Test method for determining influence of internal flow velocity on inherent frequency of pipeline

Abstract

The invention relates to the technical field of pipeline engineering, in particular to a test method for determining the influence of the internal flow velocity on the inherent frequency of a pipeline, and the method comprises the steps: S1, setting a test system; s2, determining the influence of the pressure on the inherent frequency of the pipeline when the internal flow is static; s3, determining the influence of the internal flow velocity on the inherent frequency of the pipeline when the pressure is not changed; s4, determining the influence of the internal flow velocity on the inherent frequency of the pipeline; and a data processing step. According to the method, the influence of the flow velocity of the pipeline internal flow on the inherent frequency of the pipeline can be discriminated, so that a basis is provided for economic and safe design of a pipeline system.

Description

Technical Field

[0001] The present invention relates to the technical field of pipeline engineering, and more particularly to a test method for determining the influence of internal flow velocity on the natural frequency of a pipeline. Background Art

[0002] When a pipe is filled with fluid and the fluid is at rest, the natural frequency of the pipe can be calculated. However, when the fluid is flowing, the natural frequency deviates from its resting value and changes monotonically with changes in flow velocity. Extensive research has been conducted to investigate the effects of internal flow velocity on the natural frequency of a pipe, aiming to develop a calculation method that accounts for this effect, calculating the natural frequency based on the internal flow velocity. However, two conclusions have emerged. One is that the natural frequency of a pipe decreases as the internal flow velocity increases. Therefore, for pipe designs that need to account for the influence of internal flow velocity, it is necessary to increase the stiffness of the free-span section by adding supports or by increasing the stiffness of the pipe itself to avoid resonance caused by the decrease in the natural frequency. The other is that the natural frequency of a pipe increases as the internal flow velocity increases. Therefore, pipe designs that account for the influence of internal flow velocity do not need to shorten the free-span section or increase the stiffness of the pipe itself, significantly reducing pipeline construction costs.

[0003] The two opposing conclusions above are based on theoretical analysis and have not been verified by authoritative experiments. During the test, as the internal flow velocity increases, the water pressure also increases. This increase in pressure often changes the stress in the pipe, and changes in pipe stress will also cause changes in the pipe's natural frequency. Therefore, if the effects of changes in internal flow velocity and pipe stress are contradictory, then the final test results will make it difficult to determine the law of the influence of internal flow velocity on the pipe's natural frequency.

[0004] The current test method does not consider the influence of pipeline stress. Therefore, there is no test procedure to measure the influence of pipeline stress on the natural frequency of the pipeline and no test method to control the flow pressure in the pipeline. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this paper develops a test method that can identify the impact of flow velocity on the natural frequency of pipelines, thereby providing a basis for the economical and safe design of pipeline systems. This new test method determines the effect of pipeline stress on the natural frequency of pipelines. By controlling the pressure of the pipeline flow to maintain consistency at different flow rates, the paper verifies the influence of flow velocity alone on the natural frequency of pipelines.

[0006] To achieve the above object, the present invention provides the following technical solution: a test method for determining the effect of internal flow velocity on the natural frequency of a pipeline, comprising the following steps:

[0007] S1. Set up the test system

[0008] Install a flow meter, a pressure gauge or a pressure-limiting valve, and water inlet and outlet valves at both ends of the test model pipeline, and affix a strain gauge to the middle of the test model pipeline; connect the water tank to the water pump, and the water pump to the water inlet of the test model pipeline through pipes, respectively. Use suitable pipes to connect the outlet valve of the test model pipeline and the pressure relief port of the pressure-limiting valve to the water level above the water tank, and connect the water inlet of the water pump to the bottom of the water tank.

