A method for obtaining flow resistance coefficient of control valve under cavitation conditions

Abstract

The present invention discloses a method for obtaining the flow resistance coefficient of a control valve under cavitation conditions, and relates to the technical field of valve flow capacity measurement and valve design and selection. The method comprises: (1) determining the operating parameters of the control valve and the structural characteristics of the control valve to obtain the flow resistance coefficient of the control valve under single-phase flow conditions; (2) calculating the blocking flow pressure difference according to the operating parameters and structural characteristics of the control valve to determine whether the control valve is in a blocking flow condition, and if so, proceeding to the next step, otherwise, using the flow resistance coefficient of the control valve under single-phase flow conditions; (3) calculating the maximum flow rate using the blocking flow pressure difference, and calculating the total flow resistance coefficient of the control valve under cavitation conditions based on the actual pressure difference and the maximum flow rate; (4) further calculating the additional flow resistance coefficient of the control valve caused by the cavitation phenomenon. The present invention solves the shortcomings of the prior art in that the calculation process of the flow resistance coefficient of the control valve under cavitation conditions is complex, the scope of application is vague, and the calculation error is large.

Description

Technical Field

[0001] The invention relates to the technical field of valve flow capacity measurement and valve design and selection, and in particular to a method for obtaining a flow resistance coefficient of a control valve under cavitation conditions. Background Art

[0002] The control valve flow resistance coefficient is a dimensionless coefficient of valve pressure loss. It characterizes the magnitude of local energy loss when the fluid medium flows through the valve. It is an important technical parameter for measuring the flow capacity of the control valve and is of great significance to the design and rational selection of the valve. At present, the control valve flow resistance coefficient is usually measured by experiments or CFD simulation methods, and is obtained by experiments and calculations under single-phase flow conditions according to national standards and IEC related standards. However, in the actual use of the control valve, it is often encountered that due to large pressure difference and high flow rate, flash cavitation occurs when the liquid medium flows through the valve, and the single-phase flow is transformed into cavitation vapor-liquid two-phase flow. At this time, the calculation result obtained by the calculation method of the single-phase flow control valve flow resistance coefficient does not conform to the actual working conditions and has certain errors, which has an adverse effect on the design and rational selection of the control valve.

[0003] In the prior art, patent CN202221299438.0 proposed a valve flow resistance coefficient measuring instrument, which improved the problem of water flow carrying bubbles affecting the measurement accuracy by setting a buffer water tank. Patent CN201910767104.8 proposed a modular valve flow resistance test system and test method, which improved the economic efficiency of the test system. In the prior art, the calculation methods of valve flow resistance coefficient are mostly based on non-cavitation single-phase flow conditions, and the calculation of flow resistance coefficient under cavitation multiphase flow conditions is rarely mentioned.

[0004] Therefore, it is an urgent problem for technical personnel in this field to propose a method for obtaining the flow resistance coefficient of a control valve under cavitation conditions to solve the difficulties existing in the prior art. Summary of the invention

[0005] In view of this, the present invention provides a method for obtaining the flow resistance coefficient of a control valve under cavitation conditions, so as to achieve the purpose of calculating the flow resistance coefficient of the control valve under cavitation conditions through relevant formulas during the control valve design stage.

[0006] In order to achieve the above object, the present invention adopts the following technical solution:

[0007] A method for obtaining a flow resistance coefficient of a control valve under cavitation conditions comprises the following steps:

[0008] S1. Collect control valve operating information;

[0009] S2. Determine the control valve operating parameters and control valve structural characteristics according to the control valve operating information;

[0010] S3. According to the control valve operating parameters, the control valve flow resistance coefficient ζ under non-cavitation single-phase flow conditions is obtained;

[0011] S4. Calculate the choked flow pressure difference ΔP based on the control valve operating parameters and control valve structural characteristics ch ; And determine whether the control valve is in a blocked flow condition, if so, enter S5, otherwise use the control valve flow resistance coefficient ζ under the non-cavitation single-phase flow condition obtained in S3;

[0012] S5. Using the choked flow pressure difference ΔP ch Calculate the maximum flow rate Q max ; According to the actual pressure difference ΔP and maximum flow Q max Calculation of total flow resistance coefficient ζ of control valve under cavitation conditions c .

[0013] Optionally, the control valve operating parameters in S2 include the inlet pressure P1, the actual pressure difference ΔP, the flow area A of the valve connection pipeline, the fluid density ρ, and the liquid pressure recovery coefficient F L , liquid critical pressure ratio coefficient F F , liquid saturated vapor pressure P v .

