Principles of flowmeter selection
The principle of flowmeter selection is to have a deep understanding of the structure principle and fluid characteristics of various flowmeters. At the same time, it should be selected according to the specific situation of the site and the surrounding environmental conditions. Economic factors should also be taken into account. In general, the following five aspects should be selected:
① Performance requirements of flowmeter;
② Fluid characteristics;
③ Installation requirements;
④ Environmental conditions;
⑤ The price of the flowmeter.
Performance requirements of flowmeter
The performance of flowmeter mainly includes: measuring flow (instantaneous flow) or total flow (cumulative flow); Accuracy requirements; Repeatability; Linearity; Flow range and range degree; Pressure loss; Output signal characteristics and response time of flowmeter.
(1) Flow measurement or total quantity
There are two kinds of flow measurement, namely instantaneous flow and cumulative flow. For example, for the crude oil in the distribution station pipeline belongs to trade handover or the petrochemical pipeline is subject to continuous proportioning production or process control of the production process, the total amount needs to be measured, sometimes supplemented by the observation of instantaneous flow. In some workplaces, instantaneous flow measurement is required for flow control. Therefore, it shall be selected according to the needs of on-site measurement. Some flowmeters, such as volumetric flowmeters and turbine flowmeters, are based on the measurement principle of mechanical counting or pulse frequency output to directly obtain the total amount, which has high accuracy and is suitable for measuring the total amount. If equipped with a corresponding transmission device, the flow can also be output. Electromagnetic flowmeter, ultrasonic flowmeter, etc. derive the flow by measuring the fluid velocity. They have fast response and are suitable for process control. If they are equipped with the integration function, the total flow can also be obtained.
(2) Accuracy
The accuracy level of flowmeter is specified within a certain flow range. If it is used under a specific condition or within a relatively narrow flow range, for example, if it changes only within a very small range, its measurement accuracy will be higher than the specified accuracy level. If turbine flowmeter is used to measure oil products in barrels for distribution and the valve is fully opened, the flow is basically constant, and its accuracy may be improved 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 accuracy measurement shall be considered. Generally, the flowmeter is used in the above situations, and the accuracy level is required to be 0.2. In such workplaces, metering standard equipment (such as volumetric tube) is generally equipped on site to conduct online detection on the flowmeter used. In recent years, due to the increasing tension of crude oil and the high requirements of each unit for crude oil measurement, coefficient handover has been proposed for crude oil measurement, that is, in addition to the periodic inspection of the flowmeter once every six months, the two sides of the trade handover negotiate to verify the flowmeter every one or two months to determine the flow coefficient, and the data calculated from the flowmeter’s measurement data and the flowmeter’s flow coefficient are handed over every day to improve the accuracy of the flowmeter, Also called zero error handover.
The accuracy level is generally determined according to the maximum allowable error of the flowmeter. It will be given in the flowmeter instruction provided by each manufacturer. Be sure to note whether the percentage of error refers to relative error or quotation error. The relative error is the percentage of the measured value, which is often represented by “% R”. The reference error refers to the percentage of the upper limit value or range of the measurement, usually “% FS”. Many manufacturers’ instructions are not indicated. For example, the float flowmeter generally adopts reference error, and some models of electromagnetic flowmeter also adopt reference error.
If the flowmeter is used in the flow control system instead of measuring the total amount, the accuracy of the flowmeter shall be determined under the control accuracy requirements of the whole system. Because the whole system not only has the error of flow detection, but also includes the error of signal transmission, control regulation, operation execution and other links and various influencing factors. For example, there is a backlash of about 2% in the operating system. It is uneconomical and unreasonable to determine the high accuracy (above 0.5 level) of the measuring instrument used. As far as the instrument itself is concerned, the accuracy between the sensor and the secondary instrument should also be properly matched. For example, if the designed error of the velocity averaging tube without actual calibration is between ± 2.5% and ± 4%, it is not meaningful to match the differential pressure gauge with a high accuracy of 0.2% to 0.5%.
Another problem is that the accuracy grade specified for the flowmeter in the verification regulation or manufacturer’s instructions refers to the maximum allowable error of the flowmeter. However, due to the influence of environmental conditions, fluid flow conditions and dynamic conditions when the flowmeter is used in the field, some additional errors will be generated. Therefore, the flowmeter used on site shall be the combination of the maximum allowable error and additional error of the instrument itself. This problem must be fully considered. Sometimes, the error within the field use environment may exceed the maximum allowable error of the flowmeter.
