Page 191 - 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

34 Cryogenic Process Engineering

have acquired greater commercial importance, questions tensive properties and density through accurate thermo-
have arisen relative to the quantities of these fluids trans- physical properties data.
ferred or delivered. Accordingly, the instrumentation used The case of nonhomogeneous fluids is quite differ-
must be able to indicate liquid level, density, and flow rate ent. LNG is often a mixture of five or more components
accurately. whose composition and, hence, density vary with time
and place. Accordingly, temperature and pressure mea-
surements alone will not suffice. A dynamic, direct mea-
A. Thermometry surementisrequired,embodyingoneormoreoftheliquid-
Most low-temperature engineering temperature measure- level principles used in liquid-level measurements.
ments are made with metallic resistance thermometers,
nonmetallic resistance thermometers, or thermocouples. D. Flow Measurements
In the selection of a thermometer for a specific application
Three basic types of flow meters are useful for liquid cryo-
one must consider such factors as absolute accuracy, re-
gens. These are the pressure drop or “head” type, the tur-
producibility, sensitivity, heat capacity, self-heating, heat
bine type, and the momentum type.
conduction, stability, simplicity and convenience of oper-
ation, ruggedness, and cost. Other characteristics may be
of importance in certain applications.
VIII. SAFETY

B. Fluid Measurements No discussion of cryogenic systems would be complete
without a review of some of the safety aspects associated
Liquid level is one of several measurements needed to es-
with either laboratory or industrial use of cryogenic fluids.
tablish the contents of a cryogenic container. Other mea-
Ealier discussion of the properties of cryogenic fluids and
surements may include volume as a function of depth,
the behavior of materials at low temperatures revealed
density as a function of physical storage conditions, and
that there are a number of unique hazards associated with
sometimes discerning useful contents from total contents.
cryogenic fluids. These hazards can best be classified as
Of these measurements, the liquid-level determination is
those associated with the response of the human body
presently the most advanced and can be made with an ac-
and the surroundings to cryogenic fluids and their vapors,
curacy and precision comparable to that of thermometry
and those associated with reactions between certain of the
and often with greater simplicity.
cryogenic fluids and their surroundings.
There are as many ways of classifying liquid-level
sensors as there are developers who have described them.
A convenient way to classify such devices is according A. Human Hazards
to whether the output is discrete (point sensors) or
continuous. It is well known that exposure of the human body to cryo-
genic fluids or to surfaces cooled by cryogenic fluids can
result in severe “cold burns” since damage to the skin or
C. Density Measurements
tissue is similar to that caused by an ordinary burn. The
Measurements of liquid density are closely related to severity of the burn depends on the contact area and the
quantity and liquid-level measurements since both are of- contact time; prolonged contact results in deeper burns.
ten required simultaneously to establish the mass contents Severe burns are seldom sustained if rapid withdrawal is
of a tank, and the same physical principle may often be possible.
used for either measurement, since liquid-level detectors Protective clothing is mandatory to insulate the body
sense the steep density gradient at the liquid–vapor inter- from these low temperatures and prevent “frostbite.”
face. Thus, the methods of density determination include Safety goggles, gloves, and boots are imperative for per-
the following techniques: direct weighing, differential sonnel involved in the transfer of liquid cryogens. Such
pressure, capacitance, optical, acoustic, and nuclear radi- transfers, in the interest of good safety practices, should
ation attenuation. In general, the various liquid level prin- be attempted only when sufficient personnel are available
ciples apply to density measurement techniques as well. to monitor the activity. Since nitrogen is a colorless, odor-
Two exceptions are noteworthy. In the case of homo- less, inert gas, personnel must be aware of the associated
geneous pure fluids, density can usually be determined respiratory and asphyxiation hazards. Whenever the oxy-
more accurately by an indirect measurement, namely, the gen content of the atmosphere is diluted due to spillage or
measurement of pressure and temperature which is then leakage of nitrogen, there is danger of nitrogen asphyxi-
coupled with the analytical relationship between these in- ation. In general, the oxygen content of air for breathing