CN101178347B - Narrow slit Venturi throttling set and gas-liquid two-phase flow measuring system - Google Patents

Abstract

The invention belongs to the technical field of fluid measurement, and relates to a slit Venturi throttling device, which comprises an inlet, a constriction section, a slit section, an expansion section and an outlet connected in sequence. The constriction section is a long and narrow pipe with a cross section of A long and narrow rectangle with a cross-sectional area smaller than that of the inlet and outlet. The invention also provides a two-phase flow measuring system using the throttling device. The Venturi throttling device and the two-phase flow measurement system provided by the present invention can effectively eliminate the influence of frictional pressure drop on the measurement, so that the device can adapt to different working conditions such as gas-liquid two-phase media and different pipeline materials. Factors related to friction that affect the measurement can be reduced or eliminated. Using gravity differential pressure pulsation signals and time-averaged signals perpendicular to the horizontal flow direction also avoids the influence of frictional resistance along the way, can accurately identify flow patterns and measure phase holdup, and is simple and convenient to implement.

Description

Slit Venturi throttling device and gas-liquid two-phase flow measurement system

technical field

The invention belongs to the technical field of flow measurement, and relates to a two-phase flow phase holdup and flow measurement system.

Background technique

Two-phase flow refers to a flow system composed of two-phase substances (at least one phase is a fluid). If there are more than two phases of substances in the flow system, it is called multiphase flow. Two-phase or multiphase flow is the most common viscous fluid flow involved in the process of phase change, mass transfer and reaction in chemical production. Heat transfer with phase changes, gas absorption in column equipment, liquid distillation, liquid extraction, and chemical reaction processes in stirred tanks or bubble columns all involve two-phase flow. In the oil-gas-water three-phase flow measurement in the petroleum industry, the separation device is used to initially separate the fluid to obtain a two-phase flow fluid. Two-phase flow fluid is divided into gas-liquid, gas-solid, liquid-solid, liquid-liquid, etc. For the application requirements of industrial sites, it is necessary to measure the flow rate of two-phase flow more accurately. Two-phase flow measurement methods can be divided into three types according to whether they are separated, namely, complete separation method, partial separation method, and non-separation method. The complete separation method is usually used in metering stations, the equipment is huge, and continuous online metering is not possible. Partial separation methods include fractionation and phase separation and simple separator methods. The patent No. 98113068.2 proposes the method of splitting flow and separating phases to measure two-phase flow. First, a part of the two-phase fluid is divided through the distributor, and then the separator is used to separate this part of the two-phase fluid into single-phase gas and single-phase liquid, and then the single-phase gas flowmeter and single-phase liquid flowmeter are respectively used for metering and according to the ratio The relationship is converted into the flow rate and composition of the measured two-phase fluid, and finally this part of the single-phase gas and single-phase liquid are returned to the flow pipeline of the two-phase fluid. The problem with this method is whether the gas-liquid ratio of the two-phase fluid in the sampling part is consistent with the ratio in the original flow; whether the sampling ratio (constant) itself is affected by the flow pattern and flow fluctuation. The simple separator method is to use a small separator to pre-separate the gas-liquid two-phase flow to obtain a path dominated by the gas phase and a path dominated by the liquid phase, and each path is measured by a combined instrument and a corrected correlation formula. The metered fluids are mixed together and sent back to the original pipeline. This kind of device is also bulky and has a large pressure loss. It is usually made into a vehicle-mounted metering skid, which is not conducive to online measurement. The non-separation method means that the two-phase flow does not need to be separated, and the measurement system directly measures the two-phase flow. In order to improve the measurement accuracy, a mixer is usually added to the front end. Non-separation methods are typically implemented using techniques such as conventional instrument clusters or process tomography. Although tomographic imaging technology has been developed for decades, most of it is in the stage of laboratory research, and it is rarely applied in the field. Conventional instrument combination method is an effective way to realize non-separated measurement. Differential pressure flowmeters have been used for the measurement of two-phase flow for a long time. It is recognized by the industry and academia as a flowmeter that can work stably in various flow states of two-phase flow. It is used in combination methods. preferred. Patent No. 872U04538/34-181 proposed a dual-parameter measurement method using a combination of Venturi and orifice plate, but this method is only suitable for measuring two-phase fluids that are more uniformly mixed, and the combination of the two devices is usually bulky. The flow pattern and flow parameters of two-phase flow change gradually during the flow process. This is contrary to the basic premise of the combined measurement - the basic parameters of the two-phase fluid flow through each component of the device do not change phase. If the volume of the measuring device is too large, it is difficult to ensure that the flow parameters of the two-phase fluid flowing through each component of the combined device are consistent, so there is a principle error. In addition, the differential pressure taken by the traditional differential pressure flowmeter usually includes the frictional pressure drop. In the gas-liquid two-phase flow, the frictional resistance has a significant impact on the measurement. How to measure only the accelerated pressure drop of the differential pressure flowmeter and not Not including frictional pressure drop is the key to accurate measurement.

