inner and outer container separated by a circular
insulated space (the annulus) that is packed with
a powder-type insulating material and highly
vacuum pressured to minimize heat transfer and
evaporation losses.
TRANSFER LINES
Aside from certain large fixed facility piping,
most LOX systems are made up of transfer
lines—often segmented (in sections) for ease of
demountability (reconnecting) so one transfer
system can service several pieces of equipment.
Generally, what is true for insulation of tanks is
applicable to other similar equipment with the
exception of transfer lines. Transfer lines are
cooled and warmed many times during the course
of a day. The liquid oxygen waste due to
cooldown losses can be significant. A high
vacuum insulated transfer line generally is best for
quick, frequent transfers since no insulated
cooldown mass is involved. To help reduce LOX
waste due to cooldown of transfer lines, several
wraps of multilayer insulation adds very little mass
to the system, decreases the thermal radiation, and
requires less vacuum (if any) in the insulation
annulus.
Transfer lines are most commonly constructed
of bronze, stainless steel, or aluminum. In the case
of vacuum-jacketed lines, annular spaces are
necessary to prevent the inner liquid-bearing lines
from touching the outer jacket wall. Low thermal
conductivity materials are used for this purpose,
and schemes are incorporated into the spacer
design to provide only a small area contact
wherever possible, since the heat influx must be
kept low.
Although rigid lines often are used in
stationary facility piping installations and for
some remountable applications, the predominant
type for use in the field is corrugated flexible metal
transfer hose (fig. 5-2).
Flexible metal hose is somewhat more
vulnerable to abuse than its rigid counterpart;
therefore, it usually has a braided metallic
covering or tough plastic sheath over the
external corrugations. The high working pressure
capability of the inner line requires the application
of a strengthening braid over it as well. Even
though such protection provides a degree of
reinforcement to the hose, it should not be
subjected to overpressurization in service since a
bellowslike action still may be possible if the
covering is not securely fastened at the end
connections. High pressure expands the hose
axially, causing it to grow by lengthening the
distance between each corrugation, which makes
the line less flexible.
A vital consideration in the construction of all
transfer lines is the matter of joints between line
segments. Assuming that the transfer lines are
vacuum-jacketed to the general region of the joint,
the concern with the joint and the closure of the
vacuum-jacket in close proximity to the joint is
important. The transfer line connector/coupling
(bayonet coupling) in figure 5-3 is one of the
better designs available today, and the most
expensive.
To safely and efficiently use transfer lines,
several things must be remembered. NEVER trap
liquid in a line between closed valves unless you
are absolutely certain that some type of relief
device is functionally associated with the inner
Figure 5-2.—Flexible metal transfer hose.
5-2
