In chapter 1, you learned that many factorsare considered in determining whether to use hydraulics or pneumatics as a power source in a fluid power system. Once it is determined that pneumatics will be used as the source of power, some of the same factors are considered in selecting the pneumatic gas.
The ideal fluid medium for a pneumaticsystem is a readily available gas that is nonpoisonous (nontoxic), chemically stable, free from any acids that cause corrosion of system components, and nonflammable. It also will not support combustion of other elements.
Gases that have these desired qualities may nothave the required lubricating power. Therefore, lubrication of the components of some pneumatic systems must be arranged by other means. For example, some air compressors are provided with a lubricating system, some components are lubricated upon installation or, in some cases, lubrication is introduced into the air supply line. Two gases meeting these qualities and most commonly used in pneumatic systems are com-pressed air and nitrogen.
Compressed air is a mixture of all gasescontained in the atmosphere. In this manual, compressed air is referred to as a gas when it is used as a fluid medium.
The unlimited supply of air and the ease ofcompression make compressed air the most widely used fluid for pneumatic systems. Although moisture and solid particles must be removed from the air, it does not require the extensive distillation or separation process required in the production of other gases.
Compressed air has most of the desiredproperties and characteristics of a gas for pneumatic systems. It is nonpoisonous and nonflammable but does contain oxygen, which supports combustion. One of the most undesirable qualities of compressed air as a fluid medium for pneumatic systems is moisture content. The atmosphere contains varying amounts of moisture in vapor form. Changes in the temperature of compressed air will cause condensation of moisture in the pneumatic system. This condensed moisture can be very harmful to the system, as it increases corrosion, dilutes lubricants, and may freeze in lines and components during cold weather. Moisture separators and air driers (dehydrators) are installed in the compressed air lines to minimize or eliminate moisture in systems where moisture would deteriorate system performance.
The supply of compressed air at the requiredvolume and pressure is provided by an air compressor. (For information on air compressors, refer to Naval Ships Technical Manual, chapter 551.) In most systems the compressor is part of the system with distribution lines leading from the compressor to the devices to be operated. In these systems a receiver is installed in-line between the compressor and the device to be operated to help eliminate pulsations in the compressor discharge line, to act as a storage tank during intervals when the demand for air exceeds the compressors capacity, and to enable the compressor to shut down during periods of light load. Other systems receive their supply from cylinders which must be filled at a centrally located air compressor and then connected to the system. Compressed air systems are categorized by their operating pressures as follows: high-pressure (HP) air, medium-pressure (MP) air, and low-pressure (LP) air.
High-Pressure Air Systems
HP air systems provide compressed air at anominal operating pressure of 3000 psi or 5000 psi and are installed whenever pressure in excess and high flow rates of compressed air by the addition of HP storage flasks to the system. An example of such a system is one that provides air for starting diesel and gas turbine engines. Reduction in pressure, if required, is done by using specially designed pressure-reducing stations.
MP air systems provide compressed air at anominal operating pressure of 151 psi to 1000 psi. These pressures are provided either by an MP air compressor or by the HP air system supplying air through an air bank and pressure-reducing stations.
LP air systems provide compressed air at anominal operating pressure of 150 psi and below. The LP air system is supplied with LP air by LP air compressors or by the HP air system supplying air through an air bank and pressure-reducing stations. LP air is the most extensive and varied air system used in the Navy, In addition to being used for various pneumatic applications, LP and HP compressed air are used in the production of nitrogen.
For all practical purposes, nitrogen isconsidered to be an inert gas. It is nonflammable, does not form explosive mixtures with air or oxygen, and does not cause rust or decay. Due to these qualities, its use is preferred over compressed air in many pneumatic systems, especially aircraft and missile systems, and wherever an inert gas blanket is required. Nitrogen is obtained by the fractional distillation of air. Oxygen/nitrogen-producing plants expand compressed air until its temperature decreases to 196C (320F), the boiling point of nitrogen at atmospheric pressure. The liquid nitrogen is then directed to a storage tank. A liquid nitrogen pump pumps the low-pressure liquid nitrogen from the storage tank and discharges it as a high-pressure (5000 psi) liquid to the vaporizer where it is converted to a gas at 5000 psi. Oxygen/nitrogen-producing plants are located at many naval installations and on submarine tenders and aircraft carriers.
