There are many factors that need to be seriously considered in the selection of electromagnetic flowmeters, because proper selection will bring great benefits to our use. If it is counterproductive, it will cause a lot of inconvenience. Below we Huaheng Instrument to take you to understand the problem of electromagnetic flowmeter selection.
The choice of the flow range of the electromagnetic flowmeter has a large impact on its accuracy and life. Generally, the maximum flow rate corresponding to the maximum flow during work should not be too high. The use status is divided into continuous work and intermittent work. Continuous work means more than 8 hours of work per day, and intermittent work is less than 8 hours of work per day. For continuous operation, the maximum flow rate should be selected at the lower limit of the upper limit flow of the meter, while the intermittent operation can be selected at a higher position. Typically continuous operation is to multiply the actual maximum flow rate by 1.4 as the upper limit flow rate for the flow range, while the intermittent operation is multiplied by 1.3. If the gauge diameter is inconsistent with the process pipe diameter, the pipe should be modified with a reducer and a straight pipe.
For process piping with a low flow rate, the minimum flow rate is the first consideration in selecting the gauge diameter. Usually, the actual minimum flow is multiplied by 0.8 as the lower flow of the flow range, leaving a certain margin. If the display is equipped with a piecewise linearization function, when the lower limit of the sensor flow rate cannot meet the actual minimum flow rate, the manufacturer should be required to perform flow verification at the actual minimum flow rate and its vicinity, and input the measured meter coefficient into the display device. This will reduce the flow rate lower limit of the meter and maintain the accuracy of the measurement.
In general, the choice is mainly based on its high precision. At present, the accuracy of the electromagnetic flowmeter is roughly liquid: ±0.15%R, ±0.2%R, ±0.5%R and ±1%R in the international market, ±0.5%R and ±1%R for domestic styling products; gas: The international market is ±0.5%R and ±1%R, domestic is ±1%R and ±1.5%R, and the above accuracy refers to the range of 6:1 or 10:1. Typical parameters of several electromagnetic flow sensors are shown in Table 1. Accuracy is closely related to the conditions of use, in addition to the quality of its own products.
If the range is narrowed, the accuracy can be improved; in particular, the standard flowmeter, which is a standard gauge flow standard device, can be greatly improved if it is used at a fixed point.
The higher the accuracy of the flowmeter, the more sensitive it is to changes in the field conditions. To maintain its high accuracy, special handling of the meter factor is required. One method of processing is the so-called meter coefficient floating processing method. That is, the real-time processing is performed by the following conditions: a) viscosity is affected by temperature; b) density is affected by pressure and temperature; c) sensor signal redundancy (one sensor outputs two signals, monitors the ratio; d) coefficient Long-term stability (take control chart determination) and so on.
For trade storage and transportation handover measurement, an online calibration device is often provided for periodic verification.
The accuracy of the meter listed in the manufacturer's instruction manual is the basic error. The additional error should be estimated at the site. The field error should be the synthesis of the two.
The requirements for the accuracy level of the meter should be cautious and should be considered from an economic point of view. For example, the trade settlement instrument for large-diameter oil (gas) pipelines is economically significant, and it is cost-effective to invest more in the meter. As for the small amount of transportation or the medium precision level as the process control, it is necessary to blindly pursue high precision. The intrinsically safe explosion-proof sensor is fitted with the safety barrier model and manufacturer, and the explosion-proof grade and approval number are checked. To display the mass flow (or volume flow under standard conditions), select the pressure, temperature sensor or density meter. The electromagnetic flowmeter display instrument is now included in a microprocessor-based flowmeter computer that can communicate with the host computer, and the instrument has far more functions and applications than the old-fashioned turbine flow display. Various types of flow meters currently used as trade meters tend to be equipped with direct reading display devices (as shown in Figure 6). Not only the display of the total metering, but also the compensator (a fully functional flow computer) can output the remote signal.
Electromagnetic flowmeters require fluids to be clean (or substantially clean), single-phase or low-viscosity. Common fluids are as follows: general fluids, including water, air, oxygen, high-pressure hydrogen, milk, coffee, etc.; petrochemicals: gasoline , light oil, jet fuel, light diesel oil, naphtha, ethylene, polyethylene, styrene, liquefied gas, carbon dioxide and natural gas; chemical solutions: ammonia, methanol, brine, etc.; organic liquid: alcohol, ether, benzene, toluene , xylene, butadiene, carbon tetrachloride, methylamine, acrylonitrile, etc.; inorganic liquid: formaldehyde, citric acid, caustic soda, carbon disulfide, and the like. For corrosive media, use material selection to note that impurities and abrasive media are not recommended.
