General principles of automatic instrument selection-3. Selection of flow instruments

Abstract: General principles of automatic instrument selection-3. The selection information of flow meters is provided by excellent flowmeter and flowmeter manufacturers. Selection of flow meter 1. General principles (1) Scale selection The scale of the instrument should meet the requirements of the scale modulus of the instrument. When the scale reading is not an integer, it is convenient for reading conversion, and it can also be selected according to an integer. 1) Root square scale range Maximum flow no. For more flowmeter manufacturers to select models and price quotations, you are welcome to inquire. The following is the general principle of automatic instrument selection - 3. The details of the selection of flow meters. Selection of flow meter 1. General principles (1) Scale selection The meter scale should meet the requirements of the meter scale modulus. When the scale reading is not an integer, it is convenient for reading conversion, and it can also be selected according to an integer. 1) Square root scale range The maximum flow rate is not more than 95% of the full scale; the normal flow rate is 70% to 85% of the full scale; the minimum flow rate is not less than 30% of the full scale. 2) Linear scale range The maximum flow rate is not more than 90% of the full scale; the normal flow rate is 50% to 70% of the full scale; the minimum flow rate is not less than 10% of the full scale. (2) Instrument accuracy The flowmeter used for energy measurement shall comply with the provisions of the General Rules for the Equipping and Management of Enterprise Energy Measurement Instruments (Trial). 1) For the measurement of fuel inbound and outbound settlement,±0.1%; 2) Measurement for technical and economic analysis of workshop teams and technological processes,±0.5% to 2%; 3) for industrial and civil water measurement,±2.5%; 4) for steam metering including superheated steam and saturated steam,±2.5%; 5) for the measurement of natural gas, gas and domestic gas,±2.0%; 6) Measurement of oil used for key energy-consuming equipment and process control,±1.5%; 7) Measurement of other energetic working fluids (such as compressed air, oxygen, nitrogen, hydrogen, water, etc.) used for process control±2%. (3) Flow unit: m3/h, l/h for volume flow; kg/h, t/h for mass flow; Nm3/h (0°C, 0.1013MPa) for gas volume flow under standard conditions. 2. Selection of general fluid, liquid and steam flow measuring instruments (1) Differential pressure flowmeter 1) Throttling device ①Standard throttling device For flow measurement of general fluid, standard throttling device (standard orifice, standard nozzle). The selection of standard throttling device must comply with the provisions of GB2624-8l or the international standard ISO 5167-1980. If there are new national standard regulations, the new regulations should be implemented. ②Venturi tube can be used if the non-standard throttling device meets the following conditions: a. Accurate measurement under low pressure loss is required; b. The measured medium is clean gas or liquid; c. The inner diameter of the pipeline is in the range of 100-800mm; d. The fluid pressure is within 1.0MPa. If the following conditions are met, double orifice plates can be selected: a. The measured medium is clean gas or liquid; b. The Reynolds number is greater than (equal to) 3000 and less than (equal to) 300000. If the following conditions are met, a 1/4 round nozzle can be selected: a. The measured medium is clean gas or liquid; b. The Reynolds number is greater than 200 and less than 100,000. If the following conditions are met, a round orifice plate can be selected: a. Dirty media (such as blast furnace gas, mud, etc.) that may produce sediment before and after the orifice plate; b. There must be horizontal or inclined pipes. ③The selection of the pressure taking method should take into account that the whole project should adopt a unified pressure taking method as much as possible. a. Generally, the method of corner connection or flange pressure is adopted. b. According to the conditions of use and measurement requirements, other pressure taking methods such as radial pressure taking can be used. 2) The selection of the differential pressure range of the differential pressure transmitter The selection of the differential pressure range should be determined according to the calculation. Generally, it should be selected according to the different working pressure of the fluid: Low differential pressure: 6kPa, 10kPa; medium differential pressure: 16kPa, 25kPa; High differential pressure: 40kPa, 60kPa. 3) Measures to improve measurement accuracy ① For fluids with large temperature and pressure fluctuations, temperature and pressure compensation measures should be considered; ② When the length of the straight pipe section of the pipeline is insufficient or a swirling flow occurs in the pipeline, fluid correction measures should be considered, and the corresponding pipe diameter should be selected. rectifier. 4) Special type differential pressure flowmeter ①Steam flowmeter For the flow of saturated steam, when the required accuracy is not higher than 2.5, and it is calculated locally or remotely, a steam flowmeter can be used. ②Built-in orifice flowmeter For micro flow measurement of clean liquid, steam and gas without suspended solids, when the range ratio is not greater than 3:l, the measurement accuracy is not high, and the pipe diameter is DN<When 50mm, the built-in orifice flowmeter can be selected. When measuring steam, the steam temperature is not more than 120℃. (2) Area flowmeter When the required accuracy is not higher than 1.5 and the range ratio is not more than 10:1, the rotor flowmeter can be selected. 1) The glass rotor flowmeter is the in-situ indication of the fluid flow of small and medium flow, small flow, pressure less than 1MPa, temperature lower than 100 ℃, clean, transparent, non-toxic, non-flammable and explosive, and non-corrosive and non-adhering to glass. Glass rotameter is used. 2) Metal tube rotameter ① Ordinary metal tube rotameter is not easy to vaporize, easy to condense, toxic, flammable, explosive, does not contain magnetic substances, fibers and abrasive substances, and is non-corrosive to stainless steel (1Crl8Ni9Ti). For the measurement of small and medium flow in the fluid, when local indication or remote signal transmission is required, an ordinary metal tube rotameter can be used.

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