Page 193 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

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Encyclopedia of Physical Science and Technology EN004D-156 June 8, 2001 15:28

36 Cryogenic Process Engineering

TABLE IV Flammability and Detonability Limits of dense air and cause oxygen enrichment of the liquid nitro-
Hydrogen and Methane Gas gen. The composition of air as it condenses into the liquid
Flammability Detonability nitrogen container is about 50% oxygen and 50% nitro-
Mixture limits (mol%) limits (mol%) gen. As the liquid nitrogen evaporates, the liquid oxygen
content steadily increases so that the last portion of liquid
H 2 –air 4–75 20–65
to evaporate will have a relatively high oxygen concen-
H 2 –O 2 4–95 15–90
tration. The nitrogen container must then be handled as if
CH 4 –air 5–15 6–14
it contained liquid oxygen. Explosive hazards all apply to
CH 4 –O 2 5–61 10–50
this oxygen-enriched liquid nitrogen.
Since air condenses at temperatures below ∼82 K, unin-
sulated pipelines transferring liquid nitrogen will con-
limits for these two cryogens with either air or oxygen are dense air. This oxygen-enriched condensate can drip on
presented in Table IV. Since the flammability limits are combustible materials, causing an extreme fire hazard or
rather broad, great care must be exercised to exclude oxy- explosive situation. The oxygen-rich air condensate can
gen from these cryogens. This is particularly true with saturate clothing, rags, wood, asphalt pavement, and so
hydrogen since even trace amounts of oxygen will con- on and cause the same problems associated with the han-
dense, solidify, and build up with time in the bottom of dling and spillage of liquid oxygen.
the liquid hydrogen storage container and eventually at-
tain the upper flammability limits. Then it is just a matter
D. Summary
of time until some ignition source, such as a mechanical or
electrostatic spark, accidentially initiates a fire or possibly It is obvious that the best designed cryogenic facility is
an explosion. no better than the attention paid to every potential hazard.
Because of its chemical activity, oxygen also presents Unfortunately, the existence of such potential hazards can-
a safety problem in its use. Liquid oxgen is chemically not be considered once and then forgotten. Instead, there
reactive with hydrocarbon materials. Ordinary hydrocar- must be an ongoing safety awareness that focuses on every
bon lubricants are even dangerous to use in oxygen com- conceivable hazard that might be encountered. Assistance
pressors and vacuum pumps exhausting gaseous oxygen. with identifying these safety hazards is adequately cov-
In fact, valves, fittings, and lines used with oil-pumped ered by Edeskuty and Stewart (1996).
gases should never be used with oxygen. Serious explo-
sions have resulted from the combination of oxygen and
hydrocarbon lubricants. SEE ALSO THE FOLLOWING ARTICLES
To ensure against such unwanted chemical reactions,
systems using liquid oxygen must be kept scrupulously CHEMICAL ENGINEERING THERMODYNAMICS • CRYO-
clean of any foreign matter. The phrase “LOX clean” GENICS • HEAT EXCHANGERS • METALLURGY,MECHAN-
in the space industry has come to be associated with ICAL • SUPERCONDUCTIVITY MECHANISMS • VACUUM
a set of elaborate cleaning and inspection specifications TECHNOLOGY
nearly representing the ultimate in large-scale equipment
cleanliness.
Liquid oxygen equipment must also be constructed of BIBLIOGRAPHY
materials incapable of initiating or sustaining a reaction.
Onlyafewpolymericmaterialscanbeusedinthedesignof Barron R. F. (1986). “Cryogenic Systems,” Oxford Univ. Press, London.
such equipment since most will react violently with oxy- Edeskuty, F. J., and Stewart, W. F. (1996). “Safety in the Handling of
gen under mechanical impact. Also, reactive metals such Cryogenic Fluids,” Plenum Press, New York.
Flynn, T. M. (1996). “Cryogenic Engineering,” Dekker, New York.
as titanium and aluminum should be used cautiously, since
Jacobsen, R. T., Penoncello, S. G., and Lemmon, E. W. (1997). “Thermo-
theyarepotentiallyhazardous.Oncethereactionisstarted,
dynamic Properties of Cryogenic Fluids,” Plenum Press, New York.
an aluminum pipe containing oxygen burns rapidly and Ross, R. G., Jr. (1999). “Cryocoolers 10,” Kluwer Academic/Plenum
intensely. With proper design and care, however, liquid Publishers, New York.
oxygen systems can be operated safely. Timmerhaus, K. D., and Flynn, T. M. (1989). “Cryogenic Process Engi-
Even though nitrogen is an inert gas and will not sup- neering,” Plenum Press, New York.
Van Sciver, S. W. (1986). “Helium Cryogenics,” Plenum Press, New
port combustion, there are some subtle means whereby a
York.
flammable or explosive hazard may develop. Cold traps Weisend, J. G., II (1998). “Handbook of Cryogenic Engineering,” Taylor
or open-mouth dewars containing liquid nitrogen can con- & Francis, London.