PRODUCT CATEGORIES

Vacuum Heat Treatment Furnace

Vacuum Sintering Furnace

Vacuum Brazing Furnace

Vacuum Furnaces Leaks and Detection Techniques

For the leak detection of the vacuum heat treatment furnace, it includes two parts: the furnace body and the vacuum system. After the furnace body and the vacuum system are assembled, no matter how reliable the seal is, there will always be air leakage in general. The gas leakage rate of the vacuum furnace refers to the gas flow rate entering the furnace through the leakage holes per unit time, and its unit is Pa.L/s.

At present, the air leakage rate of the vacuum heat treatment furnace is expressed by the pressure growth rate (pressure rise rate), which is generally set at 0.7Pa/h. As long as it is less than or equal to this value, the gas leakage rate of the furnace is qualified. We hope that the smaller the air leakage rate, the better, because it can affect the ultimate vacuum of the furnace chamber, and at the same time, it will also oxidize the surface of the workpiece to be processed and reduce the brightness. The air leakage rate is discussed and analyzed below.

When the vacuum pump is used to evacuate the furnace chamber, the ultimate vacuum of the cold furnace can be calculated by the following formula.

Discussed by the formula:

1) When there is no air leakage in the furnace chamber, that is, Q1=0, then P=P0, that is to say, the ultimate vacuum of the cold furnace is equal to the ultimate vacuum of the pump. In fact, this situation does not exist, because no matter how reliable the seal is, there will always be air leakage.

2) Under the condition of a certain air leakage rate Q1, the greater the effective pumping speed Se, the higher the ultimate vacuum of the cold furnace, that is, the smaller the pressure P0. It can be seen that an effective way to increase the ultimate vacuum of the cold furnace is to reduce the gas leakage rate Q1 and increase the effective pumping speed Se of the pump.

3) When the air leakage rate Q1 in the furnace chamber is relatively large, and the effective pumping speed Se of the pump is selected to be larger, it can be seen from the formula that a higher ultimate vacuum degree of the cold furnace can still be obtained. However, this kind of heat treatment furnace is prone to over-chamber wind, which oxidizes the surface of the workpiece to be processed, reduces the brightness, and produces unqualified products. Obviously, the ultimate vacuum degree of the cold furnace cannot explain the quality of the air leakage in the vacuum furnace chamber. Only by the air leakage rate can the manufacturing quality of the vacuum furnace chamber be truly judged.

The effective pumping speed Se of the vacuum pump to the furnace chamber can be calculated by the following formula

In the formula, S-the nominal pumping speed of the vacuum pump, L/s, after the pump is selected, S is a known number; C-the conductance of the pipeline between the outlet of the furnace chamber and the inlet of the vacuum pump, L/s. After the vacuum system is determined, the diameter and length of the pipe are known, and C can be calculated by the conductance formula.

It can be seen from the formula: Se<S; Se<C. If C/S<<1, then there is Se≈C; if S/C<<1, then there is Se≈S.

To sum up, the air leakage rate is an important parameter of the vacuum heat treatment furnace. How to ensure the air leakage rate is a key link in the quality of vacuum furnace manufacturing.

Manufacturing requirements and commissioning of vacuum heat treatment furnace

1 Manufacturing requirements of vacuum heat treatment furnace

There are the following requirements in the manufacture of vacuum heat treatment furnaces:

(1) The quality of the welds of the vacuum furnace chamber components and the various components of the vacuum system must be ensured. After strict leak detection, there will be no air leakage or air seepage and have reliable sealing performance.

(2) The connecting flanges, sealing rubber rings, sealing grooves, etc. of vacuum furnace chamber components and vacuum system components shall be manufactured according to the standard dimensional accuracy and smoothness requirements. In order to prevent welding deformation, the finishing of the flange should be carried out after welding with the nozzle to ensure that the sealing surface is not deformed and ensures reliable sealing after connection.

(3) All components of the vacuum furnace must be inspected for air leakage and must meet the design requirements.

2 debugging

After the installation of the vacuum heat treatment furnace chamber and the components of the vacuum system is completed, it is first necessary to carry out vacuum debugging to detect the air leakage rate and pumping time of the vacuum furnace. After these two indicators are qualified, the ultimate vacuum degree of the cold furnace is measured and used as the main parameter for each subsequent work. Only at this time can the vacuum furnace be checked and accepted.

