Addtime： 2020-07-18 Browse times： 1902
In this paper, the performance of the slide valve vacuum pump is further studied. The new concept of movable seal is proposed. The influence of exhaust velocity, size and shape of exhaust passage on the performance of the pump is studied. Another new noise source in the study of the noise mechanism of the slide valve pump, the impact of the guide rail on the sliding valve rod, is expounded. The noise transmission and shielding of the pump are analyzed, and the ways to reduce the friction work are discussed.
After years of practice and research, the main performances of the slide valve vacuum pump, such as the limit pressure, pumping rate, power consumption and noise, are further understood. Some new views and viewpoints are put forward, such as the shape and velocity of the exhaust passage, the compression force and floating valve plate of the exhaust valve spring, the impact of the guide rail on the slide valve, the impact of the slide valve on the inner wall of the pump, and the gap distribution between the high and low vacuum cylinders The influence of the passage between high and low vacuum cylinders, as well as the intake and exhaust pipes on the pump performance. As for the vibration and balance of the pump, due to the long space, it is proposed to write a separate article.
1. Vacuum degree
The vacuum degree of slide valve vacuum pump should first meet the needs of users and ensure stable operation. Generally, users seldom use it under extreme pressure, so we should not pursue vacuum degree unilaterally. The vacuum degree of pump mainly depends on the sealing degree of each seal and the quality of pump oil.
1.1 quality of pump oil
Since 1992, the oil quality standard of MKT / s oil pump has been compared with that of other oil pumps The vacuum degree measured by the company's thin film vacuum gauge is basically the same, indicating that the quality of high-quality pump oil in China has reached the level of similar pump oil abroad. But the first-class products and qualified products are slightly worse, mainly in the total pressure has a certain gap, and the partial pressure is not much difference. In terms of service quality, according to the user's reflection, the anti emulsifying ability of domestic pump oil is slightly poor, and the oil and water are not easy to be separated. Therefore, when removing the gas containing water vapor, the pump must open the gas valve.
1.2 dynamic seal
It usually refers to the seal between the shaft and the oil seal. First of all, it is required to ensure the roughness of the oil seal on the shaft, which should be 0.4 ~ 0.8 μ m, and the small value should be taken at high speed. The surface hardness of the shaft and the shaft sleeve should reach HRC (55 ~ 58), and a certain thickness must be maintained between the shaft and the oil seal lip
The lubricating oil film, it not only plays the role of lubrication, but also plays a sealing role, so the cleanliness requirements of the oil are very high, it is best not to use the working oil in the pump. Eccentricity is the main cause of abnormal wear of oil seal and the damage of sealing performance of oil seal. Eccentricity can be divided into dynamic eccentricity and static eccentricity. Dynamic eccentricity, that is, shaft runout, is caused by shaft vibration and radial runout of bearing. Generally, it should not be greater than 0.15 mm. Static eccentricity is caused by the eccentricity between the oil seal seat and the shaft or the eccentricity of the inner and outer circle of the oil seal. Two kinds of eccentricity lead to one result, which causes uneven distribution of contact pressure between oil seal lip and shaft surface, resulting in eccentric wear of shaft and oil seal. If internal leakage occurs, vacuum degree will be affected, and external leakage will cause oil leakage. Therefore, eccentricity shall be limited to a certain range, and the total eccentricity shall not exceed 0.3 mm.
1.3 movable seal
The movable seal includes the seal of exhaust valve, relief valve and the movable clearance seal between related parts in pump chamber.
1.3.1 sealing of exhaust valve and overflow valve
The sealing degree of the exhaust valve directly affects the problem that the gas slightly higher than atmospheric pressure in the oil tank flows back to the pump cavity. If it is not closed tightly, the vacuum degree of the pump will be seriously affected. The main reason for the loose closing is that the valve plate is deformed, cracked, or the valve plate is not well guided when it is beating.