(3) Repeatability
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. In general, the measurement performance requirements in the verification regulations specify not only the accuracy level of the flowmeter, but also the repeatability, which generally stipulates that the repeatability of the flowmeter shall not exceed 1/3~1/5 of the maximum allowable error specified in the corresponding accuracy level.
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 same environmental conditions and medium parameters. However, in practical applications, the repeatability of the flowmeter is often affected by changes in fluid viscosity and density parameters. Sometimes, these parameter changes have not reached the level that requires special correction, which may be mistaken for poor repeatability of the flowmeter. In this case, the flowmeter that is not sensitive to this parameter change shall be selected. For example, the float flowmeter is easily affected by the fluid density. The flowmeter with small diameter is not only affected by the fluid density, but also may be affected by the fluid viscosity; The viscosity effect of turbine flowmeter when it is used in the high viscosity range; Some ultrasonic flowmeters without correction will be affected by fluid temperature, etc. If the output of the flowmeter is nonlinear, this effect may be more prominent.
(4) Linearity
The output of flowmeter mainly includes linear and nonlinear square root. Generally, the nonlinear error of flowmeter is not listed separately, but included in the error of flowmeter. For flowmeters with a wide flow range and pulse output signal, which are used to calculate the total amount, the linearity is an important technical indicator. If a single instrument coefficient is used within the flow range, the accuracy of the flowmeter will be reduced when the linearity is poor. For example, a turbine flowmeter uses an instrument coefficient within the flow range of 10:1, and its accuracy will be low if its linearity is poor. With the development of computer technology, its flow range can be segmented, and the flowmeter can be corrected by fitting the flow instrument coefficient curve with the least square method, so as to improve the accuracy of the flowmeter and expand the flow range.
(5) Upper limit flow and flow range
The upper limit flow is also called the full flow or the maximum flow of the flowmeter. When we select the diameter of the flowmeter, we should configure it according to the flow range of the pipeline to be tested and the upper and lower flow limits of the selected flowmeter, not simply according to the pipe diameter.
Generally speaking, the maximum flow rate of the designed pipeline fluid is determined by the economic flow rate. If the selection is too low, the pipe diameter is thick, and the investment will be large; If the transmission power is too high, the operation cost will be increased. For example, the economic flow rate of low viscosity liquid such as water is 1.5~3m/s, and that of high viscosity liquid is 0.2~1m/s. The flow rate of the upper limit flow of most flowmeters is close to or higher than the economic flow rate of the pipeline. Therefore, when the flowmeter is selected, its diameter is the same as that of the pipeline, so it is more convenient to install. If they are not the same, there will not be too much difference. Generally, the specifications of the upper and lower adjacent gears can be connected by reducing pipes.
Attention shall be paid to different types of flow meters in the selection of flow meters. The upper limit flow or upper limit flow rate varies greatly due to the limitation of the measuring principle and structure of respective flow meters. Taking the liquid flowmeter as an example, the flow rate of the upper limit flow is the lowest with the glass float flowmeter, which is generally 0.5~1.5m/s, the volumetric flowmeter is 2.5~3.5m/s, the vortex flowmeter is 5.5~7.5m/s higher, and the electromagnetic flowmeter is 1~7m/s, or even 0.5~10m/s.
The upper limit of liquid flow rate also needs to consider that cavitation cannot occur due to too high flow rate. The location where cavitation occurs is generally the location where the flow rate is the highest and the static pressure is the lowest. In order to prevent cavitation, it is often necessary to control the minimum back pressure (maximum flow) of the flowmeter.
It should also be noted that the upper limit value of the flowmeter cannot be changed after ordering, such as volumetric flowmeter or float flowmeter. Once the differential pressure flowmeter, such as orifice plate of throttling device, has been designed and determined, its lower limit flow cannot be changed. The upper limit flow can be changed by adjusting the differential pressure transmitter or replacing the differential pressure transmitter. For example, for some models of electromagnetic flowmeter or ultrasonic flowmeter, some users can reset the upper limit of flow by themselves.
(6) Extent
The range is the ratio of the upper limit flow and the lower limit flow of the flowmeter. The larger the range is, the wider the flow range will be. Linear instruments have a wide range, generally 1:10. The range of nonlinear flowmeter is only 1:3. For flowmeters generally used for process control or trade handover accounting, if a wide flow range is required, do not choose a flowmeter with a small range.
