TM 1-411, Airplane Hydraulic Systems and Miscellaneous Equipment: 5 - Gases

SECTION V: GASES

37. General.-a. The properties and basic principles of fluids considered in the preceding study of aircraft hydraulic systems apply only to fluids in a liquid state. Miscellaneous aircraft equipment, however, involves fluids in a gaseous state and a study of this equipment necessitates some discussion of a few of the general properties and principles of gases and vapors.

b. The particular properties of each of the gases and vapors used in miscellaneous equipment will also be considered in this section.

38. Properties.-a. While liquids are considered practically incompressible (sec. I), fluids in a gaseous state, due to the freedom of their molecules, are very easily compressed or expanded. A consideration of this property presents the following three factors: pressure, temperature, and volume. The relationship of these three, factors when applied to a gas is governed by the general law of gases.

b. This law states-

(1) That for a given mass of gas the volume varies inversely as the pressure if the temperature is constant;

(2) That, the volume varies directly as the temperature if the pressure is constant;

(3) That the pressure varies directly as the temperature if the volume is constant.

The first and third of these laws can be applied to the storage of gases. From (1) above, if the temperature is held constant and the pressure is increased, the volume will be proportionally decreased; so that by using high pressures relatively large quantities of gas can be stored in cylinders of small volume and the amount of gas stored will be in direct proportion to the cylinder pressure. Both of these facts are employed in the storage of gases for use in miscellaneous aircraft equipment. From (3) above it is seen that the cylinder pressure depends directly on the temperature of the enclosed gas. Care then must be taken in the placement of these storage devices to prevent excessive internal pressures due to increased temperatures.

c. In the application of gases and vapors to miscellaneous equipment, consideration must also be given to the other states of matter (liquid and solid) and the methods of changing matter from one state to another. Most substances change from the solid to the liquid state when sufficiently heated; thus ice changes to water. When sufficiently cooled, liquids change back to solids, e. g., the freezing of water. By application of sufficient heat or a sufficient reduction in pressure most substances change from the liquid state to the gaseous state; thus when sufficiently heated, water changes to steam by ebullition. (boiling), and when subjected to a reduced pressure, gasoline vaporizes by evaporation. This process is also reversible, for by either decreasing the temperature or increasing the pressure, or both, substances can be changed from the gaseous state to the liquid state; e.g., the condensation of steam by cooling or the liquefaction of carbon dioxide gas by an increase in pressure.

d. It follows then that substances, which are gaseous at atmospheric pressure can be stored as liquids in an enclosed cylinder, if the temperature of the gas is sufficiently low or the storage pressure sufficiently high. The process is a reversible one so that if a liquid is subjected to a sufficiently low pressure or high temperature, it can be vaporized. Typical examples of these changes are found in the use of carbon dioxide and steam in aircraft equipment, and the formation of ice on the wings of an airplane.

e. In addition to the general properties outlined above each gas has particular chemical and physical properties which render it different from other gases. Therefore the four principal gases and vapors (carbon dioxide, oxygen, air, and steam) used in the miscellaneous equipment described herein, are taken up individually.

(1) CO₂ gas (carbon dioxide) has been adopted for use as a means of inflating various pneumatic equipment and as a fire extinguishing agent. This gas is stable, noncorrosive, nonpoisonous, noncombustible, and liquefies at a relatively low pressure which permits the filling of storage cylinders with a. greater volume than is possible with other suitable gases. The normal pressures at which CO₂ gas is stored will vary from 700 to 1,000 pounds per square inch. At these pressures from two-thirds to three-fourths of the gas is in a liquid state and, when released to the atmosphere, expands approximately 500 times its liquid volume; 1 pound of the liquefied gas expanding to approximately 9 cubic feet in the gaseous state. The freezing point of the gas is approximately minus 110° F., so that no precautions need be taken to prevent exposure of containers to low atmospheric temperatures. Being noncorrosive it is not injurious to any materials or equipment with which it may come in contact. In addition to being noncombustible, which eliminates the danger of fire hazards in its storage and use, CO₂ is effective in preventing combustion. As low as 16 percent of CO₂ in air will not permit combustion of ordinary combustible materials. This feature makes it a particularly desirable fire extinguishing agent in the case of fires with volatile liquids and gases, such as gasoline, oil, acetylene, etc. It is also a nonconductor of electricity and is safe to use, on equipment charged with high voltages. The high rate of expansion when CO₂ is released from a cylinder so lowers the temperature of the. escaping gas that carbonic snow is formed. If this snow, which has a temperature of approximately minus 110° F., comes lightly into contact with the skin, no harm will result; however, if pressed against the skin, or if the escaping gas is blown directly onto the body, a burn similar to that produced by extreme heat may result. In case of such burns, preliminary first-aid treatment should be given immediately as for other burns. Immediate application of carron oil (linseed oil and lime water) followed by treatment with picric acid spray or tannic acid is recommended, with subsequent treatment, as necessary by competent first-aid or medical personnel.

