As the flow is a dynamic quantity, the measuring instrument itself is affected by many factors, such as: pipe, diameter, shape (circular, rectangular), boundary conditions, physical properties of medium (temperature, pressure, density, viscosity, dirt, corrosivity, etc.), fluid flow state (turbulent state, velocity distribution, etc.) and installation conditions and levels. In the face of more than a dozen types and hundreds of varieties of flow instruments at home and abroad (volume type, differential pressure type, turbine type, area type, electromagnetic type, ultrasonic type and thermal flowmeter and other types developed successively), how to reasonably select the type according to the flow, flow pattern, installation requirements, environmental conditions, economy and other factors is the premise and basis for the good application of flow instruments. This paper summarizes the selection principles and methods of common flow instruments, Help you quickly master the selection and application of flow instruments.
Generally speaking, the principle of selecting flowmeter is to deeply understand the structural principle and fluid characteristics of various flowmeters, and make selection according to the specific situation of the site and the surrounding environmental conditions. Economic factors should also be taken into account. Generally, we should choose from the following five aspects:
① Performance requirements of flowmeter;
② Fluid characteristics;
③ Installation requirements;
④ Environmental conditions;
⑤ Price of flowmeter.
01 performance requirements
The performance of flowmeter mainly includes: measuring flow (instantaneous flow) or total amount (cumulative flow); Accuracy requirements; Repeatability; Linearity; Flow range and range; Pressure loss; Output signal characteristics and response time of flowmeter.
Measuring flow or total volume
There are two kinds of flow measurement, namely instantaneous flow and cumulative flow. For example, the total amount needs to be measured for the crude oil in the off load station pipeline belongs to trade handover or continuous proportioning production or process control of production process of petrochemical pipeline, and sometimes supplemented by the observation of instantaneous flow. In some workplaces, instantaneous flow measurement is required for flow control. Therefore, it should be selected according to the needs of on-site measurement. Some flowmeters, such as positive displacement flowmeter and turbine flowmeter, 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 corresponding sending device, it can also output the flow. Electromagnetic flowmeter and ultrasonic flowmeter deduce the flow by measuring the fluid flow rate, which has fast response and is suitable for process control. If equipped with the accumulation function, the total amount can also be obtained.
The accuracy grade 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, it changes only in a small range, its measurement accuracy will be higher than the specified accuracy grade. If the turbine flowmeter is used to measure the distribution of oil in barrels, when the valve is fully open, the flow is basically constant, and its accuracy may be improved from level 0.5 to level 0.25.
It is used 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, it is used for the flowmeter under the above conditions, and the accuracy level is required to be 0.2. For example, the on-line measuring equipment (such as on-line measuring pipe) is generally equipped in the workplace. In recent years, due to the high requirements of various units for crude oil measurement, coefficient handover is proposed for crude oil measurement, that is, in addition to the periodic detection of the flowmeter every half a year, the two sides of trade handover negotiate to verify the flowmeter every one or two months to determine the flow coefficient, and handover is carried out every day based on the data measured by the flowmeter and the data calculated by the flowmeter flow coefficient, so as to improve the accuracy of the flowmeter, also known as 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 instructions provided by each manufacturer. Be sure to pay attention to whether the percentage of error refers to relative error or reference error. The relative error is the percentage of the measured value, which is usually expressed by “% R”. The reference error refers to the percentage of the upper limit value or measuring range, commonly used as “% FS”. Many manufacturers do not indicate it in the instructions. For example, the float flowmeter generally adopts reference error, and some models of electromagnetic flowmeter also adopt reference error.
If the flowmeter is not simply used to measure the total amount, but is applied in the flow control system, the accuracy of detecting 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 return error of about 2% in the operating system. It is uneconomical and unreasonable to determine the excessively high accuracy (above level 0.5) of the measuring instruments 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 averaging tube error without actual calibration is between ± 2.5% and ± 4%, it is of little significance to be equipped with a differential pressure gauge with high accuracy of 0.2% ~ 0.5%.
Another problem is that the accuracy level specified for the flowmeter in the verification regulation or the manufacturer’s instructions refers to the maximum allowable error of the flowmeter. However, when the flowmeter is used on site, it will produce some additional errors due to the changes of environmental conditions, fluid flow conditions and dynamic conditions. Therefore, the flowmeter used on site should 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 scope of on-site use environment may exceed the maximum allowable error of the flowmeter.
