Due to the gravity of the earth, the air has a certain weight to form atmospheric pressure. The atmospheric pressure at a certain height is the heaviest of the entire air column on the unit area perpendicular to the ground above the point. With the increase of height, the air gradually becomes thinner and the atmospheric pressure gradually decreases. According to the actual measurement, within 3000 km of sea level, the air pressure decreases by 100 Pa for every 10 meters of height increase, and the atmospheric pressure near 31 km is 1/100 of the standard atmospheric pressure at sea level. In addition to being related to altitude, atmospheric pressure is also related to weather changes. At the same place, the pressure is high in sunny days and low in overcast days, and the pressure in winter is higher than that in summer.
In China, about 50% of the earth’s surface area is 1000 m higher than the sea level, and about 25% of the area is 2000 m higher than the sea level. It can be seen that the equipment stored, transported and used in the air at the place where the earth’s surface is higher than the sea level will inevitably encounter the low atmosphere environment and be affected by the low atmosphere active environment. For dry aerial sound products, since the highest and lowest flight altitude of the aircraft are also thousands of meters, it is generally necessary to fly nearly 10000 meters or more, up to 30 km. Therefore, the airborne equipment will bear a more severe low-pressure effect than the plateau equipment.
2.Impact of low pressure environment on equipment
The impact of low pressure environment on equipment is multifaceted: including the direct mechanical impact caused by the pressure difference caused by the reduction of air pressure, the impact of the reduction of air density on the heat dissipation and thrust of power equipment and the electrical performance of electrical equipment, the additional impact caused by the seal damage caused by the pressure difference and the harmful impact on volatile substances.
(1) Directly destroy the sealing products with shells
Under the action of low air pressure, the shell of sealing products with shell will be damaged directly due to the excessive internal and external pressure difference, and the existence of pressure difference will also lead to seal damage.
(2) Reduce electrical performance
Under normal atmospheric conditions, air is a better insulating medium, and many electrical products use air as the insulating medium. When these products are used in high altitude areas or as airborne equipment, partial discharge often occurs near the electrode with strong electric field strength due to the reduction of atmospheric pressure. More serious is that sometimes air gap breakdown occurs, which means that the normal operation of the equipment is damaged. Therefore, low pressure environment will also have an impact on the electrical performance of electrical and electronic products, especially for equipment with air as the insulation medium, low pressure has a more significant impact on it.
(3) Leading to a large temperature rise of heat dissipation products
The so-called heat dissipation product refers to the test sample whose surface hottest spot temperature differs from the ambient temperature by more than 5 ℃ after the temperature of the test sample reaches stability under free air conditions and the specified atmospheric pressure. A considerable part of electrical and electronic products are heat dissipation products, such as motors, transformers, etc. these products consume part of the electric energy in use, turning it into heat energy and raising the product temperature. The temperature rise of heat dissipation products increases with the increase of altitude (the decrease of atmospheric pressure). Temperature rise and altitude
The height is roughly linear, and its slope depends on its own structure, heat dissipation, ambient temperature and other factors.
(4) Lead to the loss of volatile substances
The decrease of pressure will reduce the boiling point of the liquid. For those liquids with high saturated vapor pressure under normal atmospheric conditions at sea level, low pressure will make them evaporate or even boil. The evaporation process of liquid is an equilibrium process, that is, the number of liquid molecules volatilized into the air by energy and the number of molecules bound by air molecules hitting the liquid surface reach equilibrium. When the atmospheric pressure decreases, the density of the air decreases, and the possibility of the volatile molecules of the liquid entering the air being knocked back to the liquid surface is greatly reduced. Therefore, under the condition of low pressure, the speed of liquid volatilization will be greatly increased. This is the case with lubricating oil or grease. The reduction of pressure will accelerate the volatilization of lubricating oil (or grease), which will aggravate the friction of moving parts and accelerate the abrasion of the surface of moving parts. Plasticizers in organic materials will also accelerate volatilization due to the reduction of air pressure. The volatilization of plasticizer promotes the aging of organic materials and changes their mechanical or electrical properties. The volatilization of volatile matter will also pollute the product and its surrounding objects, causing the product or object to be polluted or even corroded. Based on the impact of the above low-pressure environment on the equipment, the typical low-pressure environmental effects are: gas or liquid leaks from the sealed shell; Deformation, cracking or explosion of sealed containers; The physical and chemical properties of low-density materials change; Equipment malfunction or failure caused by arc or corona discharge under low voltage; Under low pressure, the heat transfer efficiency decreases, causing the equipment to overheat; Lubricant volatilization; The engine starts and burns unsteadily, and the thrust or traction decreases, and the air tight seal fails, etc.
3.1 Test Equipment
3.1 General requirements
(1) The low pressure test equipment shall be able to generate and maintain the low pressure required for the test, and be equipped with the necessary auxiliary instruments that can monitor the low pressure conditions;
(2) The low pressure chamber shall be equipped with a device for continuously recording the pressure of the test chamber:
(3) The resolution of the data readout device shall not be less than 2% of its full scale;
(4) Pay attention to prevent the air pump, valve and insulating material of the test equipment
Pollution of volatile substances such as materials to the air in the box;
(5) When re pressing, pay attention to prevent external dust and water vapor from entering the box and causing pollution.
3.2 rapid decompression explosion decompression equipment
Based on the existing low pressure test equipment, it is very difficult to enhance the evacuation capacity of the vacuum system and realize the rapid decompression test, because it requires a vacuum pump system with strong evacuation capacity, which not only requires more investment, but also it is very difficult to achieve the pressure reduction rate of the test chamber from 75 KPa to 188 KPa within 15 s or 0.1 s. At present, the auxiliary vacuum tank method is generally adopted, even if the test chamber is connected with another vacuum tank or low-pressure tank with large volume through pipeline solenoid valve, and the air in the vacuum tank or low-pressure tank is evacuated.
When the rapid pressure reduction test is required, quickly open the electromagnetic valve, the expected purpose can be achieved by connecting the test chamber with the vacuum tank and balancing the pressure of the test chamber with that of the vacuum tank.