Accurate measurement of fluid flow in pipeline transportation is a prerequisite and foundation for logistics handover, as well as an important factor in measuring a country's technological level. This article summarizes the classification of single-phase pipeline flow measurement technology and the current development status of energy measurement. It analyzes the characteristics and technical limitations of three multiphase pipeline flow measurement methods: gas-liquid separation measurement, gas-liquid mixed phase measurement, and gas-liquid split sampling measurement. It elaborates on the principles, characteristics, implementation methods, and limitations of virtual measurement. It is believed that traditional single-phase pipeline flow measurement technology has become increasingly mature, and the key technologies and challenges currently faced are: flow measurement technology under extreme parameters and complex environments, multiphase pipeline flow measurement technology under phase change conditions, bulk crude oil trade handover measurement system, complex slurry flow measurement technology, and pipeline flow value traceability. In response to the requirements for measurement in new formats and scenarios of pipelines, the following suggestions are proposed: increase research and development efforts on multiphase flow meters in new formats and scenarios, and develop standards for multiphase pipeline flow measurement;Constructing a pipeline flow quantum traceability system to achieve rapid calibration of flow measuring instruments;Accelerating the research and development of models driven by data and mechanisms, and promoting the large-scale application of virtual metrology; Constructing a pipeline flow quantum traceability system to achieve rapid calibration of flow measuring instruments; Establishing a unified and open pipeline data sharing center to promote the deep utilization of measurement data resources.

Measurement is an activity that achieves unit unity and ensures accurate and reliable measurement values. It is an important foundation for scientific and technological innovation, industrial development, national defense construction, and people's livelihood security. It is also an important support for building an integrated national strategic system and capability.Measurement plays an important leading role in technological development and product manufacturing. The industrial revolution and major technological revolutions in history were led by breakthroughs in measurement and testing technology, driven by precise verification through metrology.

Pipeline transportation, as one of the five major modes of transportation, is an important component of modern industry and energy supply systems. Accurate measurement of fluid flow in pipeline transportation is a prerequisite and foundation for logistics handover.From traditional crude oil and natural gas to new energy sources such as hydrogen and ammonia, from multiphase flow of oil, gas, and water in gathering and transportation pipelines to slurries such as coal water slurry and hydrate slurry.The current pipeline transportation medium is becoming increasingly complex and diverse, covering single-phase gases, single-phase liquids, multiphase fluids of oil, gas, and water, as well as slurries. At the same time, the pipeline topology has also shifted from a single pipeline to a complex network, and new businesses and scenarios have brought new challenges to pipeline flow measurement.

The development of pipeline flow measurement technology is facing an era of great change, integration and innovation. New technological revolutions such as the Internet, blockchain and artificial intelligence have made it possible to solve the measurement needs of complex transmission media, real-time working conditions and extreme environments.In response to the characteristics and development needs of pipelines, it is necessary to develop a batch of specialized measurement and testing equipment, form a series of specialized measurement and testing methods and standard specifications, enhance the ability and level of independent and controllable measurement, cultivate new measurement formats, and develop new measurement models.

1. Progress in Single-phase Pipeline Flow Measurement Technology

1.1 Single Phase Pipeline Flow Measurement Technology

Currently, single-phase pipeline flow measurement technology mainly applies the response relationship between fluid flow and physical parameters such as force, heat, sound, electricity, light, and atomic energy for measurement, including differential pressure, volumetric, velocity, ultrasonic flow measurement, as well as direct mass flow measurement and other measurement methods.

Differential pressure flow measurement obtains flow by measuring the differential pressure before and after the throttling element. Typical instruments include orifice flow meters, V-cone flow meters, Venturi nozzle flow meters, Venturi tube flow meters, etc.The throttling meter has good adaptability to the medium and can be used for single-phase liquid, gas, and steam flow measurement, but the flow coefficient needs to be calibrated before use.Positive displacement flow meters mainly come in various forms such as waist wheels, scrapers, gears, rotors, etc. They rotate once to discharge a constant volume, and the flow rate can be obtained by measuring the number of revolutions per unit time. The volumetric flowmeter has high accuracy and high measurement precision for high viscosity fluids, but when measuring small flow rates, the error will increase due to the increase in leakage ratio.Velocity flow meters commonly include turbine, vortex street, electromagnetic flow meters, etc. Depending on the flow rate, the corresponding vortex speed, vortex street shedding frequency, and induced electromotive force of the electromagnetic flow meter also change. Electromagnetic flow meters cannot measure non-conductive media.

