Abstract: Based on the analysis of cavitation of centrifugal pump and the comparison of several schemes to improve the cavitation resistance of centrifugal pump, the problem of pump cavitation should be solved on the premise of not affecting the normal production. Induction wheel. After the actual transformation, achieved good results.
Key words: Centrifugal pump; Cavitation; Induction wheel
Abstract: The cavitation in the centrifugal pumps is analyzed and the kinds of the schemes are compared. In the regular production, installing a inducer is a better choose of resistance to the cavitation. The improvement of installing a inducer achieved good results.
Key word: centrifugal pump; cavitation; inducer
1 Introduction
The m-cresol unit of Yanhua Chemicals Group is one of the largest cresol production plants in the world and has undergone many revisions due to its inherent deficiencies. During the process of transformation, the process parameters of the pump changed greatly, resulting in severe cavitation of several pump units in the unit, among which 143-J / JA and 217-J / JA were the most serious ones. The disintegration inspection found that the impeller and pump suction port have serious pitting corrosion, while these pumps in operation, not only vibration, noise, and significantly decreased efficiency. Table 1 is the operation of these two groups of equipment questionnaire.
Table 1 143-J / JA, 217-J / JA operational status questionnaire
Model Amplitude (mm)
Noise (dB)
effectiveness(%)
Maintenance cycle (days)
Pitting maximum pit depth (mm)
143-J / JA
0.46
49
43.0
8.5
2.3
217-J / JA 0.47
52
46.5
7.3
2.7
As can be clearly seen from the above table: The two sets of equipment have been a serious cavitation phenomenon, which caused frequent maintenance equipment inspection, spare parts consumption increases, the cost increases, which greatly threaten the safety of the device, stable, long Full, excellent production. Therefore, it is inevitable to solve this actual problem that affects production.
2, cavitation phenomenon and solution
2.1 cavitation phenomenon
Since the liquid pressure near the impeller vane inlet is less than or equal to the saturated vapor pressure at the liquid transport temperature, the liquid evaporates and gas dissolved in the liquid may escape to form a large number of bubbles. Height and instantaneous condensation collapse. At the moment when the bubbles condense and collapse, the liquid around the bubbles quickly flushes into the cavities formed by the bubbles to form a strong local high-frequency and high-pressure water hammer, and the metal surface is denuded due to fatigue. At the same time, the electrochemical corrosion of the metal surface occurs due to the presence of an active gas (such as oxygen) and the local high temperature generated when the bubbles condense. This process is called cavitation.
2.2 Factors affecting cavitation
Factors that affect fluid pressure and saturated vapor pressure all affect cavitation.
2.2.1 Factors Affecting
â‘ pump inlet structural parameters: including the shape of the impeller suction inlet, the width of the blade inlet edge and the edge of the blade inlet edge and the shape of the front cover.
â‘¡ pump operating conditions: It includes the pump flow, head and speed and so on.
â‘¢ pump installation location: It includes the pump suction pipe hydraulic loss and installation height.
â‘£ environmental factors: it includes the installation of atmospheric pressure at the pump site.
2.2.2 Factors Affecting
It includes the nature of the media and the media operating temperature.
2.3 to solve the problem of centrifugal pump cavitation several programs
Based on the above analysis of factors affecting cavitation, we can get the following solution to the problem of cavitation in centrifugal pumps:
â‘ improve the pump inlet structural parameters
This solution is suitable for centrifugal pump design and manufacturing stage, the method is rarely used in the production site.
â‘¡ In the pump inlet induction wheel installation induction
Addition of induction wheel, to improve the anti-cavitation centrifugal pump performance, to solve the problem of cavitation, the effect is significant. And its simple structure easy to manufacture and install, easy operation and maintenance, low cost, without prejudice to the premise of production can be installed and debugged, especially suitable for promotion and application in the production site.
â‘¢ Reasonable design suction pipe and adjust the installation height
Although this method can completely eliminate the cavitation problem, it is seldom used in the production site. This is because the pump suction pipe and installation height adjustment, large quantities, high construction costs, and by the construction environment constraints, only when the device parking or overhaul can be carried out at the same time, due to process conditions, adjust the pump Inhalation pipe and installation height will affect the follow-up process, with a chain reaction.
