Ring type natural gas flow standard system and method

Abstract

The invention provides a loop type natural gas flow standard system and method, and relates to the technical field of natural gas flow instrument verification and calibration, and the system comprises a closed loop pipeline, a piston volume pipe primary standard device, a circulating fan, a cooling device, a buffer tank, a flow adjusting unit and a control unit; the inlet end of the closed loop pipeline is connected with the outlet of the circulating fan, and the outlet end of the closed loop pipeline is connected with the inlet of the circulating fan; the piston volume pipe is connected in series in the loop pipeline, the buffer tank and the cooling device are sequentially arranged at the upstream, and the detected flowmeter mounting station is connected at the downstream; the flow regulating unit comprises a backflow regulating valve and a flow-limiting regulating valve which are arranged in parallel, an inlet of the backflow regulating valve is connected with an outlet of the circulating fan, and an outlet is connected with an inlet of the buffer tank; an inlet of the flow-limiting regulating valve is connected with the downstream of the detected flowmeter station, and an outlet is connected with an inlet of the cooling device. The system is wide in pressure coverage, flexible in flow adjustment, accurate in temperature control and suitable for the hydrogen environment.

Description

Technical Field

[0001] This invention relates to the field of natural gas flow meter calibration technology, specifically to a loop-type natural gas flow standard system and method. Background Technology

[0002] The statements in this section are merely background information relating to this disclosure and do not necessarily constitute prior art.

[0003] In natural gas trade transactions, the accuracy of flowmeters directly affects the fairness of trade. With the large-scale construction of high-pressure gas transmission pipelines, the number of flowmeters used for trade measurement with high operating pressure and wide flow range is increasing. To ensure the accuracy and reliability of flowmeter measurement results, it is necessary to establish traceable primary natural gas standard devices. These devices serve as the source of the metrological value transfer system, providing the most accurate metrological value traceability basis for secondary and working-level standard devices.

[0004] In existing technologies, the primary standard device for natural gas flow rate is usually based on the High Pressure Piston Prover (HPPP method) to reproduce the flow rate value: a fixed volume of gas is discharged by the piston moving at a constant speed in a standard volume tube, and the volumetric flow rate is calculated by combining the movement time, tracing back to the length and time reference, with an uncertainty of 0.05%.

[0005] Although the above technologies have been applied in testing stations both domestically and internationally, the following technical problems still exist: (1) Insufficient adaptability to pressure and flow: The direct discharge process relies on the pressure difference between upstream and downstream pipelines. For example, the Tianjin branch requires (6.7–9.2) MPa, which cannot cover the low-pressure calibration requirements. The Jinan branch requires (2.5–5.5) MPa. Although the loop process can adjust the pressure, the fan is susceptible to hydrogen embrittlement under high pressure, and the existing equipment is not adapted to hydrogen / hydrogen-doped media.

[0006] (2) The efficiency and accuracy of value transfer are limited: Although the primary standard of the HPPP method can be directly transferred to the working standard (the uncertainty of the large flow standard device at the Wuhan branch station is 0.16%), the flow upper limit of the existing primary standard devices at home and abroad is ≤500m³ / h, which cannot meet the direct traceability requirements of large flow (such as 12000m³ / h at the Tianjin branch station). Summary of the Invention

[0007] To address the aforementioned problems, this invention provides a loop-type natural gas flow standard system and method, which for the first time applies a loop device to the calibration of primary natural gas flow standards, providing a loop-type primary flow standard calibration system with wide pressure coverage, flexible flow regulation, precise temperature control, and adaptability to hydrogen-blended natural gas.

[0008] The first aspect of the present invention provides a loop-type natural gas flow standard system, including a closed loop pipeline, a piston volume tube primary standard device, a circulating fan, a cooling device, a buffer tank, a flow regulating unit, and a control unit; The closed loop pipeline forms a gas circulation loop, with its inlet end connected to the outlet of the circulating fan and its outlet end connected to the inlet of the circulating fan. The piston volume tube is connected in series in the loop pipeline, with a buffer tank and a cooling device installed upstream of it, and the downstream is connected to the installation station of the flow meter under test. The flow regulation unit includes a reflux regulating valve and a flow limiting regulating valve arranged in parallel. The inlet of the reflux regulating valve is connected to the outlet of the circulating fan, and the outlet is connected to the inlet of the buffer tank. The inlet of the flow limiting regulating valve is connected to the downstream of the station of the flow meter under test, and the outlet is connected to the inlet of the cooling device. The control unit is electrically connected to the piston volume tube, the circulating fan, the cooling device, and the flow regulation unit.