(2) Oxygen is essential in definite quantities for the proper functioning of the human body and since at high altitudes the supply in the air is not sufficient for this purpose auxiliary oxygen must be carried in aircraft to make up this deficiency. Oxygen should be used at all times by every one participating in flights above 15,000 feet. Oxygen should be used when remaining at an altitude below 15,000 feet but in excess of 12,000 feet for periods of 2 hours or longer and in excess of 10,000 feet for periods of 6 hours or longer duration. The normal requirements for oxygen at the various altitudes are listed in table 1. These amounts of oxygen will permit the individual to maintain his mental and physical efficiency, prevent fatigue, and prevent injury to the body cells, particularly those of the nervous system which are very susceptible to  oxygen want. The amount of oxygen absorbed in the body becomes progressively less during ascent to high altitudes. The result of this shortage of oxygen in the body depends on many factors, the most important of which are as follows:

Altitude attained.

Duration of flight at that altitude.

Amount of physical effort performed.

Inherent altitude tolerance of the individual.

Gaseous oxygen is colorless, odorless, and tasteless and is obtained from the atmosphere by liquefaction or fractional distillation. It is placed through a purifying process before being placed as a charge in the oxygen cylinders for use in aircraft breathing equipment. Breathing oxygen is nonpoisonous, has no deleterious effect on the teeth or dental restorations, and there are no ill effects in taking more than the prescribed amount other than to deplete unnecessarily the available supply. An individual moving about in an airplane and doing light work will require approximately twice the amount of oxygen as when seated quietly. The administration of oxygen, when needed, should be continuous since there is no storage of this gas in the body, except a small amount in the lungs and blood which is normally expended within approximately 40 seconds. Table I takes the above facts into consideration, the smaller value of flow being the average amount necessary for a pilot or observer during normal flying, while the greater value is the average amount needed for a pilot carrying out maneuvers or for an observer swinging a gun, camera, etc.

(3) Air is used in aircraft de-icing systems to periodically inflate rubber shoes for the purpose of breaking up ice formations on airplane wings, tail surfaces, radio loop and masts, and for the conduction of heat in aircraft heating systems. Air, in the atmosphere, contains a small percent of water vapor and may contain moisture in the form of droplets. Water vapor and droplets may exist in their respective states (gaseous and liquid) at air temperatures below freezing and, depending on certain temperature and turbulence conditions, may produce solids upon contact with stationary or moving objects. These solids may be classified as follows: Frost, rime or granular ice, and glaze or clear ice. The presence of any one of these on the airfoils of an airplane may seriously affect the aerodynamic characteristics of the airplane, or cause excessive vibration. The formation of frost is a, direct transformation of water vapor to the solid state (sublimation) and depends on amount of water vapor present in the air, temperature of the, air, and temperature of the surface on which the frost is formed. Rime is formed, when an under-cooled water particle hits a solid object and immediately freezes, the rate of deposition depending on the degree, of under-cooling. Since this action is the freezing of the individual droplets, the result is an opaque granular formation building up on the leading edges of the structure. The formation of glazed or clear ice is due to freezing temperature aided by evaporative cooling.

(4) Steam used in aircraft heating systems is produced by raising the temperature of water above, its vaporization or boiling point. When heat is applied to an enclosed vessel or boiler having a small steam outlet and containing water at atmospheric pressure, steam bubbles will be formed when the temperature of the water reaches 212° F. As steam is generated faster than it escapes, the pressure and temperature of both water and steam will rise. This increased pressure serves to force the steam to surfaces where it is utilized in transferring heat to the air at a. lower temperature. This transfer of heat cools the steam and returns it to a liquid state by condensation, when its temperature drops to a given value.