Repeatability is determined by the principle and manufacturing quality of the flowmeter. It is an important technical index in the use of the flowmeter and is closely related to the accuracy of the flowmeter. Generally, in the metrological performance requirements in the verification regulation, the flowmeter has not only the accuracy level, but also the repeatability. The general provision is 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 when the environmental conditions and medium parameters are unchanged. However, in practical application, the repeatability of the flowmeter will often be affected by the changes of fluid viscosity and density parameters. Sometimes these parameters do not reach the level that needs special correction, which will be mistaken for the poor repeatability of the flowmeter. In view of this situation, the flowmeter insensitive to the change of this parameter should be selected. For example, the rotameter is easily affected by the fluid density, and the small-diameter flowmeter is not only affected by the fluid density, but also may be affected by the fluid viscosity; Viscosity effect of turbine flowmeter if used in high viscosity range; Some ultrasonic flowmeters without correction will be affected by fluid temperature and so on. If the output of the flowmeter is nonlinear, this effect may be more prominent.
The output of flowmeter mainly includes linear and nonlinear square root. Generally speaking, the nonlinear error of flowmeter is not listed separately, but included in the error of flowmeter. For the flowmeter with a wide flow range and pulse output signal, linearity is an important technical index. If a single instrument coefficient is used within its flow range, the accuracy of the flowmeter will be reduced when the linearity is poor. For example, the turbine flowmeter adopts an instrument coefficient in the flow range of 10:1, and its accuracy will be low when the linearity is poor. With the development of computer technology, its flow range can be segmented, and the flow instrument coefficient curve can be fitted by the least square method to correct the flowmeter, so as to improve the accuracy of the flowmeter and expand the flow range.
Upper limit flow and flow range
The upper limit flow is also known as the full flow or maximum flow of the flowmeter. When we choose the diameter of the flowmeter, we should configure it according to the flow range used by the tested pipeline and the upper and lower limit flow of the selected flowmeter, and we can’t simply match it according to the pipe diameter.
Generally speaking, the maximum flow rate of design pipeline fluid is determined according to the economic flow rate. If the selection is too low and the pipe diameter is thick, the investment will be large; If it is too high, the transmission power is large and the operation cost is 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 flow meters 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 different, there will not be too much difference. Generally, the specifications of the upper and lower adjacent gears can be connected by reducer.
In the selection of flow meters, attention should be paid to different types of flow meters. The upper limit flow or upper limit flow rate is greatly different due to the limitation of the measurement principle and structure of their 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 between 0.5 ~ 1.5m/s, the positive displacement flowmeter is between 2.5 ~ 3.5m/s, the vortex flowmeter is higher between 5.5 ~ 7.5m/s, and the electromagnetic flowmeter is between 1 ~ 7m / S, even between 0.5 ~ 10m / s.
The upper limit flow rate of liquid also needs to be considered, and cavitation phenomenon cannot occur due to too high flow rate. The place where cavitation phenomenon occurs is generally at the place with the maximum flow rate and the lowest static pressure. In order to prevent the formation of 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 positive displacement flowmeter or float flowmeter. Once the design of differential pressure flowmeter such as orifice plate of throttling device is determined, the lower limit flow cannot be changed, and 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.
The range degree is the ratio of the upper limit flow and the lower limit flow of the flowmeter. The larger the value, the wider the flow range. Linear instruments have a wide range, generally 1:10. The range of nonlinear flowmeter is small, only 1:3. Generally used for process control or trade handover accounting, if the flow range is required to be wide, do not choose the flowmeter with small range.
At present, in order to publicize the wide flow range of their flowmeter, some manufacturers raise the flow rate of the upper limit flow very high in the operation manual, such as raising the liquid to 7 ~ 10m / S (generally 6m / s); Increase the gas 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 lower limit flow rate to meet the needs of measurement. Therefore, a wide range flowmeter with low lower limit flow rate is more practical.
The pressure loss generally refers to the unrecoverable pressure loss of the flow sensor due to the static or movable detection element set in the flow channel or changing the flow direction, and its value can sometimes reach tens of kPa. 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 the fluid flow, resulting in excessive pressure loss and affect the circulation efficiency. For some liquids (high vapor pressure hydrocarbon liquid), attention should also be paid to the excessive pressure drop, which may cause cavitation and liquid-phase vaporization, reduce the measurement accuracy and even damage the flowmeter. For example, for the flowmeter for water transmission with a pipe diameter greater than 500mm, the increased pumping cost caused by excessive energy loss caused by pressure loss shall 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 expensive price.
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 velocity; ④ Point velocity. Some flow meters output analog quantity (current or voltage), while others output pulse quantity. Analog output is generally considered to be suitable for process control and more suitable for connection with control loop units such as regulating valve; Pulse output is more suitable for total amount and high accuracy flow measurement. The pulse output of long-distance signal transmission has higher transmission accuracy than the 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.
