sure is discharged into a flexible hose which connects to the charging valve and the adapter. The charging valve controls oxygen flowing away from the servicing trailer, and the adapter connects the recharging equipment to the aircraft filler valve.
On many aircraft a chart is located adjacent to the filler valve which shows the safe maximum charging pressure for the ambient temperature. This must be observed when charging the system.
It is common practice to have a warning placard cautioning against using oil or grease on the filler connections. Oxygen ground equipment should be maintained to a standard of cleanliness comparable to that of the aircraft system.
Leak Testing Gaseous Oxygen Systems This test is performed at different times, depending on the inspection requirements for the particular type of aircraft. The system is allowed to cool, usually 1 hr., after filling before the pressures and temperatures are recorded. After several hours have elapsed, they are recorded again. Some manufacturers recommend a 6-hr. wait, others a 24-hr. wait.
The recorded pressures are then corrected for any change in temperature since filling. Figure 14-62 is typical of the graphs provided in the aircraft main-
• 50 - 25 45
- 20 CHANGE IN TEMPERATURE-°C +15.
- 10 5 10 15 20 25 30 35 40
CHANGE IN PRESSURE-PSI
CORRECTION OF PRESSURE DURING LEAKAGE
TEST FOR CHANGE IN TEMPERATURE
ADD PRESSURE CHANGE IF TEMP.
ERATURE RISES:
SUBTRACT PRESSURE CHANGE IF TEMP.
ERATURE FALLS FicURE 14-62. Pressure/temperature correction chart.
tenance manual to aid in making pressure/temperature corrections.
As an example for using the chart, assume that the oxygen system was recently charged. An hour later, the oxygen pressure gage read 425 p.s.i. at a temperature of 72° F. When the pressure gage was read 6 hrs. later, the pressure was 430 p.s.i. with a temperature of 79° F. By referring to figure 14-57, we see that the 7° temperature rise should have caused the pressure to increase 5 p.s.i., making the pressure gage read 430 p.s.i.
When oxygen is being lost from a system through leakage, the gage reading will be less than shown on the pressure/temperature correction chart. Leakage can often be detected by listening for the distinct hissing sound of escaping gas. If the leak cannot be located by listening, it will be necessary to soap-test all lines and connections with a castile soap and water solution or specially compounded leak-test material. To make this check, apply the test solution to areas suspected of leakage. Watch for bubbles. Make the solution thick enough to adhere to the contours of the fittings.
Any leak no matter how small must be found and repaired. A small leak may not cause trouble but if leak continues over a period of time, the surroundings and atmosphere become saturated Such conditions are especially dangerous because personnel may not be aware that oxygen-enrichment exists. Oxygen-enriched conditions are almost always present in poorly ventilated areas.
NO ATTEMPT SHOULD BE MADE TO
TIGHTEN A LEAKING FITTING WHILE THE SYSTEM IS CHARGED.
Draining the Oxygen System When it is necessary to drain the system, it can be done by inserting a filler adapter into the filler valve and opening the shutoff valves. Do not drain the system too rapidly as this will cause condensa tion within the system. An alternate method of draining the system is opening the emergency valve on the demand oxygen regulator. Perform this job in a well-ventilated area and observe all fire precautions.
Cleaning the Oxygen System Always keep the external surfaces of the components of the oxygen system, such as lines, connections, and mounting brackets, clean and free of corrosion and contamination with oil or grease.
As a cleaning agent, use anhydrous (waterless) ethyl alcohol, isopropyl alcohol (anti-icing fluid), 599
or any other approved cleaner. If mask-to-regulator hoses are contaminated with oil or grease, the hoses should be replaced.
Cleaning Compound, Oxygen System An approved cleaning formula for use on oxygen systems is available. This mixture of chlorinated, fluorinated hydrocarbons (freon) and isopropyl alcohol is safe for cleaning oxygen system components in aircraft, and for rinsing, flushing, and cleaning oxygen lines. Skin contact and prolonged inhalation of vapors should be avoided.
Purging the Oxygen System An oxygen system needs to be purged if: (1) It has been depleted and not re-charged within 2 hrs.,
(2) if any line or component is replaced, requiring the draining or opening of the system for more than 2 hrs., or (3) it is suspected that the system has been contaminated.
The main cause of contamination in the system is moisture. Moisture in the system may be due to damp charging equipment. In very cold weather the small amount of moisture contained in breathing oxygen can cause contamination, due to repeated Although the introduction of moisture into the aircraft oxygen system can be considerably reduced by using the correct charging procedure, cumulative condensation in the system cannot be entirely avoided. There have been instances where oxygen systems, unused for long periods, have developed an unpleasant odor which necessitated purging to clear the system of moisture.
The procedure for purging may vary somewhat with each aircraft model. Generally speaking, on aircraft having the filler lines and the distribution lines commonly connected to one end of the storage cylinder, the system can be purged by filling the system with oxygen and then draining it at least three times. On aircraft that have the filler lines connected on one end of the cylinder and distribution lines connected to the opposite end of the cylinder, purge the system as follows: With all the regulator emergency palves open, pass oxygen at a pressure of 50 p.s.i. at the filler valve through the system for at least 30 min. Perform this job in a well-ventilated area and observe all fire precautions.
Dry nitrogen and/or dry air may also be used to purge oxygen systems. All open lines must be capped after use, also the system lines must be purged of the nitrogen by use of oxygen.
PREVENTION OF OXYGEN FIRES OR EXPLO SIONS Many materials, particularly oils, grease, and non-metallic materials, are likely to burn when exposed to oxygen under pressure. To avoid fire or an explosion it is essential that all oxygen equipment be kept clean and free from oil or grease.
An oxygen fire or explosion depends on a combination of oxygen, a combustible material, and heat.
The danger of ignition is in direct ratio to the concentration of oxygen, the combustible nature of the material exposed to the oxygen, and the temperature of the oxygen and material. Oxygen itself does not burn but it supports and intensifies a fire with any combustible material.
When working on an oxygen system it is essential that the warnings and precautions given in the aircraft maintenance manual be carefully observed. In general, before any work is attempted on an oxygen system the following fire precautions should be taken:
(1) Provide adequate fire-fighting equipment.
(2) Display "NO SMOKING" placards.
(3) Avoid checking aircraft radio or electrical systems.
(4) Keep all tools and oxygen servicing equipment free from oil or grease.
Oxygen System Inspection and Maintenance Oxygen system inspection and maintenance should be accomplished according to these precautionary measures and any in addition to the manufacturer's instructions.
1. Never attempt maintenance until oxygen supply is turned off.
2. Fittings should be unscrewed slowly to allow residual pressure to dissipate.
3. Plug or cap all open lines immediately.
4. Do not use masking tape to seal openings:
use caps or plugs designed for that purpose.
5. Maintain at least 2 inches clearance between oxygen lines and all moving equipment/ parts within the aircraft to prevent the possibility of wearing oxygen lines.
6. Maintain at least 2 inches clearance between oxygen lines and all electrical wiring in the aircraft.
7. Provide adequate clearance between oxygen lines and all hot ducts, conduits and equip- 600
ment to prevent heating of the oxygen system.
8. Maintain at least 2 inches clearance between oxygen lines and all oil, fuel, hydraulic, or other fluid lines to prevent contamination.
* U.S. G.P.O.: 1996-873-S/20104 601
9. Do not use lubricants unless specifically approved for oxygen system use.
10. A pressure and leak check must be performed each time the system is opened for maintenance.