36. Storage.-a. Steel cylinders are used for the storage of oxygen and carbon dioxide. The identifying colors of the cylinders and the, stencil colors are given in the following table:

b. These cylinders are of seamless steel and are supplied in various weights, shapes, and capacities depending on the type of gas being stored, the use to be made of the. gas, and the particular installation in which the cylinder is to be placed. They are constructed to withstand pressures of several thousand pounds and will therefore hold fairly large quantities of gas. They may be classified as lightweight for installation in aircraft, and a heavier weight for ground use. Cylinders for aircraft use can be identified by their hemispherical ends, while those for ground use have flat bases. Discussion will be limited to cylinders of the first type, in which carbon dioxide and oxygen are stored for use in aircraft equipment. As the storage of these two gases present different problems, they will be treated separately.

c. CO₂ cylinders, in addition to the markings required by Interstate Commerce Commission regulations, are marked to provide the following information:

Weight empty (including valve or safety disk bushing).

Weight fully charged.

Rated CO₂ capacity.

Name of gas (carbon dioxide).

Type of syphon tube installed.

In carbon-dioxide cylinders from two-thirds to three-fourths of the charge is in a liquid state, the remainder being in a gaseous form at the highest point in the cylinder. Syphon tubes attached to the body bushing and running to the lowest part of the cylinder are used to allow only liquid to pass through the release valve and thus prevent the formation of carbonic snow and the consequent clogging of passages. There are three types of syphon tubes in general use as shown in figure 37:

(1) The straight rigid type which is used on cylinders mounted in a vertical position.

(2) The short flexible type which is used on cylinders mounted in a horizontal position.

(3) The curved rigid type used on cylinders having no fixed mounting position.

In order to identify readily the type of syphon tube furnished with each cylinder, the following markings are stamped on the main body bushings of each cylinder: "S" means that the cylinder is equipped with a straight rigid syphon tube that extends to the bottom of the cylinder. As a further identification the word "vertical" is also stamped on the cylinder. "S.F." means that the cylinder is equipped with a short flexible syphon tube that extends only to the end of the upper spherical portion of the cylinder. As a further identification the word "horizontal" is also stamped on the cylinder.

d. All CO₂ cylinders are equipped with safety disks designed to rupture at a pressure of 2,200 to 2,800 pounds per square inch, which is below the test pressures to which each cylinder is subjected. As the pressure of C0₂ in the cylinder rises rapidly with increases in temperature, charged cylinders should not be exposed to the direct rays of the sun, or where the temperature will exceed 130° F. It is therefore advisable to enclose charged cylinders in suitable containers during shipment to prevent exposure to the direct rays of the sun. The various type valves employed on CO₂ cylinders will be discussed with the particular equipment with which they are used. The cylinder charge or amount of CO₂ contained in the cylinder can be determined only by weighing. If a cylinder shows a loss of charge when weighed for inspection, it is an indication that a leak exists and the cylinder is removed from service and replaced. The leaking cylinder can be returned to a commercial supply house or service-control depot for recharging. The following table shows the CO₂ charge in pounds with the permissible variation for the various aircraft installations presented in this manual.

e. The oxygen cylinder for use with high altitude breathing apparatus consists of the lightweight seamless steel cylinder described above with a double-seat hand shut-off type oxygen valve. The, main body of the valve is provided with an outlet for coupling to the system and a secondary outlet fitted with a plug of fusible metal which will melt at 212° F., also a frangible gold safety disk which is designed to rupture at 2,350 to 2,900 pounds per square inch pressure. A syphon tube on the bottom of the valve extending part way into the cylinder helps to prevent rust or foreign particles from getting onto the valve seats. As the charge of oxygen is completely in a gaseous state the amount of gas in the cylinder is directly proportional to the pressure. The state of charge can therefore be determined at any time by noting the cylinder pressure. The following are examples:

Repairs to aircraft type gas cylinders or the removal of body bushings or valve bodies which are threaded directly into the cylinders should be done by the manufacturer or a service-control depot. Repainting of cylinders to renew the finish is done as needed. All cylinders used for the storage of gases are subject to a 5-year or quinennial hydrostatic test. Since those used for the storage of CO₂ and oxygen in aircraft are frequently transferred between airplanes and activities, the only record of the time of this test is a marking stamped on the cylinder; e. g., 6-40. The time of the next quinquennial test for these cylinders is checked on installation and at periodical inspections. Cylinders that require this test are reported to the control depot 3 months, in the case of oxygen, and 6 months, in the case of the CO₂ in advance of their due date. They are removed from service and replaced. The date of each weight (charge) inspection of aircraft type CO₂ cylinders is stenciled on the cylinder in black letters not to exceed 3/4 inch high. The date of the initial installation inspection is preceded by "Inst" and that of the 6-month inspection by "6 month Insp." In case the inspection date, as stenciled on cylinder is obscured, due to mounting position of cylinder, the inspection record is maintained on a card fastened to the cylinder.