If the exhaust valve is not closed in time, it will also affect the vacuum degree of the pump. For example, in order to reduce the power consumption and noise in high vacuum, the single-stage pump will not have problems when using the composite exhaust valve combined with the compression valve and the floating valve. When the exhaust valve is used in the double-stage pump, because the pressure difference between the low vacuum cavity and the oil tank is relatively small, especially when the speed is high, the floating valve will not be able to work The phenomenon of closing in time affects the vacuum degree of the pump. If floating valve is used in relief valve of double-stage pump with high vacuum cavity, the problem will be more serious because of smaller pressure difference between two sides.
The spring force of the exhaust valve can be smaller on the premise of ensuring the valve sealing. If the compression force is too large, the power consumption will be increased. In the past, the compression force of some imitated pump springs reached 1.2 × 105 Pa. after repeated tests, we found that 1.02 ~ 1.1 × 105 PA was more suitable, which not only reduced the power consumption, but also reduced the noise.
1.3.2 clearance sealing between parts in pump chamber
The clearance between the parts of pump chamber is most important, especially the clearance between guide rail and slide valve (rod), guide rail and pump body. The gas flowing back through these two gaps flows back to the pump inlet at a state slightly higher than atmospheric pressure, so it has the greatest impact on vacuum degree. When the split guide rail is used, the influence of gas backflow is greater, because when the split guide rail exhausts the pump, the two gaps between the guide rail and the slide valve (rod) and between the guide rail and the pump body are concentrated on a gap between the guide rail and the pump body (as shown in Fig. 1). However, the integral guide rail still keeps two gaps when the pump exhausts (as shown in Fig. 2). Therefore, from the perspective of improving the vacuum degree and pumping speed of the pump, it is better to adopt the integral guide rail.
Experiments have proved that the same is true. We have done a comparative test of the integral guide rail and the split guide rail on a 150 l / s pump. In order to ensure the same gap between the two, the split guide rail is disassembled from the integral guide rail. The measured results show that the vacuum degree of the pump is lower and the noise is about 2 dB (a) higher when the guide rail is separated. It must be pointed out that the split guide rail is easy to bite, so the gap should be properly enlarged.
Further study on the performance of slide valve vacuum pump
The clearance between the two sides of the slide valve, the pump cover and the middle clapboard also has an obvious influence on the vacuum degree. If the clearance is too large, especially after the pump temperature increases, the viscosity of the oil will be reduced, the sealing performance will be poor, and the vacuum degree of the pump will be reduced. Small clearance is beneficial to vacuum degree, but too small clearance will cause difficulties to the formation of oil film, which is not conducive to the improvement of vacuum degree. Therefore, the appropriate clearance should be selected according to the working conditions, such as the size and amount of dust, the influence of the extracted gas on the pump oil and the working temperature of the pump.
The clearance between the eccentric wheel and the inner circle of the slide valve (ring) has no direct impact on the vacuum degree of the pump, but the space composed of the eccentric wheel, especially the open eccentric wheel and the inner circle of the slide valve (ring), is filled with air after the pump stops to destroy the vacuum. When the pump starts again, the air in the above space can only be slowly extracted through the gap between the eccentric wheel and the inner circle of the slide valve (ring), and the pump oil is also from this This replacement process can last for several hours, which affects the rising speed of the vacuum degree of the pump. Therefore, an oil guide groove must be opened at the gap between the eccentric wheel and the inner circle of the slide valve (ring) to facilitate the exchange of oil and gas.
The clearance between the outer circle of the slide valve (ring) and the inner wall of the pump cylinder determines the sealing degree between the suction chamber and the compression chamber. If the gap is too large, the sealing oil film is easy to be broken down, and if the gap is too small, the oil film is not easy to form, which will affect the vacuum degree. However, the above clearance does not exist alone. It is also restricted by the clearance between the eccentric wheel and the inner circle of the slide valve (ring). The total clearance formed by these two gaps must ensure that the outer circle of the slide valve (ring) will not collide with the inner wall of the pump cylinder.