[0009] Furthermore, the buffer tank has a volume of ≥2m³ and a design pressure of ≥10MPa, and is used to stabilize the gas temperature and pressure.

[0010] Furthermore, when the buffer tank stores hydrogen-blended natural gas, a magnetically coupled fan is selected as the circulating fan.

[0011] Furthermore, the magnetically coupled fan is designed with a pressure ≥10MPa, an inlet-outlet pressure difference ≥400kPa, and a range ratio ≥1:10.

[0012] Furthermore, the piston volume tube is a stainless steel cylinder, which is divided into an acceleration section, a metering section and a buffer section along the airflow direction, with a length ratio of 2:3:1; three position sensors are installed on each of the a2, a3 and a4 sections of the metering section to trigger the timer.

[0013] A second aspect of the present invention provides a calibration method using the above-described natural gas primary flow rate standard calibration system, comprising: Natural gas is introduced into the closed-loop pipeline, and the pressure is adjusted to the target test pressure range. The control unit starts the circulating fan and selects the adjustment mode according to the target flow rate; After the flow rate stabilizes, the gas flows through the piston volume tube. The piston movement triggers the timing of position sensors a2-a4. The standard volume flow rate Qv=V / t is calculated using the standard volume tube V and time t. Using Qv as the reference value, calibrate the indication error of the flow meter under test in the loop.

[0014] Furthermore, the adjustment mode is selected based on the target flow rate, specifically including: When the target flow rate is greater than or equal to the preset flow rate threshold, the flow limiting valve is closed, and the opening of the return flow regulating valve and the fan speed are adjusted; when the target flow rate is less than the preset flow rate threshold, the opening of the return flow regulating valve, the flow limiting valve and the fan speed are adjusted simultaneously.

[0015] Furthermore, the specific process of piston movement triggering timing is as follows: The timer starts when the piston passes through section a2 and stops when it passes through section a4. The average value of the timing results from the three position sensors a2, a3, and a4 is taken as time t.

[0016] Furthermore, the testing method also includes: During the flow regulation process, the control unit activates the cooling device, which exchanges heat with the gas in the closed loop through an intermediate cooling medium to stabilize the gas temperature within the target calibration temperature preset value ±0.2℃.

[0017] Furthermore, after the verification is completed, the circulating fan is turned off, the emergency vent valve of the loop pipeline is opened, and the venting rate is controlled by the flow limiting orifice plate.

[0018] Compared with the prior art, the circular natural gas flow standard system and method provided by the present invention have the following beneficial effects: (1) In view of the problems of gas source dependence and insufficient pressure coverage mentioned in the background technology, the present invention provides a closed loop design, which constructs an independent gas source system through a circulating fan and a buffer tank, gets rid of the dependence of the direct discharge process on the pressure difference between upstream and downstream, and achieves a wide pressure coverage of (2.5~9.2) MPa, meeting the calibration requirements of high, medium and low pressure calibration stations for the entire pressure range.

[0019] (2) In view of the problem that the loop process design in the background technology has difficulty in adjusting the small flow rate, the present invention provides a flow rate adjustment unit. Through the coordinated control of the parallel return flow regulating valve and the flow limiting regulating valve, the flow limiting valve is closed when the target flow rate is greater than or equal to the preset flow rate threshold (target flow rate ≥ 100 m³ / h), and only the return flow valve and the fan speed are adjusted; when the target flow rate is less than the preset flow rate threshold (target flow rate < 100 m³ / h), the flow limiting valve is linked to generate a throttling pressure difference of ≥ 0.5 MPa to ensure the piston moves at a uniform speed, thus solving the problem of stable control of the extremely low flow rate of 8 m³ / h.

[0020] (3) The cooling device and buffer tank provided by the present invention can stabilize the gas temperature within a preset value of ±0.2℃ through dynamic heat exchange of intermediate cooling medium, thereby achieving high-precision temperature control.

[0021] (4) When the buffer tank stores hydrogen-blended natural gas, the circulating fan provided by the present invention is a magnetically coupled fan to avoid the risk of hydrogen embrittlement, so that the system is compatible with hydrogen-blended natural gas (3%~30% hydrogen content) and pure hydrogen testing, supporting the strategic layout of the green hydrogen industry. Attached Figure Description

[0022] The accompanying drawings, which form part of this disclosure, are used to provide a further understanding of this disclosure. The illustrative embodiments of this disclosure and their descriptions are used to explain this disclosure and do not constitute an undue limitation of this disclosure.

