Design of a Crude Oil ultrasonic flowmeter sensor

introduction

At present, one of the difficult problems faced in the extraction of crude oil in oil fields is the online measurement of the extracted crude oil. The main reason is that the composition of crude oil is very complex. The crude oil contains oil, water, gas and other impurities. It is a multi-phase and complex fluid, and single-well crude oil flows intermittently, so general ultrasonic flow meters cannot meet the requirements. This paper designs a measurement system based on ultrasonic wave correlation flow calculation, which solves the problem of crude oil non-contact online measurement.

Principle of ultrasonic flowmeter

The correlation method uses related techniques to measure fluid flow. The measurement accuracy of ultrasonic flowmeter transducer has nothing to do with the speed of sound in the fluid, and the measurement accuracy is high. It is suitable for the measurement of multi-phase flow and large interference fluid. When the fluid flows in the pipeline, if it contains other impurities, there will be a variety of random disturbances inside, which will generate flow signals related to the flow conditions and have certain statistical characteristics. The structure of the correlation method flowmeter is shown two sets of ultrasonic transmitting and receiving transducers, and L is the distance between the upstream transducer and the downstream transducer. When the ultrasonic signal passes through the pipeline, the ultrasonic signal will be modulated by the noise in the fluid. The modulated ultrasonic signal contains the information of the fluid velocity field. The ultrasonic signal is analyzed to extract the flow signal A( t) and B(t), and perform correlation operations on A(t) and B(t).

2 Single-well crude oil ultrasonic related flow measurement system structure

The single-well flow measurement system is mainly composed of three parts: gas-liquid separation pretreatment, ultrasonic detection, and signal processing. The three parts are analyzed separately below.

2.1 Gas-liquid separation pretreatment part

The structure of the preprocessing part is shown. The function of the pretreatment part is to carry out liquid-gas separation (the crude oil pumped by the pumping unit contains gas and other impurities in addition to the oil mixture. The gas will bring greater error to the oil measurement, so the oil Gas-liquid separation must be carried out during measurement); the second is to solve the problem of full pipe measurement during intermittent flow (the crude oil pumped out each time when the pumping unit is working is not equal, and it flows intermittently, so that the oil in the pipeline may not be full, or Bring great measurement error). For these reasons, pre-treatment should be carried out before the oil measurement. After pre-treatment, the gas-liquid separation and the oil-filled pipe pass through the metering oil pipe to reduce the measurement error. The working principle is: the crude oil enters the oil storage tank from the oil inlet through the sedimentation tank, and the oil and gas are separated in the oil storage tank. The separated gas is output from the valve (outlet valve) on the oil storage tank through the gas pipeline. When the oil  reaches a certain height, the floating ball floats up to open the lower valve (oil outlet valve), and at the same time the upper valve blocks the gas port to build up pressure. The oil in the oil storage tank flows to the oil outlet through the measuring pipeline under the action of pressure. When the oil storage tank drops to a certain level, the floating ball sinks to block the lower valve and opens the upper valve, so that the repeated work completes the gas-liquid separation.

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2.2 Ultrasonic testing part

The detection part is mainly composed of two pairs of ultrasonic sensors. The detection of ultrasonic sensors is done by transmitting and receiving energy of ultrasonic waves. The core of  transducer (converting ultrasonic energy into electric energy or converting electric energy into ultrasonic energy. Reversible transducer means that the two forms of The energy of the transducer is converted into each other). Common ultrasonic transducers include piezoelectric crystal vibrators, magnetostrictive vibrators and so on. Ultrasonic waves used for related flow measurement generally have two forms: sine wave and pulse wave. The related flowmeters of pulsed ultrasonic and sine wave integrate the velocity information of the cross section of the flow field to obtain the flow velocity. This design uses a piezoelectric crystal ultrasonic sensor with a center frequency of 200 kHz. In order to overcome the influence of standing waves, ultrasonic uses a phase-locked loop pulse signal generator.

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2.3 Signal processing part

The signal processing part is mainly composed of ultrasonic receiving transducer.The signal conditioning circuit is composed of a receiving transducer, a three-stage amplifier circuit, a filter circuit and an envelope detection circuit. The preamplifier is composed of MAX410 instrument amplifier module, the secondary amplifier and final amplifier are composed of INA128 precision low power instrument amplifier; the filter circuit is a band pass filter composed of MAX275 analog integrated filter, the center frequency is 200 kHz, a low-pass filter is formed by TL14, and the signal after detection is mainly taken out. The envelope detection circuit is composed of a diode and a capacitor to form a peak detector.

