The measurement of natural gas flow is an indirect measurement in the process of natural gas flow, and the accuracy of measurement depends on the quality of the whole process of reasonable design, construction, operation and maintenance of the whole metering system. In order to ensure the total quality management of the metering system according to the unified technical requirements and the accuracy of natural gas metering, it is necessary to formulate scientific and reasonable natural gas metering standards. Among the relevant standards of natural gas measurement, the flow measurement standard is the main one, and it should also include the measurement and calculation standards of natural gas density, composition, calorific value, compression factor and other related parameters, as well as instrument, design and safety standards. Natural gas metering involves design, construction, commissioning, operation, maintenance, inspection, verification, safety and environmental protection. , so its related standards are very extensive. American petroleum industry is developed, natural gas metering technology is advanced, and there are strict and perfect regulations, standards and advanced metering equipment. 1978, the United States passed the natural gas law, which unified the price of natural gas between the states and the federal government, stipulated that the actual energy content per cubic foot should be used as the basis of natural gas price, and changed the traditional measurement method of natural gas. This new metering method combines two independent metering systems, natural gas mass metering and calorific value metering, and produces a new natural gas energy metering system.

(1) Volumetric flowmeter A volumetric flowmeter is equivalent to a container with a standard volume, which continuously measures the flowing medium. The greater the flow rate, the more measurement times and the higher the output frequency. The principle of positive displacement flowmeter is relatively simple, and it is suitable for measuring fluids with high viscosity and low Reynolds number. According to the different shapes of rotating bodies, the products currently produced are divided into: elliptical gear flowmeter, waist wheel flowmeter (Roots flowmeter), rotary piston flowmeter and scraper flowmeter, which are suitable for measuring liquid flow; Servo volumetric flowmeter, diaphragm flowmeter and simplified flowmeter are suitable for measuring gas flow.

(II) Impeller flowmeter The working principle of the impeller flowmeter is that the impeller is placed in the measured fluid and rotated by the impact of the fluid flow, and the flow rate is reflected by the rotating speed of the impeller. Typical impeller flowmeters include water meters and turbine flowmeters, and their structures can be mechanical transmission output or electric pulse output. Generally, the accuracy of water meter output by mechanical transmission is low, with an error of about 2%, but it has simple structure and low cost. It has been mass-produced, standardized, generalized and serialized in China. The accuracy of turbine flowmeter with electric pulse signal output is high, and the general error is 0.2% ~ 0.5%.

(3) Differential pressure flowmeter (pressure drop flowmeter) The differential pressure flowmeter consists of a primary device and a secondary device. The primary equipment is called a flow measuring element, which is installed in the pipeline of the measured fluid and produces a pressure difference proportional to the flow rate (flow rate) for the secondary equipment to display the flow rate. Secondary equipment is called display instrument. It receives the differential pressure signal generated by the measuring element and converts it into the corresponding flow for display. The main device of differential pressure flowmeter is usually throttling device or dynamic pressure measuring device (pitot tube, pressure equalizing tube, etc.). ). The secondary equipment is all kinds of mechanical, electronic and combined differential pressure meters with flow display instruments. The differential pressure sensitive elements of differential pressure gauge are mostly elastic elements. Because of the square root relationship between pressure difference and flow, the flow display instrument is equipped with a square root device to linearize the flow scale. Most meters are also equipped with a flow totalizer to display the accumulated flow of economic accounting. This method of measuring flow by pressure difference has a long history and is relatively mature. It is generally used in more important occasions around the world, accounting for about 70% of all kinds of flow measurement methods. This instrument is used to measure the flow of main steam, feed water and condensate in power plant. At present, the products are divided into orifice flowmeter, wedge flowmeter, Venturi flowmeter and averaging pitot tube.

(4) The float of the variable area flowmeter (constant pressure drop flowmeter) is placed in the upper and lower conical flow channels and moves under the force of the fluid flowing from bottom to top. When this force is balanced with the "displayed weight" of the float (the weight of the float itself minus the buoyancy of the fluid it bears), the mooring device is at rest. The height of the float at rest can be used to measure the flow. Because the cross-sectional area of the flowmeter changes with the height of the float, the pressure difference between the upper and lower parts is equal when the float is at rest, so this flowmeter is called variable area flowmeter or equal pressure drop flowmeter. The typical instrument of this flowmeter is the rotor (float) flowmeter.

