Fault Analysis of Vacuum System of Vacuum Furnaces

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1. Vacuum heat treatment

With the development of science and technology, the application of vacuum heat treatment technology has spread throughout all walks of life. The more advanced the field, the stronger the requirements for vacuum heat treatment technology. Now, the technology has become a key factor in aviation, spaceflight, nuclear energy and electronics. One of the indispensable heat treatment methods in the development of the industry. Its main advantage is that it can complete the heat treatment process of the workpiece under the conditions of no oxidation, no decarburization and small deformation; the surface of the workpiece after this treatment is smooth, the mechanical properties are high, and the service life is long; at the same time, it has good degassing effect and no pollution. environment and ease of automation.

Vacuum heat treatment technology was adopted earlier in industrially developed countries, and the technology is relatively mature. In order to improve product quality, enhance market competitiveness and change the outdated and backward situation of heat treatment equipment, this paper analyzes and studies this, so that the problem can be properly solved. There are three common faults, that is, the pumping effect of the vacuum system is poor, the vacuum system leaks and the vacuum system deflates, which are described below.

2.The pumping effect of the vacuum system is poor

The poor pumping effect of the vacuum system is a problem in the pumping system itself. There are two main reasons: one is that the backing pump cannot reach the ultimate vacuum degree, and the other is that the diffusion pump is faulty.

2.1 The backing pump cannot pump the ultimate vacuum

When it is found that the vacuum degree of the foreline pump is not high, it should be checked one by one according to the following steps and eliminated.

2.1.1 The pump temperature is too high

(1) Check whether the set value of the thermostat that controls the pump temperature is in compliance with the regulations. If not, it should be reset.

(2) The cooling water flow rate of the pump becomes smaller due to the blockage of solid scale or other dirt. causing the pump to rise. Because the pump oil must have a certain viscosity to ensure the sealing of the pump, if the viscosity is too small, the sealing performance will deteriorate. The viscosity of pump oil decreases with increasing temperature. As a result, the seal of the pump is deteriorated, the saturated vapor pressure of the oil increases, and the degree of vacuum decreases. The solution is to dredge the waterway and increase the flow of cooling water so that the temperature of the pump oil should not exceed the specified value of 100°C, generally around 0°C.

(3) Improper assembly of components causes wear and tear and should be reassembled.

2.1.2 Oil pump problem

(1) The oil level is too low. The oil should be replenished to the position specified in the manual.

(2) The pump oil is aging or polluted. The service life of pump oil is generally 2000h. When used for a long time or contaminated due to improper use, the pump oil will change from light yellow, clean and transparent to darker milky. An increase in the viscosity of the pump oil will reduce the performance of the pump. It is generally stipulated that the viscosity of the pump oil should not exceed 0.3Pa·s (at a temperature of 25°C), otherwise it should be replaced.

(3) The pump oil contains water. Open the gas ballast valve 15-30 mm to increase the vacuum degree. The reason is: when the pumped gas contains condensable gases such as steam, they will be compressed after entering the working chamber of the pump, thus condensing into water and other liquids. These condensates not only reduce the vacuum. It will also deteriorate the pump oil, which will greatly reduce the performance of the pump. In order to avoid the impact of condensate on the performance of the pump, a gas ballast is installed at the compression exhaust position of the pump. When the pump compresses the gas, a little air is introduced to increase the pressure in the working chamber. The condensable gas in the mixed gas is discharged out of the pump when the saturated vapor pressure is not reached, so as to achieve the purpose of removing moisture and other condensable gases, not polluting the pump oil and improving the vacuum degree.

(4) The selected pump oil specification is wrong

2.1.3 The filter port is blocked

The filter at the air inlet and the gas ballast valve should be removed, cleaned, and reinstalled.

2.1.4 The pump itself leaks air

(1) Check whether the sealing rubber ring is aging, failing and permanently deformed, and if any problem is found, it should be replaced.

(2) Gas ballast valve gasket is damaged or not tightened, it should be replaced or tightened in time.

(3) The exhaust sheet is damaged and the seal is not good, and the exhaust sheet should be replaced.

2.1.5 When the pump has abnormal noise, noise, or difficult rotation during operation, it should be dealt with in the following ways:

(1) The pump temperature is too high.

(2) The filter is damaged, causing metal shavings, ceramics, fiber shavings, etc. to fall into the pump, hindering its operation. Disassemble the pump for inspection, remove debris, replace screens and worn parts, and assemble it.

(3) If the pump oil deteriorates and the viscosity increases, the pump oil should be replaced.

(4) Noise is generated due to improper assembly of the alkali shock device. Should be reassembled.

(5) Motor failure produces noise. The motor should be repaired or the motor bearings should be lubricated:

2.1.6 The pump is difficult to start. There are three main reasons why the pump is difficult to start, namely:

(1) The indoor temperature is too low, causing the viscosity of the mascara to increase, which makes it difficult to start the pump. The indoor temperature of the pump room should be kept above 12°C.

(2) The pump oil has deteriorated, which increases the viscosity of the pump oil and makes it difficult to start the pump. The pump oil should be replaced in time.

(3) Motor power failure. The power supply should be checked.

2.2 The diffusion pump is faulty

When the backing pump has reached the ultimate vacuum and there is no air leakage, if the vacuum still cannot be pumped up. Then the diffusion pump should be checked. Inspection can be carried out item by item according to the following steps.