Contents of the invention

The purpose of the present invention is to overcome the above-mentioned problems of the prior art, propose a kind of Venturi throttling device suitable for two-phase flow measurement, and propose a kind of two-phase flow measurement system that adopts this kind of Venturi throttling device, the This kind of throttling device and measurement system does not need to separate the two-phase flow, does not need a mixer, can directly identify the flow pattern according to the gravity pressure drop signal, and uses different measurement models for flow measurement according to different flow patterns.

The present invention first proposes a slit Venturi throttling device, which includes an inlet, a constriction section, a slit section, an expansion section and an outlet connected in sequence. The slit section is a long and narrow pipe with a long and narrow cross-section. The cross-sectional area is smaller than that of the inlet and outlet.

In the above technical scheme, the ratio of the length to the width of the rectangular section of the slit segment should be between 6:1 and 20:1, preferably 10:1; the contraction angle from the contraction segment to the slit segment should not be greater than 25° , the expansion angle is not greater than 25°; the contraction angle from the slit segment to the expansion segment is not greater than 25°, and the expansion angle is greater than 25°.

The slit Venturi throttling device of the present invention preferably has a symmetrical shape with the cross-section in the middle of the slit section as a plane of symmetry.

The present invention also proposes a gas-liquid two-phase flow measurement system using the above-mentioned Venturi throttling device, including a Venturi throttling device, first, second, third differential pressure transmitters, a pressure transmitter, data Acquisition and processing unit, display unit, characterized in that the Venturi throttling device includes sequentially connected inlet, constriction section, slit section, expansion section and outlet, the section of the slit section is a long and narrow rectangle, which The cross-sectional area is smaller than the cross-sectional area of the inlet and the outlet; the two pressure-taking holes of the first differential pressure transmitter are respectively arranged at the upper and lower ends of the slit section, and are used to obtain the pressure drop signal of the heavy position; the second differential pressure The two pressure-taking holes of the pressure transmitter are arranged at both ends of the constriction section 17; the two pressure-taking holes of the third differential pressure transmitter are arranged at both ends of the expansion section 19; the pressure-taking holes of the pressure transmitter Holes are provided at the entry or exit.

In the above gas-liquid two-phase flow measurement system, the first differential pressure transmitter is preferably a high-frequency differential pressure transmitter; a temperature transmitter should also be installed on the Venturi throttling device.

The beneficial effects and advantages of the present invention are that the slit Venturi throttling device adopts a symmetrical structure design, which can effectively eliminate the influence of frictional pressure drop on measurement, so that the device can adapt to different gas-liquid two-phase media, different pipeline materials, etc. Working conditions, all factors related to friction that affect the measurement can be reduced or eliminated. Using gravity differential pressure pulsation signals and time-averaged signals perpendicular to the horizontal flow direction also avoids the influence of frictional resistance along the way, can accurately identify flow patterns and measure phase holdup, and is simple and convenient to implement. Different solution schemes are adopted according to different flow patterns. The purpose of measuring gas-liquid two-phase flow without separation is realized. The small size of the device can be easily connected to the pipeline for continuous on-line measurement.

Description of drawings

Fig. 1 is the front view of slit Venturi throttling device of the present invention;

Fig. 2 is the top view of the slit Venturi throttling device of the present invention;

Fig. 3 is the structural principle diagram of the slit Venturi gas-liquid two-phase flow measurement system of the present invention;

Fig. 4 is a working flow chart of the slit venturi gas-liquid two-phase flow measurement system of the present invention.