[0014] Optionally, the specific content of determining whether the control valve is in a blocked flow condition in S4 is:

[0015] If the actual pressure difference ΔP ≤ the choked flow pressure difference ΔP ch , it can be determined as a single-phase flow condition, then the control valve flow resistance coefficient ζ under the non-cavitation single-phase flow condition obtained by S3 is used; if ΔP>ΔP ch , then the operating condition is determined to be a blocking flow condition, and the process goes to S5.

[0016] Optionally, the choked flow pressure difference ΔP is used in S5 ch Calculate the maximum flow rate Q max The specific formula is:

[0017]

[0018] Among them, A1 is the effective flow area of ​​the upstream chamber of the valve seat throttling, σ is the flow area ratio, C d is the throttling coefficient.

[0019] Optionally, in S5, the actual pressure difference ΔP and the maximum flow rate Q max Calculation of total flow resistance coefficient ζ of control valve under cavitation conditions c The specific formula is:

[0020]

[0021] Optionally, it also includes S6. Using the total flow resistance coefficient ζ of the control valve c, calculate the additional flow resistance coefficient ζ′ of the control valve caused by cavitation conditions.

[0022] Optionally, the specific formula of the additional flow resistance coefficient ζ′ of the control valve in S6 is:

[0023]

[0024] It can be seen from the above technical solution that, compared with the prior art, the present invention provides a method for obtaining the flow resistance coefficient of a control valve under cavitation conditions, which has the following beneficial effects:

[0025] (1) The shortcomings of the prior art in the control valve flow resistance coefficient calculation process under cavitation conditions, such as complex calculation process, vague application scope, and large calculation error, are solved.

[0026] (2) Based on the choking flow theory, the flow resistance coefficient of the control valve under cavitation conditions can be calculated through relevant formulas during the control valve design stage. It is derived based on the correlation theory between choking flow and orifice outflow, with fewer assumptions and clearer applicable conditions, which can effectively reduce the error of the calculation results.

[0027] (3) Compared with other methods, the present invention requires fewer technical parameters and a simpler calculation process. At the same time, it can be performed with the help of CFD simulation tests or only theoretical calculations, thus saving test costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying creative work.

[0029] Figure 1 A flow chart of a method for obtaining a flow resistance coefficient of a control valve under cavitation conditions provided by the present invention;

[0030] Figure 2 A schematic diagram of the relationship between the flow rate and the pressure difference of the control valve under cavitation conditions provided by an embodiment of the present invention;

[0031] Figure 3 A simplified schematic diagram of a control valve throttling structure provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0032] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only 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 creative work are within the scope of protection of the present invention.

[0033] Reference Figure 1 As shown, the present invention discloses a method for obtaining a flow resistance coefficient of a control valve under cavitation conditions, comprising the following steps:

[0034] S1. Collect control valve operating information;

[0035] S2. Determine the control valve operating parameters and control valve structural characteristics according to the control valve operating information;

[0036] S3. According to the control valve operating parameters, the control valve flow resistance coefficient ζ under non-cavitation single-phase flow conditions is obtained;

[0037] S4. Calculate the choked flow pressure difference ΔP based on the control valve operating parameters and control valve structural characteristics ch ; And determine whether the control valve is in a blocked flow condition, if so, enter S5, otherwise use the control valve flow resistance coefficient ζ under the non-cavitation single-phase flow condition obtained in S3;

[0038] S5. Using the choked flow pressure difference ΔP ch Calculate the maximum flow rate Q max ; According to the actual pressure difference ΔP and maximum flow Q max Calculation of total flow resistance coefficient ζ of control valve under cavitation conditions c .

[0039] Furthermore, the control valve operating parameters in S2 include the inlet pressure P1, the actual pressure difference ΔP, the flow area A of the valve connecting pipeline, the fluid density ρ, and the liquid pressure recovery coefficient F L , liquid critical pressure ratio coefficient F F , liquid saturated vapor pressure P v .

[0040] Furthermore, the specific content of determining whether the control valve is in a blocked flow condition in S4 is:

[0041] If the actual pressure difference ΔP ≤ the choked flow pressure difference ΔP ch , it can be determined as a single-phase flow condition, then the control valve flow resistance coefficient ζ under the non-cavitation single-phase flow condition obtained by S3 is used; if ΔP>ΔP ch , then the operating condition is determined to be a blocking flow condition, and the process goes to S5.

