Method for determining protection range of runner chamber under cavitation oscillation condition of large tubular pump

Abstract

The invention belongs to the technical field of hydraulic mechanical engineering, and provides a method for determining the protection range of a runner chamber under the cavitation oscillation condition of a large tubular pump, and the implementation process is as follows: firstly, introducing a life expansion curve and a life atrophy curve to respectively describe the growth and collapse processes of a cavitation area under the cavitation oscillation condition of the large tubular pump; then, according to a birth threshold value and a death threshold value, key tripod nodes are calculated step by step; according to the protection threshold value, sequentially solving mark values of the upper protection node and the lower protection node; and finally, determining the optimal protection range of the runner chamber by combining the mark values of the holding node, the upper protection node and the lower protection node. The empirical estimation method which follows the cavitation oscillation rule and achieves economic protection of the runner chamber is given for the first time, the optimal circumferential protection interval of the runner chamber can be rapidly and accurately predicted, it can be guaranteed that the large tubular pump has the necessary cavitation erosion resistance, and additional economic losses caused by full-range excessive protection can be avoided.

Description

Technical Field

[0001] The invention belongs to the technical field of hydraulic machinery engineering, and in particular relates to a method for determining the protection range of a runner chamber under cavitation oscillation conditions of a large-scale tubular pump. Background Art

[0002] In engineering, tubular pumps with impellers exceeding 3m in diameter are called "large tubular pumps". Large tubular pumps have a significant gravity effect during operation, which is manifested as a hydrostatic pressure difference between the top and bottom of the impeller flow channel that is equivalent to the pump head. This can easily induce cavitation oscillation, which is manifested as significant cavitation at the top of the impeller flow channel and no cavitation at the bottom, and the cavitation zone will evolve periodically with the rotation of the impeller. Under this special cavitation oscillation condition, the inner wall of the impeller chamber will be subjected to the periodic impact of the cavitation zone carried by each blade, which is prone to cavitation damage and affects the healthy operation of the tubular pump device. Therefore, it is very necessary to protect the impeller chamber. However, it is not clear in the current engineering to what extent the circumferential range of protection is most appropriate, because different areas will be subjected to different degrees of cavitation impact. For example, the top of the flow channel should be protected intensively, while the bottom of the flow channel generally does not need to be protected, and excessive protection of the entire range will bring high economic costs. Summary of the invention

[0003] 1. Technical issues to be resolved

[0004] The present invention aims to solve the problem of how large the circumferential range is to be the most economical for protecting the inner wall of the impeller chamber under the impact of cavitation oscillation induced by gravity effect for a large-scale tubular pump with an impeller diameter exceeding 3m.

[0005] (II) Technical solution

[0006] The present invention provides a method for determining the protection range of a runner chamber under cavitation oscillation conditions of a large-scale tubular pump, and the implementation process includes the following steps:

[0007] Step 1: Given the “life expansion curve” V according to the following formula + , which is used to characterize the growth process of the cavitation zone under the cavitation oscillation conditions of a large-scale tubular pump. The specific expression is:

[0008]

[0009] In the formula, e is a natural constant, x is a node coordinate variable used to mark the circumferential position of the runner chamber, α1 is an upper limit value used to characterize the maximum cavitation degree of the cavitation zone growth process, α2 is a slope value, and α3 is a milestone point, which together determine the growth speed of the "life expansion curve";

[0010] Step 2: Given the “life shrinkage curve” V according to the following formula -, which is used to characterize the collapse process of the cavitation zone under the cavitation oscillation condition of a large-scale tubular pump. The specific expression is:

[0011]

[0012] In the formula, e is a natural constant, x is a node coordinate variable used to mark the circumferential position of the runner chamber, α4 is an upper limit value used to characterize the maximum cavitation degree of the cavitation zone collapse process, α5 is a slope value, and α6 is a milestone point, which together determine the decay speed of the "life shrinkage curve";

[0013] Step 3: Let V + Equal to the birth threshold δ B , and the birth node is x = A; let V - Equal to the death threshold δ D , and the dead node is obtained by reverse calculation as x = B;

[0014] Step 4: Calculate the peak node value T according to the following formula, that is, determine the circumferential position where the runner chamber cavitation reaches the maximum destructive capacity. The specific expression is:

