Federal Specification WW-T-799, Type N. Low-pressure oxygen systems
use a larger diameter aluminum tubing with flared aluminum fittings.
Only in case of an emergency can copper tubing with the same diameter
and wall thickness of the aluminum tubing be used to replace it. It
must then conform to Federal Specification WW-T-799, Type N.
Steel
tubing must not be used to replace high-pressure oxygen system copper
tubing because it loses ductility and becomes brittle at low
temperatures.
ROUTING OF LINES
If a damaged line is discovered, the first step for repair is to
determine the cause of the damage.
If it was caused by chafing
structural members of the aircraft or poor layout planning, the
condition must be corrected. If the line was defective and the same
layout is acceptable, carefully remove the damaged tube and use it as
a pattern for fabrication of the replacement tube.
Generally, replacement lines follow the path of the original line;
however, when the line must be rerouted use the standards that are
discussed in the paragraphs that follow.
Number of Bends.
When fluid flows around a bend, it creates
friction which generates heat and causes an overall loss in system
efficiency.
With this in mind, tubing layout must always follow a
path that results in gradual bends. On the other hand, a path with
no bends is likely to result in even more problems. First, to cut a
replacement line to an exact length is virtually impossible.
This
can result in a mechanical strain being exerted on the tube when the
attaching nut is drawn up on the fitting.
Because the greatest
amount of strain is already concentrated on the flared portion of the
tube as a result of the flaring operation, this additional strain is
likely to weaken the tube beyond tolerances. Second, if the tube has
no bends it cannot flex when subjected to vibrations.
This lack of
flexibility promotes fatigue of the tubing metal and makes it more
susceptible to failure.
Third, a straight line installation allows
no provision for the normal contraction and expansion of the tubing
caused by temperature change. Examples of correct and incorrect tube
layout are shown in Figure 2-3.
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AL0907
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