[0042] Furthermore, in S5, the blocking flow pressure difference ΔP is used chCalculate the maximum flow rate Q max The specific formula is:

[0043]

[0044] Among them, A1 is the effective flow area of ​​the upstream chamber of the valve seat throttling, σ is the flow area ratio, C d is the throttling coefficient.

[0045] Specifically, In the formula: μ is the throttling correction coefficient, μ=0.64.

[0046] Furthermore, in S5, according to the actual pressure difference ΔP and the maximum flow rate Q max Calculation of total flow resistance coefficient ζ of control valve under cavitation conditions c The specific formula is:

[0047]

[0048] Further, it also includes S6. Using the total flow resistance coefficient ζ of the control valve c , calculate the additional flow resistance coefficient ζ′ of the control valve caused by cavitation conditions.

[0049] Furthermore, the specific formula of the additional flow resistance coefficient ζ′ of the control valve in S6 is:

[0050]

[0051] In a specific embodiment, taking a control valve as an example, the specific embodiment steps are as follows:

[0052] Step 1. Calculate the control valve flow resistance coefficient ζ under single-phase flow conditions;

[0053] First, determine the operating parameters and structural characteristics of the control valve, query the flow resistance coefficient data provided by the manufacturer, or calculate the flow resistance coefficient ζ under single-phase flow conditions according to relevant standards such as GBT 30832 or IEC 60534-2-3. The test or numerical simulation must be carried out under non-cavitation single-phase flow conditions.

[0054] Step 2: Determine whether the control valve is in a blocked flow condition under actual working conditions;

[0055] Calculate the choked flow pressure difference ΔP based on the control valve operating parameters and structural characteristics ch , the calculation formula is:

[0056]

[0057] Where: F L is the liquid pressure recovery coefficient; F F is the liquid critical pressure ratio coefficient, P vis the saturated vapor pressure of the liquid; if the actual pressure difference ΔP ≤ the blocking flow pressure difference ΔP ch , it can be determined as a single-phase flow condition, and the flow resistance coefficient ζ obtained under the single-phase flow condition is directly used. If ΔP>ΔP ch , then the operating condition is determined to be a blocking flow condition and the next step of calculation must be performed.

[0058] Step 3. Calculate the total flow resistance coefficient ζ of the control valve under cavitation conditions c ;

[0059] First, calculate the maximum flow rate Q under the choked flow condition max .

[0060] like Figure 2 As shown in Figure 1, the characteristic of choking flow under cavitation conditions is that when the actual pressure difference is greater than the choking flow pressure difference, the flow rate through the valve no longer increases with the increase of the pressure difference and reaches the maximum flow rate Q max After that, it basically remains stable. Assuming that cavitation does not occur, the theoretical flow rate that can be achieved is calculated based on the flow capacity of the valve under single-phase flow conditions as Q th , then the theoretical loss flow Q loss is the theoretical flow rate Q th With the maximum flow Q max The difference, Q loss =Q th -Q max .

[0061] Q max It can be obtained by experiment or CFD numerical calculation, or by the relevant theory of thick-wall orifice outflow. Figure 3 The simplified model of the control valve throttling structure is analyzed and calculated, Q max The calculation formula is:

[0062]

[0063] Where: A1 is the effective flow area of ​​the upstream chamber of the valve seat throttling, σ is the flow area ratio, C d is the throttling coefficient, ρ is the test fluid density; σ=A0 / A1, A0 is the effective flow area of ​​the valve seat throttling channel; C d The calculation formula is:

[0064]

[0065] In the formula: μ is the throttling correction coefficient, μ=0.64.

[0066] Secondly, calculate the total flow resistance coefficient ζ of the control valve under cavitation conditions c ;

[0067] like Figure 2As shown in the figure, under cavitation conditions, when the actual pressure difference of the control valve is ΔP, the actual flow rate of the control valve is the maximum flow rate Q under the blocking flow condition. max , at this time the total flow resistance coefficient of the control valve ζ c for:

[0068]

[0069] Where: A is the flow area of ​​the valve connecting pipeline.

[0070] ζ c It can also be expressed as: c =ζ+ζ′.