[0015]

[0016] Step 5: Let V + is equal to the protection threshold β, and the mark value x=M of the upper protection node is obtained by inverse calculation, that is, the circumferential position where the cavitation in the runner chamber begins to have destructive power is determined; let V - Equal to the protection threshold β, the mark value x=N of the lower protection node is obtained by reverse calculation, that is, the circumferential position where the runner chamber cavitation finally loses its destructive ability is determined;

[0017] Step 6: Establish a circumferential coordinate system, including defining the counterclockwise rotation direction of the impeller based on the inlet direction of the tubular pump, defining the top point of the tubular pump runner chamber as the position of θ=90° and the bottom point of the tubular pump runner chamber as the position of θ=270° based on the gravity direction;

[0018] Step 7: Calculate the circumferential coordinates θ of the upper protection node according to the following formula: + and the circumferential coordinates of the lower protection node θ - , according to the circumferential coordinate θ + and the circumferential coordinate θ - Determine the optimal protection range of the runner chamber in the circumferential interval [θ + ,θ - ]Inside;

[0019]

[0020] For a method for determining the protection range of a runner chamber under cavitation oscillation conditions of a large-scale tubular pump provided by the present invention, in step 1, the life expansion curve V+ The values ​​of parameters α1, α2, and α3 in the expression are: α1=0.9902, α2=-44.9196, α3=1.5461.

[0021] For the method for determining the protection range of the runner chamber under the cavitation oscillation condition of a large-scale tubular pump provided by the present invention, in step 2, the life atrophy curve V - The values ​​of parameters α4, α5, and α6 in the expression are: α4=0.9608, α5=182.5196, α6=1.9537.

[0022] For the method for determining the protection range of the impeller chamber under the cavitation oscillation condition of a large-scale tubular pump provided by the present invention, in step 3, the values ​​of the birth threshold and the death threshold are δ B =δ D =0.0012.

[0023] For the method for determining the protection range of the impeller chamber of a large-scale crossflow pump under cavitation oscillation conditions provided by the present invention, in step 5, the protection threshold β is set to 0.5.

[0024] (III) Compared with the prior art, the beneficial effects of the present invention

[0025] The present invention proposes a method for determining the protection range of the impeller chamber of a large-scale crossflow pump under cavitation oscillation conditions. The method has the advantages of providing for the first time an empirical estimation method that follows the law of cavitation oscillation and realizes economical protection of the impeller chamber. The method is suitable for large-scale crossflow pumps with an impeller diameter exceeding 3 m, and can quickly and accurately predict the optimal circumferential protection range of the impeller chamber. The method can ensure that the large-scale crossflow pump has the necessary anti-cavitation ability and avoid the additional economic losses caused by full-range excessive protection, thereby providing technical guidance for realizing the reasonable guarantee of the anti-cavitation ability of large-scale crossflow pumps. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The present invention will be further explained below in conjunction with the accompanying drawings and specific embodiments.

[0027] Figure 1 It is a schematic diagram of the circumferential coordinate system in step 6 of the present invention.

[0028] Figure 2 The circumferential protection range of the impeller chamber of a large-scale cross-flow pump with an impeller diameter of 4.2m determined according to the determination method provided by the present invention and its comparison with the actual measured value.

[0029] Figure 3 The present invention provides a flowchart for the method for determining the protection range of the impeller chamber of a large-scale throughflow pump under cavitation oscillation conditions. DETAILED DESCRIPTION

[0030] In order to make the purpose, technical solutions and advantages of the implementation of the present invention clearer, the relevant technical solutions will be described in more detail below in conjunction with the accompanying drawings. In the accompanying drawings, the same or similar reference numerals throughout represent the same or similar elements or elements with the same or similar functions. The described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work and without departing from the essence and spirit taught by the present invention belong to the scope of protection of the present invention.