[0009] S2. Determine the effect of pressure on the natural frequency of the pipe when the internal flow is stationary

[0010] S21. Remove the pressure-limiting valve from the test model pipeline, connect the water pump outlet to the port of the pressure-limiting valve, connect the strain gauge to the dynamic strain gauge, and connect the dynamic strain gauge to the data acquisition instrument; start the water pump to fill the test model pipeline with water, then close the water pump and the inlet and outlet valves of the test model pipeline; start and debug the strain and pressure testing system, and record initial data;

[0011] S22, exciting the test model pipeline to generate lateral vibration, and recording the strain data of the vibration of the test model pipeline;

[0012] S23, increasing the pressure of the test model pipeline to the initial value plus 0.5 MPa, and recording the strain and pressure data;

[0013] S24, repeat steps S22-S23 until the difference between the pressure and the pressure bearing capacity of the test model pipeline is less than 0.5 MPa;

[0014] S3. Determine the effect of internal flow velocity on the natural frequency of the pipeline when the pressure remains constant

[0015] S31, connecting the water pump outlet to the water inlet valve of the test model pipeline, connecting the pressure limiting valve to the model pipeline, setting the pressure value of the pressure limiting valve to the maximum value, and opening the inlet and outlet valves of the test model pipeline;

[0016] S32. Turn on the water pump and adjust the flow rate of the water pump until the flow rate in the test model pipeline reaches 0.2 m / s, and record the strain, pressure, and flow rate values;

[0017] S33, adjusting the pressure of the pressure-limiting valve to the pressure value recorded in step S32, and repeating step S22;

[0018] S34. Adjust the flow rate of the water pump to increase the flow rate of the test model pipeline by another 0.2m / s, and record the strain, pressure and flow rate values.

[0019] S35, repeat steps S22 and S34 until the flow rate reaches 2m / s;

[0020] S4. Determine the effect of internal flow velocity on the natural frequency of the pipe

[0021] S41. Set the pressure value of the pressure-limiting valve to the maximum value, start the water pump, and adjust the flow rate of the water pump until the flow rate in the test model pipeline reaches 0.2 m / s;

[0022] S42, record the strain, pressure and flow rate values, repeat step S22, adjust the flow rate of the water pump, and increase the flow rate of the test model pipeline by another 0.2 m / s;

[0023] S43. Repeat step S42 until the flow rate reaches 2 m / s.

[0024] Furthermore, four pairs of strain gauges are provided, and the four pairs of strain gauges are arranged at equal intervals along the circumferential direction of the test model pipeline, and each pair of strain gauges includes a longitudinal strain gauge and a transverse strain gauge.

[0025] Furthermore, the method also includes a data processing step, in which the time history data of the longitudinal strain gauge obtained by the above-mentioned test method are subjected to time-frequency transformation analysis to obtain the natural frequency of the model pipeline under different pressure conditions when the internal flow is stationary, the natural frequency of the model pipeline under different internal flow velocities when the internal flow pressure is constant, and the natural frequency of the model pipeline when the internal flow is flowing normally.

[0026] Furthermore, the recording duration in step S22 is determined according to the sampling frequency.

[0027] Furthermore, the recording duration in step S22 is more than 5 minutes.

[0028] In summary, the present invention includes at least one of the following beneficial technical effects: the present invention establishes a new test method, which solves the problem that the current test method does not consider the influence of pipeline stress. It can reliably conduct experimental research on the influence of internal flow velocity on the natural frequency of the pipeline, and measure the influence of pipeline stress on the natural frequency of the pipeline, providing a reliable basis for the economical and safe design of the pipeline system. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 It is a schematic diagram of the test model pipeline of the present invention;

[0030] Figure 2 It is a schematic diagram of the test system and connection relationship of the present invention;

[0031] Figure 3 It is a cross-sectional schematic diagram of the test bench of the present invention.

[0032] In the figure, 1. flow meter; 2. pressure gauge; 3. pressure limiting valve; 4. inlet and outlet valves; 5. transverse strain gauge; 6. longitudinal strain gauge; 7. water tank; 8. water pump; 9. support; 10. test model pipeline; 11. dovetail groove; 12. test bench. DETAILED DESCRIPTION

[0033] The present invention will be further described in detail below with reference to the accompanying drawings.