When applied to pulsating flow, attention should be paid to the response of flowmeter to flow step change. Some applications require the output of the flowmeter to follow the fluid flow, while others require a slower response output to obtain a comprehensive average. The instantaneous response is often expressed by time constant or response frequency. The former ranges from a few milliseconds to a few seconds, and the latter is below hundreds of Hz. The use of display instruments may significantly prolong the response time. It is generally believed that the asymmetry of dynamic response will accelerate the increase of flow measurement error when the flow of flowmeter increases or decreases.
02 fluid characteristics
In flow measurement, because various flowmeters are always affected by one or several parameters of fluid physical properties, the physical properties of fluid will affect the selection of flowmeters to a great extent. Therefore, the selected measurement method and flowmeter should not only adapt to the properties of the measured fluid, 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 fluid parameters under service conditions and the adaptability of the selected flowmeter. But there are also some physical properties that cannot be found. Such as corrosivity, scaling, blockage, phase transformation and miscible state.
Temperature and pressure of fluid
Carefully analyze the working pressure and temperature of the fluid in the flowmeter, especially the excessive density change caused by the change of temperature and pressure when measuring the gas, and the selected measurement method may have to be changed. For example, when temperature and pressure affect the accuracy of flow measurement, temperature or pressure correction should be made. In addition, the structural strength design and material of the flowmeter shell also depend on the temperature and pressure of the fluid. Therefore, the maximum and minimum values of temperature and pressure must be accurately known. 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 its volume flow value is the temperature and pressure under working conditions or the temperature and pressure under standard conditions.
Density of fluid
For liquids, the density is relatively constant in most applications. Unless the temperature changes greatly, the density correction can not be carried out generally. In gas applications, the range and linearity of the flowmeter depend on the gas density. Generally, the values under standard state and working condition should be known for selection. There are also some methods to convert the value of flow state to some recognized parameter ratios, which are widely used in oil storage and transportation. Low density gas is difficult for some measurement methods, especially instruments that use gas momentum to drive the detection sensor (such as turbine flowmeter).
The viscosity of various liquids varies greatly and varies significantly due to temperature changes. The gas is different. The viscosity difference between various gases is small, and its value is generally low. And there will be no significant change due to the change of temperature and pressure. Because the viscosity of liquid is much higher than that of gas. For example, at 20 ℃ and 100KPA, the dynamic viscosity of water is pa · s, while that of air is pa · s, so the influence of viscosity must be considered for liquid, and the viscosity of gas is not as important as that of liquid.
The influence of viscosity on various flowmeters is different. For example, the flow values of electromagnetic flowmeter, ultrasonic flowmeter and Coriolis mass flowmeter are in a wide viscosity range, which can be considered not to be affected by liquid viscosity; The error characteristic of positive displacement flowmeter is related to viscosity and may be slightly affected; However, when the viscosity of rotameter, turbine flowmeter and vortex flowmeter exceeds a certain value, it will have a great impact and can not be used.
The characteristics of some flowmeters are described by the pipe Reynolds number as a parameter variable, and the pipe Reynolds number is a function of fluid viscosity, density and pipe velocity. Therefore, the viscosity still has an impact on the characteristics of the instrument.
Viscosity is also a parameter to judge Newtonian fluid or non-Newtonian fluid. Most flow measurement methods and flow meters are only applicable to Newtonian fluid. All gases are Newtonian fluids. Most liquids and liquids containing small amounts of spherical particles are also Newtonian fluids. The measurement method and flowmeter only applicable to Newtonian fluid will affect the measurement if applied to non-Newtonian fluid. Therefore, Newtonian fluid is an important condition for the normal use of fluid flow measurement.
The influence trend of viscosity on the range of different types of flowmeter is different. Generally, the viscosity of positive displacement flowmeter increases and the range expands. On the contrary, the viscosity of turbine flowmeter and vortex flowmeter increases and the range decreases. Therefore, when evaluating the adaptability of flowmeter, we should master the temperature viscosity characteristics of liquid.
The flow state of some non Newtonian fluids (such as drilling mud, pulp, chocolate and paint) is complex and it is difficult to judge their properties. Care must be taken when selecting flow meter.
Chemical corrosion and scaling
1) Chemical corrosion problem
The chemical corrosion of fluid sometimes becomes the decisive factor for us to choose measurement method and use flowmeter. For example, some fluids will corrode the parts contacted by the flowmeter, deposit or precipitate crystals on the surface, and produce electrolytic chemistry on the surface of metal parts. The occurrence of these phenomena will reduce the performance and service life of the flowmeter. Therefore, in order to solve the problems of chemical corrosion and scaling, the manufacturer has adopted many methods, such as selecting anti-corrosion materials or taking anti-corrosion measures on the structure of the flowmeter, for example, the orifice plate of the throttling device is made of ceramic materials, and the metal float flowmeter is lined with corrosion-resistant engineering plastics. However, for the flowmeter with complex structure, such as positive displacement flowmeter and turbine flowmeter, it is impossible to measure the corrosive fluid. Some flowmeters have corrosion resistance in principle and structure or are easy to take corrosion resistance measures. The transducer probe of ultrasonic flowmeter is installed on the outer wall of the pipeline and does not contact with the measured fluid, which is essentially anti-corrosion. The electromagnetic flowmeter only has the lining of the measuring tube and a pair of electrodes with simple shape in contact with the liquid. In recent years, some designs do not contact the electrode with the liquid, which is also an anti-corrosion measure.