[0031] See also Figure 1-3 The method for determining the protection range of the impeller chamber under the cavitation oscillation condition of the large-scale tubular pump provided by the present invention is implemented according to the following process:

[0032] Step 1: Given the “life expansion curve” V according to the following formula + , which is used to characterize the growth process of the cavitation zone under the cavitation oscillation conditions of a large-scale tubular pump. The specific expression is:

[0033]

[0034] In the formula, e is a natural constant, x is a node coordinate variable used to mark the circumferential position of the runner chamber, α1 is an upper limit value used to characterize the maximum cavitation degree of the cavitation zone growth process, α2 is a slope value, and α3 is a milestone point, which together determine the growth speed of the "life expansion curve";

[0035] Step 2: Given the “life shrinkage curve” V according to the following formula - , which is used to characterize the collapse process of the cavitation zone under the cavitation oscillation condition of a large-scale tubular pump. The specific expression is:

[0036]

[0037] In the formula, e is a natural constant, x is a node coordinate variable used to mark the circumferential position of the runner chamber, α4 is an upper limit value used to characterize the maximum cavitation degree of the cavitation zone collapse process, α5 is a slope value, and α6 is a milestone point, which together determine the decay speed of the "life shrinkage curve";

[0038] Step 3: Let V + Equal to the birth threshold δ B , and the birth node is x = A; let V - Equal to the death threshold δ D , and the dead node is obtained by reverse calculation as x = B;

[0039] Step 4: Calculate the peak node value T according to the following formula, that is, determine the circumferential position where the runner chamber cavitation reaches the maximum destructive capacity. The specific expression is:

[0040]

[0041] Step 5: Let V + is equal to the protection threshold β, and the mark value x=M of the upper protection node is obtained by inverse calculation, that is, the circumferential position where the cavitation in the runner chamber begins to have destructive power is determined; let V - Equal to the protection threshold β, the mark value x=N of the lower protection node is obtained by reverse calculation, that is, the circumferential position where the runner chamber cavitation finally loses its destructive ability is determined;

[0042] Step 6: Establish a circumferential coordinate system, including defining the impeller as counterclockwise rotating direction based on the inlet direction of the tubular pump, defining the top point of the tubular pump runner chamber as θ=90° and the bottom point of the tubular pump runner chamber as θ=270° based on the gravity direction;

[0043] Step 7: Calculate the circumferential coordinates θ of the upper protection node according to the following formula: + and the circumferential coordinates of the lower protection node θ - , according to the circumferential coordinate θ + and the circumferential coordinate θ - Determine the optimal protection range of the wheel chamber in [θ + ,θ - ], calculate the circumferential coordinate θ of the upper protection node + and the circumferential coordinates of the lower protection node θ - The specific expression is:

[0044]

[0045] In the above embodiment, the lifeline includes a life expansion curve and a life contraction curve.

[0046] In this example, the life expansion curve V in step 1 + The values ​​of parameters α1, α2, and α3 in the expression are: α1=0.9902, α2=-44.9196, α3=1.5461.

[0047] In this example, the life shrinkage curve V in step 2 - The values ​​of parameters α4, α5, and α6 in the expression are: α4=0.9608, α5=182.5196, α6=1.9537.

[0048] In this example, the birth threshold and death threshold in step 3 are set to δ B =δ D =0.0012.

[0049] In this example, in step 5, the value of the protection threshold β is 0.5.

[0050] Taking a large-scale tubular pump with an impeller diameter of 4.2m as an example, the method for determining the protection range of the impeller chamber under the cavitation oscillation conditions of the large-scale tubular pump is described in detail below.

[0051] First, the birth node A is calculated as:

[0052]

[0053] The dead node B is calculated as:

[0054]

[0055] Then the peak node T is calculated as:

[0056]

[0057] Then the mark value M of the upper protection node is calculated as:

[0058]

[0059] Then the mark value N of the next protection node is calculated as:

[0060]

[0061] Finally, the circumferential coordinate θ of the upper protection node is calculated + for:

[0062] θ + =[1-4(TM)]·90°≈1.48°

[0063] Calculate the circumferential coordinate θ of the lower protection node - for:

