What is the mathematical relationship between the valve stem lift and the pressure ratio before and after the valve?

Regulating valve is a typical mechanical or electromechanical product, but the biggest difference between it and manual valve is that it can be accurately adjusted by field general route technology after combining with modern information technology, which greatly improves the control system and important position of regulating valve. The main function of the control valve is to change the area of the circulation part and then change the parameters such as pressure, temperature and flow after the valve to meet the needs of different working conditions. Under some working conditions, the strong unstable fluid flow in the control valve may affect the stability of the valve and even cause the vibration of the valve. As far as the safety of control valve is concerned, accidents such as valve stem vibration and fracture occur from time to time. These phenomena are basically related to fluid-induced valve instability, that is, the instability of gas (liquid) flow in the regulating valve leads to valve vibration, especially the vibration of valve stem-spool. In this paper, the micro high-frequency dynamic pressure sensor and its acquisition system are used to collect, process and analyze the working condition data that cause the vibration or instability of the control valve rod. The influence of flow field in the valve on the working stability of the valve is studied through the dynamic pressure change and vibration characteristics test of the valve stem and the corresponding results.

2。 Regulating valve model and its testing system

The inner cavity of the control valve is complex and the flow is tortuous. Experiments were carried out at different pressure ratios and relative lift.

The pressure ratio is defined as

ε=P 1/P0

Where: p 1- pressure behind the valve, Mpa.

P0—— Pressure before valve, Mpa

Relative lift is defined as

L (relative) = l/dn

Where: L-lifting height of control valve stem, mm

Dn-matching diameter between valve core and valve seat, mm

When the valve stem lift is large or fully open, the smallest passage in the valve is the throttle section of the valve seat. If the valve stem lift is small, the annular channel area formed by the valve core and the valve seat may also be smaller than the flow area of the throttle part of the valve seat. Generally, the annular channel formed by the upper part of the valve core and the valve seat is called the first nozzle channel. When the lift is small, the area of the annular channel is the smallest channel. The valve seat is called the second nozzle channel, and its throttling cross section is the smallest area in the second nozzle channel.

In order to fully understand the complex flow characteristics in the valve, measuring points are set at the inlet of the valve cavity, the top of the valve cavity, the throttle section of the valve seat, the gradually expanding section of the valve seat and the head of the valve core, and a plurality of measuring points are also set at the throttle section of the valve seat and the head of the valve core. By measuring each measuring point, processing the measured data of each measuring point and analyzing the correlation of the results, the flow characteristics in the valve under different working conditions can be obtained.

The medium used in the test system is air. In order to make the intake air flow uniform, the air from the high-pressure air source enters the regulating valve after passing through the diffusion section, the pressure stabilizing section and the convergence section, and the air flow passes through the annular channel between the valve core and the valve seat, and then is injected into the valve seat, and then gradually compressed by the valve seat and then enters the exhaust pipeline, and the exhaust pipeline is introduced into the underground exhaust chamber to reduce noise. The air flow in and out direction is 90 degrees. There are special gas flow, pressure and temperature measuring tubes in the test.

3。 Dynamic pressure sensor and data acquisition system

(1) miniature dynamic pressure sensor

In order to minimize the interference of contact measurement on the smooth flow in the control valve, the piezoresistive dynamic pressure sensor produced by Kulite sensor company of the United States is adopted. The sensor integrates silicon sensitive elements and is made into micron scale by photolithography technology. It has high natural frequency, low hysteresis, excellent thermal and environmental performance, excellent static and dynamic performance, and is durable.

When calibrating the pressure sensor, there are generally two calibration methods: static calibration and dynamic calibration. Static calibration can only be carried out if the sensor is linear. However, it is difficult to give some standard dynamic pressure, so static standards are still used for dynamic pressure measurement at present. Experience shows that only when the response frequency of the whole pressure measuring system is high enough and the dynamic pressure is measured by the static calibration pressure measuring system, the result is accurate enough.

(2) Data acquisition system

High-frequency dynamic acquisition and analysis system can carry out multi-channel parallel dynamic acquisition, which has the advantages of high speed, large capacity and transient digitization. It is a high-performance comprehensive measurement system integrating measurement, analysis and result output. It has highly stable circuit design and instrument structure design, as well as excellent modularization characteristics of software and hardware. It can be conveniently used in testing fields such as transient acquisition, dynamic process monitoring and recording, and can also be used for parallel multi-channel data acquisition. Each acquisition channel stores data in its own buffer, and the internal computer processes these data through a unified bus.

