Page 192 - 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

Cryogenic Process Engineering 35

purposes should never be below 16%. Whenever proper resulting final pressure. Relief valves and burst disks are
air ventilation cannot be ensured, air-line respirators or a normally used to relieve piping systems at a pressure near
self-contained breathing apparatus should be used. the design pressure of the equipment. Such relief should
An oxygen-enriched atmosphere, on the other hand, be provided between valves, on tanks, and at all points of
produces exhilarating effects when breathed. However, possible (though perhaps unintentional) pressure rise in a
lung damage can occur if the oxygen concentration in piping system.
the air exceeds 60%, and prolonged exposure to an atmo- Overpressure in cryogenic systems can also occur in a
sphere of pure oxygen may initiate bronchitis, pneumonia, more subtle way. Vent lines without appropriate rain traps
or lung collapse. An additional threat of oxygen-enriched can collect rainwater. Which when frozen can block the
air can come from the increased flammability and explo- line. Exhaust tubes on relief valves and burst disks like-
sion hazards. wise can become inoperable. Small-necked, open-mouth
dewars can collect moisture from the air and freeze closed.
Entrapment of cold liquids or gases can occur by freez-
B. Materials Compatibility
ing water or other condensables in some portion of the
Most failures of cryogenic systems can generally be traced cold system. If this occurs in an unanticipated location,
to an improper selection of construction materials or a the relief valve or burst disk may be isolated and afford no
disregard for the change of some material property from protection. Such a situation usually arises from improper
ambient to low temperatures. For example, the ductility operating procedures and emphasizes the importance of
property of a material requires careful consideration since good operating practices.
low temperatures have the effect of making some con- Another source of system overpressure that is fre-
struction materials brittle or less ductile. This behavior quently overlooked results from cooldown surges. If a
is further complicated because some materials become liquid cryogen is admitted to a warm line for the purpose
brittle at low temperatures but still can absorb consider- of transfer of the liquid from one point to another, severe
able impact, while others become brittle and lose their pressure surges will occur. These pressure surges can be
impact strength. Brittle fracture can occur very rapidly, up to 10 times the operating or transfer pressure and can
resulting in almost instantaneous failure. Such failure can even cause backflow into the storage container. Protection
cause shrapnel damage if the system is under pressure, against such overpressure must be included in the overall
while release of a fluid such as oxygen can result in fire or design and operating procedures for the transfer system.
explosions. In making an accident or safety analysis, it is always
Low-temperature equipment can also fail because of wise to consider the possibility of encountering even more
thermal stresses caused by thermal contraction of the serioussecondaryeffectsfromanycryogenicaccident.For
materials used. In solder joints, the solder must be able example,anyoneofthefailuresdiscussedpreviously(brit-
to withstand stresses caused by differential contraction tle fracture, contraction, overpressure, etc.) may release
where two dissimilar metals are joined. Contraction in sizable quantities of cryogenic liquids, causing a severe
long pipes is also a serious problem; a stainless-steel fire or explosion hazard, asphyxiation possibilities, fur-
pipeline 30 m long will contract ∼0.085 m when filled ther brittle fracture problems, or sharpnel damage to other
with liquid oxygen or nitrogen. Provisions must be made flammable or explosive materials. In this way the situation
for this change in length during both cooling and warming can rapidly and progressively become much more serious.
of the pipeline by using bellows, expansion joints, or flex-
ible hose. Pipe anchors, supports, and so on likewise must C. Flammability and Detonability
be carefully designed to permit contraction and expansion
to take place. The primary hazard of failure due to thermal Almost any flammable mixture will, under favorable con-
contraction is spillage of the cryogen and the possibility ditions of confinement, support an explosive flame propa-
of fire or explosion. gation or even a detonation. When a fuel–oxidant mixture
All cryogenic systems should be protected against over- of a composition favorable for high-speed combustion is
pressure due to phase change from liquid to gas. Systems weakened by dilution with an oxidant, fuel, or an inert
containing liquid cryogens can reach bursting pressures, substance, it will first lose its capacity to detonate. Fur-
if not relieved, simply by trapping the liquid in an enclo- ther dilution will then cause it to lose its capacity to burn
sure. The rate of pressure rise depends on the rate of heat explosively. Eventually, the lower or upper flammability
transfer into the liquid. In uninsulated systems, the liq- limits will be reached and the mixture will not maintain its
uid is vaporized rapidly and pressure in the closed system combustion temperature and will automatically extinguish
can rise very rapidly. The more liquid there is originally itself. These principles apply to the combustible cryogens
in the tank before it is sealed off, the greater will be the hydrogen and methane. The flammability and detonability