2. Vacuum degree
According to JB / T 1246-2007 "vacuum technology - slide valve vacuum pump", the geometric pumping speed (theoretical pumping speed) of slide valve vacuum pump should be 1 ~ 1.2 times of the nominal pumping speed. The geometric pumping speed multiplied by the pumping efficiency is the actual pumping speed. There is a very close relationship between the pumping efficiency and the clearance between the parts of the pump. Under the conditions of fully considering the composition of the extracted gas, working temperature and dust, properly reducing the clearance is conducive to the improvement of the pumping efficiency. Other factors such as exhaust velocity, pumping and exhaust pipe, pump oil quality will also affect the pumping efficiency. The clearance between the relative moving parts of the pump cavity has a significant influence on the pumping speed of the pump. As stated in the article "re exploration of the performance of the slide valve vacuum pump - limit vacuum degree", everything that is conducive to the improvement of the vacuum degree is also conducive to the improvement of the pumping speed.
2.1 clearance
It should be specially pointed out that the vacuum degree of high vacuum cylinder of double-stage pump is high, so the clearance should be smaller. In fact, this is not the case. It is precisely because the vacuum degree of high vacuum cylinder is high and the free path of gas molecules is large. Under the same gap, gas molecules are not easy to penetrate the gap, while in the low vacuum cylinder, due to the small free path of gas molecules, the gas is easier to penetrate the gap. Therefore, the clearance of the low vacuum cylinder must be strictly controlled, but the clearance of the high vacuum cylinder can be appropriately relaxed. We have done a comparative test on a 15 L / s two-stage pump, which proves that this viewpoint is correct and beneficial to improve the pumping efficiency. We have detected that the side clearance of high vacuum cylinder of E2M40 pump (nominal pumping speed of 11.8 L / s) is 0.095 mm and that of low vacuum cylinder is 0.06 mm; the side clearance of high vacuum cylinder of d-650k pump (nominal pumping speed of 10.67 L / s) is 0.07 ~ 0.08 mm and that of low vacuum cylinder is 0.05 mm.
2.2 exhaust velocity
The exhaust speed should be appropriate, generally no more than 30 m / s. if the exhaust speed is too high, the compressed gas will flow back from the compression chamber to the suction chamber, which will not only affect the vacuum degree of the pump, but also lead to the decrease of the pumping rate. For the two-stage slide valve pump, the channel area between high vacuum cylinder and low vacuum cylinder is also very important. If the channel area is too small, the gas flow rate will be accelerated, resulting in the low vacuum cylinder can not fully absorb the gas discharged from the high vacuum cylinder, resulting in gas backflow, which will seriously affect the pumping speed of the high vacuum cylinder. Since this possibility has been considered in the design, the above phenomenon has not occurred in our two-stage slide valve pump. However, the above phenomenon has occurred in the 2xz-2 and 2xz-4 double-stage vane pumps commissioned by our brothers. Most parts of the 2xz-2 and 2xz-4 pumps are universal, but the length of the pump cylinder is different, and the channel area between the high vacuum cylinder and the low vacuum cylinder is the same, so it is sufficient for the 2 L / s pump, but not enough for the 4 L / s pump. It is found that at 1.5 kPa, the pumping efficiency of 2 L / s pump is 85%, while that of 4 L / s pump is only 77%, which can not meet the requirements of industry standard. After our suggestion, the channel area of 4 L / s pump is expanded, and the pumping efficiency of 4 L / s pump at 1.5 kPa is more than 85%.
2.3 quality of pump oil
The quality of the pump oil has a certain influence on the pumping speed in the high vacuum region, especially the full pressure pumping speed. The pump oil with poor quality volatilizes more oil vapor in the high vacuum region, which accounts for a certain proportion of the extracted gas. However, the saturated steam pressure of the high-quality pump oil is low, and the oil vapor volatilized in the high vacuum region is very small. Therefore, the actual pumping gas is more, and the pumping efficiency is higher.