If during the evacuation test, the ultimate vacuum degree of the cold furnace does not meet the design requirements, it does not necessarily mean that the air leakage rate is unqualified. At this time, the reason must be found first. For a more complex vacuum system, the factors that affect the decline of the system’s ultimate vacuum degree need to be analyzed one by one. By analyzing the formula, it can be seen that firstly, if the air leakage rate Q1 increases, the ultimate vacuum degree of the cold furnace can be reduced. Secondly, if the outgassing amount Q2 on the inner surface of the furnace increases (or water and outgassing substances enter the furnace), the pressure will also increase and the ultimate vacuum degree will decrease. The third is that when the effective pumping speed Se of the vacuum pump becomes smaller, the ultimate vacuum degree of the cold furnace will also decrease. The fourth is that if the ultimate vacuum degree of the vacuum pump decreases, that is, P0 increases, the ultimate vacuum degree of the cold furnace will also decrease. Fifth, if the measuring instrument is inaccurate, it will also affect the measurement of the ultimate vacuum degree of the cold furnace.

First of all, check whether the instrument is calibrated and qualified, and then check whether the ultimate vacuum and pumping speed of the vacuum pump are qualified. For example, you can check whether the oil volume of the vacuum pump is sufficient, whether the cooling water pipe is blocked, etc. Outgassing material. If no problem is found through the above inspections, and the furnace chamber is evacuated for a long time, the amount of gas released on the surface can be set to approximately zero. At this time, the ultimate vacuum degree of the cold furnace cannot meet the requirements, it is likely that the air leakage rate Q1 of the vacuum furnace chamber has increased, and a comprehensive leak detection of the vacuum system should be carried out at this time.

Vacuum system: 1. Vacuum heat treatment furnace chamber 2.5.9.11. Regulation 3.6.12. Venting valve 4. Pre-vacuum valve 7. Mechanical pump 8. Pre-stage valve 10. Maintenance pump inlet pipeline valve 13. Maintenance mechanical pump 14. Oil booster pump 15. Mechanical trap 16. High vacuum valve

For leak detection in vacuum systems, the air leakage rate can generally be checked by section, and each closed area must be inspected individually. Taking the vacuum furnace system shown in the figure as an example, to detect leaks in each closed area, first check the air leakage rate in the closed area of the furnace chamber. Start the vacuum pump to evacuate the furnace chamber for a long time, so that the surface outgassing is approximately zero. Then close valve 4 and valve 16 immediately, and record the pressure and time in the furnace chamber at this time, after waiting for a period of time, record the pressure and time again, divide the pressure difference by the time difference, you can get the pressure growth rate Leakage rate value (pressure rise rate). If this value exceeds the specified value, it means that the air leakage hole exists in this closed area of the furnace chamber, and then further search for the exact location of the air leakage hole. It is difficult to find leaks in such a large space, so analytical methods must be used to check for leaks. Because in the manufacturing process, all parts of the vacuum furnace are qualified after the air leakage rate inspection, so it is only necessary to check the dynamic seal and static seal of those joints during assembly. First, check whether the dynamic sealing rubber ring is damaged, whether it is compressed, whether there is a longitudinal scratch line on the dynamic sealing shaft, whether there is lubricating grease at the dynamic sealing rubber ring, whether the structure of the dynamic sealing is reasonable, etc. The inspection method is to seal the dynamic seal with vacuum sealant and then measure the air leakage rate. You can also disassemble the dynamic seal for inspection, and check the air leakage rate after reassembly. If there are no problems with these dynamic seals, then further search for other hidden dangers of air leakage. Secondly, check whether the removable static sealing rubber ring is pressed tightly, whether the rubber ring is damaged, whether the sealing structure is reasonable, etc. The inspection method is the same as that of the dynamic seal. The third is that if there is no problem with static sealing, it is necessary to continue to search for air leakage at weak links, such as whether there are cracks in the observation hole and the regulatory glass, which can be visually inspected. Leaks were found after inspection and were resolved. After the air leakage rate was qualified, the ultimate vacuum of the cold furnace was measured, which was the main parameter that the vacuum furnace should first achieve in the future.