[0034] To facilitate those skilled in the art to understand the present invention, specific embodiments of the present invention are described below with reference to the accompanying drawings.

[0035] The present invention discloses a test method for determining the influence of internal flow velocity on the natural frequency of a pipeline. The test method of the present invention is implemented by the following test system, referring to Figure 1-3 .

[0036] The test model of the present invention can be made of PVC, PPR or plexiglass pipes. The pipe diameter can be adjusted according to the test conditions, but shall not exceed 1 inch to avoid a significant gradient in the internal flow velocity in the cross section of the model. The model length is set to 2.0m-2.5m, and the pipe pressure bearing capacity is not less than 2.5MPa.

[0037] A flow meter, pressure gauge or pressure limiting valve, and inlet and outlet valves are installed at both ends of the test model pipeline. Four pairs of strain gauges are attached to the middle position of the test model pipeline along its circumference. Each pair of strain gauges is set at equal intervals, that is, each pair of strain gauges is 90 degrees apart along the circumference. Each pair of strain gauges consists of a longitudinal strain gauge and a transverse strain gauge. The installation position diagram of the strain gauge, flow meter, pressure gauge, pressure limiting valve, and inlet and outlet valves is shown in the figure below. Figure 1 shown.

[0038] Test system such as Figure 2 As shown, the test system also includes a water tank, a water pump, and pipes connecting the water tank, water pump, and test model pipeline. Pipes connect the water tank to the water pump, and the water pump to the water inlet of the test model pipeline. The outlet valve of the test model pipeline and the pressure relief port of the pressure-limiting valve are connected to the water level above the water tank using suitable pipes to ensure that water flowing out of the pressure-limiting valve and the outlet flows naturally into the water tank. The pressure relief port of the pressure-limiting valve can be introduced into the water tank using a hose (not shown in the figure). The water inlet of the water pump is connected to the bottom of the water tank.

[0039] The test model pipe is supported on a test bench with a dovetail groove by a pipe clamp support. A dovetail slider is installed at the bottom of the support so that the support can move in the dovetail groove to adjust the length of the model caused by the change of flow pressure in the model. The cross section of the test bench is shown as follows: Figure 3 shown.

[0040] Embodiment 1 of the present invention comprises the following steps:

[0041] S1. Set up the test system

[0042] Install a flow meter, a pressure gauge or a pressure-limiting valve, and water inlet and outlet valves at both ends of the test model pipeline, and stick a strain gauge on the middle of the test model pipeline; connect the water tank and the water pump, and the water pump and the water inlet of the test model pipeline through pipes, respectively. Use adaptive pipes to connect the outlet valve of the test model pipeline and the pressure relief port of the pressure-limiting valve to the water level line above the water tank, and use the water inlet of the water pump to the bottom of the water tank.

[0043] S2. Determine the effect of pressure on the natural frequency of the pipe when the internal flow is stationary

[0044] S21. Remove the pressure-limiting valve from the test model pipeline, connect the water pump outlet to the port on the pressure-limiting valve, connect the strain gauge to the dynamic strain gauge, and connect the dynamic strain gauge to the data acquisition instrument. If an electronic flow meter and a non-direct-reading pressure gauge are used, connect them to their respective amplifiers, and connect the amplifier to the data acquisition instrument. Start the water pump to fill the test model pipeline with water, then close the water pump and the inlet and outlet valves of the test model pipeline. Start and debug the strain and pressure testing system, and record initial data.

[0045] S22. Excite the test model pipeline to generate lateral vibration. Any excitation method may be used. In this embodiment, the model pipeline is manually touched to generate lateral vibration. Record the strain data of the pipeline vibration. The recording time is determined according to the sampling frequency to ensure sufficient data for frequency analysis. For example, if the sampling frequency is 100 Hz, the sampling time should be at least 5 minutes.