[0023] Figure 1 This is a schematic diagram of the direct discharge process of the primary standard device for the HPPP method provided in Embodiment 1 of the present invention. Figure 2 Add a flow chart for reflux regulation and current limiting regulation to the loop process scheme provided in Embodiment 1 of the present invention; Figure 3 The flow chart for adding reflux regulation and flow limiting regulation to the loop process scheme provided in Embodiment 1 of the present invention under high flow conditions is shown below. Figure 4 The flowchart for adding reflux regulation and flow restriction regulation to the loop process scheme provided in Embodiment 1 of the present invention under low flow conditions is shown below. Figure 5 This is a simplified schematic diagram of the gas piston device (HPPP) provided in Embodiment 1 of the present invention; Figure 6 This is a flowchart of the calibration method based on the natural gas primary flow standard calibration system provided in Embodiment 2 of the present invention; Figure 7 This is a schematic diagram of the ring cooling system provided in Embodiment 2 of the present invention. Detailed Implementation

[0024] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the invention. Unless otherwise specified, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0025] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the exemplary embodiments of the present invention. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. Furthermore, it should be understood that the terms “comprising” and “having”, and any variations thereof, are intended to cover non-exclusive inclusion, for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0026] Where there is no conflict, the embodiments and features in the embodiments of the present invention can be combined with each other.

[0027] All data acquisition in this embodiment is carried out in accordance with laws and regulations and with user consent, and the data is used legally.

[0028] Example 1 This invention provides a natural gas primary flow standard calibration system based on a closed loop, including a closed loop pipeline, a piston volume tube (HPPP method) primary standard device, a circulating fan, a cooling device, a buffer tank, a flow regulation unit, and a control unit; The closed loop pipeline forms a gas circulation loop, with its inlet end connected to the outlet of the circulating fan and its outlet end connected to the inlet of the circulating fan. The piston volume tube is connected in series in the loop pipeline, with a buffer tank and a cooling device installed upstream of it, and the downstream is connected to the installation station of the flow meter under test. The flow regulation unit includes a reflux regulating valve and a flow limiting regulating valve arranged in parallel. The inlet of the reflux regulating valve is connected to the outlet of the circulating fan, and the outlet is connected to the inlet of the buffer tank. The inlet of the flow limiting regulating valve is connected to the downstream of the station of the flow meter under test, and the outlet is connected to the inlet of the cooling device. The control unit is electrically connected to the piston volume tube, the circulating fan, the cooling device, and the flow regulation unit.

[0029] In one specific embodiment, such as Figure 1 As shown, the direct discharge calibration process mainly includes: calibration gas supply, filtration and separation, heating, large differential pressure regulation, pressure stabilization regulation, small flow working stage standard device, HPPP method primary standard device, transfer standard, flow limiting nozzle device (secondary standard device), back pressure valve, and supporting temperature and pressure detection equipment, pipeline switching electric switch valve, etc.

[0030] The direct discharge process connects the upstream of the primary standard unit to a high-pressure pipeline and the downstream to a low-pressure pipeline. Testing is conducted using the flow created by the pipeline pressure. Currently, all existing HPPP method primary standard units both domestically and internationally employ this method, which is technically mature. Specifically at the Tianjin branch, gas is first drawn from a high-pressure gas source, heated, and then its pressure is adjusted to the calibration pressure using a pressure regulating valve assembly and kept stable. The natural gas enters the primary standard unit, passes through a flow-limiting nozzle device, and is then discharged as low-pressure gas to the downstream low-pressure export pipeline. A diversion valve is installed to control the back pressure and total flow rate of the nozzle. The process flow is as follows: Figure 1 The advantages are that flow is generated by pressure difference, avoiding the use of high-pressure blowers and saving energy. The disadvantages are that it requires a stable gas source and low-pressure natural gas users, and the operating pressure is limited by upstream and downstream conditions.

[0031] In the primary standard device of the HPPP method, in order to ensure that the piston can move at a constant speed during operation, a flow-limiting nozzle device is set downstream of the volume tube. The mass flow rate of the flow-limiting nozzle is constant under critical flow conditions, which can effectively achieve the flow-limiting effect. Different flow rates can be switched by combining flow-limiting nozzles of different specifications.