[0071] Step 4. Calculate the additional flow resistance coefficient ζ′ of the control valve caused by cavitation;

[0072] Depend on Figure 2 Analysis shows that under the condition of single-phase flow without cavitation, the control valve flow resistance coefficient is a constant and does not change with the working conditions. The valve flow capacity is only related to the structural parameters. When the pressure difference continues to increase and cavitation occurs, the single-phase flow is transformed into a cavitation blocking flow condition, and the control valve flow resistance coefficient will increase with the increase in pressure difference. This is because the cavitation phenomenon will cause additional energy loss, and the valve flow capacity will decrease with the increase in the degree of cavitation. The valve flow capacity under cavitation conditions is related to the working conditions. The corresponding control valve additional flow resistance coefficient ζ′ calculation formula can be expressed as:

[0073]

[0074] Step 5. Verification of the method for obtaining the control valve flow resistance coefficient under cavitation conditions:

[0075] The reliability of the flow resistance coefficient acquisition method proposed in the present invention is verified by using the experimentally verified CFD cavitation flow numerical calculation model. The comparison data and errors are shown in Table 1. The numerical calculation is carried out using a simplified control valve throttling model. The main structural parameters are: the valve seat throttling channel diameter is 10mm and the length is 15mm; the throttling upstream and downstream chambers have a diameter of 40mm and lengths of 25mm and 90mm respectively. The upstream and downstream inlet and outlet pipe sections are extended by 2 and 6 times the pipe diameter respectively to allow the flow to fully develop. The calculated control valve throttling model under single-phase flow conditions has ζ=421.62, and the blocking flow pressure difference ΔP ch =0.55MPa.

[0076] Table 1 Verification of the method for obtaining the flow resistance coefficient of the control valve under cavitation conditions

[0077]

[0078] It can be seen from the data listed in Table 1 that under the cavitation flow conditions of working conditions 1-5, the ζ obtained by the method for obtaining the flow resistance coefficient under cavitation conditions proposed by the present invention is cIt shows good consistency with the numerical calculation data. c The data have the same trend as the pressure difference increases. Taking the numerical calculation data as the benchmark, the maximum relative error between the two is 6.60%, which is within the acceptable range. The error is caused by the simplification of the model from single-phase flow to cavitation multiphase flow. The comparative verification results show that the method for obtaining the flow resistance coefficient of the control valve under cavitation conditions proposed in this invention is feasible.

[0079] The above description of the disclosed embodiments enables one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for obtaining the flow resistance coefficient of a control valve under cavitation conditions, characterized in that: The following steps are involved: S1. Collect control valve operating information; S2. Determine the control valve operating parameters and control valve structural characteristics according to the control valve operating information; S3. According to the control valve operating parameters, the control valve flow resistance coefficient ζ under non-cavitation single-phase flow conditions is obtained; S4. Calculate the choked flow pressure difference ΔP based on the control valve operating parameters and control valve structural characteristics ch ; And determine whether the control valve is in a blocked flow condition, if so, enter S5, otherwise use the control valve flow resistance coefficient ζ under the non-cavitation single-phase flow condition obtained in S3; S5. Using the choked flow pressure difference ΔP ch Calculate the maximum flow rate Q max ; According to the actual pressure difference ΔP and maximum flow Q max Calculation of total flow resistance coefficient ζ of control valve under cavitation conditions c ; In S5, the choked flow pressure difference ΔP is used ch Calculate the maximum flow rate Q max The specific formula is: Among them, A1 is the effective flow area of ​​the upstream chamber of the valve seat throttling, σ is the flow area ratio, C d is the throttling coefficient; S5 is based on the actual pressure difference ΔP and the maximum flow rate Q max Calculation of total flow resistance coefficient ζ of control valve under cavitation conditions c The specific formula is: Also includes S6. Using the total flow resistance coefficient ζ of the control valve c , calculate the additional flow resistance coefficient ζ′ of the control valve caused by cavitation conditions, where ζ c =ζ+ζ′; The specific formula of the additional flow resistance coefficient ζ′ of the control valve in S6 is:

2. The method for obtaining the flow resistance coefficient of a control valve under cavitation conditions according to claim 1, characterized in that: The control valve operating parameters in S2 include the inlet pressure P1, the actual pressure difference ΔP, the flow area A of the valve connecting pipeline, the fluid density ρ, and the liquid pressure recovery coefficient F L , liquid critical pressure ratio coefficient F F , liquid saturated vapor pressure P v .

3. The method for obtaining the flow resistance coefficient of a control valve under cavitation conditions according to claim 1, characterized in that: The specific content of determining whether the control valve is in a blocked flow condition in S4 is: If the actual pressure difference ΔP ≤ the choked flow pressure difference ΔP ch , it can be determined as a single-phase flow condition, then the control valve flow resistance coefficient ζ under the non-cavitation single-phase flow condition obtained by S3 is used; if ΔP>ΔP ch , then the operating condition is determined to be a blocking flow condition, and the process goes to S5.

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