Principle of flowmeter selection
The principle of selecting a flowmeter is first to profoundly understand the structural principles and fluid characteristics of various flowmeters, and also to select according to the specific conditions of the site and the surrounding environmental conditions. Also consider the economic factors. In general, you should mainly choose from the following five aspects:
①performance requirements of the flow meter;
② fluid characteristics;
⑤ the price of the flow meter.
Flowmeter performance requirements
The performance aspects of the flow meter mainly include: measuring flow (instantaneous flow) or total (cumulative flow); accuracy requirements; repeatability; linearity; flow range and range; pressure loss; output signal characteristics and flowmeter response time Wait.
(1) Measurement flow or total amount
There are two kinds of flow measurement, namely instantaneous flow and cumulative flow. For example, the crude oil of the pipeline of the sub-transmission station belongs to the custody transfer or the process of continuous control of the petrochemical pipeline or the process control of the production process, etc. Observation. Instantaneous flow measurement is required to control flow at some workplaces. Therefore, it is necessary to make a selection according to the needs of on-site measurement. Some flowmeters, such as volumetric flowmeters, turbine flowmeters, etc., measure the principle directly by mechanical count or pulse frequency output, which has a high accuracy and is suitable for the total amount of measurement, such as with a corresponding signaling device. It can also output traffic. Electromagnetic flowmeters, ultrasonic flowmeters, etc. are used to measure the flow rate of the fluid to derive the flow rate. The response is fast and suitable for process control. The total amount can also be obtained if the integrated calculation function is used.
The accuracy level of the flowmeter is specified within a certain flow range. If it is used under a certain condition or a narrow flow range, for example, only in a small range, the measurement accuracy will be Higher than the specified accuracy level. If the turbine meter is used to measure the oil and the barrel is distributed, when the valve is fully opened, the flow rate is basically constant, and the accuracy may be increased from 0.5 to 0.25.
For trade accounting, storage and transportation handover and material balance, if the measurement accuracy is required to be high, the durability of the accuracy measurement should be considered. It is generally used for the flowmeter under the above conditions, and the accuracy level requirement is 0.2. In such a workplace, it is generally equipped with on-site measurement standard equipment (such as volumetric tubes) to perform on-line inspection of the flowmeter used. In recent years, due to the increasing tension of crude oil and the high requirements of various units for the measurement of crude oil, the implementation of the coefficient of crude oil is proposed, that is, in addition to a cycle test of the flowmeter every six months, the two parties negotiated each month or two. Monthly verification of the flowmeter to determine the flow coefficient, daily calculation of data according to the flow meter measurement data and flowmeter flow coefficient to improve the accuracy of the flowmeter, also known as zero error handover.
The accuracy level is generally determined based on the maximum allowable error of the flow meter. It will be given in the flowmeter instructions provided by each manufacturer. It is important to note that the percentage of error is the relative error or the reference error. The relative error is the percentage of the measured value and is usually expressed as "%R". The reference error is the percentage of the upper limit or range measured, commonly used as "%FS". Not stated in many manufacturer's instructions. For example, float flowmeters generally use reference errors, and some models of electromagnetic flowmeters also use reference errors.
If the flowmeter is not simply metered, but is applied to the flow control system, the accuracy of the flowmeter should be determined under the overall system control accuracy requirements. Because the whole system not only has the error of flow detection, but also includes the error of signal transmission, control adjustment, operation execution and other influencing factors. For example, there is a 2% difference in the operating system, and it is uneconomical and unreasonable to determine the excessive accuracy (above 0.5) for the measuring instrument used. As far as the meter itself is concerned, the accuracy between the sensor and the secondary meter should also be properly matched. For example, the average speed tube error designed without actual calibration is between ±2.5% and ±4%, with 0.2%. ~0.5% high accuracy differential pressure gauge is of little significance.
A further problem is that the accuracy level specified for the flowmeter in the verification protocol or manufacturer's instructions refers to the maximum allowable error of the flowmeter. However, due to changes in environmental conditions, fluid flow conditions, and dynamic conditions when the flowmeter is used in the field, additional errors will occur. Therefore, the flowmeter used in the field should be the synthesis of the maximum allowable error and additional error of the instrument itself. The problem must be fully considered. Sometimes the error within the operating environment of the field may exceed the maximum allowable error of the flowmeter.
Repeatability is determined by the flowmeter principle itself and manufacturing quality. It is an important technical indicator in the use of the flowmeter and is closely related to the accuracy of the flowmeter. Generally, in the measurement performance requirements in the verification regulations, the flowmeter not only has the accuracy level specified, but also the repeatability is specified. Generally, the repeatability of the flowmeter must not exceed the maximum allowable error specified by the corresponding accuracy level. 1/3 to 1/5.
Repeatability is generally defined as the consistency of multiple measurements in the same direction for a certain flow value in a short time under the condition that the environmental conditions and the media parameters are constant. However, in practical applications, the repeatability of the flowmeter is often affected by changes in fluid viscosity and density parameters. Sometimes these parametric changes have not yet reached the level of special correction, which is mistaken for the reproducibility of the flowmeter. . In view of this situation, a flow meter that is not sensitive to this parameter change should be selected. For example, float flowmeters are susceptible to fluid density, and small-diameter flowmeters are not only affected by fluid density, but may also be affected by fluid viscosity; turbine flowmeters are used for viscosity effects in high viscosity ranges; some are not corrected The treated ultrasonic flow meter is affected by the temperature of the fluid and the like. If the output of the flowmeter is non-linear, this effect may be more pronounced.
