1 orifice differential pressure flow meter working principle
Standard throttling orifices are throttling devices that measure the differential pressure of the flow, and can be used with a variety of differential pressure gauges or transmitters to measure the flow of various fluids. Throttle orifice meter orifice device includes annular orifice orifice, nozzle, etc. See Figure 1 for structure. Throttle orifice flowmeters are used in conjunction with transmitters to measure the flow of liquids, vapors, and gases. Orifice flowmeters are used in power petroleum and metallurgy chemical and light industry sectors. An orifice plate throttling device is installed in the process pipeline. The inner diameter of the pipe is much larger than that of the orifice plate. When the measured medium passes through the orifice plate throttling device, the channel suddenly shrinks, which means that the stream becomes smaller and the flow velocity becomes faster. The medium is behind the orifice plate. The static pressure is reduced and static pressure difference (differential pressure) is generated on both sides of the orifice plate and downstream.
Figure 1 Standard orifice plate structure
2 Simple orifice plate flowmeters for simple analysis and installation considerations
Orifice flow meter It is more intuitive and easy to maintain and other characteristics, in the enterprise's general process control flow meter will choose orifice meter. Differential pressure flowmeters are based on the flow continuity principle and the Bernoulli equation to derive the relationship between differential pressure and flow rate. The differential pressure is proportional to the square of the flow. The fluid in the pipeline passes through the orifice flowmeter to generate differential pressure. The orifice flowmeter measures the flow according to mass flow, volume flow, flow coefficient, flow bundle expansion coefficient, orifice opening cross-sectional area, pressure difference before and after the orifice, medium density, etc. The relationship between the calculated.
2.1 and other flowmeters have advantages over orifice flowmeters
(1) Orifice plate flowmeter is low in price, simple and firm in structure, its performance is stable and reliable, and its service life is long;
(2) Other types of flowmeters are not widely used in orifice flowmeters;
(3) The throttling element and the transmitter, the display instrument can be produced by different manufacturers.
2.2 Disadvantages
(1) narrow range, generally only 3:1 to 4:1
(2) High requirements for on-site installation conditions
(3) The measurement accuracy is generally low
(4) Large pressure loss (finger plates, nozzles, etc.)
3 Differential pressure orifice meter normal conditions of use
(1) The process media must continue to pass through the orifice flowmeter and must be filled with a pressure-conducting tube, otherwise the flow rate will not be measured. For example, there is no indication of the flow rate of the bar mill water supply, and the instrumentation worker has to check the transmitter to open the sewage. A small amount of water flows out. It is considered that the pressure guiding pipe is clogged and blown with nitrogen. It is found to be open. After careful inspection, it is found that the throttling component is installed in the Near the outlet (atmospheric pressure), and the pipe is again high (11m from the ground). The water is not full of process piping. It is recommended to displace the orifice flowmeter and then move the orifice plate to a cross-section of about 7 meters from the ground. At this time, the water flow can indicate normal flow.
(2) The measured media must be homogeneous and thermodynamically homogeneous single-phase fluids.
(3) No phase change may occur when the measured medium flows through the throttling device; even if the phase change occurs in the pressure-transmitting tube, it will not work.
(4) The throttling device measures that the measured medium must be a stable flow, or the measured medium is a steady and slowly changing one, and is not suitable for pulsating flow and critical flow measurement.
(5) The stream bundle should be parallel to the pipeline and must not have swirling flow.
4 Installation of throttling device
(1) The centerline of the throttling element and the pipe should be as vertical as possible.
(2) The centerline of the orifice plate of the throttling element should coincide with the centerline of the process pipe.
(3) The principle of determining the length of the straight section on the upstream and downstream side of the throttling device is: when the design of the construction drawing is clearly stipulated, it shall be performed in accordance with the design provisions; when the design of the construction drawing is not specified, the straight section before and after the throttling piece must be It is straight, it must not be visible to the naked eye and it should be lubricated. If it is not lubricated, the flow coefficient should be multiplied by the roughness correction factor. A straight pipe section of 10 times the diameter is required before the orifice plate, and a straight pipe section with a length of 5 times the diameter is required after the orifice plate.
(4) The position of the pressure-reducing port of the standard throttling device installed on the vertical pipeline shall be taken in the direction of easy maintenance and shall not interfere with the maintenance of other equipment and the operation of the process; the pressure-reducing port installed on the throttling device of the horizontal pipeline Position the gas up to 45° vertically upwards.
The liquid level goes down 45° and the steam level goes up 45°. Under special circumstances, you may not press this way. For example, the flow rate of the grey water can be taken down 45° horizontally, and can be taken vertically upwards, so that some of the solid impurities do not easily settle into the pressure pipe as a blockage. See Figure 2 for the installation diagram.
Figure 2 Installation Diagram
(5) If the measured medium is corrosive, then the pressure guiding tube must be made of a material that is resistant to corrosion, and the pressure level of the pressure guiding tube must meet the requirements. The inclination of the ordinary medium pressure guiding tube must be greater than or equal to 1:12. When the flow rate of high fluids is increased, the inclination of the pressure guiding tube should be increased; the inner diameter of the pressure guiding tube should not be less than 6 mm, and it is preferable that the length thereof be within 16 m. The length of the pressure guiding pipe should be as short as possible. When the site conditions do not allow close installation. When the length is more than 30m, install it in sections and install exhaust valves and air collectors at the highest points. Drain valve, settler; positive and negative pressure guide pipe height should be the same high, and as close as possible to lay, easy to crystallize or water and other media also need to match heat insulation.
(6) In order to prevent the metal scrap, oxides, welding slag, etc., from damaging the throttle during operation, the piping must be flushed with high-pressure steam before installing the throttling device.
(7) The surface of the throttling device cannot be wiped with hard objects so as not to damage the throttling element. (8) In order to avoid affecting the accuracy, when lifting the throttling device, the wire, wire, hook, etc., cannot penetrate into the throat of the throttling element to prevent damage to the orifice of the throttling element.
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