefied oxygen soaked into sheet metal-clad wooden floors. Part of the flooring had been carbonized from a previous unknown smouldering fire which had been caused by repairs to the torn metal cladding several days before. A minor fire resulted in a detonation which killed 15 people and severely damaged the plant. The cryogenic form of oxygen with its ability to penetrate and persist can produce detanable mixtures with organic materials.
Fires and Explosions For a combustible cryogen such as LH? or LNG (liquefied natural gas) to bum it must be mixed with an oxidant (usually air) within appropriate limits, called flammable ranges, and be initiated by an ignition source. Thus the three conditions of the classical fire triangle (fuel, air, ignition) are to be satisfied for a fire or explosion t,o occur. Normal control me* sures seek to eliminate two of the three conditions, namely, by avoiding ignition sources and by controlling the release, or accumulation, of the fuel. I n some unusual circumstances where large pipes with high flow rates must go through confined spaces and even heroic ventilation measures could not safely dilute a. release of fuel than one might control the third side of the triangle, namely, exclude air by introducing inerting g a ~ such aj nitrogen or argon. Methane forms flammable mixtures with mom air over the concentration range 5 to 15 volume percent. The minimum value of electrical spark energy reqoirement far ignition is ahout 0.3 millijoules. Hydrogen forms flammable mixtures with room d r over a wide concentration range 4 to 7.5 volume percent. The minimum value of electrical spark energy reauirement for ienitian is about 0.02 mil&
Control of Ignition Sources Smoking, open lights and sparking devices ahohould obviously he avoided in sreas where hydrogen or methane might he present. Whenever work is to bc performed in areas where the continued absence of combustible gas cannot be confidently (Continued a page A4781 Volume 47, Number 7, July 7 970
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assured, t,he need for doing this work and the methods employed must be scrutinized by the appropriate aiit,horit.y, most likely the operating supervisors. The me of "nonsparking" tools cannot be relied upon to avoid ignition if gas is present. They offer little safety value compared to t,he eliminat,ion of fuel wherever tools are used. An aluminum painted iron surface can readily spark when struck, one should woid such Thermit combinations in closed areas. The National Fire Code, Volume 5, "Electrical," one of the ten volumes published by the National Fire Protection Association, covers the safe installation and use of eleotrieal equipment. Articles 500-517 provide standards for the use of electrical equipment in hazardous meas. For atmospheres containing methane Group D (Class 1) fixtures are specified. Because of the high explosion pressure and small quenching dist,ance approved equipment for hydrogen is difficult to build and therefore expensive when available. Some ingenuity may be employed to control the electrical sparking problem without the necessity of the mare expensive devices approved far hssardous areas, pa,rticularly in hydrogen service. If electrical devices spark with less energy necessary for the ignition of a gas/air mixt,ure they are intrinsically safe and may be used in the hazardous zone. Such devices are being developed
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a t an increasing rate. An alternate to intrinsically safe or explosion proof is to place the equipment in an enclosure purged above ambient pressure with an inert ga3 such as nitrogen or with air from an uncontaminated source. Locating electrical equipment ont,side of hazardous area? is a good choice if i t can be done. Light fixtures, switches, outlet? and instruments are examples of devices that can often be located in a leis hasmd011s or nonhazardous environment.
Deflagration and Detonation Ignition of combustible ga~/air mixtures by the previously discussed sources usually result in ordinary reactions of cambustian or deflagratians. These reactions continue through t,hat portion of the unburned mixture within the app~apriate range, a t significantly less t,han sonic speeds. However, under some circumstances, usually of confinement, a deflagration may change into a det,onatian wherein the reaction propagates into the unburned mixture a t supersonic rates. If cambustible gss/air mixtures can occur in confined volum&?, the destructive effect of explosions can he avoided by providing pressure relief of sufficient area. and ease of opening so that the progression from deflegrntion to detonation will he interrupted as the products of combustion and the nnbnmned mixture can expand into free space. Where possible the introduction of an inert diluent or sufficient quantities of dihting air can also minimize the possibility of a destructive explosion.
The energy available in combustible cryagens and in t,heir ga? p h a w is enor-. mom when related t,o the energy available in eandcnsed chemical euplosivoa. Met,l~ane has 12 times the energy of an equal weight of trinitrotoluene; hydrogen h a available 29 fold the energy of equal T N T weight. Nevertheless, in a combustible gas/air explosion not all the available heat e m he converted into apressure wave. I n actual events the rate of energy release bears largely on the effect produced. Mixing is not always uniform nor within an approximately stoiohiometric range so only a small percentage of the available fuel is usually involved at a giveninstant. There is some evidence that accidental hydrogen explosions in free space can do damage but this damage has been observed only in light structures. Reider and 0thers described sn experiment wherein hydrogen was released into the air a t the rate of 120 lh/sec; inadvertent ignition yielded a pressure wave equivalent to 100 miles/hr wind. The R-38 (hydrogen lift) dirigible disaster over Hull, England in 1921 is reported to have shattered windows over a two mile radius. Cassntt and others were able to get an explosive yield in staichiometrio hydrogenair mixtures in s. 5 f t diameter balloon only when using a.strong initiating source of 2 g of pentolite. Using a blasting cap initiztor, which is approximately g of explosive, reduced the yield 95%, while flame sources, sparks, and hot wires gave only combustion of the gases with no meawrable pressures.