3. Power consumption
The main power consumption of slide valve vacuum pump is compression work, friction work, mechanical impact and hydraulic impact loss.
Compression work
The compression work is inevitable and can not be reduced artificially. It only changes with the inlet pressure. In order to reduce the useless compression work, only the loss caused by air leakage can be reduced and eliminated. The power consumed by gas ballast is also compression work, which depends on the magnitude of gas ballast. The amount of gas stabilization depends on the amount of condensable vapor in the extracted gas. Therefore, it is necessary to adjust the amount of condensable steam in the extracted gas, so that the condensable steam can be completely removed and the loss of compression work can be properly controlled.
3.2 friction work
Generally, the friction work accounts for about 20% ~ 25% of the total power, especially at low pressure and extreme pressure, the power consumed by the pump is mainly the friction work, so the loss of friction work should be reduced as much as possible.
3.2.1 mechanical friction loss
The mechanical friction loss includes the friction loss between all rotating parts and static parts, and between rotating parts and rotating parts. In order to reduce the friction loss, the gap can be appropriately relaxed and the friction contact area can be reduced on the basis of improving the wear resistance of parts. For example, reducing the contact area between the inlet side of the slide valve (rod) and the guide rail, reducing the contact surface between the inner circle of the slide valve (ring) and the outer circle of the eccentric wheel, adding retaining rings on both sides of the eccentric wheel, so as to prevent the friction between the two sides of the eccentric wheel and the pump cover and the middle diaphragm. Reduce the quality of parts, for example, reduce the quality of the spool valve
It is also beneficial to reduce the friction between slide valve (rod) and guide rail, between slide valve (ring) and eccentric wheel, between guide rail and small cylinder of pump body.
3.2.2. Bearing friction loss
Bearing friction loss also belongs to mechanical friction loss, but it has its particularity. First of all, to use the regular bearing products, beware of counterfeit products, so as to avoid unnecessary friction loss. Bearing lubrication is a very important part, less oil or lack of oil will lead to unnecessary friction loss and bearing damage, so separate oil supply or grease should be used as far as possible.
The fit interference of bearing inner and outer rings and related parts should be appropriate. If it is too large, the raceway deformation of bearing inner and outer rings will be caused, resulting in excessive extrusion, heating and noise, which will greatly increase the friction loss. The shaft of slide valve pump is mostly a slender shaft, which is easy to cause insufficient stiffness, or the inclination of the center line of the two bearings and the center line of the pump shaft will cause inclination of the axis lines of the inner and outer rings of the bearing. For those non self-aligning bearings, extra friction loss and noise will be generated. Therefore, for the bearings with small load, self-aligning bearings or those with a slightly larger angle difference can be used Bearing. In addition, the mass of some rotating parts, such as shaft, slide valve, eccentric pulley, belt pulley and balance wheel, act on the bearing, so reducing their mass directly reduces the friction work of the bearing. The tension of the belt also acts on the bearing, so the belt tension should be appropriate.
3.2.3 friction loss of liquid
Vacuum pump oil in the pump plays the role of sealing, lubrication, cooling and opening the exhaust valve, so the pump oil participates in compression and friction, and reduces the friction between the parts. For the slide valve pump without oil pump, as long as the clearance between the slide valve and the pump cover and the middle clapboard is appropriate, the oil intake of the pump will be automatically adjusted. However, if an oil pump is equipped, the flow rate of the oil pump must be carefully calculated, usually (7 ~ 10) × 10-4s (s is the pumping speed L / s of the pump). If the oil volume is too large, the vacuum pump will become an oil pump after entering the pump, which will not only increase the compression work, but also increase the friction loss sharply.