Because each channel has its own A/D and buffer, the maximum acquisition rate or storage depth will not be reduced because of the channel expansion, and the whole sampling channel is conducted in parallel, so the time difference between channels can be ignored. Its basic working mode is to work in the order of acquisition, processing, re-acquisition and re-processing. In dynamic analysis, it mainly uses its own depth buffer to store enough data for processing. The maximum sampling rate of the system is 1.25MPa, and the sampling accuracy is 12bit, which can respond to the parameters and changes of unsteady flow in the valve in time.

(3) Pressure signal regulator

The pressure signal regulator is an instrument for regulating the pressure signal. The output signal obtained by the regulator can be used for display and data acquisition, and it can be used as a preamplifier of high frequency dynamic acquisition system in the experiment. The regulator mainly consists of five parts: pressure sensor, sensor power supply, measuring instrument amplifier, limiting circuit and whole machine power supply. It can condition 12 pressure signals at the same time, which can not only meet the requirements of different types of pressure sensor signals, but also meet the requirements of other voltage signals. In order to reduce the interference of power frequency AC signal, its output part is equipped with a limiting wave line with a limiting wave frequency of 50±5Hz, thus greatly improving the anti-interference ability of the whole regulating circuit.

The micro dynamic pressure sensor first amplifies the mV signal through a high-frequency preamplifier, and then inputs it into a high-frequency dynamic acquisition system for fast parallel acquisition and storage. After all kinds of signal processing in time domain, frequency domain and filtering, the really useful signal is obtained, and finally its characteristic curve is drawn, and then the unsteady flow characteristics in the valve are obtained.

4。 Static pressure measurement acquisition and spectrum analysis

(1) Attitude pressure measurement and acquisition system

The static pressure measurement and acquisition system consists of 305 1CD-BC intelligent pressure transmitter,1565438 series pressure transmitter, 3595 1C data acquisition board, 35954B data acquisition interface board and computer. In the test, the parameters such as inlet and outlet flow and static pressure of the regulating valve are mainly measured. Due to real-time acquisition, the measured data of pressure and other parameters can be averaged in time, which reduces the measurement error.

(2) Spectrum analysis

The spectrum analysis system consists of computer, printer, display, signal amplifier, filter, data collector and analysis software. Through the computer acquisition system, the system converts the vibration characteristics of the parts under the impact of external force into digital signals, and analyzes the frequency spectrum to obtain the harmonic frequency of the vibration signals, thus obtaining the natural frequencies of each order. Because the vibration form of control valve is mainly the vibration of valve stem-spool, the frequency spectrum analysis system is used to analyze the vibration signal of valve stem-spool in the experiment.

5。 Dynamic signal processing

The flow in the control valve has typical unsteady characteristics, and dynamic mixing can accurately and timely determine the instantaneous and time-varying size of its internal flow field. Data processing and analysis in dynamic testing are extensive and involve many problems. In order to find out the law, we must get real and reliable data and results, among which spectrum analysis and waveform analysis are the most important and basic methods in dynamic data processing. Spectrum analysis and waveform analysis are independent and closely related, and there are obvious differences between them, so they can be transformed by Fourier transform. Spectrum and waveform analysis and random data processing methods have become the most commonly used methods in signal analysis.

6。 label

Combining the experimental data processing results and numerical simulation, we can draw the following conclusions:

(1) Due to the development and use of a set of testing systems for studying the working stability of control valves, including testing equipment and technologies such as high-frequency dynamic pressure testing platform for control valves and miniature piezoresistive high-frequency dynamic pressure sensors, the mechanism of liquid-induced vibration in valve bodies can be studied.

(2) In the test, the micro-sensor is directly inserted into the key parts of the valve body, such as the throttle section of the valve seat and the head of the valve core, and the high-frequency dynamic acquisition system is used for multi-working range and multi-direction measurement. The high-frequency dynamic pressure test data in the valve are processed and analyzed by frequency spectrum analysis and correlation analysis. This method is simple, practical and reliable.

(3) In the test, the vibration of the valve stem-spool has complicated reasons and forms, which are related to the pulsation of unstable airflow in the valve. According to the vibration mode, there are horizontal and axial vibrations in equal and vertical directions. According to the nature of vibration, there are * * * vibration and forced vibration. Judging from the factors that cause vibration, there are vibrations induced by vortex shedding and combinations of these vibrations with different properties.