[0032] For the flow-limiting nozzle to reach the critical flow condition, a certain pressure difference is required between the upstream and downstream sides, typically 15% of the upstream pressure. Therefore, a significant pressure drop occurs during operation of the volumetric pipe, necessitating the intake of gas from a high-pressure source and its discharge into a low-pressure external pipeline. The operating pressure of the low-pressure external pipeline is 5 MPa. If calculated based on an inlet pressure of 5.2 MPa, and considering a maximum station pressure difference of 0.5 MPa, the minimum upstream pressure of the flow-limiting nozzle is approximately 6.7 MPa. Therefore, the operating pressure of the HPPP primary standard unit under direct discharge process operation is (6.7~9.2) MPa.

[0033] The HPPP primary standard unit requires a larger pressure differential to ensure uniform piston movement. To prevent uneven movement within the volume tube due to excessively low pressure differential in the annulus at low flow rates, a flow-limiting regulating valve is added to assist in low-flow regulation. The process flow diagram is shown below. Figure 2 As shown.

[0034] At high flow rates, flow regulation is achieved through frequency conversion and recirculation, and the operation mode is as follows: Figure 3 At low flow rates, the operation is adjusted through a combination of frequency conversion, reflux, and current limiting, as follows: Figure 4 The reflux regulation method is commonly used in loop flow meter calibration devices. When the flow rate is low, a large amount of natural gas flows directly back to the loop inlet, resulting in a very small pressure difference within the entire loop. Adding a flow-limiting regulating valve helps maintain the pressure difference within the loop, creating a throttling effect that increases the pressure difference and meets the requirements for pushing the volume tube piston. By cooperating with a buffer tank, the stability of the loop flow meter is ensured, resulting in a small pressure difference within the loop. Overall, the technical difficulty of implementation is relatively low.

[0035] Specifically, the buffer tank has a volume of ≥2m³ and a design pressure of ≥10MPa. Since the overall pipe volume of the loop process is very small, temperature and pressure fluctuations are likely to occur. Therefore, a buffer tank is added upstream of the volume pipe to stabilize the temperature and pressure of the natural gas entering the volume pipe.

[0036] Specifically, when the buffer tank stores hydrogen-blended natural gas, a magnetically coupled fan is selected for the circulating fan, with a design pressure ≥10MPa, an inlet-outlet pressure difference ≥400kPa, and a range ratio ≥1:10.

[0037] Advantages of magnetic coupling fans: Small motor and fan size, facilitating transportation and installation; fan motor cooling utilizes ambient medium and temperature, reducing the load on the cooling system. Disadvantages of magnetic coupling fans: Higher equipment cost.

[0038] In one specific embodiment, for the primary standard device loop process, the motor blades and connecting parts of the second-generation specially designed built-in motor circulating fan are all wrapped in the medium environment, and the blades of the specially designed external motor circulating fan (magnetic coupling fan) are processed as a whole and connected to the motor in the air environment through a magnetic coupling.

[0039] Considering the characteristics and affected areas of hydrogen embrittlement, the journals, keyways, splines, and other areas of the shaft of the second-generation specially designed built-in motor circulating fan are easily affected by hydrogen embrittlement, which may lead to unpredictable risks. The specially designed external motor circulating fan (magnetically coupled fan) effectively avoids hydrogen embrittlement by placing the coupling externally in the air environment. Magnetic coupled fans are used in hydrogen-related circulation systems abroad.

[0040] Specifically, the piston volume tube is a stainless steel cylinder, which is divided into an acceleration section, a metering section and a buffer section along the airflow direction, with a length ratio of 2:3:1; three position sensors are installed on each of the a2, a3 and a4 sections of the metering section to trigger the timer.

[0041] In one specific embodiment, the main body of the entire HPPP is Figure 5 The cylinder is a precision-machined stainless steel cylinder with cross-sectional views. A1-A2 is the stable section of piston movement; during the movement from A1 to A2, the piston's speed increases and stabilizes. A2-A4 is the metering section; during the movement from A2 to A4, the piston's speed remains stable. Position switches are installed at A2, A3, and A4, with three switches installed on each cross-section. The final measurement time is the average of the three timer readings. The times measured at A2, A3, and A4 can be cross-checked, ensuring good accuracy and correctness. A4-A5 is the buffer section of piston movement, approximately 1 meter long. During the movement from A4 to A5, the piston's speed begins to decrease, allowing it to come to a stop at the right end. Therefore, HPPP measurement is achieved during the piston's movement from left to right. The entire metering process can be divided into the following sections: Figure 5 The three stages shown: 1) such as Figure 5 As shown in (a), first open valve 2, close valve 1 and check valve, adjust the gas source pressure to the required pressure point, and the gas flows through the piston cylinder via the bypass.

