Various flow meter selection principles and methods

First, the principle of flow meter selection principle The principle of selecting the flow meter is to first understand the knowledge of various flow meter structure principles and fluid properties, and also to select according to the specific circumstances of the scene and the surrounding environmental conditions. . Also take into account economic factors. Under normal circumstances, the main choice should be from the following five aspects: 1 flowmeter performance requirements; 2 fluid characteristics; 3 installation requirements; 4 environmental conditions; 1. Performance requirements of the flow meter The performance of the flow meter mainly includes: measurement flow (instantaneous flow) or total flow (accumulated flow); accuracy requirement; repeatability; linearity; flow range and range; pressure loss; output signal Characteristics and response time of the flowmeter. (1) The measurement of flow or total flow includes two kinds, namely instantaneous flow and cumulative flow. For example, the crude oil for the branch pipelines is continuously controlled for custody transfer or petrochemical pipelines, or the process control of the production process needs to be measured. Total amount, with or without observation of instantaneous flow. To control the flow in some workplaces requires an instantaneous flow measurement. Therefore, choose according to the needs of on-site measurement. Some flow meters such as volumetric flow meters, turbine flow meters, etc., the measurement principle is to directly obtain the total amount with mechanical counting or pulse frequency output, its accuracy is higher, and it is suitable for measuring the total amount, if equipped with a corresponding transmitting device Output flow can also be output. Electromagnetic flowmeters, ultrasonic flowmeters, etc. are based on measuring the flow rate of the fluid to derive the flow, fast response, suitable for process control, if you add the total function can also get the total amount. (2) Accuracy The accuracy of the flowmeter is specified within a certain flow range. If it is used under a certain condition or in a relatively narrow flow range, for example, only in a very small range, this time Its measurement accuracy will be higher than the specified accuracy level. Such as the use of turbine flowmeters to measure the distribution of oil barrels, the use of the valve in the case of full open, the flow is basically constant, its accuracy may increase from 0.5 to 0.25. For trade accounting, storage and handling handover, and material balance If accuracy of measurement is required to be high, the durability of the accuracy measurement should be considered. Generally used for flowmeters in the above cases, the accuracy level requirement is 0.2. In such workplaces, field measurement equipment (such as volume tubes) is usually provided on site to perform on-line inspection of the used flow meter. In recent years, due to the increasingly tight crude oil and high demand for crude oil metering by various units, the implementation of coefficient transfer is proposed for crude oil metering. That is, in addition to a one-cycle inspection of the flow meter once every six months, the parties to the trade transfer will negotiate every one or two months. Monthly inspection of the flow meter to determine the flow coefficient, according to the flow meter measurement data and flow meter flow coefficient calculated data transfer, in order to improve the accuracy of the flow meter, also known as zero error transfer. The accuracy level is generally determined based on the maximum allowable error of the flow meter. The flowmeter specifications provided by each manufacturer will be given. It must be noted that the percentage of error is relative error or reference error. The relative error is the percentage of the measured value, which is often expressed as "% R". Reference error refers to the percentage of the upper limit value or range, commonly used in "% FS". Many manufacturers do not specify the instructions. For example, floater flowmeters generally use reference errors, and some models of electromagnetic flowmeters also use quoted errors. If the flow meter is not simply a total amount, but is used in a flow control system, the accuracy of the flow meter is determined under the overall system control accuracy requirement. Because the entire system not only has the error of flow detection, it also contains errors and various influencing factors such as signal transmission, control adjustment, and operation execution. For example, there is a backlash of about 2% in the operating system, and it is uneconomical and unreasonable to determine excessively high accuracy (0.5 or more) for 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, the average velocity tube error that is designed without actual calibration is between ±2.5% and ±4%, which is accompanied by 0.2%. A differential pressure meter with a high accuracy of 0.5% does not make much sense. There is also a problem that the accuracy level specified for the flow meter in the verification protocol or the manufacturer's specification refers to the maximum allowable error of the flow meter. However, due to changes in environmental conditions, fluid flow conditions, and dynamic conditions when the flowmeter is used in the field, there will be some additional errors. Therefore, the flowmeter used in the field should be the combination of the maximum allowable error and additional error of the instrument itself. It must be fully taken into account that this problem may sometimes exceed the maximum allowable error of the flowmeter within the field's use environment. (3) Repeatability The repeatability is determined by the flowmeter principle itself and the 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 procedures, the flowmeter not only has an accuracy level specification, but also specifies the repeatability. The general rule is: the repeatability of the flowmeter must not exceed the maximum allowable error specified by the corresponding accuracy level. 