Scaling or crystallization on the flowmeter cavity and flow sensor will reduce the clearance of moving parts in the flowmeter and reduce the sensitivity or measurement performance of sensitive elements in the flowmeter. For example, in the application of ultrasonic flowmeter, the scaling layer will hinder the ultrasonic emission. In the application of electromagnetic flowmeter, the non-conductive scaling layer insulates the electrode surface, which will make the flowmeter unable to work. Therefore, some flow meters often use heating outside the flow sensor to prevent precipitation and crystallization, or install a descaler.
The result of chemical corrosion and scaling is to change the roughness of the inner wall of the test pipeline, and the roughness will affect the flow velocity distribution of the fluid. Therefore, it is suggested that users should pay attention to this problem. For example, the pipeline used for many years should be cleaned and descaled.
Corrosion and scaling affect the change of flow measurement value, which will vary according to the type of flowmeter. The following takes ultrasonic flowmeter and electromagnetic flowmeter as examples to illustrate the results of the influence of pipeline scaling. For example, if the inner wall of the pipeline with an inner diameter of 50mm is scaled or deposited by 0.1 ~ 0.2mm, the measured pipeline area will be reduced by 0.4% ~ 0.6%, and the resulting error will be a deviation that can not be ignored for the flowmeter with a class of 0.5 ~ 1.0.
Coefficient of compressibility
The gas compressibility coefficient Z is the ratio of the actual specific volume to the “ideal specific volume” of a certain mass of gas at the same temperature and pressure. In general, z = 0 for gas; The actual gas Z may be greater than or less than 1. The value of Z deviation from 1 indicates the degree to which the actual gas deviates from the ideal gas. The value of gas compressibility Z depends on the type or composition, temperature and pressure. Therefore, the fluid density in the working state must be obtained through the compressibility coefficient in gas measurement. If the component of the fluid is fixed, the density is calculated by temperature, pressure and compressibility. If the fluid is multi-component (such as the measurement of natural gas) and works near (or in) supercritical area, it is necessary to equip online densitometer to measure the density online.
03 installation of flowmeter
Problems needing attention during installation
The installation problem has different requirements for flow meters with different principles. For some flowmeters, such as differential pressure flowmeter and velocity flowmeter, a certain length or longer straight pipe section shall be equipped at the upstream and downstream of the flowmeter according to the regulations to ensure the full development of fluid flow in front of the inlet end of the flowmeter. While other flowmeters, such as positive displacement flowmeter and float flowmeter, have no or low requirements for the length of straight pipe section.
Some flowmeters have certain errors due to the influence of installation. For example, Coriolis mass flowmeter will bring great errors to use due to the influence of installation stress. The problems of tracing flowmeter in use may not be all due to the problems of flowmeter itself, and many conditions are caused by poor installation. Common problems are as follows:
① Reverse the inlet surface of the orifice plate of the differential pressure flowmeter;
② The flow sensor is installed in the place with poor velocity distribution profile;
③ Unwanted phase exists in the impulse piping connected to the differential pressure device;
④ The flowmeter is installed in a harmful environment or inaccessible place;
⑤ The flow direction of flowmeter is installed incorrectly;
⑥ The flowmeter or electric signal transmission line is placed under strong electromagnetic field;
⑦ Install the flowmeter easily disturbed by vibration on the pipeline with vibration;
⑧ Lack of necessary protective accessories.
During the use of the flowmeter, attention shall be paid to the adaptability and requirements of installation conditions, mainly from the following aspects, such as the installation direction of the flowmeter, the flow direction of fluid, the configuration of upstream and downstream pipelines, valve position, protective accessories, pulsating flow influence, vibration, electrical interference and the maintenance of the flowmeter.
1) Field pipeline wiring
Attention shall be paid to the installation direction of the flowmeter during on-site pipeline wiring. Since the installation direction of the flowmeter is generally divided into vertical installation mode and horizontal installation mode, there are differences in flow measurement performance between the two installation modes. For example, the vertical downward flow of fluid will bring additional force to the flowmeter sensor, affect the performance of the flowmeter, and reduce the linearity and repeatability of the flowmeter. The installation direction of the flowmeter also depends on the physical properties of the fluid. For example, solid particles may precipitate in the horizontal pipeline, so the flowmeter measuring this state is best installed in the vertical pipeline.