Detailed ways

The present invention will be described in further detail below with reference to the accompanying drawings and embodiments.

Referring to Figure 1 and Figure 2, Figure 1 is a front view and Figure 2 is a top view. The slit Venturi throttling device of the present invention includes an inlet 9 , a constriction section 17 , a slit section 18 , an expansion section 19 and an outlet 10 . The slit section 18 in the middle is a long and narrow pipe at one end, and its cross-section is a long and narrow rectangle whose length is much greater than its width. From the front view, the width of the slit section 18 is greater than that of the shrinkage section 17 and the expansion section 19 , and viewed from a top view, the width of the slit segment 18 is much larger than the width of the constriction segment 17 and the expansion segment 19 . The cross-sectional area of the tube of the slit section 18 is smaller than that of the constriction section 17 and the expansion section 19. From this point of view, the present invention is still a throttling device. This embodiment is a symmetrical structure, and the shape of the constricted section 17 is an expansion transition from the upper and lower parts of the inlet 9 to the part connected to the slit section 18, and from the middle part of the inlet 9 to the part connected to the slit section 18. The position is in contraction transition, and the expansion section 19 is symmetrical to the contraction section 17. When used, it is placed horizontally, and the inlet and outlet are externally connected to the pipeline that needs to measure the two-phase flow through flanges.

The gas-liquid two-phase flow measurement system of the present invention adopts the above-mentioned slit Venturi throttling device, utilizes the gravity differential pressure pulse signal and time-average signal perpendicular to the horizontal flow direction to identify the flow pattern and measure the phase holdup, and uses the contraction section 17 and the differential pressure signal of the expansion section 19 to measure the flow rate, and then obtain the phase-splitting flow rate of the two phases. See Figure 3, in the figure, 1. Slit Venturi throttling device, 2. Pressure transmitter, 3. Contraction differential pressure transmitter, 4. Slit high-frequency differential pressure transmitter, 5. Expansion differential pressure Pressure transmitter, 6. Temperature sensor, 7. Data acquisition and processing unit, 8. Display unit.

When the gas-liquid two-phase fluid enters the slit Venturi throttling device 1, a heavy pressure drop will be generated between 11 and 12 of the slit section 18, and the vertical height between the pressure-taking holes at the slit of the device is 100mm. The positive pressure terminal 12 of the high frequency differential pressure transmitter 4 . The time-averaged value of the repositioning pressure drop signal is directly related to the holdup of the two phases. The pulsation value and the time-average value of the gravity pressure drop are measured by the high-frequency differential pressure transmitter 4 , and the two-phase flow state in the device 1 can be identified by using these two signals. Here, we divide the gas-liquid two-phase flow state of the horizontal pipe into three categories. The first category is the gas-liquid two-phase flow dominated by the gas phase, including the so-called annular flow, stratified flow and wavy stratified flow. The volume flow rate of the gas phase in the first type of flow is much larger than that of the liquid phase, and the flow rate of the gas phase is higher than that of the liquid phase; the second type is the gas-liquid two-phase flow dominated by the liquid phase, including bubbly flow and plug flow. The volume flow rate of the liquid phase in this type of flow is much larger than that of the gas phase, and the flow rate of the gas phase is lower than that of the liquid phase. The third category is intermittent flow, mainly including slug flow. In this type of flow, the gas phase and the liquid phase intermittently scour the tube wall. According to different flow patterns, different solution schemes are determined later. When the two-phase flow in the tube is dominated by the gas phase, the time-average signal of the gravity pressure drop is low, no more than 300Pa; when the two-phase flow in the tube is dominated by the liquid phase, the time-average signal The signals are all high, not lower than 500Pa, and when there is intermittent two-phase flow in the pipe, the time-average signal is in a fluctuating state of high and low, and the three flow states can be distinguished by the size of the time-average signal value. Combined with the high-frequency pulsation signal, it can more accurately distinguish whether the gas-liquid flow is separate or the gas-liquid two-phase flow. When the gas phase is the main two-phase flow in the pipe, flow-related information can be obtained through the two differential pressure transmitters 3 and 5 of the constriction section 17 and expansion section 19 of the slit Venturi throttle device 1 . The differential pressure of the contracting section 17 is obtained between the two pressure-taking holes 13 and 14, the positive pressure terminal of the differential pressure transmitter is connected to 13, and the differential pressure of the expanding section 19 is obtained between the two pressure-taking holes 15 and 16, the differential pressure Positive pressure terminal connection 16 of the pressure transmitter. The differential pressure in the constriction section 17 is equal to the sum of the acceleration pressure drop and the friction pressure drop, and the pressure drop in the expansion section 19 is equal to the difference between the acceleration pressure drop and the friction pressure drop. The acceleration pressure drop can be obtained by adding and taking the average of these two differential pressure signals without including the friction pressure drop. There is a clear relationship between the acceleration pressure drop and the two-phase flow rate and phase holdup. There is also a definite functional relationship between the ratio of the two differential pressure signals of the differential pressure transmitters 3 and 5 and the phase holdup, and the two-phase flow can be obtained by solving the flow and holdup equations simultaneously. When the two-phase flow in the pipe is dominated by the liquid phase, the phase holdup can be obtained according to the time-average value of the gravity pressure difference, and the two differential pressure transmissions of the constriction section 17 and the expansion section 19 of the throttling device 1 3, 5 to obtain the flow information, simultaneous solution can obtain the two-phase flow. When the pipe is in an intermittent flow state, the flow dominated by the gas phase and the flow dominated by the liquid phase appear alternately, which can also be identified by the gravity pressure drop pulsation value and time-average value measured by the high-frequency differential pressure transmitter 4 The time for these two flows to alternate, and then use the corresponding solution scheme to solve it. The temperature transmitter 6 is used to correct the density of gas and liquid. All signals enter the data acquisition and processing unit 7 for processing. Displayed via the display unit 8 . The data acquisition and processing unit 7 and the display unit 8 can also be realized by a computer system. The specific working process of the system is shown in the work flow chart 4.