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Burning Hydrogen Leaks Burning hydrogen exhibits such low emissivity that leaks (audible as well as inaudible) may be aflame and yet not be visible. This nonluminous flame may make i t difficult to detect a. fire from a small lesk. Special devices (ultraviolet, infrared and television type detectors) have been developed but are not in common use. The use of s. broom has been recommended as s. physical detection device since the burning straws held out a t a safe distanoe can be readily seen. But people have received painful, if not serious burns, while searching far leaks or inadvertently finding one. Of course, large leaks may well be audihle and this would be a warning.
Safety Training Every employee involved with cryagens must be conscious of misadventure potential and therefore safety-minded. This consciousness can and should be developed by regular and specific training including cryogenics among other safety subjects. Circumtances of actual and near accidents should be m d e known; incidents may be found in the literature. Safety papers may be assembled in topical collections and made available to employees at d l technical levels to enhance their knowledge of an interest in the safe use of cryogens.
Operating Procedures In potentially hasardous operations a standard procedure should be used. Such sproeedure may be developed by the ufiing personnel but should be approved by a competent higher authority. Deviations should he permitted only by the approving authority. Cheok lists may be used along with such procedures. Procedures should be reviewed a t intervals determined to he appropriate on the basis of frequency of use, change in programs or facilities, development of experience or new information, and personnel changes. I t has been found useful for persons outside the chain of responsibility, such as safety or consulting authorities, to contribute to these reviews. Significmt procedural or facility changes should be carefully reviewed for possibly adverse influences on safety.
Emergency Plans An understanding of the principal hazards and procedures to be followed in an emergency by the personnel required to work in a hazardous area is generally accepted to be of major import from the standpoint of safety. Elaborate emergency plans and emergency equipment that bear little resemblance to credible accidents should be avoided for fear thrtt they could detract tram the important elements of emergencies. I t has been common practice in the technologies potentially of high hazard to go to considerable effort to acquaint employees with the comequences of misadventure.
With s. reasonable knowledge of such consequences the employee can he additionally motivated toward safety from the natural interest of self-preservation. Not only is a reasonable understanding of risk fundamental to the enhancement of self-motivation for safety, but also that same understanding of risk might relieve uneasiness about a new and presumably exotic risk and permit a more rational approach to emergency situations and to safety improvements. Fire brigade or other personnel who might be oalled upon to respond to an emergenoy should dso he specifically trained and made aware of the unique problems of the cryogenic facility. If they represent an independent ituthority, they should be encouraged to make their own plans for emergency response, subject to technical counsel or approval. Significant changes in the facility or the cryogenic operations require a. reevaluation of emergency plans. The burning gas evolving from a. leak of combustible cryogenic fluid should not necessarily be promptly extinguished. A continning gas leak might be more dangerous; therefore, shutting off the supply is an important emergency consideration. Liquefied natural gas (methane) authorities insist "that a major fire should be controlled-not extinguished!'
Driver Training The person accompanying and responsible far the shipment of 8. cryogen should be aware of the nature and consequences of a. mishap involving his load. IIe should receive training in emergency aotian required and be provided with specific instructions. These instructions should fire and first aid measures. loadinrhrde -ing and travel requirements as necessary, and sources for additional aid. ~~~~~~~~~~
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Labeling Containers and pipelines should be labeled with the common name of the contents. Shipping containers that might meet with an emergency evoking response by public, and perhaps not fully aware, protective forces, should also be labeled with the nature of the risk-flammable, toxic, corrosive. Jargon should be avoided even within an isolated establishment; if color codes or numerical or letter codes m G t be used they should be accompanied by appropriate legend. I n this literate age there is little need to retain often misunderstood code designations.
Liquid Nitrogen Irradiation Problems The use of liquefied nitrogen around high radiation fields has resulted in a unique hazard. The radiative generation of ozone or nitrogen oxides has been s u e gested as the mechanism for explosions that occur with the evaporation of the nitrogen. The most significant control measure is the avoidance of contaminating the nitrogen with oxygen condensed from the atmosphere. Liquid nitrogen used around radiation fields should have a maximum allowable concentration of 20 ppm 0% and 5 ppm oxidizable hydrocarbons. Circle No. 112 01 Readers' Service Card
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