[0042] 2) Once the system's flow rate, pressure, and temperature have stabilized, such as Figure 5 As shown in (b), valve 1 is open and valve 2 is closed. The flow rate being measured pushes the piston to start moving in the pipe. After passing through the stable section a1-a2 where the piston moves, the piston triggers three timers located at the cross section at a2. The timers start timing and stop timing when the piston moves to a4, and the measurement ends.

[0043] 3) The piston moves to the top right side, as... Figure 5 As shown in (c), the check valve opens, and gas flows back from the check valve to the detection pipeline.

[0044] 4) such as Figure 5 As shown in (d), the direction of the four-way valve on the bottom left changes. With the flow state at the gauge unchanged, gas enters the piston from the right pipe, pushing the piston from right to left to the top left. During this process, valve 1 is in the open state and valve 2 is in the closed state.

[0045] In one specific embodiment, the key equipment in the primary standard device includes: a. Piston-type standard volume tube Piston-type standard volume tubes are used to generate stable instantaneous flow rates. The piston is made of aluminum, and the cylinder is made of stainless steel, requiring sufficiently high machining precision. PTFE sealing material is used between the cylinder and the piston. The effective cylinder volume is traceable to the unit of length, and after precise verification, the uncertainty of the geometric volume should be better than 0.02%.

[0046] b. Four-way valve Four-way valves are used for switching the flow direction of natural gas fluids and require good switching and sealing performance.

[0047] c. High-precision timing equipment The high-precision timing device is used to record the start (a2 point) and end (a4 point) times of passing through the effective volume. Hardware timing should be used, and software timing via PLC should not be used. The timing device should be able to output hardware fast-linkage switches for the system to record the effective pulses of the turbine value transfer table.

[0048] Example 2 like Figure 6 This invention provides a calibration method using the natural gas primary flow rate standard calibration system provided in Example 1, comprising the following steps: Natural gas is introduced into the closed-loop pipeline, and the pressure is adjusted to the target test pressure range. The control unit starts the circulating fan and selects the adjustment mode according to the target flow rate; After the flow rate stabilizes, the gas flows through the piston volume tube. The piston movement triggers the timing of position sensors a2-a4. The standard volume flow rate Qv=V / t is calculated using the standard volume tube V and time t. Using Qv as the reference value, calibrate the indication error of the flow meter under test in the loop.

[0049] Specifically, the adjustment mode is selected based on the target flow rate, including: When the target flow rate is greater than or equal to the preset flow rate threshold, the flow limiting valve is closed and the return flow regulating valve is adjusted. Opening degree and fan speed; when the target flow rate is less than the preset flow rate threshold, the backflow regulating valve and limit valve are adjusted synchronously. The opening degree of the flow regulating valve and the speed of the fan.

[0050] Specifically, the process by which the piston movement triggers the timing is as follows: The timer starts when the piston passes through section a2 and stops when it passes through section a4. The average value of the timing results from the three position sensors a2, a3, and a4 is taken as time t.

[0051] Specifically, the verification method further includes: during the flow regulation process, the control unit starts the cooling device, which exchanges heat with the gas in the closed loop through an intermediate cooling medium to stabilize the gas temperature within the range of the target verification temperature preset value ±0.2℃.

[0052] In one specific embodiment, the cooling system's function is to remove the heat generated by the natural gas used for verification during the operation of the circulating fan in the loop device. JJG1037-2008 "Turbine Flowmeter" Verification Procedure 7.1.2.3 states regarding the verification gas (5): "During each verification process at each flow point, the temperature change of the verification gas should not exceed ±0.5℃." Based on engineering experience and the requirements of metrological technology development, the actual temperature change of the verification gas at home and abroad generally does not exceed ±0.2℃. Because the temperature change range of the verification gas is small, it is required that the heat transfer performance of the cooling system remain constant throughout the equipment's service life.

[0053] The cooling system consists of a shell-and-tube heat exchanger, a circulating water pump, an intermediate coolant storage tank, and an outdoor refrigeration system. The shell-and-tube heat exchanger is located at the fan outlet and exchanges heat with natural gas through a cooling intermediate medium, stabilizing the natural gas temperature and removing heat from the natural gas. The integrated refrigeration unit then cools the intermediate medium. Its schematic diagram is shown below. Figure 7 As shown.