The gas electromagnetic flowmeter mainly considers the influence of fluid density on the meter factor, and the influence of density is mainly in the low flow area, as shown in Fig. 14. The increase in density (i.e., the increase in pressure) causes the linear portion of the characteristic curve to expand toward the lower flow rate region, the range of the sensor is expanded, and the linearity is improved. If the working pressure of the measured medium is different when the gas electromagnetic flowmeter is used in the normal pressure air, the lower limit flow rate is calculated by the following formula qvmin, qvamin- respectively the measured medium under pressure p and pressure pa (101.325 kPa) The lower limit of the volumetric flow rate of air, m3/h; p, pa- is the working pressure (absolute pressure) and atmospheric pressure (101.325 kPa), kPa; d-the relative density of the measured medium, dimensionless.
The liquid electromagnetic flowmeter is a viscosity-sensitive flowmeter. When the viscosity of the liquid increases, the linear region of the meter coefficient becomes narrower, the lower flow rate increases, and when the viscosity increases to a certain value, even a wireless region. The condition of the spiral blade is much better than that of the straight blade.
For liquids, the sensor is usually calibrated with water. When the accuracy is 0.5, the liquid can be below 5×10-6 mm 2 /s without regard to the influence of viscosity. When the fluid viscosity is higher than 5 x 10-6 mm2/s, a liquid of a comparable viscosity can be used without a viscosity correction. In addition, some measures can be taken to compensate for the effects of viscosity. If the scope of use is reduced, increase the flow downline value or meter factor multiplied by the Reynolds number correction factor.
The effect of viscosity on the meter factor is related to the type of sensor structure and the size of the parameters. There are several ways to express the influence of viscosity on the meter factor: the relationship between the meter factor and the Reynolds number, the relationship between the meter factor and the output frequency, the relationship between the meter factor and the output frequency divided by the ratio of the year of the exercise, and so on. Some of these materials are prepared by the manufacturer, but not all manufacturers have this information.
The electromagnetic flowmeter measures the actual volume flow. Regardless of the material balance or energy metering, the medium flow rate (ie the volume flow rate under standard conditions) must be measured. This should be converted by the following formula.
Where qv,qvn- are the volumetric flow in the working state and the standard state, respectively, m3/h;
p, T, Z- are the absolute pressure (Pa), thermodynamic temperature (K) and gas compression coefficient in the working state;
Pn, Tn, Zn- are absolute pressure (Pa), thermodynamic temperature (K) and gas compression coefficient under standard conditions;
The use of electromagnetic flowmeters for high precision applications, the economic factors should be considered in many ways. The purchase cost of the instrument is only a part of the cost. The following aspects should also be considered: the auxiliary equipment cost for installation (such as air eliminator, filter, etc.) or the bypass branch pipe including valves; the calibration fee must be frequent in order to maintain high precision. Verification, even installing an online calibration device on site, the cost is considerable; maintenance costs, replacement of consumable parts for electromagnetic flowmeters, he is necessary to maintain high performance.
Fluids containing a lot of impurities, such as circulating cooling water, river water, sewage, fuel oil, etc.; places where the flow rate changes drastically, such as boiler water supply system, air supply system with air hammer, etc.; when measuring liquid, the pipeline pressure is not high and the flow rate is Larger, the pressure on the downstream side of the instrument may be close to the saturated vapor pressure, and there is a danger of generating air pockets. For example, liquid ammonia can freely flow from the high position groove, and it is not suitable to be installed at the discharge port; electric welder, motor, relay with contacts In the vicinity of the area, there are places with severe electromagnetic interference; the length of the upstream and downstream straight pipe sections is seriously insufficient, such as in the engine room of the ship; if the boiler automatic water supply system frequently pumps and stops the pump, it will impact the impeller and cause the sensor to be damaged quickly; Care should be taken when selecting corrosive or abrasive media. Contact the manufacturer for advice.
1) Confirm the available measurement objects as described above.
2) Select the type. According to the physical properties of the fluid, the gas and the liquid are respectively of a gas type and a liquid type, and are not universal. In the working state, the viscosity of the liquid exceeds 5mPa.s, and the high viscosity type should be selected (there is no fixed product in China). The acid corrosive liquid is acid-resistant (there is no domestic product).
According to the environmental conditions, select the appropriate instrument according to the ambient temperature and humidity. If there is an explosive flammable atmosphere around it, select the explosion-proof sensor. According to the pipe connection method, the sensor has two installation modes, horizontal and vertical. Flange, threaded and clamped connections to the pipe connection during horizontal installation. The medium diameter is flanged, the small diameter and high pressure pipe are threaded, and the clamped connection is only suitable for low pressure medium and small pipe diameter. Vertical mounting is only a threaded connection.
3) Select specifications. According to the conditions of use in the field, such as flow range, pipe diameter, fluid pressure and temperature, installation position, etc. and performance requirements, such as accuracy, repeatability, display mode, etc., refer to the manufacturer's selection sample or instruction manual to select specific specifications and models. May not find a suitable one, so I have to choose another flow meter.
Due to the variety of electromagnetic flowmeter types, especially the quality of different manufacturers, so we must try to collect the manufacturer and related standards and other data for repeated investigation and comparison before deciding.