Flowmeter selection must be considered

The continuous development of the domestic economy and the series of measures taken by the state to save energy consumption have further promoted the demand for flow meters. Especially in the measurement of urban natural gas, gas, industrial steam, and other types of media, as well as internal internal cost accounting, measurement instruments with wide range, high accuracy, and high reliability are urgently needed. The differential pressure flowmeter has the advantages of its mature technology, simple structure, no movable parts, stable and reliable, and wide application area.
Differential pressure flow meter composition
The current differential pressure flowmeter has the following three kinds of composition methods:
1 Integral differential pressure flowmeter integrates throttling device, differential pressure, pressure, temperature, and calculation display into one integral flowmeter. The flow rate can be displayed directly from the meter, and the measured value can also be transmitted remotely via a 4-20 mA current.
2 Integral differential pressure flow transmitter, the differential pressure transmitter is directly installed on the throttling device, output 4-20mA electrical signal representing the size of differential pressure signal, flow calculation and temperature and pressure compensation are configured by the user.
3 It consists of a throttling device, a differential pressure transmitter, a pressure transmitter, a temperature transmitter, and a flow calculation display device. Also known as split type differential pressure flowmeter.
The third type is the traditional composition of differential pressure flowmeters, and its use in recent years has dropped significantly.
The second method cannot be called a flowmeter. It does not have a flow display or output. It is usually called an integrated differential pressure flow transmitter. It is suitable for use as a field instrument in a control system. The 4-20mA differential pressure signal is sent to the control system. After the control system calculates, the measured flow rate is obtained.
The first is a true differential pressure flowmeter. This integrated differential pressure flowmeter contains all the necessary elements for a flowmeter. Also represents the development direction of differential pressure flowmeter.
General principles for selecting flow meters
To correctly and effectively select flow measurement methods and meters, you must be familiar with the instrument and the fluid properties being measured. At the same time, economic factors must also be taken into account. Overall there are five main factors: performance requirements, fluid characteristics, installation requirements, environmental conditions, and cost levels. There may be multiple options for the instrument that can be used in a certain place of use. If you choose only to rely on past experience, or simply consider a certain factor and make hastily make a decision, you may lose the possibility of choosing the most suitable instrument. However, it is not a simple matter to combine these factors to propose the optimal solution. Based on our many years of accumulation, we will give you some suggestions for everyone to refer to when choosing a flow meter.
4 Accuracy
Accuracy is a very important indicator of a flow measurement instrument. It is also an important indicator that is easily overlooked during the selection process. What is easy to ignore here is that users do not take a deeper look at the accuracy of the manufacturer's proposal when selecting a table. For the accuracy of a table marked by the manufacturer, the questions that need further consideration when selecting a table are:
5 The percentage of error is the guide error (measurement upper limit or percentage of range) or relative error. In addition, changes in temperature, pressure, and environmental conditions used in the field can generate additional errors. The integrated system error of the flowmeter should be pursued when it is selected, and it cannot be superstitious to a single precision index.
6 The accuracy level marked by the manufacturer is reasonable. Some manufacturers set plug-in meters for measuring flow rates at plug-in vortex, turbine, venturi, etc., to ±1% or ±1.5%. Obviously it is unreasonable to belong to the excessive exaggeration of the accuracy of the measurement of the flow velocity of the insertion head to the flow measurement accuracy.
3. All flow meters except the standard throttling device are non-standard flow meters. Actual flow calibration must be performed before delivery to obtain the applicable flow coefficient. Therefore, the standard device must be understood. The ideal flow field of the flowmeter working in the industrial field is to fully develop the turbulence. Therefore, the instrument constants and accuracy levels obtained by real-flow calibration on a standard device that fully develops turbulence have practical significance.
4. When selecting a flow meter, it should pursue its integrated system error instead of superstitious a single precision index. For example, when the same factory is calibrated with a 1% flowmeter, what is its accuracy for steam measurement? It is a difficult question to answer. If you use different manufacturers' tables, the results will be very different. The main problem is that in the calculation of steam density, manufacturers and various types of flowmeters are very different. Some flowmeters do not even treat superheated steam and saturated steam differently and mix the two into a pool. Accuracy.
The worst is the calculation of a given density value. It is not surprising that operating temperature and pressure fluctuations cause tens of percent of error. The other is that the straight line formula for calculating the density can only be implemented in a very narrow range. It is easy to bring about a 10% error after the operating temperature and pressure fluctuations exceed this range. There is also a multi-line (partial linearization) method that can achieve 3-5% accuracy over the full range. To use a factory-calibrated 1% flowmeter to achieve a steam measurement accuracy of 1.5%, density values ​​must be calculated using look-up tables, but such flowmeters or flow totalizers are few.
Above, we use steam measurement as an example. In fact, there is a density calculation problem for the measurement of compressible fluids. The measurement of liquid media also has a density compensation (mainly a temperature change) problem. Therefore, when selecting a table, it is necessary to understand the comprehensive error index of the selected table. There is a big difference between the same type of flow meters produced by different manufacturers.