[0054] The main parameters of the cooling device include: Function: Reduces the temperature of natural gas in the loop caused by the work done on it by the circulating fan pressurization.

[0055] Composition: Integrated refrigeration unit, heat exchanger, circulating water pump, cold water tank, etc.

[0056] Design pressure for natural gas medium: 10 MPa.

[0057] Temperature stability requirement: (20±0.2)℃.

[0058] Design parameters: The intermediate cooling medium temperature is designed to be (7~12)℃.

[0059] Specifically, after the calibration is completed, the circulating fan is turned off, the emergency vent valve of the loop pipeline is opened, and the venting rate is controlled by the flow limiting orifice plate.

[0060] While the present invention has been disclosed above, its scope of protection is not limited thereto. Those skilled in the art can make various changes and modifications without departing from the spirit and scope of the present invention, and all such changes and modifications will fall within the scope of protection of the present invention.

Claims

1. A loop-type natural gas flow standard system, characterized in that, This includes closed-loop piping, piston volume tube primary standard device, circulating fan, cooling device, buffer tank, flow regulation unit and control unit; The closed loop pipeline forms a gas circulation loop, with its inlet end connected to the outlet of the circulating fan and its outlet end connected to the inlet of the circulating fan. The piston volume tube is connected in series in the loop pipeline, with a buffer tank and a cooling device installed upstream of it, and the downstream is connected to the installation station of the flow meter under test. The flow regulation unit includes a reflux regulating valve and a flow limiting regulating valve arranged in parallel. The inlet of the reflux regulating valve is connected to the outlet of the circulating fan, and the outlet is connected to the inlet of the buffer tank. The inlet of the flow limiting regulating valve is connected to the downstream of the station of the flow meter under test, and the outlet is connected to the inlet of the cooling device. The control unit is electrically connected to the piston volume tube, the circulating fan, the cooling device, and the flow regulation unit.

2. The loop-type natural gas flow standard system as described in claim 1, characterized in that, The buffer tank has a volume of ≥2m³ and a design pressure of ≥10MPa, and is used to stabilize the gas temperature and pressure.

3. The loop-type natural gas flow standard system as described in claim 1, characterized in that, When the buffer tank stores hydrogen-blended natural gas, a magnetically coupled fan is selected as the circulating fan.

4. The loop-type natural gas flow standard system as described in claim 3, characterized in that, The magnetically coupled fan is designed with a pressure ≥10MPa, an inlet-outlet pressure difference ≥400kPa, and a range ratio ≥1:

10.

5. The loop-type natural gas flow standard system as described in claim 1, characterized in that, The piston volume tube is a stainless steel cylinder, which is divided into an acceleration section, a metering section and a buffer section along the airflow direction, with a length ratio of 2:3:1; three position sensors are installed on each of the a2, a3 and a4 sections of the metering section to trigger the timer.

6. A verification method based on the system according to any one of claims 1-5, characterized in that, include: Natural gas is introduced into the closed-loop pipeline, and the pressure is adjusted to the target test pressure range. The control unit starts the circulating fan and selects the adjustment mode according to the target flow rate; After the flow rate stabilizes, the gas flows through the piston volume tube. The piston movement triggers the timing of position sensors a2-a4. The standard volume flow rate Qv=V / t is calculated using the standard volume tube V and time t. Using Qv as the reference value, calibrate the indication error of the flow meter under test in the loop.

7. The testing method as described in claim 6, characterized in that, Select the adjustment mode based on the target flow rate, specifically including: When the target flow rate is greater than or equal to the preset flow rate threshold, the flow limiting valve is closed, and the opening of the return flow regulating valve and the fan speed are adjusted; when the target flow rate is less than the preset flow rate threshold, the opening of the return flow regulating valve, the flow limiting valve and the fan speed are adjusted simultaneously.

8. The testing method as described in claim 6, characterized in that, The specific process of piston movement triggering timing is as follows: The timer starts when the piston passes through section a2 and stops when it passes through section a4. The average value of the timing results from the three position sensors a2, a3, and a4 is taken as time t.

9. The testing method as described in claim 6, characterized in that, The verification method further includes: during the flow regulation process, the control unit starts the cooling device, which exchanges heat with the gas in the closed loop through the intermediate cooling medium to stabilize the gas temperature within the range of the target verification temperature preset value ±0.2℃.

10. The testing method as described in claim 6, characterized in that... After the calibration is completed, turn off the circulating fan, open the emergency vent valve of the loop pipeline, and control the venting rate through the flow limiting orifice plate.

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