Ultrasonic flow meters have the advantages of wide range, non intervention, no pressure loss, easy installation and use, and low maintenance costs. Recently, they have attracted much attention and are widely used for flow measurement in large-diameter natural gas pipelines.According to the principle, it can be divided into time difference type, phase difference type, frequency difference type, and Doppler ultrasonic flowmeter (suitable for measuring two-phase flow). Ultrasonic flow meters are affected by the acoustic characteristics of the fluid, and the temperature, density, viscosity, and suspended particles of the fluid also affect their measurement accuracy, requiring calibration.

Mass flow meters can directly measure the mass of pipeline fluids with high measurement accuracy. There are mainly two types: Coriolis mass flow meters and thermal mass flow meters.The advantage of Coriolis mass flowmeter is high accuracy, which can directly measure mass flow rate and density, allowing users to not only know the volume of the fluid but also its composition; The disadvantage is that it has a large pressure loss and is easily affected by fluid temperature and pressure.Thermal mass flow meters use the principle of heat transfer, that is, the heat exchange relationship between the fluid in the flow and the heat source, for measurement. The advantages are that there are no moving parts and low pressure loss, but the disadvantages are that they are easily affected by the composition of the medium.

1.2 Energy Metering Technology

In terms of energy gas measurement, there are mainly three methods: volume measurement, mass measurement, and energy measurement. Volume measurement can only reflect the quantity of fluid transported through pipelines, and cannot reflect its quality. Energy measurement can reflect the energy properties of fuel.Taking natural gas as an example, there are significant differences in the composition of natural gas from various sources, origins, and types. The difference in heat generation between different sources can exceed 20%. Currently, China is attempting to transport natural gas mixed with hydrogen.Based on the current natural gas consumption in China, when the hydrogen blending ratio reaches 20%, more than 10 million tons of hydrogen can be consumed, which is conducive to solving the problem of resource mismatch and peak shaving demand in the east and west of China.Natural gas is mainly used as fuel, and calorific value is the most important indicator to measure the value of natural gas. The difference in calorific value between methane and hydrogen is significant, and the traditional volumetric measurement method cannot reflect the difference in calorific value between different gas sources, which is no longer suitable for the current comprehensive gas source handover measurement needs.

Energy metering has gradually been promoted in Western Europe and North America since the early 1980s, and has now become the dominant measurement method in handover metering.On May 24, 2019, the "Measures for the Fair and Open Supervision of Oil and Gas Pipeline Network Facilities" jointly issued by the National Development and Reform Commission and four other ministries and commissions explicitly required the implementation of natural gas energy metering, and required the formal implementation of a natural gas energy metering and pricing system from May 24, 2021.

The rapid development of China's natural gas industry has formed a diversified supply and sales pattern, including domestically produced conventional gas, unconventional gas (shale gas, tight sandstone gas, coalbed methane, etc.), coal to natural gas, imported liquefied natural gas (LNG), and imported pipeline gas.According to the latest annual report from the International Energy Agency, China has become the world's largest importer of LNG by 2023. At present, the natural gas volumetric measurement system is still widely used in China, and the implementation of energy measurement is slow.The Chinese natural gas energy measurement standard system involves 80 items, including 25 items for flow measurement, 33 items for heat generation measurement, and 22 items for verification regulations and calibration specifications. But the flow measurement under this system belongs to the National Petroleum and Natural Gas Standardization Technical Committee, the calorific value measurement belongs to the National Natural Gas Standardization Technical Committee, and the verification regulations and calibration standards belong to the National Flow Measurement Technical Committee. Therefore, the coordination mechanism between technical organizations in different specialized fields needs to be further optimized.

