The purpose of pneumatics is to do work in a controlled manner. The control of pneumatic power is accomplished through the use of valves and other control devices that are connected together in an organized circuit. The starting point in this organized cir cuit is the air compressor, where the air is pressurized.
The pressurized air goes to an air receiver for storage and then is processed for use by passing through filters, dryers, and, in some cases, lubricators. This pressurized air is normally classified as instrument air when it is used in control systems. This air must be free of moisture and oil to prevent the control devices from clogging up.
The law states that any pressurized air system must be fitted with a pressure relief valve. This valve prevents the system from being overpressurized and becoming a hazard to personnel or damaging equipment. A pressure switch is an electro-pneu matic control device that is installed on the air receiver to regulate the output of the air compressor. When the air pressure reaches its maximum set point, the regulator is activated and transmits a signal to a solenoid valve on the air compressor. This sole noid valve opens to direct lubricating oil to hydraulically keep the suction valves shut on both the low- and high-pressure cylinders on the air compressor. The air compres sor will remain in this mode until the pressure drops to the lower set point and deacti vates the pressure regulator. This, in tum, de-energizes the solenoid valve on the air compressor, causing it to release the lubricating oil pressure on the low- and high pressure suction valves. The air compressor returns to normal operation, pumping air into the receiver until the maximum pressure set point is reached. When this happens, the control cycle starts again. Using a pressure switch prevents the air compressor from running continuously.
Figure 12-1 shows a typical compressed air supply system. The following describes the functions ofthe system components:
• Compressor. Compresses the air.
• Pressure switch. Turns the air compressor on and off.
• Pressure relief valve. Relieves air pressure at 110 percent of the operating maximum pressure. This device is fined to the receiver by law.
• Check valve. Permits the compressed air to flow away from the compressor and will not allow any air to return to the compressor .
• Air receiver. The receiver stores the pressurized air. By law, it must have the following fittings installed on it:
(a) Pressure relief valve
(b) Pressure gauge
(c) Access hand-hole
(d) Drain valve.
• Pressure regulator. Controls the system pressure to the manifold.
• Pressure gauge. Indicates internal pressure of the system. Must be fitted by law.
• Filter. This device cleans the air of dirt and contaminants.
• Lubricator. This device adds a small amount of oil to the air in order to lubricate equipment.
Note: This is only installed when needed. Most systems are oil-free.
• Pressure manifold. This distributes the air to the various pressure ports.
• Needle valves. These control the airflow to the various systems that are to be operated.
HAZARDS OF COMPRESSED AIR
People often lack respect for the power in compressed air because air is so common, and it is viewed as harmless. At sufficient pressures, compressed air can cause damage if an accident occurs. To minimize the hazards of working with compressed air, all safety precautions should be followed closely. Reasons for general precautions fol low.
Small leaks or breaks in the compressed air system can blow minute particles at surprisingly high speeds. Always wear safety glasses when working in the vicinity of any compressed air system. Goggles in place of glasses are recommended if contact lenses are worn.
Compressors can make an exceptional amount of noise while running. The noise of the compressor, in addition to the drain valves lifting, creates enough noise to require hearing protection. The area around compressors should always be posted as a hear ing protection zone.
Pressurized air can do the same type of damage as pressurized water. Treat all opera tions on compressed air systems with the same care taken with liquid systems. Closed valves should be slowly cracked open and both sides allowed to equalize prior to opening the valve further.