Claims (5)

1. A slit Venturi throttling device, comprising successively connected inlets, constriction sections, slit sections, expansion sections and outlets, the slit section is a long and narrow pipeline, and its cross section is a long and narrow rectangle with a cross-sectional area of Smaller than the cross-sectional area of the inlet and outlet; the ratio of the length to width of the rectangular section of the slit section is between 6:1 and 20:1; the contraction angle from the contraction section to the slit section is not more than 25°, and the expansion angle Not more than 25°; the contraction angle from the slit section to the expansion section is not more than 25°, and the expansion angle is greater than 25°; the slit Venturi throttling device has a symmetrical shape with the middle section of the slit section as a symmetrical plane. 2. The slit Venturi throttle device according to claim 1, characterized in that the ratio of the length to the width of the rectangular section of the slit section is 10:1. 3. A gas-liquid two-phase flow measurement system, including a Venturi throttling device, the first, second, and third differential pressure transmitters, a pressure transmitter, data acquisition and processing unit, display unit, its characteristics In that, the Venturi throttling device includes an inlet, a contraction section, a slit section, an expansion section, and an outlet connected in sequence, and the section of the slit section is a long and narrow rectangle, and its cross-sectional area is smaller than that of the inlet and the outlet; The two pressure-taking holes of the first differential pressure transmitter are respectively arranged at the upper and lower ends of the slit section, and are used to obtain the pressure drop signal after repositioning; the two pressure-taking holes of the second differential pressure transmitter It is arranged at both ends of the contraction section; the two pressure-taking holes of the third differential pressure transmitter are arranged at both ends of the expansion section; the pressure-taking holes of the pressure transmitter are arranged at the inlet or outlet; the slit The ratio of the length to width of the rectangular section of the segment is between 6:1 and 20:1; the contraction angle from the contraction segment to the slit segment is not greater than 25°, and the expansion angle is not greater than 25°; from the slit segment to the expansion segment The contraction angle is not greater than 25°, and the expansion angle is greater than 25°; the slit Venturi throttling device takes the cross section of the middle part of the slit section as a symmetrical plane, and has a symmetrical shape. 4. The gas-liquid two-phase flow measurement system according to claim 3, wherein the first differential pressure transmitter is a high frequency differential pressure transmitter. 5. The gas-liquid two-phase flow measurement system according to claim 3, characterized in that a temperature transmitter is also arranged on the Venturi throttling device.

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