The kinematic viscosity of pump oil varies greatly with temperature. For example, the viscosity of V100 oil in a factory is 104.62 mm2 / s at 40 ℃ and 11.63 mm2 / s at 100 ℃. Therefore, under the condition of ensuring the working vacuum degree, properly increasing the pump temperature is of great benefit to reduce the power consumption. We have proved through experiments that the pump temperature can be increased from 50 ℃ to 70 ℃ and the power consumption can be reduced by 7%, which is more conducive to the extraction of condensable steam and the consumption of cooling water. Vacuum pump oil will generally solidify at - 12 ℃, so it is not advisable for some units in the north to put the pump outdoors. This is not conducive to the start of the pump, and the pump in a short time after starting, some parts in the state of oil shortage or oil shortage, easy to cause pump damage.
3.3 pipeline and others
For users, the diameter of the suction pipe should be the same as the diameter of the pump port, and the length should be as short as possible. Too many elbows and too long pipeline will not only affect the pumping speed, but also affect the power consumption. Exhaust pipe is also the same, exhaust pipe is the most easily ignored by users, too long pipe will increase exhaust resistance, increase gas backflow, affect pumping speed, and increase power consumption.
The oil mist eliminator of slide valve pump developed by us is connected to the exhaust port of the pump, which can meet the requirements of gas chamber discharge. Of course, the oil mist eliminator will inevitably lose part of the power. After careful design and repeated improvement, not only the exhaust resistance is small, but also the effect is very good, and the increase of power is not obvious. It should be noted that the core of the oil mist eliminator must be replaced regularly according to the use condition.
4. Noise
The noise of slide valve vacuum pump is mainly produced by impact, including the impact of oil on the slide valve and pump cavity wall, the impact of exhaust valve plate and valve seat (or pump body), the impact of slide valve (ring) and pump cavity wall. The collision between slide valve (rod) and guide rail is another new noise source discovered recently, which is our latest research achievement. The size of the noise is not only related to the speed of the pump, but also related to the sound conduction.
4.1. Oil impact
There are two cases of oil impingement. First, the oil entering the pump cavity, driven by the slide valve, rushes to the exhaust port at high speed, which impacts the pump cavity wall and exhaust passage; at the end of exhaust and the moment the exhaust valve is closed, the compression chamber turns into a vacuum chamber, and the oil at the exhaust port returns to the vacuum chamber at high speed and collides with the slide valve and the flow passage. The two kinds of noise caused by oil impingement are related to the vacuum degree, rotating speed, oil quantity, flow passage shape and exhaust velocity.
The pump has high vacuum degree, high speed, large amount of oil, high flow resistance, high exhaust speed and high noise. The noise can be obviously reduced by properly controlling the oil intake, improving the shape of the exhaust passage and reducing the exhaust velocity. We have studied these factors comprehensively, and the improved design of a pump has achieved a significant effect of noise reduction of (8 ~ 10) dB (a).
4.2 impact of exhaust valve
The exhaust valve should be closed quickly at the end of exhaust, otherwise the backflow of gas will be increased and the vacuum degree and pumping speed will be affected. Therefore, when the exhaust valve is closed, the impact noise between the valve plate and the valve seat (or pump body) will be generated, which is related to the speed. In order to verify the noise, we used wool felt as valve plate to reduce the impact and noise. Of course, wool felt can not be used as valve plate for a long time. After several hours of impact, the texture gradually becomes firm, and the noise gradually rises.
4.3. Impact between slide valve and pump chamber wall
When the pump is running, with the rotation of the eccentric wheel, the slide valve (ring) rolls along the pump cavity wall. After passing through the exhaust port to the junction of the pump cylinder and the guide rail hole, there is a gap in the pump cylinder. At this time, each rotation of the slide valve (ring) has 0.01 s It takes about time to leave the pump cavity wall. As the eccentric wheel continues to rotate, the slide valve (ring) contacts with the pump cavity wall at the inlet again. Due to the high speed, the mechanical impact will be more serious. Generally, the noise will increase by (5 ~ 7) dB (a). It can be found that there is an obvious impact trace on the slide valve (ring) near the inlet of the slide valve (rod). In order to eliminate the noise, the gap between the slide valve (ring) and the pump cavity wall must be strictly controlled to keep it in a small range. The above clearance is related to the clearance between the eccentric wheel and the inner circle of the slide valve (ring), and must be controlled accordingly.
