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Cause gas flowmeter measurement error analysis of the main factors

by:KAIDI     2020-07-08
Began to be put into use in the market in the early 1930 s, subsequently won the mass is widely used in production areas, so far, orifice meter manufacturing and using the technology is mature, with the help of a long-term research and practice on the domestic experience, our country made the natural gas industry standard of the gas flow standard orifice metering method '( SY / 6143 - 1996). , the regulation of standard orifice plate structure, technical requirements, the throttle device of pressure method, using method and flow rate calculation and the uncertainty of information, provide a standard basis for natural gas flow metering. Vision is rich natural gas resources in our country power, natural gas reserves of 3. 8*1013m3( 38 trillion m3) 1, has proven reserves. 6*1012m3( 1. 6 trillion m3) , natural gas is not only a kind of high efficient, clean fuel, is also a kind of chemical raw materials, natural gas flow metering has become the important restriction factor of the development of gas industry, is the basis of natural gas supply and demand both sides trade settlement, and the efficiency of the gas production department technical indicators, is the use of natural gas in China urgently needs to solve an important problem, orifice flow meter is composed of three parts: the throttle device, the differential pressure transmitter and flow display. Orifice meter is easy to deviate from the standard of the reason is that the structure and working principle of the instrument itself characteristic, the instrument itself error are produced by the manufacture, installation and use is in the installation or use for a long time due to the fluid medium corrosion, wear, contamination and so on. Therefore, we should install the flow measurement system, in strict accordance with the technical requirements to eliminate installation error. In use process, the operator should do system maintenance, maintenance, maintenance, prolong the service life and reduce the measurement error. 1. 1 meter itself using gas flowmeter measurement, measuring error caused by the requirements for orifice plate processing technology is very high, must conform to the relevant provisions of the standard, otherwise, will bring great error for the flow measurement results. Orifice plate eccentricity and bending is orifice plate manufacture installation and use of the main factors influencing the measuring instrument accuracy. 1. 1 orifice meter production and installation factors impact of 1. 1. 1 orifice error introduced improperly installed although installation specification are generally able to master, but some of the detail is often not taken seriously, result in large measurement error. Such as sealing gasket inner hole is not according to the size of ring room processing, gasket ring chamber, interfere with the stability of fluid flow; Guide tube to unreasonable, differential pressure not smooth conduction; Balancer is not level, direct measurement error, etc. 1. 1. 2 the error due to eccentric orifice plate and pore size than the size according to the requirements of standard orifice plate and straight pipe of the throttle device. Generally speaking, aperture ratio & beta; The higher the value, the greater the eccentricity ratio effect on measurement accuracy. Another formula: according to the volume flow uncertainty from formula ( 1) Can be calculated from the aperture ratio & beta; And flow relationship of uncertainty: basic flow uncertainty as & beta; And increase or decrease, increase or decrease when & beta; < 0. 6, the uncertainty of change is very small, basic is a straight line; As & beta; Increases quickly, with the increasing of uncertainty, when & beta; = 0. 75, achieve big uncertainty. So should avoid using aperture than large orifice. 1. 1. 3 orifice plate bending into the tube in the measurement error caused by the throttle device of gasket, weld, etc. Can also cause measurement error, error of the size and position of prominence, orifice plate and the distance of the protrusions and so on. Related experiments show that when & beta; = 0. 7, the protrusion is located in the upper orifice diameter 2 times, produce great error; If the protrusion is located at upper orifice, the error is 16% ~ 50%. If located in the downstream, the influence on measurement accuracy is relatively small. 1. 2 use condition change the measuring error caused by the error is mainly by the pulsating flow phenomenon and super range, process and other issues caused by frequent fluctuations. Sudden changes in the gas flow rate and pressure in the pipeline, cause the pulsating flow, differential pressure fluctuation, when the orifice flowmeter throttle device appeared obvious pulsing flow, may produce large error of measurement, the relevant literature points out that the pulsating flow caused by the great error could reach 20% ~ 30%. Currently, the main trunkline pulsating source: one is that natural gas for gas water well with production, these Wells measuring differential pressure change; Second, long-distance pipeline or gas pipeline fluid differential pressure fluctuation; Three is that between gas well interference caused by the differential pressure fluctuation; Four is, due to the small user gas uneven, gas instantaneous change sharply, air supply valve popping or relations will lead to differential pressure fluctuation. Outrange most problem is due to the different season, the amount of change, especially the steam metering, large flow in winter, the summer flow is small, probably more than the range for the selected instrument, causing the error. At the same time, and no peak shaving of city fuel gas facilities of the metering device, when flow frequent fluctuations over time, will directly affect the natural gas measurement accuracy. 1. 3 medium conditions caused by measurement error 1. 3. 1 the impurity in the natural gas from the formation of natural gas mining, although after separation, dust or filtering, but due to processing incomplete or set of gas pipeline and gas transmission trunk line internal corrosion, impact, has resulted in a mixed with a small amount of liquid or gas solid impurities, easy to gather these impurities in the orifice section shrinkage, flow velocity mutation orifice sharp edge, and the gas flow meter for orifice sharp edges are very sensitive, the change of cross section and streamline. In addition, the impurity in the natural gas will be on the orifice plate corrosion and scouring, especially the measurement of rectangular entrance hole plate edge and tube wall erosion is very serious, this will affect the orifice Angle inlet edge arc radius and measure the tube wall relatively rough, the provisions of the standard orifice outflow coefficient will change, and the measurement accuracy can not meet the requirements.
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