2) Flow direction of fluid
This problem is similar to the installation direction of the flowmeter. Because some flowmeters can only work in one direction, the reverse flow will damage the flowmeter. When using a similar flow meter, it is also considered that there may be reverse flow in case of no operation, so measures need to be taken, such as installing a check valve to protect the flowmeter. Even if the flowmeter can be used in two directions, there may be some differences in the measurement performance between the forward and reverse directions, and it should be used in accordance with the requirements specified by the manufacturer.
3) Straight pipe sections upstream and downstream of flowmeter
As the flowmeter will be affected by the flow state at the inlet of the pipeline, the pipe fittings will also introduce flow disturbance. The flow disturbance generally has vortex and velocity distribution profile distortion. The existence of vortex is generally caused by two or more spatial (three-dimensional) bends. The distortion of velocity profile is usually composed of local obstruction of pipeline fittings (such as valves) or bends. These effects need to be improved with an appropriate length of upstream straight pipe or the installation of flow regulators. In addition to considering the influence of flowmeter connecting fittings, the influence of upstream pipeline fitting combination may also be considered, because they may produce different disturbance sources, so the distance between each disturbance source must be opened as far as possible to reduce its influence. For example, a valve that is partially opened immediately after a single elbow.
A straight pipe section is also required downstream of the flowmeter to reduce the impact of downstream flow.
Positive displacement flowmeter and Coriolis mass flowmeter are not affected by asymmetric flow profile; When the turbine flowmeter is used, the vortex shall be reduced as much as possible; Electromagnetic flowmeter and differential pressure flowmeter shall limit the vortex in a very small range.
Cavitation and condensation are caused by unreasonable pipeline layout, so as to avoid sharp changes in pipeline diameter and direction. Poor pipeline layout will also produce pulsation.
4) Pipe diameter and pipe vibration
The pipe diameter range of some types of flowmeter is not very wide, so too large or too small will limit the selection of flowmeter varieties. When measuring the flow of low flow rate or high flow rate, the pipe diameter of the flowmeter with different pipe diameter can be selected, and the reducer can be used to connect to make the flowmeter operate within the specified range. If the flow exceeds the range, if the flow rate is too low, the flowmeter error will increase, and if the flow rate is too high, the flowmeter error may also increase. At the same time, the flow sensor will overspeed or the pressure drop will be too large, which will damage the use of the flowmeter.
Some flowmeters, such as vortex flowmeter of piezoelectric detection parts and Coriolis mass flowmeter, are sensitive to mechanical vibration and are easy to be disturbed by pipeline vibration. Attention should be paid to the support design on the front and rear pipelines of the flowmeter. In addition to using pulsation eliminators to eliminate pulsation effects, all installed flow meters shall be kept away from vibration or pulsation sources.
5) Installation position of valve
Control valve and isolation valve are installed in the pipeline where the flowmeter is installed. In order to avoid some flow velocity distribution disturbance and cavitation caused by the valve affecting the measurement of the flowmeter, the general control valve should be installed downstream of the flowmeter. The control valve installed downstream of the flowmeter can also increase the back pressure of the flowmeter, so as to reduce the possibility of cavitation inside the flowmeter.
The purpose of isolation valve installation is to isolate the flowmeter from the fluid of the pipeline for maintenance. The upstream valve shall be sufficiently far away from the flowmeter. When the flowmeter is running, the upstream valve shall be fully open to avoid disturbance such as flow velocity distribution distortion.
6) Protective accessories
The installation of protective accessories is a protective measure to ensure the normal operation of the flowmeter. For example, in positive displacement flowmeter and turbine flowmeter, some necessary equipment such as filter shall be installed at the upstream. The installation of all these equipment shall not affect the use of flowmeter.
7) Electrical connections and electromagnetic interference
At present, most flow measurement systems, whether the flowmeter itself or its accessory connection, have electronic equipment, so the power supply used should be matched with the flowmeter. When the output level of the flowmeter is low, the preamplifier suitable for the environment shall be used. The output signal of some types of flow meters is easy to be disturbed by high-power switching devices, so that the output pulse of the flow meter fluctuates and affects the performance of the flow meter. For example, the signal cable should be as far away from the power cable and power source as possible to reduce the impact of electromagnetic interference and RF interference.