If no gas leakage is found after the above inspection, it means that the leakage hole exists in the welds of various components in the furnace chamber (it may also be gas leakage from the steel plate itself). At this time, the sealing performance of the weld should be rechecked. You can use the pressure filling method, the soap bubble method, and the leak detector to detect leaks until you find the leak hole and make the air leakage rate qualified.

If the air leakage rate in the furnace chamber is qualified, but the cold furnace still cannot reach the required ultimate vacuum degree, the air leakage must appear in the vacuum system pipeline. It is necessary to check whether the air leakage rate of each enclosed section is acceptable. If it fails, the detachable static seal connection in the enclosed section should be inspected for air leakage. If there is no air leakage in these sealed connections, it means that the air leakage occurs at the welding part of the pipeline components. Use the above method to find leaks in the weld until the leak is found.

If after inspection and maintenance, the air leakage rate of the vacuum furnace is qualified, then the ultimate vacuum degree of the cold furnace is measured and used as the main parameter of the furnace operation.

Analysis of gas leakage during use of vacuum heat treatment furnace

Although the vacuum heat treatment furnace has undergone sealing performance inspection during manufacturing and debugging, its air leakage rate has been qualified. However, air leakage will occur in the vacuum furnace during operation, which will reduce the ultimate vacuum degree of the cold furnace. At this time, it is necessary to stop work for inspection, which will not only affect the work, but also cause economic losses. When looking for leaks, you can use leak detectors and other methods to find leaks, and you can also find leaks through analysis. According to the time and conditions of the air leakage failure, after careful analysis, sometimes the leakage hole can be found directly, which is fast and economical.

In addition to external air leakage, vacuum heat treatment furnaces also have internal air leakage. These two problems are introduced below.

1 Analysis and inspection of air leakage outside the furnace shell

According to the time and conditions of the leak, analyze and find the leak in several situations.

(1) After the vacuum heat treatment furnace worked for a long period of time, it was found that the ultimate vacuum of the cold furnace gradually decreased, and the reason was analyzed and found. It can be seen from the formula that there are two possibilities: one is that the air leakage rate Q1 increases, and the other is that the ultimate vacuum degree of the vacuum pump decreases. It can be seen from the conditions of air leakage that if the system has passed the leak detection test and the air leakage rate is qualified, and the vacuum furnace has not been overhauled during this period. Then it can be preliminarily judged that there will be no air leakage in the weld seam according to these two items, and there will be no air leakage in the detachable sealed joints. Typically, this leak occurs at a dynamic seal connection. After working for a period of time, the dynamic sealing rubber ring wears out, the gap increases, and the sealing grease loses its effect, resulting in an increase in the air leakage rate Q1. In addition, it may be that the effective pumping speed Se of the pump is reduced, and the ultimate pressure P0 of the pump is increased, which reduces the ultimate vacuum of the cold furnace. That is, after the vacuum furnace has been working for a period of time, the dynamic seal of the mechanical vacuum pump will leak air, which will reduce the ultimate vacuum degree. The solution is to overhaul the dynamic sealing rubber ring of the vacuum pump, replace the rubber ring and put the sealing grease again. Overhaul the vacuum pump to restore its pumping performance.

(2) If the vacuum heat treatment furnace worked normally on the previous working day, but it was found that the required ultimate vacuum degree could not be reached when it was evacuated the next day, find out the reason.

Based on this condition, it can be analyzed that there may be factors that cause a sudden change in the vacuum degree in the furnace: 1) If it is in summer, when the furnace door was opened last night when the workpiece basket was taken out, the indoor temperature was high and the humidity was high, while the furnace shell continued to flow. The cooling water causes the inner furnace wall in contact with the atmosphere to “sweat”, and a large number of water droplets condense on the inner wall, which becomes the gas source when it is evacuated again. 2) When loading the basket into the furnace chamber, the organic matter is brought into the vacuum chamber and becomes a source of outgassing. 3) The cooling water of the vacuum pump is cut off, or the working voltage is low, resulting in a decrease in the power of the vacuum pump. 4) At the end of the last working day, the dynamic sealing rubber ring broke, but this possibility is very small.