[0064] θ - =[1+4(NT)]·90°≈147.89°

[0065] From the circumferential coordinates of the lower protection node and the upper protection node obtained by the above calculation, it can be seen that the optimal protection range of the runner chamber is within the circumferential range of 1.48° to 147.89° (the circumferential coordinate system is shown in the attached figure). Figure 1 As shown in Figure 2 ). Actual observations show that for a large-scale tubular pump with an impeller diameter of 4.2 m, with the gravity direction as a reference, during the cavitation growth process from the bottom to the top of the impeller, at θ + ≈3°, the cavitation zone reaches a semi-vaporized state, that is, the vapor volume fraction is about to exceed 50%. In engineering, this is considered to be the starting point for entering the cavitation protection zone. In the process of cavitation collapse from the top to the bottom of the impeller, at θ -When ≈145°, the cavitation zone reaches a semi-vaporized state, that is, the vapor phase volume fraction is about to be less than 50%. In engineering, this is considered to be the end point where the cavitation protection zone can be exited. Obviously, the estimation results of this patent are in good agreement with the actual observation results, and the optimal circumferential protection interval of the runner chamber can be predicted quickly and accurately. In contrast, traditional engineering experience cannot infer this accurate economic interval, and can only take irrational measures such as semi-circular protection of the top of the runner chamber or overall full circumferential protection. For large-scale cross-flow pumps with huge sizes, this will significantly increase the protection cost. In summary, this patent gives for the first time an empirical estimation method that follows the law of cavitation oscillation and realizes economical protection of the runner chamber. It can quickly and accurately predict the optimal circumferential protection interval of the runner chamber, which can not only ensure that large cross-flow pumps have the necessary anti-cavitation ability, but also avoid additional economic losses caused by excessive protection over the entire range.

[0066] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the protection scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for determining the protection range of the impeller chamber of a large-scale tubular pump under cavitation oscillation conditions, characterized in that: The method comprises the following steps: Step 1: Given the "life expansion curve" V according to the following formula + , which is used to characterize the growth process of the cavitation zone under the cavitation oscillation conditions of a large-scale tubular pump. The specific expression is: Where, e is a natural constant, x is a node coordinate variable used to mark the circumferential position of the runner chamber, α1 is the upper limit value, α2 is the slope value, and α3 is the mileage point; Step 2: Given the "life shrinkage curve" V according to the following formula - , which is used to characterize the collapse process of the cavitation zone under the cavitation oscillation condition of a large-scale tubular pump. The specific expression is: Where, e is a natural constant, x is a node coordinate variable used to mark the circumferential position of the runner chamber, α4 is the upper limit value, α5 is the slope value, and α6 is the mileage point; Step 3: Let V + Equal to the birth threshold δ B , and the birth node is x = A; let V - Equal to the death threshold δ D , and the dead node is obtained by reverse calculation as x = B; Step 4: Calculate the value T of the peak node according to the following formula. The specific expression is: Step 5: Let V + Equal to the protection threshold β, the mark value x of the protection node is obtained inversely, and V - Equal to the protection threshold β, the mark value x of the next protection node is obtained inversely. Step 6: Establish a circumferential coordinate system, including defining the counterclockwise rotation direction of the impeller based on the inlet direction of the tubular pump, defining the top point of the tubular pump runner chamber as the position of θ=90° and the bottom point of the tubular pump runner chamber as the position of θ=270° based on the gravity direction; Step 7: Calculate the circumferential coordinates θ of the upper protection node according to the following formula: + and the circumferential coordinates of the lower protection node θ - , according to the circumferential coordinate θ + and the circumferential coordinate θ - Determine the optimal protection range of the wheel chamber in [θ + ,θ - ] in the circumferential range, the specific expression is:

2. The method for determining the protection range of the impeller chamber of a large-scale tubular pump under cavitation oscillation conditions according to claim 1 is characterized in that: In step 1, the life expansion curve V + The values ​​of parameters α1, α2, and α3 in the expression are: α1=0.9902, α2=-44.9196, α3=1.5461.

3. The method for determining the protection range of the impeller chamber of a large-scale tubular pump under cavitation oscillation conditions according to claim 1 is characterized in that: In step 2, the life shrinkage curve V - The values ​​of parameters α4, α5, and α6 in the expression are: α4=0.9608, α5=182.5196, α6=1.9537.

4. The method for determining the protection range of the impeller chamber of a large-scale tubular pump under cavitation oscillation conditions according to claim 1 is characterized in that: In step 3, the birth threshold and death threshold are set to δ B =δ D =0.0012.

5. The method for determining the protection range of the impeller chamber of a large-scale tubular pump under cavitation oscillation conditions according to claim 1 is characterized in that: In step 5, the protection threshold β is set to 0.5.

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