As in hydraulic systems, fluid contaminationis also a leading cause of malfunctions in pneumatic systems. In addition to the solid particles of foreign matter which find a way to enter the system, there is also the problem of moisture. Most systems are equipped with one or more devices to remove this contamination. These include filters, water separators, air dehydrators, and chemical driers, which are discussed in chapter 9 of this manual. In addition, most systems contain drain valves at critical low points in the system. These valves are opened periodically to allow the escaping gas to purge a large percentage of the contaminants, both solids and moisture, from the system. In some systems these valves are opened and closed automatically, while in others they must be operated manually. Complete purging is done by removing lines from various components throughout the system and then attempting to pressurize the system, causing a high rate of airflow through the system. The airflow will cause the foreign matter to be dislodged and blown from the system.
NOTE:If an excessive amount of foreign matter, particularly oil, is blown from any one system, the lines and components should be removed and cleaned or replaced. In addition to monitoring the devices installed to remove contamination, it is your responsibility as a maintenance person or supervisor to control the contamination. You can do this by using the following maintenance practices:
1. Keep all tools and the work area in a clean,dirt-free condition.
2. Cap or plug all lines and fittingsimmediately after disconnecting them.
3. Replace all packing and gaskets duringassembly procedures.
4. Connect all parts with care to avoidstripping metal slivers from threaded areas. Install and torque all fittings and lines according to applicable technical instructions.
5. Complete preventive maintenance asspecified by MRCs. Also, you must take care to ensure that the proper cylinders are connected to systems being supplied from cylinders. Cylinders for compressed air are painted black. Cylinders containing oil-pumped air have air have one green stripe. Oil-pumped air indicates that the air or nitrogen is compressed by an oil-lubricated compressor. Air or nitrogen com-pressed by a water-lubricated (or nonlubricated) compressor is referred to as water pumped. Oil-pumped nitrogen can be very dangerous in certain situations. For example, nitrogen is commonly used to purge oxygen systems. Oxygen will not burn, but it supports and accelerates combustion and will cause oil to burn easily and with great intensity. Therefore, oil-pumped nitrogen must never be used to purge oxygen systems. When the small amount of oil remaining in the nitrogen comes in contact with the oxygen, an explosion may result. In all situations, use only the gas specified by the manufacturer or recommended by the Navy. Nitrogen cylinders are painted gray. One black stripe identifies cylinders for oil-pumped nitrogen, and two black stripes identify cylinders for water-pumped nitrogen. In addition to these color codes, the exact identification of the contents is printed in two locations diametrically opposite one another along the longitudinal axis of the cylinder. For compressed air and nitrogen cylinders, the lettering is white.
All compressed gases are hazardous. Compressedair and nitrogen are neither poisonous nor flammable, but should not be handled carelessly. Some pneumatic systems operate at pressures exceeding 3000 psi. Lines and fittings have exploded, injuring personnel and property. Literally thousands of careless workers have blown dust or harmful particles into their eyes by the careless handling of compressed air outlets. Nitrogen gas will not support life, and when it is released in a confined space, it will cause asphyxia (the loss of consciousness as a result of too little oxygen and too much carbon dioxide in the blood). Although compressed air and nitrogen seem so safe in comparison with other gases, do not let overconfidence lead to personal injury.
To minimize personal injury and equipmentdamage when using compressed gases, observe all practical operating safety precautions, including the following:
1. Do not use compressed air to clean partsof your body or clothing, or to perform general space cleanup in lieu of vacuuming or sweeping.
2. Never attempt to stop or repair a leak whilethe leaking portion is still under pressure. Always isolate, repressurize and danger tag out the portion of the system to be repaired. For pressures of 1000 psi or greater, double valve protection is required to prevent injury if one of the valves should fail.
3. Avoid the application of heat to the airpiping system or components, and avoid striking a sharp or heavy blow on any pressurized part of the piping system.
4. Avoid rapid operation of manual valves.The heat of compression caused by a sudden high-- pressure flow into an empty line or vessel can cause an explosion if oil is present. Valves should be slowly cracked open until airflow is noted and should be kept in this position until pressures on both sides of the valve have equalized. The rate of pressure rise should be kept under 200 psi per second, if possible. Valves may then be opened fully.
5. Do not discharge large quantities ofnitrogen into closed compartments unless adequate ventilation is provided. 6. Do not subject compressed gas cylinders to temperatures greater than 130F. Remember, any pressurized system can be hazardous to your health if it is not maintained and operated carefully and safely