8) Pulsating flow and unsteady flow
As mentioned earlier, for the impact of pulsation flow, in addition to using pulsation eliminators, attention should also be paid to keep all installed flowmeters away from pulsation sources. The most common sources of pulsation are hydraulic oscillations such as constant displacement pumps, reciprocating compressors, oscillating valves or regulators, vortex trains and so on. Generally, the differential pressure flowmeter has pulsating flow error, and the turbine flowmeter and vortex flowmeter will also produce pulsating flow error. Unsteady flow refers to the flow that changes with time, and slow pulsation is a special case of unsteady flow. For example, the slow pulsation caused by the operation of an oversized control valve.
The flowmeter can deal with the impact of pulsation on on the flow sensor and secondary display instrument respectively. Install the flow sensor far away from the pulsation source, or install low-pass filters such as impulse buffer (for liquid) or choke (for gas) in the pipeline system to reduce the pulsation degree. For the secondary display instrument, the flowmeter with good response characteristics (such as electromagnetic flowmeter and ultrasonic flowmeter) can be selected to increase the damping, and the pulsation parameters can be measured to estimate the additional error of pulsation.
04 requirements for environmental conditions
In the process of selecting flowmeter, the surrounding conditions and relevant changes, such as ambient temperature, humidity, safety and electrical interference, shall not be ignored:
1) Ambient temperature
The change of ambient temperature will affect the electronic part and flow sensor part of the flowmeter. For example, the change of temperature will affect the change of sensor size, change the fluid density and viscosity through the heat transfer of flowmeter shell, etc. When the ambient temperature affects the electronic components of the display instrument, the component parameters will be changed. The flow sensor and secondary display instrument should be installed in different places. For example, the secondary display instrument should be installed in the control room to ensure that the electronic components are free from the influence of temperature. It should be said that the influence of ambient temperature should not be one of the main influence quantities of uncertainty when estimating the total uncertainty of flow measurement.
2) Ambient humidity
Atmospheric humidity in the environment is also one of the problems affecting the use of flowmeter. For example, high humidity will accelerate atmospheric corrosion and electrolytic corrosion and reduce electrical insulation, and low humidity will induce static electricity. Rapid changes in ambient temperature or medium temperature will cause humidity problems, such as surface condensation.
The flowmeter shall be selected according to relevant specifications and standards to adapt to the explosive hazardous environment, and the site shall be inspected according to the explosion-proof standards.
4) Electrical interference
Power cables, motors and electrical switches will produce electromagnetic interference. If relevant measures are not taken, it will become the cause of error in flow measurement.
Consider the cost of purchasing flowmeter from the economic aspect
When purchasing a flow meter, the impact of different types of flow meters on the economy of the whole measurement system shall be compared. For example, a flowmeter with a small range needs multiple flow meters in parallel and multiple pipelines to cover the same measurement range than a flowmeter with a wide range. Therefore, in addition to the flow meters, many auxiliary equipment, such as valves and pipeline accessories, need to be added. Although on the surface, the cost of flowmeter is less, but other costs are increased, which is not cost-effective. For example, the cost of installing orifice flowmeter and differential pressure gauge is relatively cheap, but the cost of forming measurement circuit, including fixed accessories of orifice, may exceed that of basic parts.
When purchasing flow meter, not only the purchase cost of flowmeter, but also other costs, such as accessory purchase cost, installation and commissioning cost, maintenance and regular detection cost, operation cost and spare parts cost, shall be considered.
For example, many flow meters should be equipped with relatively long upstream straight pipe section to ensure their measurement performance. Therefore, the correct installation requires the arrangement of additional pipelines or the provision of bypass pipelines for regular maintenance. Therefore, the installation cost should be considered reasonably in many aspects. For example, it should also include the auxiliary costs such as stop valve and filter required for operation.
The operation cost of the flowmeter mainly refers to the energy consumption during operation, including the internal power consumption of the electric instrument or the energy consumption of the air source of the pneumatic instrument, as well as the energy consumed by pushing the fluid through the instrument in the measurement process, that is, the pumping energy consumption to overcome the pressure loss of the instrument due to measurement, etc. For example, a large part of the differential pressure generated by the differential pressure flowmeter is unrecoverable, and the positive displacement flowmeter and turbine flowmeter also have considerable resistance. Only the full channel and unimpeded electromagnetic flowmeter and ultrasonic flowmeter are basically free of this cost. Since the plug-in flowmeter is used for large pipe diameter and the blockage ratio is small, its pressure loss can also be ignored.
It is estimated that the pumping energy consumption of differential pressure orifice flowmeter with diameter of 100mm in one year is equivalent to the purchase cost of flowmeter. If electromagnetic flowmeter is replaced, the purchase cost is only equivalent to the energy consumption of differential pressure orifice flowmeter for more than 4 years. A larger share of the pumping cost is envisaged. It is generally believed that the flowmeter with low pressure loss and no pressure loss should be selected as far as possible for the flowmeter with a diameter of more than 5000mm. For example, the traditional differential pressure flowmeter used in water supply engineering rarely uses orifice plate and uses low-pressure loss venturi. Now it is updated to electromagnetic flowmeter and ultrasonic flowmeter.
