breaking the vacuum. Over pressurization also
expands the hose axially, causing it to grow by
lengthening the distance between corrugations. This
makes the line less flexible (in much the same way a
charged fire hose gets stiff) and can possibly lead to
breaking the seal between the joints.
Another important consideration is to never allow
liquid to be trapped in a line between closed valves,
unless there is a relief device, such as a pressure-relief
valve, protecting the inner line. Trapped liquid
vaporizes, warms, and exerts excessive pressure on the
walls of the line.
As another consideration, you should avoid
coupling two or three transfer sections together where
one will do the job. If you use more hose than is
required, more line material has to be cooled and more
liquid is lost to vaporization. If that happens, more
liquid is required to satisfy a given transfer demand.
Valves
Low-temperature valves used in LOX systems
often cause more trouble than any other element of the
system. Liquid oxygen valves must be able to function
properly at extremely low temperatures without
causing excessive boil-off losses. The best method for
meeting the thermal considerations for such valves is
through insulation. Several materials have been used
ASf10004
for insulation, but the most effective method is to
Figure 10-4.--Vacuum insulated cryogenic globe valve.
vacuum-jacket the valve. The valve illustrated in figure
10-4 is typical of this type.
LIQUID OXYGEN SERVICING TRAILERS
(LOX CARTS)
LOX carts are used to service oxygen converters
used in naval aircraft. Although there may still be some
older carts in use at some activities, the two most
commonly used LOX carts are the TMU-27/M and the
TMU-70/M, shown in figure 10-5.
The TMU-27/M LOX cart is called a standard
trailer. Standard trailers vent gaseous and liquid
ASf10005
oxygen overboard through the vent fitting of the LOX
converter as it is being filled. The TMU-70/M, on the
Figure 10-5.--TMU-70/M low-loss, closed-loop, liquid oxygen
other hand, is called a low-loss, closed-loop (LLCL)
servicing trailer.
trailer. It recaptures the vented gases and returns them
back to the storage tank for recovery.
and can only service converters that have been
removed from the aircraft.
The TMU-27/M has a long service hose so it can
service aircraft LOX converters in the aircraft. The
The TMU-70/M LOX servicing trailer, shown in
TMU-70/M, on the other hand, has a short service hose
figure 10-5, is manufactured by the Cryogenic
10-4