The output of the flowmeter is mainly linear and nonlinear square root. In general, the nonlinearity error of the flowmeter is not listed separately, but is included in the error of the flowmeter. For a generally wide flow range, the output signal is pulsed, used as a total flow meter, linearity is an important technical indicator, if a single meter factor is used in its flow range, when the linearity difference is Will reduce the accuracy of the flow meter. For example, a turbine flowmeter uses a meter factor in a 10:1 flow range. When the linearity is poor, its accuracy will be low. With the development of computer technology, the flow range can be segmented and fitted by least squares method. The flow-meter factor curve corrects the flowmeter to improve the accuracy of the flowmeter and extend the flow range.
（5）Upper limit flow and flow range
The upper limit flow is also referred to as the full flow or maximum flow of the flow meter. When we choose the diameter of the flowmeter, it should be configured according to the flow range used by the pipeline to be tested and the upper and lower flow rates of the selected flowmeter. It cannot be simply used according to the pipeline diameter.
In general, the maximum flow rate for designing a pipe fluid is determined by the economic flow rate. If the choice is too low, the pipe diameter is thick, the investment will be large; if it is too high, the transmission power is large, and the running cost is increased. For example, a low-viscosity liquid such as water has an economical flow rate of 1.5 to 3 m/s, and a high-viscosity liquid of 0.2 to 1 m/s. The flow rate of most flow rate upper flow rates is close to or higher than the economical flow rate of the pipeline. Therefore, when the flowmeter is selected, its diameter is the same as that of the pipeline, and the installation is convenient. If they are different, they will not be too different. Generally, the specifications of the upper and lower ones can be connected by different diameter pipes.
Different types of flowmeters should be noted in the selection of flowmeters. The upper or upper flow rates are greatly different due to the measurement principle and structure of the respective flowmeters. Taking the liquid flow meter as an example, the flow rate of the upper limit flow rate is the lowest with a glass float flowmeter, generally between 0.5 and 1.5 m/s, and the volumetric flowmeter is between 2.5 and 3.5 m/s, and the vortex flowmeter is higher. Between 5.5 and 7.5 m/s, the electromagnetic flowmeter is between 1 and 7 m/s, and even between 0.5 and 10 m/s.
The upper limit flow rate of the liquid also needs to be considered. The cavitation phenomenon cannot be generated because the flow rate is too high. The place where cavitation occurs is generally at the position where the flow rate is the highest and the static pressure is the lowest. In order to prevent the formation of cavitation, it is often necessary to control the minimum flowmeter. Back pressure (maximum flow).
It should also be noted that the upper limit of the flowmeter cannot be changed after ordering, such as a volumetric flowmeter or a float flowmeter. After the differential pressure flowmeter is designed and determined, the lower limit flow rate cannot be changed. The upper limit flow fluctuation can be changed by adjusting the differential pressure transmitter or replacing the differential pressure transmitter. For example, some types of electromagnetic flowmeters or ultrasonic flowmeters, some users can reset the flow upper limit by themselves.
The range is the ratio of the upper flow rate to the lower flow rate of the flow meter, and the larger the value, the wider the flow range. Linear meters have a wide range, typically 1:10. The range of nonlinear flowmeters is only 1:3. Flowmeters generally used for process control or custody transfer accounting. If the flow range is required to be wide, do not select a flowmeter with a small range.
At present, some manufacturers are promoting the flow range of their flowmeters. In the instruction manual, the flow rate of the upper limit flow is raised very high, for example, the liquid is increased to 7-10 m/s (generally 6 m/s); the gas is increased to 50~ 75m / s (typically 40 ~ 50) m / s); in fact, such a high flow rate is not used. In fact, the key to the wide range is to have a lower lower flow rate to meet the measurement needs. Therefore, a wide range of flow meters with a low lower flow rate is more practical.
(7) Pressure loss
Pressure loss generally refers to a flow sensor that produces a non-recoverable pressure loss as a function of flow due to static or active sensing elements placed in the flow path or changes in flow direction, sometimes up to tens of kilopascals. Therefore, the flow meter should be selected based on the allowable pressure loss of the maximum flow rate, such as the pumping capacity of the piping system and the inlet pressure of the flow meter. Improper selection can limit excessive pressure loss caused by fluid flow and affect circulation efficiency. Some liquids (high vapor pressure hydrocarbons) should also be aware that excessive pressure drop may cause cavitation and liquid phase vaporization, reducing measurement accuracy and even damaging the flow meter. For example, a flow meter for water delivery with a diameter greater than 500 mm should take into account the increased pumping cost due to excessive energy loss caused by pressure loss. According to relevant reports, the pumping cost of the flowmeter with a large pressure loss for several years often exceeds the purchase cost of the low-pressure loss and the more expensive flowmeter.
(8) Output signal characteristics
The output and display of the flow meter can be divided into:
1 flow (volume flow or mass flow); 2 total; 3 average flow rate; 4 point flow rate. Some flowmeter outputs are analog (current or voltage) and others output pulses. The analog output is generally considered to be suitable for process control, and is more suitable for connection with control loop units such as regulating valves; the pulse output is more suitable for total and high-accuracy flow measurement. Long-distance signal transmission pulse output has higher transmission accuracy than analog output. The manner and magnitude of the output signal should also be compatible with other devices, such as control interfaces, data processors, alarm devices, open circuit protection circuits, and data transfer systems.
(9) Response time
When applied to pulsating flow, attention should be paid to the flowmeter's response to flow step changes. Some applications require that the flow meter output follow a change in fluid flow, while others require a slower response output to achieve a composite average. The transient response is often expressed in terms of a time constant or response frequency, the former being from a few milliseconds to a few seconds and the latter being below a few hundred Hz. Using a display meter can considerably increase the response time. It is generally believed that the dynamic response asymmetry when the flow rate of the flow meter increases or decreases accelerates the increase of the flow measurement error.
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