2.2.1 The vacuum degree of the system cannot reach the limit value

(1) The pump oil is too little or the pump oil is contaminated. Under normal circumstances, the pump oil is white, clean, and transparent liquid. If it changes color or becomes turbid, it means that the pump oil has been polluted, and the pump must be disassembled for thorough cleaning. And replace with new oil. The pump oil should be replaced once every two years in normal use. If the pump oil is not polluted, but the amount is small, then just add the pump oil of the same specification to the oil mark position.

(2) Insufficient heating of the pump oil. The power supply and fuses should be checked for faulty pump heater operation. If there is a fault, it should be eliminated in time.

(3) The cooling of the pump is insufficient, so that the oil vapor in the pump cannot be completely and quickly condensed, and the condensed oil evaporates again before returning to the cold trap, and may flow back to the vacuum system and pollute the heating chamber. The elimination method should increase the cooling water flow.

(4) The flow of cooling water is too large, which reduces the surface temperature of the pump wall, increases the backflow of oil vapor, and affects the ultimate vacuum. When the diffusion pump is working, the outlet temperature of the cooling water should be around 26C (no more than 30C).

(5) Improper assembly of the oil vapor deflector in the diffusion pump. It should be checked whether the geometric dimensions, positions and clearances of the nozzles at all levels are normal and whether they are inclined or not. If there is a problem, it should be reassembled.

2.2.2 The diffusion pump does not work

(1) The vacuum system leaks seriously. Check the furnace door, electrodes, thermocouples and other sealing parts that are prone to leakage, find out the location of the leakage and eliminate it.

(2) The heating of the diffusion pump is insufficient. The diffusion pump should be preheated 30 minutes in advance, so that the pump oil can be heated to the working temperature before it can start working.

(3) The foreline pump is faulty or there is leakage in the foreline pipeline, which causes the pressure in the foreline pipeline of the diffusion pump to be too high, making it difficult to start the diffusion pump. Troubleshooting can refer to Section 2.1 of this article. If the foreline pump is not faulty but the pressure of the foreline pipeline is too high, it means that there is a leak in the foreline pipeline, and the location of the leak should be found and eliminated.

3 Vacuum system leaks

Air leakage in the vacuum system is one of the main factors that cause the vacuum degree of the vacuum system to fail to increase. In order to ensure that the vacuum system maintains good sealing performance, the pressure rise rate should be tested regularly. Once the pressure rise rate is greater than 0.27Pa/h (2×10 mmH8/h). The vacuum system should be checked for leaks. The location of vacuum equipment air leakage is usually easy to occur in the places where it is often active, such as the seal of furnace doors, thermocouples, electrodes and vacuum gauges. The methods used for column leakage include injection method, helium mask method or a combination of the two. Practice has proved that it is faster and more accurate to first use the helium mask method to make a rough inspection to determine the approximate location of the leak, and then use the blowing method to carefully find the exact location of the leak. After finding the exact location of the leak, follow the steps below to find out the cause of the leak and eliminate it.

(1) Check whether the cavity ring matches the sealing groove. If the two do not match, leakage will easily occur, and the cavity ring with appropriate specifications should be replaced.

(2) Check whether the cavity ring is permanently deformed or micro-cracked due to long-term use or overheating. If these phenomena have occurred, they should be replaced in time.

(3) Check whether the sealing flange and seedlings are rusted, scratched, deformed, etc. If so, the rust, scratches, etc. should be ground off, or the deformation should be corrected until the new sealing flange and Mushroom pieces.

(4) Check whether the sealing elements are assembled properly. When assembling, the bolts of the sealing flange should be tightened evenly and symmetrically, so as to prevent one side from being loose and the other side from being tight, so that the sealing effect is not good. The assembly pressing force should not be too large to avoid permanent deformation of the apron.

(5) Check whether the sealing element is clean. Its surface shall be free from oil stains and contamination by fibers or fines of any substance. If the sealing elements are contaminated, they should be cleaned thoroughly.

4 Bleed the vacuum system

Because the surface of each part in the vacuum system, especially those parts with low molecular density, such as graphite, ceramics, fiber felt, etc., has a large amount of gas and water adsorbed, the adsorbed gas and water are under external conditions (such as temperature) , pressure) changes, it will escape from the surface of these parts, resulting in a decrease in the vacuum of the vacuum system. Without taking any other measures, it is very difficult to desorb all the gases, especially water vapor, adsorbed on the surfaces of the components in the vacuum system.

In order to prevent the vacuum furnace from absorbing a large amount of gas and moisture, the vacuum furnace should also be in a vacuum state when it is not working. For fixtures and parts to be processed, they must be thoroughly cleaned and dried before entering the furnace to keep their surfaces clean and dry. In addition, the quenching fixture of the vacuum oil quenching furnace is not allowed to be manufactured by ordinary sand casting process. Because the clamps made by this method, especially the large ones with complex shapes, are not only loose in structure, but also have a large number of defects such as blisters, pores, and micro-cracks. When they are quenched into the oil together with the workpiece, the quenching oil will remain. Inside, it becomes very difficult to remove the oil. After vacuum heating, the oil infiltrated into it will volatilize and pollute the inside and parts of the vacuum furnace.

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