7 Range
Usually the measurement range of a flow measuring instrument refers to the ratio of the maximum instantaneous flow rate to the minimum instantaneous flow rate that the flowmeter can measure under the premise of ensuring the accuracy. Some manufacturers have indicated that they have a wide range, and the flow rate of the upper limit flow is very high in the indicator. If the liquid flow rate refers to 5 to 10 m/s and the gas refers to 35 to 75 m/s, the actual high flow rate is generally unusable. The wide range of flow measurement range is to have a lower lower flow rate to meet the actual measurement needs of the site. Therefore, a wide-range flow meter with a lower limit flow rate is practical. It is recommended to use the liquid flow rate of 5m/s and the gas flow rate of 35m/s as the upper limit to account for the scope of the flowmeter.
8 Pressure loss
The permanent pressure loss of flow measuring instruments is also an important indicator. The direct consequence of excessive permanent pressure loss (irreversible pressure loss) is the waste of power and energy. The comprehensive indicator selects the pressure loss between 2-30 kpa, which can meet the requirements of most industrial sites.
9 Long-term reliability
The most important thing of the flow measurement instrument is the stability and reliability of long-term work. When selecting the flowmeter, it should be selected according to the conditions of the use environment and the conditions of use. For long-term use, flowmeters with no moving parts, insensitivity to dirt, dust, etc., and no special requirements for the use environment (vibration, electrical interference, temperature change, etc.), will put long-term stability in the first place. However, many are pursuing high performance indicators and functional diversity. In terms of reliability theory, the more complex the structural functions are, the lower the reliability is. "Simple is the best." Therefore, from the actual need to consider the performance and function of the flow meter, choose the best performance-cost ratio meter is the correct principle.
10 Selection of Flowmeter Types
There are many types of flowmeters and they have their own merits. It's not an easy task to choose a flow meter that fits your particular application. However, comprehensive consideration, differential pressure flowmeter has five major characteristics: mature technology, simple structure, no moving parts, stable and reliable, wide adaptability, there is no flowmeter can compare with it, so differential pressure type Flowmeters are still at the top of the single-source flow meter with absolute advantages of 60 to 70% at home and abroad.
One of the five features of the differential pressure flowmeter is its wide adaptation area, which makes it suitable for almost all field requirements. This is also not possible with other types of flow meters. For example, the selection of flowmeters such as vortex street, turbine, precession vortex, etc. requires the user's on-site flow range to adapt to the requirements of the flowmeter. Because the characteristic size of these types of flowmeters is fixed, the scope of the flowmeters of the same diameter is the same, so users cannot use the flowrate beyond the site, unless the process piping is modified (reduced or expanded). ).
The differential pressure flowmeter is the instrument to adapt to the user's on-site flow range. Each differential pressure flowmeter is specially designed and manufactured according to the working condition parameters of the use point, and it will not be used in bad condition. And there are many types of throttles for differential pressure flowmeters that can be used for almost any field requirement. For example, there may be insufficient straight V-shaped cones, optional wedges for dirt and stickiness, optional nozzles for high flow rates, V-shaped cones, nozzles, venturis for low pressure losses, and standard throttling devices for trade settlement measurements.
Differential pressure flowmeter factory configuration
There are three kinds of factory configuration, please pay special attention when selecting.
1 differential pressure flow sensor (only throttling device part)
Only provides the differential pressure signal forming part (nozzle, orifice plate, V-cone, wedge type, etc. any kind of throttling device), does not provide differential pressure transmitter and flow display part, the output signal is the pressure of both ends of the throttle difference. It is the user's own configuration.
2 differential pressure flow transmitter
It consists of throttling device (nozzle, orifice plate, V-cone, wedge type, etc.) and differential pressure transmitter. The output signal is a differential pressure signal of 4-20mA current, and other devices calculate the flow value based on the differential pressure signal. There are two types of split-type installation and one-piece installation.
The split differential pressure flow transmitter consists of an independent throttling device (nozzle, orifice plate, V-cone, wedge type, etc.) and differential pressure transmitter. The connection of the pressure pipe between the throttle device and the differential pressure transmitter is done by the user himself.
Integral installation is to connect the differential pressure transmitter and the throttling device together when the product is shipped. After the user purchases the integrated differential pressure flow transmitter, it is not necessary to connect the pressure pipe after use. But must match the corresponding flow calculator, pressure transmitter and temperature transmitter.
3 differential pressure flow meter
There are two types of split-type installation and one-piece installation:
Split type installation consists of independent throttling devices (any type of throttling device such as nozzle, orifice plate, V-cone, wedge type, etc.), differential pressure, pressure, temperature transmitter, flow rate calculator, globe valve, etc. Made. The connection between the various parts is completed by the user himself. There are explosion-proof and ordinary type two.
The integrated differential pressure flowmeter consists of the HX2003 intelligent display device and throttling device (nozzle, orifice plate, V-cone, wedge type, etc.). It comes with a high-quality differential pressure sensor, pressure sensor, thermal resistance temperature sensor. HX2003-type intelligent display instrument not only automatically adapts to the range of differential pressure sensor, but also a variety of compensation coefficients, such as: outflow coefficient C, flow expansion coefficient ε, etc., are all calculated online, truly achieving the expansion of the range while ensuring the accuracy of measurement . The large-screen LCD displays the cumulative flow, instantaneous flow, instantaneous pressure, and instantaneous temperature on the same screen without manual switching. With RS485 communication port or 4-20mA two-wire instantaneous flow output. Two types of intrinsically safe explosion-proof products and ordinary products.
 

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