The detection cost shall be determined according to the verification cycle of the flowmeter. For the detection of crude oil or refined oil generally used for trade settlement, a standard volume tube is often set on site to conduct on-line verification of the flowmeter.
Maintenance cost and spare parts cost, etc
The maintenance cost is the cost required to keep the measuring system working normally after the flowmeter is put into use, mainly including the cost of maintenance and spare parts. The flowmeter with moving parts needs more maintenance work, such as regular replacement of easily worn bearings, shafts, runners, transmission gears, etc; The flowmeter without moving parts also needs to be inspected, such as the most common orifice flowmeter with geometric measurement method.
The cost of spare parts will increase with the improvement of the performance of the flowmeter. When selecting flow meter, the purchase cost of spare parts shall be increased at the same time, especially the flowmeter imported from abroad. Sometimes the whole flowmeter will be replaced due to the difficulty of vulnerable spare parts.
06 selection of measurement method and flowmeter
Selection of slurry flow measurement
From the list of flowmeter selection, it can be found that the optional flowmeters applied to the slurry containing granular fiber include: differential pressure flowmeter, including elbow, wedge tube, electromagnetic flowmeter, Doppler ultrasonic flowmeter, vortex flowmeter, target flowmeter, Coriolis mass flowmeter, etc. According to the current use of domestic flowmeters and the measurement performance of various flowmeters, electromagnetic flowmeters are preferred for measuring slurry flow. Other flowmeters can only be selected unless the measured slurry is non-conductive or contains ferromagnetic particles, and the measurement pipeline system is not allowed to be cut off to connect to the flow sensor. It is reported that the electromagnetic flowmeter is the best for many years of application experience in measuring the flow of coal water slurry with pulverized coal content up to 65%.
The differential pressure flowmeter can be used to measure the differential pressure sensor of slurry. In addition to elbow, wedge-shaped pipe and annular pipe, circular missing orifice plate and eccentric orifice plate can also be used when there is less solid phase. Venturi tube also has examples for measurement.
Doppler ultrasonic flowmeter can be measured without cutting off the pipe and clamping ultrasonic transducer (probe) outside the pipe, but the measurement accuracy is not high.
Vortex flowmeter can only measure a small amount of powdery solids, and more solids or fibers will produce noise and cannot be used.
The target flowmeter is used for liquid flow such as heavy oil or residue containing pulverized coal, and the strain target flowmeter is adopted.
Coriolis mass flowmeter has experience in slurry measurement abroad. Generally, its straight pipe measuring pipe is suitable, but there is little application experience in China.
Selection of liquid mass flow measurement in closed pipeline
The large flow here does not refer to the “relatively large flow” when the flow rate of a certain pipe diameter is high, but the large flow of the absolute value of the flow. Since the flow rate of liquid transported by pipeline has a certain range, and the common economic flow rate of low viscosity liquid is 1 ~ 3m / s, the “large flow” measurement here refers to large pipeline flow measurement.
Generally speaking, flowmeters with pipe diameter below DN300 are called small and medium pipe diameter flowmeters, those with pipe diameter above DN300 ~ dn400 are called large pipe diameter flowmeters, and those with pipe diameter above DN1200 are called extra large pipe diameter flowmeters. Usually, the liquid flow measurement of super large pipe diameter is mainly water, and there are petroleum products in addition to water. Generally, large diameter flowmeter includes differential pressure flowmeter, electromagnetic flowmeter, ultrasonic flowmeter and plug-in flowmeter, and DN300 ~ DN500 also include volumetric flowmeter and turbine flowmeter.
1) Installation conditions
The installation conditions are mainly based on whether the measurement method is allowed to cut off the pipe flow, suspend the operation, punch holes in the pipe, and cut off the pipe flow, and install the flow sensor.
If it is allowed to cut off pipe flow and install flow sensor, electromagnetic flowmeter, ultrasonic flowmeter with measuring pipe section, positive displacement flowmeter and turbine flowmeter can be selected.
If it is allowed to drill holes in the pipeline, the ultrasonic flowmeter with external transducer and plug-in flowmeter can be selected.
If the above requirements are not allowed, only external clamped transducer ultrasonic flowmeter can be selected.
2) Measurement accuracy requirements
For non-conductive liquid with high measurement accuracy required for trade handover, ultrasonic flowmeter with measuring pipe section, multi-channel ultrasonic flowmeter, volumetric flowmeter and turbine flowmeter can be selected, and electromagnetic flowmeter can also be selected for conductive liquid.