At present, some manufacturers, in order to publicize the wide flow range of their flowmeters, have raised the flow rate of the upper limit flow very high in the operating instructions, such as raising the liquid flow rate to 7~10m/s (generally 6m/s); The gas is increased to 50~75m/s (generally 40~50) m/s); In fact, such a high flow rate is useless. In fact, the key to a wide range is to have a lower lower limit flow rate to meet the measurement needs. Therefore, the flowmeter with wide range and low lower limit flow rate is more practical.
(7) Pressure loss
Pressure loss generally refers to the unrecoverable pressure loss that changes with the flow due to the static or active detection elements set in the flow channel or the change of the flow direction of the flow sensor, and its value can sometimes reach tens of kilopascals. Therefore, the flowmeter shall be selected according to the allowable pressure loss of the maximum flow determined by the pumping capacity of the pipeline system and the inlet pressure of the flowmeter. Improper selection will limit fluid flow and cause excessive pressure loss, which will affect the flow efficiency. For some liquids (hydrocarbon liquids with high vapor pressure), it should also be noted that excessive pressure drop may cause cavitation and liquid phase vaporization, reduce measurement accuracy and even damage the flowmeter. For example, if the pipe diameter is greater than 500mm, the increased pumping cost due to excessive energy loss caused by pressure loss should be considered. According to relevant reports, the pumping cost of flowmeter with large pressure loss for measurement in recent years often exceeds the purchase cost of flowmeter with low pressure loss and high price.
(8) Output signal characteristics
The output and display quantity of flowmeter can be divided into:
① Flow (volume flow or mass flow); ② Total amount; ③ Average flow velocity; ④ Point velocity. Some flowmeters output analog quantity (current or voltage), while others output pulse quantity. Analog output is generally considered to be suitable for process control and is more suitable for connection with control loop units such as control valves; Pulse output is more suitable for flow measurement of total amount and high accuracy. Long distance signal transmission pulse output has higher transmission accuracy than analog output. The mode and amplitude of output signal shall also have the ability to adapt to other equipment, such as control interface, data processor, alarm device, open circuit protection circuit and data transmission system.
(9) Response time
The response of flowmeter to flow step change shall be noticed when it is applied to pulsating flow. In some applications, the flowmeter output is required to follow the fluid flow, while in others, the output with slow response is required to obtain the comprehensive average value. Instantaneous response is often expressed in terms of time constant or response frequency. The former value ranges from a few milliseconds to a few seconds, while the latter value is below hundreds of Hz. The response time may be considerably prolonged by the use of display instruments. It is generally believed that the asymmetry of dynamic response will accelerate the increase of flow measurement error when the flow rate of the flowmeter increases or decreases.
Fluid properties
In flow measurement, various flowmeters are always affected by one or several parameters of fluid physical properties, so the physical properties of fluid will greatly affect the selection of flowmeters. Therefore, the selected measurement method and flowmeter should not only adapt to the properties of the fluid to be measured, but also consider the influence of the change of one parameter of fluid physical properties on another parameter in the measurement process. For example, the effect of temperature change on liquid viscosity.
The common physical properties of fluid include density, viscosity, steam pressure and other parameters. These parameters can generally be found in the manual to evaluate the adaptability of fluid parameters and flowmeter selection under service conditions. However, some physical properties cannot be found. Such as corrosivity, scaling, plugging, phase change and miscibility.
(1) Fluid temperature and pressure
Carefully analyze the working pressure and temperature of the fluid in the flowmeter, especially the excessive density change caused by the temperature and pressure change when measuring the gas, and the selected measurement method may need to be changed. For example, when temperature and pressure affect performance such as flow measurement accuracy, temperature or pressure correction shall be made. In addition, the structural strength design and material of the flowmeter housing also depend on the temperature and pressure of the fluid. Therefore, the maximum and minimum values of temperature and pressure must be known exactly. When the temperature and pressure change greatly, it should be selected carefully.
It should also be noted that when measuring gas, it is necessary to confirm whether the volume flow value is the temperature and pressure under working conditions or the temperature and pressure under standard conditions.
(2) Density of fluid
For liquids, the density is relatively constant in most applications. Unless the temperature changes greatly, the density correction is generally not required. In gas applications, the range and linearity of the flowmeter depend on the gas density. Generally, the values under standard and working conditions should be known for selection. There are also methods to convert the value of flow state to some recognized reference values, which are widely used in oil storage and transportation. Low density gas is difficult for some measurement methods, especially for instruments that use gas momentum to drive detection sensors (such as turbine flowmeter).
Post time: 16-12-22