[0046] S23, increasing the pressure of the test model pipeline to the initial value plus 0.5 MPa, and recording the strain and pressure data;

[0047] S24. Repeat steps S22-S23 until the difference between the pressure and the pressure bearing capacity of the test model pipeline is less than 0.5 MPa.

[0048] S3. Determine the effect of internal flow velocity on the natural frequency of the pipeline when the pressure remains constant

[0049] S31, connecting the water pump outlet to the water inlet valve of the test model pipeline, connecting the pressure limiting valve to the model pipeline, setting the pressure value of the pressure limiting valve to the maximum value, and opening the inlet and outlet valves of the test model pipeline;

[0050] S32. Turn on the water pump and adjust the flow rate of the water pump until the flow rate in the test model pipeline reaches 0.2 m / s, and record the strain, pressure, and flow rate values;

[0051] S33, adjusting the pressure of the pressure-limiting valve to the pressure value recorded in step S32, and repeating step S22;

[0052] S34. Adjust the flow rate of the water pump to increase the flow rate of the test model pipeline by another 0.2m / s, and record the strain, pressure and flow rate values.

[0053] S35. Repeat steps S22 and S34 until the flow rate reaches 2 m / s.

[0054] S4. Determine the effect of internal flow velocity on the natural frequency of the pipe

[0055] S41. Set the pressure value of the pressure-limiting valve to the maximum value, start the water pump, and adjust the flow rate of the water pump until the flow velocity in the test model pipeline reaches 0.2 m / s;

[0056] S42, record the strain, pressure and flow rate values, repeat step S22, adjust the flow rate of the water pump, and increase the flow rate of the test model pipeline by another 0.2 m / s;

[0057] S43. Repeat step S42 until the flow rate reaches 2 m / s.

[0058] S5. Data Processing

[0059] The time-frequency transformation analysis of the time history data of the longitudinal strain gauge obtained by the above test method can obtain the natural frequencies of the model pipeline under different pressure conditions when the internal flow is stationary, the natural frequencies of the model pipeline under different internal flow velocities when the internal flow pressure is constant, and the natural frequencies of the model pipeline when the internal flow is flowing normally.

[0060] Current testing methods do not consider the influence of pipeline stress. Consequently, there are no test procedures for measuring the effect of pipeline stress on the pipeline's natural frequency, nor are there any test methods for controlling the flow pressure within the pipeline. This new test method, developed in the present invention, measures the effect of pipeline stress on the pipeline's natural frequency. By controlling the flow pressure within the pipeline to maintain consistency at different flow rates, this method verifies the influence of flow velocity alone on the pipeline's natural frequency. This method can identify the influence of flow velocity on the pipeline's natural frequency, thus providing a basis for the economical and safe design of pipeline systems.

[0061] In the description of the present invention, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., indicating orientations or positional relationships, are based on the orientations or positional relationships shown in the accompanying drawings and are intended solely to facilitate and simplify the description of the present invention. They are not intended to indicate or imply that the devices or components referred to must have, be constructed, or operate in a specific orientation, and therefore should not be construed as limitations on the present invention. Furthermore, the terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0062] In the description of the present invention, it should be noted that, unless otherwise expressly specified or limited, the terms "mounted," "connected," and "connected" should be understood in a broad sense. For example, they may refer to fixed, detachable, or integral connections; mechanical or electrical connections; direct or indirect connections through an intermediate medium; and internal communication between two components. Those skilled in the art will understand the specific meanings of the above terms in the present invention based on the specific circumstances.

[0063] The above-described embodiments of the present invention do not limit the scope of protection of the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the claims of the present invention.

