Flow meters generally have the following types: differential pressure flow meters, turbine flow meters, fluid vibration flow meters, volumetric flow meters, ultrasonic flow meters, target flow meters, and Coriolis mass flow meters. 1. Differential pressure flowmeter: including the detector is a standard throttle device and a constant velocity tube. It is a kind of flowmeter with a long history of application, rich and mature practical experience, perfect standard and standard (except for average speed tube) and complete specifications. It may be the only natural gas flowmeter before the 1950s. It has many shortcomings: such as measurement accuracy, high requirements for on-site installation conditions, large pressure loss, narrow range, etc. In response to the above shortcomings, domestic manufacturers have introduced a series of improved methods in recent years, such as integrated differential pressure flowmeters. Fixed-rate throttles, interchangeable orifice throttling devices, standard nozzles, etc., plus two components of differential pressure flowmeters after the 1980s: differential pressure transmitters and flowmeters have made outstanding progress. It is estimated that such flow meters will still occupy an important position in the future. 2. Turbine Flowmeter: In Europe and the United States, the turbine flowmeter is the second legal natural gas flowmeter after the orifice flowmeter. It has been developed into a multi-variety, full-series, multi-standard, mass-produced natural gas flowmeter. The standard specifications are also very complete, but there are also some shortcomings: the requirements for the cleanliness of the measuring medium are high and the service life is limited. However, according to the foreign use department, it has been used for periodic verification of 240 flow meters using 8 to 15 years, and it is found that the deviation of the accuracy of the meter is still within the specified range. 3. Vibrating Flowmeters: There are two types of such flowmeters: vortex flowmeters and precession (vortex precession) flowmeters. 4. Volumetric flowmeters: including lumbar flowmeters, membrane flowmeters (home gas meters), etc. It is a natural gas flowmeter with a long history and a large amount of use, and has a complete standard specification. Moderate accuracy, wide range (150:1), suitable for medium and small flow range, direct reading, no external energy and no straight pipe sections. 5. Ultrasonic Flowmeter: In the 1990s, gas ultrasonic flowmeters began to be used in the natural gas industry. It has attracted great international attention due to its excellent characteristics, and has become an important natural gas flowmeter after orifice plate and vortex street. Its characteristics are: high precision, wide range, can adapt to very low flow rate (less than 0.5m / s), short length of straight pipe installation (multi-channel variety). The current shortcomings: only applicable to the medium and large caliber, the price is expensive. 6. Target Flow Meter: The new target flow meter uses a strain gauge force sensor with many excellent features. Such as high precision (total measurement up to ±0.5% ~ ±0.1%), wide range (10:1), suitable for small and medium diameter (DN15 or more), no moving parts, simple structure, easy maintenance, and can be dried Verification (hanging method) simplifies cycle verification and the like. 7. Coriolis Mass Flowmeter: The use of compressed natural gas as a fuel for vehicles must address the measurement problems of automotive refueling stations. The successful application of Coriolis mass flowmeters in liquids has prompted people to promote them to gases, but encountered a problem in that the gas density is low and cannot be adapted to the characteristics of the instrument. High-pressure natural gas has greatly increased the gas density to meet this requirement. Faced with such a wide variety of natural gas flow meters, the selection of general users has become a problem. How to scientifically and objectively select the best flow meter is a problem that needs attention. We believe that the selection should follow the appropriate rules, try to avoid the misleading propaganda of the manufacturer, find a just right flow meter for yourself, it is your ideal flow meter. The selection can be made in five aspects: instrument performance, fluid properties, installation conditions, environmental conditions, and economic factors. The various considerations are as follows: 1. Instrument performance: accuracy, repeatability, linearity, range, pressure loss, upper and lower flow, signal output characteristics, response time, etc. 2. Fluid properties: fluid pressure, temperature, density, viscosity, lubricity, chemical properties, wear, corrosion, fouling, fouling, gas compression coefficient, isentropic index, specific heat capacity, electrical conductivity, sound velocity, mixed phase flow, pulsating flow, etc. ; 3. Installation conditions: pipe layout direction, flow direction, upstream and downstream pipe length, pipe diameter, maintenance space, pipe vibration, grounding, power supply, auxiliary equipment (filtration, sewage), explosion-proof, etc.; 4. Environmental conditions: ambient temperature, humidity, safety, electromagnetic interference, etc.; 5. Economic factors: purchase cost, installation fee, maintenance fee, calibration fee, service life, operating cost (energy consumption), spare parts, etc. The selection steps are as follows: 1. The type of instrument available for primary selection based on five factors; 2. Use the elimination method to select 2-3 types in the comparison, the discharge order; 3. Repeatedly evaluated in five aspects, and finally eliminated to one type of instrument. The success of the selection depends to a large extent on the exact knowledge of the quality of the instrument and the characteristics of the object being measured. Special attention should be paid to the manufacturer's false propaganda and misleading components in terms of instrument performance quality. The exact understanding of the measurement object is very important, not the user has an accurate understanding of their own measurement objects, many of the failures of the selection is due to inaccurate parameters. Some object characteristics require an in-depth investigation to make it clear. 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In addition, a high-efficiency rectifying rectifier and a turbine flow sensor are coupled into a structurally integrated gas turbine flowmeter. Only the 2D straight pipe length is required under the condition that the upstream side baffle is an elbow or has a half-opening stop valve. It is a very prominent feature for the narrow installation space of the city.
Vortex flowmeter has many characteristics, such as output pulse frequency signal, linear relationship between signal and flow, convenient for total metering, wide range, small pressure loss, no moving parts, simple structure, easy installation and maintenance, etc. In order to have a high regularity requirement for the flow field, a long straight pipe section is required, and the stress type is sensitive to the mechanical vibration of the pipe, and should not be used in a strong vibration place. It has been proven for many years that the selection of vortex flowmeters (appropriate measurement range) is essential, and installation requirements should be paid special attention. Sometimes appropriate adjustments on site are necessary, which are the key to improving the use of the results.
The characteristics of the precession flowmeter are basically the same as those of the vortex street, but there are two differences: the flowmeter has a much larger pressure loss and a shorter straight pipe length requirement. The production of direct-reading (on-site display) flowmeters by the manufacturer is welcomed by users. By the way, all urban natural gas should develop direct-reading field display flowmeters. It should be one of the basic varieties, and it is a total amount. Main instrument.
Flowmeter type and selection