In addition, the energy value of natural gas is usually determined by multiplying the volumetric flow rate or mass flow rate per unit time by the heat generation, and accurate detection of the heat generation of natural gas is a prerequisite for implementing energy metering. The direct method obtains direct heat generation by burning natural gas, which is difficult to monitor in real-time online.Online gas chromatography is commonly used abroad for indirect detection through composition analysis. The online chromatographic analyzers used on nearly 120000 km of natural gas pipelines in China are mainly imported products.At present, the National Petroleum and Natural Gas Pipeline Network Group Co., Ltd. has organized domestic research and manufacturing forces to develop a high-performance online chromatographic analyzer, and held a press conference in 2023.

2.Progress in Multiphase Pipeline Flow Measurement Technology

Gas liquid multiphase flow is widely present in various industrial pipeline transportation fields such as petroleum, chemical, power, and nuclear energy.For example, in offshore oil and gas development, the underwater wellhead products are multiphase oil and gas flows that need to be mixed and transported through pipelines to platforms or onshore terminals; In steam injection heavy oil production, the flowing medium in the steam pipeline network is also steam water two-phase flow.The flow measurement of multiphase fluids is of great significance for production management, flow assurance, and trade handover.

Due to the complexity and randomness of gas-liquid two-phase flow, accurate measurement of two-phase flow rate has always been a challenge. According to different principles of gas-liquid flow measurement, current research on multiphase pipeline flow measurement is mainly divided into three methods: gas-liquid separation measurement, gas-liquid mixed phase measurement, and gas-liquid split sampling measurement.

2.1 Gas Liquid Separation Metering Technology

Gas liquid separation metering technology is the application of separation equipment to separate gas-liquid mixtures into single-phase gas and single-phase liquid, and then measure them through ordinary single-phase flow meters.This method converts the measurement of two-phase flow into single-phase flow measurement, which has the advantages of reliable operation, high measurement accuracy, wide measurement range, and is not affected by changes in gas-liquid two-phase flow patterns.Common metering separators use vertical or horizontal containers. To ensure separation efficiency, the diameter of the separator is usually 5-10 times the diameter of the pipeline, and it has a large volume. The Gas Liquid Cylindrical Cyclone (GLCC) proposed by the University of Tulsa in the United States is a typical representative of complete separation metering.

The gas-liquid separation metering technology has the following difficulties. ① The separation equipment is bulky and expensive. ② Due to the large buffer capacity, there is a measurement delay and poor real-time performance. ③ It is necessary to establish specialized measurement stations and testing pipelines, which greatly increases the cost of measurement.

2.2 Gas Liquid mixed Phase Metering Technology

Gas liquid mixed phase metering technology is currently the mainstream method for flow measurement of multiphase pipelines in industrial sites. This technology does not require the use of separation devices, therefore it has a small volume and compact structure. The total gas-liquid flow rate is usually measured using throttling flow meters such as Venturi tubes, while the phase fraction is measured using phase fraction sensors such as capacitance, conductivity, and radiation. In recent years, the laboratory has begun to use tomographic imaging technology to measure gas-liquid phase fractions, mainly including capacitance imaging, conductivity imaging, ultrasound imaging, magnetic resonance imaging, etc.In the measurement of multiphase flow phase separation parameters, due to the significant differences in the physical properties of the oil, gas, and water phases, the interfacial contact between them is very complex, and the slip effect is significant, resulting in non-uniform distribution of the oil, gas, and water phases in both the pipeline cross-section and axial direction, which poses great challenges to the accurate measurement of phase separation parameters.In recent years, there have been reports of using real mass flow meters to directly measure the mass flow rate of multiphase fluids. However, the flow resistance loss of mass flow meters is large, and the flow pattern and composition of each phase have an impact on the measurement accuracy.The gas-liquid mixed phase metering technology has the following difficulties. ①Directly facing gas-liquid two-phase fluids, the error can even exceed 10% due to the complexity of gas-liquid two-phase flow. ②The use of throttling flow meters such as Venturi tubes for gas-liquid mixing measurement results in significant resistance losses. ③The phase separation rate sensor is affected by factors such as pipeline flow pattern and salt content in the fluid medium, and requires regular calibration. Radiation based methods also pose a risk of radioactive contamination.