Claims

1. A test method for determining the effect of internal flow velocity on the natural frequency of a pipeline, characterized in that: The following steps are involved: S1. Set up the test system Install a flow meter, a pressure gauge or a pressure-limiting valve, and water inlet and outlet valves at both ends of the test model pipeline, and affix a strain gauge to the middle of the test model pipeline; connect the water tank to the water pump, and the water pump to the water inlet of the test model pipeline through pipes, respectively. Use suitable pipes to connect the outlet valve of the test model pipeline and the pressure relief port of the pressure-limiting valve to the water level above the water tank, and connect the water inlet of the water pump to the bottom of the water tank. S2. Determine the effect of pressure on the natural frequency of the pipe when the internal flow is stationary S21. Remove the pressure-limiting valve from the test model pipeline, connect the water pump outlet to the port of the pressure-limiting valve, connect the strain gauge to the dynamic strain gauge, and connect the dynamic strain gauge to the data acquisition instrument; start the water pump to fill the test model pipeline with water, then close the water pump and the inlet and outlet valves of the test model pipeline; start and debug the strain and pressure testing system, and record initial data; S22, exciting the test model pipeline to generate lateral vibration, and recording the strain data of the vibration of the test model pipeline; S23, increasing the pressure of the test model pipeline to the initial value plus 0.5 MPa, and recording the strain and pressure data; S24, repeat steps S22-S23 until the difference between the pressure and the pressure bearing capacity of the test model pipeline is less than 0.5 MPa; S3. Determine the effect of internal flow velocity on the natural frequency of the pipeline when the pressure remains constant S31, connecting the water pump outlet to the water inlet valve of the test model pipeline, connecting the pressure limiting valve to the model pipeline, setting the pressure value of the pressure limiting valve to the maximum value, and opening the inlet and outlet valves of the test model pipeline; S32. Turn on the water pump and adjust the flow rate of the water pump until the flow rate in the test model pipeline reaches 0.2 m / s, and record the strain, pressure, and flow rate values; S33, adjusting the pressure of the pressure-limiting valve to the pressure value recorded in step S32, and repeating step S22; S34. Adjust the flow rate of the water pump to increase the flow rate of the test model pipeline by another 0.2m / s, and record the strain, pressure and flow rate values. S35, repeat steps S22 and S34 until the flow rate reaches 2m / s; S4. Determine the effect of internal flow velocity on the natural frequency of the pipe S41. Set the pressure value of the pressure-limiting valve to the maximum value, start the water pump, and adjust the flow rate of the water pump until the flow rate in the test model pipeline reaches 0.2 m / s; S42, record the strain, pressure and flow rate values, repeat step S22, adjust the flow rate of the water pump, and increase the flow rate of the test model pipeline by another 0.2 m / s; S43. Repeat step S42 until the flow rate reaches 2 m / s.

2. The test method for determining the effect of internal flow velocity on the natural frequency of a pipeline according to claim 1, characterized in that: The strain gauges are provided in four pairs, and the four pairs of strain gauges are arranged at equal intervals along the circumferential direction of the test model pipeline, and each pair of strain gauges includes a longitudinal strain gauge and a transverse strain gauge.

3. A test method for determining the effect of internal flow velocity on the natural frequency of a pipeline according to any one of claims 1-2, characterized in that: The method also includes a data processing step, in which the time history data of the longitudinal strain gauge obtained by the above test method are subjected to time-frequency transformation analysis to obtain the natural frequencies of the model pipeline under different pressure conditions when the internal flow is stationary, the natural frequencies of the model pipeline under different internal flow velocities when the internal flow pressure is constant, and the natural frequencies of the model pipeline when the internal flow is flowing normally.

4. The test method for determining the effect of internal flow velocity on the natural frequency of a pipeline according to claim 1, characterized in that: The recording duration in step S22 is determined according to the sampling frequency.

5. The test method for determining the effect of internal flow velocity on the natural frequency of a pipeline according to claim 1, characterized in that: The recording duration in step S22 is more than 5 minutes.

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