(5) Momentum flowmeter A flowmeter that measures the momentum of a fluid to reflect the flow rate is called a momentum flowmeter. Since the momentum p of the flowing fluid is directly proportional to the square of the fluid density and velocity v, that is, pv2, when the flow cross section is determined, V is directly proportional to the volume flow q, so p Q2. Let the proportional coefficient be a, then Q = A. Therefore, the measured P can reflect the flow Q. This kind of flowmeter mostly uses detection elements to convert momentum into pressure, displacement or force, and then measures the flow. Typical instruments of this flowmeter are target flowmeter and rotary vane flowmeter.

(VI) Pulse flowmeter The flowmeter that uses the pulse theorem to measure the flow is called a pulse flowmeter, which is mostly used to measure the flow of granular solid media, as well as the flow of slurry, crystallization liquid and abrasive. Flow measurement ranges from several kilograms per hour to nearly ten thousand tons. A typical instrument is a horizontal component pulse flowmeter. Its measuring principle is that when the measured medium falls freely from a certain height h to the detection plate at an inclined angle, an impulse is generated, and the horizontal component of the impulse is proportional to the mass flow, so measuring this horizontal component can reflect the mass flow.

(VII) Electromagnetic flowmeter The electromagnetic flowmeter is made by applying the principle that the conductor moves in a magnetic field to generate an induced electromotive force proportional to the flow, and reflects the flow of the pipeline by measuring the electromotive force. Its measurement accuracy and sensitivity are high. It is widely used in industry to measure the flow of water, pulp and other media. The maximum measurable pipe diameter is 2m, and the pressure loss is extremely small. However, media with low conductivity, such as gas and steam, cannot be used. Electromagnetic flowmeter is expensive, and its signal is easily disturbed by external magnetic field, which affects its wide application in industrial pipeline flow measurement. To this end, the products are constantly improved and updated, and are developing towards computerization.

(VIII) Ultrasonic flowmeter Ultrasonic flowmeter is designed according to the principle that the propagation speed of ultrasonic wave in flowing medium is equal to the geometric sum of the average velocity of the measured medium and the velocity of sound wave itself. It also reflects the flow rate by measuring the flow rate. Although the ultrasonic flowmeter only appeared in the 1970s, it is very popular and has a broad prospect because it can be made into a non-contact type and can be linked with the ultrasonic water level gauge to measure the opening flow without causing disturbance and resistance to the fluid. Classification of ultrasonic flowmeter: (1) Doppler ultrasonic flowmeter: transducer 1 emits ultrasonic signal with frequency of f 1. After passing through the suspended particles or bubbles in the liquid in the pipeline, the frequency shifts and is reflected to the transducer 2 with the frequency f2, which is Doppler convergence, and the difference between f2 and f 1 is the Doppler frequency difference fd. Let the fluid velocity be V, the ultrasonic velocity be C, and the Doppler frequency shift fd is directly proportional to the fluid velocity V. When the pipeline conditions, transducer installation position, emission frequency and sound velocity are determined, C, f 1 and θ are constants, and the fluid velocity is directly proportional to the Doppler frequency shift. By measuring the frequency shift, the fluid velocity can be obtained, and then the fluid flow can be obtained. (2) Time-difference ultrasonic flowmeter: Time-difference ultrasonic flowmeter measures the fluid flow by using the principle that the time difference between the downstream propagation and the countercurrent propagation of sound waves in the fluid is proportional to the fluid flow.

(9) Fluid oscillation flowmeter The fluid oscillation flowmeter is designed based on the principle that fluid will oscillate when it flows under specific channel conditions, and the frequency of oscillation is directly proportional to the flow rate. When the cross section of the channel is fixed, the flow rate is proportional to the volume flow rate. Therefore, the flow can be measured by measuring the oscillation frequency. This flowmeter was developed in the 1970s. Because it has the advantages of no rotating parts and pulse digital output, it has a good development prospect. At present, the typical products are vortex flowmeter and precession vortex flowmeter.

(10) Mass flowmeter Because the volume of fluid is affected by temperature, pressure and other parameters, it is necessary to give the parameters of the medium when expressing the flow by volume flow. When the medium parameters are constantly changing, it is often difficult to meet this requirement, which leads to the distortion of the instrument display value. Therefore, mass flowmeter has been widely used and valued. Mass flowmeter can be divided into direct type and indirect type. Direct mass flowmeter uses the principle directly related to mass flow to measure. At present, there are mass flowmeters such as calorimetry, angular momentum, vibrating gyro, Magnus effect and Coriolis force. Indirect mass flowmeter directly multiplies the densimeter with the volume flow to obtain the mass flow. There are also various weir flow meters and trough flow meters suitable for open channel flow measurement; Plug-in flowmeter suitable for large diameter flow measurement; Laminar flow meter for measuring laminar flow; Related flowmeters suitable for two-phase flow measurement: laser method, nuclear magnetic vibration flowmeter, various tracing methods and dilution methods.

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