The other part is a data acquisition and processing circuit composed of modules. This circuit of ultrasonic water flow sensor selects the TMS320F2812DSP chip of TI Company. In the current process control field, it is the most advanced DSP microprocessor. Compared with traditional single-chip microcomputers, it has outstanding performance such as strong functions, rich resources, and low power consumption. It has perfect performance and the best integrated peripheral interface. It integrates flash memory, high-speed A/D converter, high-performance CAN module, etc.

During measurement, the upstream and downstream transmitter transducers emit high-frequency ultrasonic waves. When the ultrasonic waves propagate in the fluid, the flow signal will modulate the ultrasonic waves in amplitude, phase and frequency. The high-frequency modulated signal received by the transducer is received and filtered. After demodulation and amplification, the flow signal is obtained and sent to the A/D converter for data collection, and the collected information is sent to the related processing to obtain the flow rate of the fluid.

3 System programming

The software system includes initialization, calculation module, flow display, interrupt processing module and other parts.The main program flow chart is shown. After the main program is initialized, it enters a loop program to process the sampled data, and responds to external A/D interrupt requests, serial communication interrupt requests and timer interrupt requests at any time. Determining whether the flow rate display timing is reached. The main program responds to the above interrupt requests and calls each corresponding processing program to complete data collection and processing.

Initialization is on the one hand to set up the working environment of the DSP, and on the other hand to prepare for the subsequent signal processing. The system initialization program includes the internal initialization that affects the operation of the DSP chip CPU and the peripheral initialization that affects the work of each peripheral, as well as the initialization of peripheral programmable devices (such as A/D, D/A, etc.), specifically including the following functions: set the clock generator, set the timer, initialize the status registers, open interrupts, etc.

The interrupt processing module includes three interrupts: the timer interrupt processing module is used to start the A/D converter and control the sampling frequency, the serial communication interrupt processing module is used to communicate with the upper computer, and the A/D interrupt processing module is used to read The A/D converter samples the data, and its flowchart is shown in the figure .

 The display module refreshes the meter regularly to display the instantaneous flow value and the cumulative flow value.The system processing process is to set the timing period, the timer generates an interrupt, this interrupt starts the A/D converter, after the conversion, the A/D converter requests the DSP to read the data interrupt, and the DSP responds to the A/D converter interrupt request , call the A/D interrupt processing module, read the sampled data, and send it to the data buffer. Since the fluid flows intermittently, after the DSP receives the N-point data of the upstream and downstream signals, it performs fourier analysis on the data to determine whether the fluid is flowing. If it is flowing, the calculation program is called to perform related operations on the sampled data and find the related functions. Determine the transit time T, and obtain the instantaneous flow value and cumulative flow value according to the instrument parameters and temperature compensation, and store the result in the data storage unit for display by the display instrument.

In the correlation flow measurement, one of the key issues of brass ultrasonic flowmeter sensor is the calculation method of the correlation function, which requires high-speed and accurate completion of the acquisition of a large number of random modulation signals, correlation integration calculations and the peak search of the correlation function. The correlation function algorithm mainly has two kinds of polarity repetition method and zero crossing method. In order to improve the operation speed, this system adopts correlation operation in the frequency domain. After the input data is transformed by FFT, the correlation operation in the frequency domain can be obtained. Then the correlation result in the time domain can be obtained through IFFT, which can be used for peak search.

4 Conclusion

On the basis of analyzing the working conditions of a single well in the oil field and the principle of related flow measurement, a device suitable for single well crude oil metering was designed. The field test has achieved good results with an error of less than 2%. However, there are still the following problems: First, the signal fluctuates greatly, which mainly has the crude oil contains gas and impurity. Therefore, the signal difference is large, and the detection circuit needs to increase the AGC circuit. The second has the difficulty of setting the correction coefficient. Different wells have different water content and oil viscosity. At the same time, the fluidity of the oil varies greatly at different temperatures, so it must be adjusted by multiple times under different environments. The correction factor brings inconvenience to use. Third, the error is relatively large when the flow rate is low. These are the areas to be improved in future research.