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introduction
The thermal vacuum test chamber is an environmental simulation test device that simulates the space environment on the ground. In recent years, thermal vacuum test equipment has played an important role in the development and finalization of aerospace products. Due to the complicated structure of the hot vacuum test chamber, some abnormal phenomena sometimes appear during operation. If it cannot be eliminated in time, the test period will be extended, which will affect the development of the product. To this end, we briefly explain the basic working principle of the test chamber, and how to analyze and judge the general failure of the thermal vacuum test chamber.
1 hot vacuum test box working principle
The following is a brief introduction to the working process of the HS-2P-ZQ thermal vacuum simulation equipment. The test chamber consists of a main pump turbomolecular pump, mechanical pump, pre-pumping valve, main pumping valve, etc., which vacuums the vacuum chamber to obtain a relatively clean high vacuum test environment. The heat sink or the liquid nitrogen heats or cools the heat sink, and forms a relatively uniform temperature field in the heat sink of the box by means of radiation conduction, thereby obtaining a vacuum and a wide temperature range of -100 ° C. 125°C space simulation environment. After the hot vacuum test is completed, open the bleed valve and fill it with nitrogen to remove the test piece. The working principle of the test box is shown in Figure 1.
2 judgment analysis of failure
Since the test chamber is a relatively complex electromechanical combination system, once the equipment is abnormal, the entire equipment must be inspected and comprehensively analyzed in a comprehensive and orderly manner. For the occurrence of fault phenomena, the principle of first and second is generally firstly performed, that is, the external factors, such as cooling water and power supply, are first excluded. After the external factors are completely excluded, the cause of the fault is found by the electromechanical correlation method according to the fault phenomenon. Find electrical system and computer software problems according to electrical schematics, and finally check
Look for mechanical system problems, as shown in Table 1.
3 analysis and diagnosis of typical faults
3. 1 Failure phenomenon
For example, when a test piece is subjected to a thermal vacuum test, the temperature is required to drop to -55 °C at a vacuum of 2.X10-3 Pa and kept for 2 hours. Normally, the vacuum at this time will be better than 2X10-3Pa, but After cooling for 5 minutes, there was a phenomenon that the vacuum suddenly dropped to 2.5X10-2Pa, as shown in Figure 2. For this we will check according to Table 1.
3. 2 Analysis, diagnosis and elimination of fault phenomena
(1) Disconnect the refrigeration control, the refrigeration valve is closed, check whether the vacuum gauge communication interface is loose, whether the molecular pump rotation speed has changed, and the inspection is normal, indicating that the vacuum unit problem can be eliminated.
(2) Due to the abnormal vacuum curve in the low temperature cooling stage, first observe whether the high vacuum curve can reach the high vacuum requirement under normal temperature and high temperature state. The curve shows that the vacuum degree can reach 2X10-3Pa or more, and the work is normal, indicating from the main power supply. The electrical wiring to each solenoid valve is normal and there is no problem with the electrical system.
(3) Although there is no problem with the electrical system, when liquid nitrogen cooling is applied under vacuum conditions of 2X10-3Pa, this failure indicates that the failure should occur in the refrigeration of the test chamber. First, tighten the screws of each suction port of the box, and fill the nitrogen gas inlet with nitrogen gas of about 0.5kg. Observe that there is a slight change in the value of the high vacuum gauge.
(4) Further confirm the cause of the failure in conjunction with the control process of the test chamber. Because the heat sink of this equipment uses a 1.5mm copper plate as the cavity material, the internal liquid nitrogen coil is made of a long copper tube. Due to thermal expansion and contraction, there is no weld in the vacuum chamber, so it is judged that liquid nitrogen enters. There may be a leak in the external joint of the heat sink. At this point, it can be considered that the cause of the failure is that air leakage occurs at the junction of the solenoid valve and the metal bellows connected to the vacuum chamber.
(5) Disassemble the metal bellows connected to the vacuum chamber and open the solenoid valve to find that the plastic gasket inside it is aged and cracked due to long time high and low temperature test. The solenoid valve stem is re-welded, and the test chamber is heated under vacuum. The liquid is filled with liquid nitrogen, and the degree of vacuum shows that it is above 2X10-3Pa, and the test chamber works normally.
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