â‘£ optimize the process operating conditions
Under the conditions of the process conditions, to change the pump flow, head, speed and media operating temperature and other operating parameters, to avoid the occurrence of cavitation. However, due to the limitation of the process conditions, the optimization of the process operation conditions has great limitations, and the effect is not significant in most cases. Therefore, this method can be used as a secondary solution to cavitation problems.
2.4 program to determine
Through the analysis and comparison of the above several programs, without affecting the normal production under the premise of solving pump cavitation problems, it should be preferred to install inducer at its entrance.
3, induction wheel design
When liquid flows through the induction wheel, the induction wheel does work on the liquid, which is equivalent to pressurizing the liquid entering the impeller behind to increase the pressure. Although this method increases the load on the motor, the power of the motor is usually relatively large (usually 20-30% larger than the shaft power of the centrifugal pump), and the size of the inducer is affected by the diameter of the suction port. The supercharging range limited. The motor can still meet the general requirements, do not need to replace the motor.
Due to the current understanding of induction wheel is still in the exploratory stage, there is no unified view on some theoretical issues. Therefore, the induction wheel design is largely based on experience, combined with the actual structure of the pump carried out. The 217-J / JA (model D1011 3 × 11/2 × 8) pump (Figure 1) will be taken as an example to introduce the design method of the inducer and provide some references for solving the pump cavitation problems in the field.
3.1 Characteristics and operating parameters (Table 2)
Table 2 characteristic parameters and operating parameters
Impeller diameter flow rate of head speed than the number of revolutions
Characteristic parameters 173
41
81.5
3550
51.0
Operating parameters 30
51.4
2960
51.4
3.2 induction wheel design calculations [1]
Assuming that the outer diameter of the induction wheel is equal to the diameter of the hub, the outer diameter of the induction wheel is φ, the diameter of the hub is φ, the number of blades is φ, the distance between the outer circumferences of the blades is φ, the length of the blades at the outer circle is φ, The angle at which the blade is placed is the angle of the blade at the outer circumference of the inducer; the radius of the edge of the blade at the entrance is the axial length of the blade at the outer circumference of the inducer; the blade angle at the hub is Circle at the minimum thickness of the blade.
4, after the induction wheel mounted anti-cavitation performance calculation
4.1 induced wheel cavitation margin [2]
Induction wheel rotation is the amount of leakage at the induced rim clearance, which is the amount of pump leakage that affects the induced wheel flow.
Induction wheel cavitation than the speed of which is the inlet flow coefficient, induced wheel inlet axial velocity, rim import circumferential velocity.
Therefore, induction wheel NPSH is
4.2 After the introduction of induction wheel wheel main cavitation performance analysis
Generally induced wheel lift coefficient, then the induced wheel actual lift is 217-J / JA pump without induction wheel, when the flow rate is Q = 30m3 / h, NPSH is. After the induction wheel is added, the NPSH of the main impeller is reduced after the induction wheel is added.
According to the above calculation results of the transformation of the equipment, as shown in Figure 1, the transformation of its operation after the investigation and analysis, the results in Table 3.
Table 3 before and after the transformation of the operating table comparison
After the transformation of the project before the transformation
Amplitude (mm)
0.47
0.02
Impeller pitting situation impeller entrance spongy impeller impeller surface no significant Ma Hang
Maintenance cycle (days)
7.3
> 90
Motor current (A)
6.9 ~ 7.0
7.0 ~ 7.2
Through the above calculation, it can be seen that adding an inducer at the suction inlet of the pump significantly improves the cavitation resistance of the pump, prevents the cavitation phenomenon, greatly improves the operation period of the equipment, thereby stabilizing the production of the equipment and lowering the equipment Inspection and maintenance costs, improve the economic efficiency of enterprises.
5 Conclusion
The installation of induction wheel has become one of the important measures to improve the anti-cavitation performance of centrifugal pumps, especially in the production site, not only because of its remarkable effect, simple structure, easy manufacture and installation, more importantly, its low cost, universal Strong, easy maintenance.
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