Differential pressure venturi and ultrasonic flowmeter with external clamped transducer can be selected for controlling the proportion and requiring lower measurement accuracy. Plug in flowmeter can be selected for low measurement accuracy.
3) Pressure loss (pumping energy consumption)
The pumping energy consumption cost of large flow measurement accounts for a considerable proportion in the operation cost of flow measurement. The larger pressure loss and (pumping energy consumption cost) are differential pressure venturi, positive displacement flowmeter and turbine flowmeter. The smaller one is the plug-in flowmeter, and the one without pressure loss is the electromagnetic flowmeter.
Selection of steam flow measurement
Steam flow measurement can be divided into two types in terms of measurement technology, one is superheated steam and saturated steam with high dryness (dryness x = above 0.9), and the other is saturated steam with low dryness. The former can be treated as single-phase fluid, while the latter is two-phase flow. At present, all flowmeters are only applicable to single-phase fluid, so the saturated steam with low dryness needs to be further studied.
1) Flow measurement of superheated steam and high dryness saturated steam
The commonly used flowmeter is throttling differential pressure flowmeter, which is still the main instrument for measuring steam flow. In order to meet the needs, there has also been some technical development,. For example, the throttling device, differential pressure transmitter and three valve group are integrated into an integrated throttling flowmeter, which solves the disadvantage of easy failure of differential pressure signal pipeline. In addition, the standard orifice plate is replaced by the standard nozzle, because compared with the orifice plate, the outflow coefficient of the nozzle is stable, the outflow coefficient will not change due to the blunt edge acute angle, and the pressure loss is also lower than the orifice plate. Generally, the pressure loss is about 30% ~ 50% of the orifice plate at the same flow and value.
Vortex flowmeter measures medium temperature, i.e. below 200 ℃. It should be said that it has become mature when applied to steam. It is a kind of flowmeter commonly used in steam measurement at present. However, it must be noted that the medium with low dryness will make its instrument coefficient deviate from the detection value and increase the measurement error.
Averaging tube flowmeter and split rotor flowmeter can be used in internal management distribution with low accuracy requirements, because they are relatively cheap and concise, and are suitable for the measurement of medium and small flow steam.
For the target flowmeter, the electric and pneumatic target flow transmitter developed in China in the 1970s is the detection instrument of the electric and pneumatic unit combination instrument. At that time, the force converter directly adopted the force balance mechanism of differential pressure transmitter, so it brought many deficiencies caused by the force balance mechanism itself. For example, low measurement accuracy, zero drift, poor reliability and stability of lever mechanism, etc. Therefore, the original JJG 461-1986 specification of target flow transmitter was formulated in 1986 for 25 years. Now, electric and pneumatic target flow transmitters are basically no longer produced and used. The original regulations are no longer suitable for use, so the new regulations for target flowmeter are revised.
The structure of target flowmeter is composed of measuring tube, target plate, force sensor and signal processing unit. The force sensor is a strain gauge sensor. The signal processing display can directly read and display locally or output standard signals. The force sensor is composed of a cylindrical elastomer and a force strain gauge, which can be internal and external. When the elastomer deforms under the action of force, it destroys the balance of the electric bridge composed of force and strain gauge and produces an electrical signal squared with the flow.
Its working principle is to set a target plate perpendicular to the flow beam direction in the straight pipe section with constant section. When the fluid passes around the target plate, the target plate is subjected to thrust, and the thrust is directly proportional to the kinetic energy of the fluid and the area of the target plate. In a certain range of Reynolds number, the flow through the flowmeter is directly proportional to the force on the target plate. The force on the target plate is detected by the force sensor.
Due to the new structure and measurement principle of the new strain target flowmeter, it has a relatively superior application prospect in steam measurement and is suitable for the measurement of medium and small flow steam.
2) Flow measurement of low dryness saturated steam
The saturated steam produced by general industrial boilers is saturated steam with high dryness (above 0.95) at the outlet. However, in the process of long-distance transportation, due to poor insulation or unbalanced intermittent steam consumption and many other factors, the dryness decreases continuously, and even becomes wet steam with high water content, that is, gas and water two-phase fluid. The flow characteristics of this two-phase fluid are essentially different from that of single-phase flow. The meter coefficient or outflow coefficient of flowmeter detected in single-phase flow cannot be used for two-phase flow measurement. For example, the outflow coefficient in the two-phase flow test of orifice flowmeter must be corrected for dryness. Therefore, the dryness parameter must be measured in the flow measurement of low dryness saturated steam. Unfortunately, there is no mature dryness meter yet. In addition, the dryness correction of instrument coefficients of other types of flow meters has not been deeply studied. Only by solving this problem can the flow of saturated steam with low dryness be measured.
Post time: 13-04-22