4.4 impact between slide valve and guide rail
The reason why the impact noise of slide valve (rod) and guide rail has not been found for a long time is that it is relatively low noise, and more importantly, it is difficult to separate from the total noise of the pump, so it is easy to neglect. It is found that the impact noise between the slide valve (rod) and the guide rail does exist, and the split guide rail is (2 ~ 3) dB (a) higher than that of the integral guide rail. By analyzing the reasons, we think that there are two times of impact between the split guide rail and the slide valve (rod) in each rotation, and because the gap between the two is slightly larger, the impact noise is larger, while the impact of the overall guide rail in each rotation is only once, and the gap between the two is smaller, and the impact noise is also smaller. The above noise exists objectively and can not be eliminated, but it can be reduced by controlling the gap.
4.5 transmission and shielding of noise
Whether it is the impact of pump oil or mechanical impact, the noise generated is related to the pump speed. With the development trend of pump speed to high speed, this problem is more prominent. Therefore, it is necessary to reduce the noise from the aspects of sound conduction and shielding.
Compared with steel tank, the noise of cast iron tank can be reduced by (2 ~ 3) dB (a). The main reason is that the structure of cast iron is a little looser than that of steel plate, so it has a certain sound absorption effect. Moreover, the cast iron oil tank is thicker than the steel tank, and the sound insulation effect is better. If a layer of film is sprayed on the inner wall of the oil tank, the effect will be better. The structural shape of the inner surface of the oil tank should be designed to be asymmetric to avoid resonance sound.
The exhaust valve cover is connected with the oil-gas separator, which has the function of sound insulation and noise elimination. If the exhaust valve cover is removed, the pump noise will be much higher. If the oil-gas separator is properly designed, it will also have a certain noise reduction effect. For example, the "secondary cyclone oil-gas separator" designed by us has two collisions through the oil-gas separator in the transmission process, resulting in a certain loss of sound energy, which is conducive to noise elimination.
4.6 cavitation noise
Cavitation exists objectively on the slide valve pump, and there is noise if there is cavitation. However, due to the low speed of general slide valve pump, cavitation has not reached the level of damaging parts, so it has not been found. In the trial production of h150ab pump with speed up to 920r / min, we found abnormal noise with high noise frequency and sound level. The sound power was as high as 100dB (a). After disassembly and inspection, it was found that many shallow pinholes appeared on the slide valve (ring) near the exhaust port. After removing it, the above phenomenon occurred again after running for several hours. Later, many tests confirmed that this was cavitation. But the operation time is short, the cavitation damage phenomenon only appears on the surface.
5. Others
Some manufacturers have simply changed the motor of h-150 slide valve pump from 15kw to 7.5kW, which seems to be energy-saving, but it is actually very dangerous. If there is no scientific improvement test, reducing the motor power will have adverse consequences. Taking h150 slide valve pump as an example, when the room temperature is 6 ℃, the instantaneous starting power is as high as 36.5kw. If 7.5kW motor is used, the pump can not be started in winter. Moreover, when the pressure is 7.5kW, the pump can only work under 8 × 102 Pa. if the pressure is exceeded, the motor will be overloaded and the motor will be damaged for a long time.
6. Conclusion
As a traditional low vacuum acquisition equipment, slide valve vacuum pump has a considerable market share and high product maturity, but it still has potential in the aspects of pumping efficiency, energy consumption, vibration noise and lightweight design. Through the efforts of the majority of scientific and technological personnel, scientific research methods are used to improve and improve the cost performance of the series of products, which will make it have a better market The prospect is to create better economic and social benefits for national economic construction.
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