(3) During the normal operation of the vacuum heat treatment furnace, the vacuum degree suddenly drops rapidly. The reasons for the analysis may be as follows: 1) The water-cooled electrode is short-circuited, resulting in a large current, which burns the electrode sealing rubber ring and leaks air. 2) The heating wire of the steam flow pump heater is burned out, or the cooling water is cut off. 3) The glass at the weak link in the vacuum system may crack due to temperature and other effects and cause air leakage.

2 Analysis of air leakage inside the vacuum heat treatment furnace

The so-called internal air leakage refers to the leakage of the valve cover sealing ring of the vacuum system, which causes the high vacuum pump to perform cyclic evacuation, which is called internal air leakage. The following example illustrates the phenomenon of internal air leakage in a vacuum furnace. Suppose there is a vacuum quenching furnace with a vacuum system as shown in the figure. The oil booster pump 14 is the main pump, and the mechanical pump 7 is a backing pump and pre-pump. If the vacuum heat treatment furnace worked normally on the previous working day, but it was found during work today that when using a mechanical pump to evacuate, it can reach a vacuum of 1.33Pa as before. However, when the oil booster pump is started to evacuate, the vacuum degree of the vacuum furnace still stays at 1.33Pa, which cannot reach the limit vacuum degree of 1.33×10^-1Pa required by the cold furnace. Analyze and find the cause of the fault.

First of all, the vacuum furnace worked normally yesterday, which means that all the static seals will not leak air, and the leaking parts can only be the dynamic seals that worked yesterday. Second, if a mechanical pump is used to evacuate as in normal work, it can reach 1.33Pa, which shows that there is no external air leakage in the entire vacuum system, which further reduces the suspicion of air leakage. The third is that there is no change in pressure when the oil booster pump is used to evacuate, which shows two points: one is that the oil booster pump is not working; the other is that the oil booster pump is working, but the gas is not discharged. First of all, you should check whether the oil booster pump is working, because it worked normally yesterday, so there will be no problem with the oil loading of the oil booster pump, only the heater of the pump is broken, or the cooling water stops flowing. If there are no problems after inspection, it means that the oil booster pump is working normally. This only shows that the oil booster pump is working, but it fails to discharge the gas into the atmosphere. After analysis, it is determined that there is a problem with the sealing of the pre-pumping valve 4 in the figure (such as the sealing rubber ring of the pre-pumping valve is broken), so the gas discharged from the oil booster pump enters the oil booster pump from the pre-pumping valve 4 At the inlet end, a gas circulation is formed and the ultimate vacuum degree of the cold furnace cannot be reached. This leak is exactly the internal leak.

There is another situation that looks like an internal air leak, but in fact it is not an internal air leak, but a failure of the air extraction system. The following example illustrates.

There is a high vacuum annealing furnace system, and its vacuum system is similar to that shown in the figure. Just change the main pump to a diffusion pump. If the vacuum system of the vacuum furnace has been inspected and cleaned, re-evacuate and test it. When a mechanical pump is used to evacuate, the vacuum degree is 1.33Pa as in normal operation. When a diffusion pump is used to evacuate, the vacuum degree is still 1.33Pa. Analyze the cause of the failure.

When the vacuum furnace is evacuated by a mechanical vacuum pump, the vacuum degree reaches 1.33Pa as in normal operation, which indicates that there is no external air leakage in the vacuum system, but a fault or internal air leakage. One is that there is air leakage at 4 locations of the pre-pumping valve on the vacuum system, causing gas circulation. The other is that the diffusion pump is not working. At this time, you can check whether the oil content of the diffusion pump is normal, whether the heater is working, and whether the cooling water flow is normal. If the above is normal and the rubber ring of pre-pumping valve 4 is in good condition when assembling the system. It can be concluded that there was a problem when the diffusion pump was reassembled. It can be checked by disassembling the diffusion pump. If the nozzle is installed incorrectly and the gap is uneven when assembling the pump, the steam jet flow cannot form the pumping capacity, and the diffusion pump cannot work normally.

From the above discussion, it can be seen that when the vacuum system fails, it is not necessarily caused by air leakage. It is necessary to analyze the conditions under which the accident occurred and make correct judgments. Determine whether it is a pump failure or an external or internal air leak in the furnace, and then resolve it appropriately.