1/3 to 1/5. Reproducibility is generally defined as the consistency of multiple measurements in the same direction for a given flow value in a short period of time with constant environmental conditions and media parameters. However, in practical applications, the repetitiveness of the flowmeter is often affected by changes in fluid viscosity and density parameters. Sometimes these parameter changes have not yet reached the level required for special correction, and they are mistaken for the poor repeatability of the flowmeter. . In view of this situation, flowmeters that are insensitive to this parameter change should be selected. For example, the float flow meter is easily affected by the fluid density. The small-diameter flow meter is not only affected by the fluid density, but may also be affected by the viscosity of the fluid; the viscosity of the turbine flow meter if used in the high viscosity range; some are not corrected The treated ultrasonic flow meter is affected by the temperature of the fluid and so on. If the output of the flowmeter is non-linear, this effect may be more pronounced. (4) Linearity The output of the flow meter is mainly linear and non-linear square root. In general, the non-linearity error of the flowmeter is not listed separately but is included in the error of the flowmeter. For a generally wider flow range, the output signal is pulsed and used as a totalizer flowmeter. Linearity is an important specification. If a single meter factor is used within its flow range, the linearity difference Will reduce the accuracy of the flowmeter. For example, a turbine meter uses a meter factor in a 10:1 flow rate range. When the linearity is poor, the accuracy will be lower. With the development of computer technology, the flow range can be segmented and fitted by least square method. The outflow-meter coefficient curve corrects the flowmeter to increase the accuracy of the flowmeter and the extended flow range. (5) Upper limit flow and flow range The upper limit flow is also referred to as full flow or maximum flow of the flow meter. When we choose the diameter of the flowmeter, we shall configure the flow range used by the pipeline under test and the upper and lower flow rates of the selected flowmeter. It cannot be simply matched by the pipeline diameter. In general, the maximum flow rate for designing pipeline fluids is determined by the economic flow rate. If the choice is too low and the diameter is too large, the investment will be large; if it is too high, the transmission power will be large and the operating cost will be increased. For example, low viscosity liquids such as water have an economic flow rate of 1.5 to 3 m/s, and highly viscous fluids of 0.2 to 1 m/s. Most flow meters have an upper flow rate close to or higher than the flow rate of the pipe. Therefore, when the flowmeter is selected, its caliber is the same as the pipeline, and the installation is more convenient. If they are not the same, they will not differ too much. Generally, the specifications of the upper and lower adjacent ones can be connected by different diameter pipes. In the selection of the flow meter, different types of flow meters should be noted. The upper limit flow rate or the upper limit flow rate is greatly different due to the limitations of the measurement principle and the structure of the respective flow meter. Taking a liquid flow meter as an example, the flow rate of the upper flow rate is the lowest for a glass float flow meter, which is generally 0.5-1.5 m/s, and the volume flow meter is 2.5-3.5 m/s. 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 flow rate of the liquid also needs to be considered that cavitation cannot occur due to high flow velocity. The location where cavitation occurs is generally at the position where the flow velocity is the maximum 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 positive displacement flowmeter or a float flowmeter. Differential pressure flowmeters, such as throttling device orifices, are designed and determined so that the lower limit flow rate cannot be changed, and the upper limit flow rate can be changed by adjusting the differential pressure transmitter or replacing the differential pressure transmitter. For example, some models of electromagnetic flowmeters or ultrasonic flowmeters, some users can reset their own flow upper limit. (6) Range Range The ratio is the ratio of the upper limit flow rate to the lower limit flow rate of the flow meter. The larger the value, the wider the flow rate range. Linear meters have a wide range of degrees, typically 1:10. The range of nonlinear flow meters is only 1:3. For flowmeters that are generally used for process control or custody transfer accounting, do not select a flowmeter with a small range if a wide flow range is required. At present, some manufacturers have advertised their flowmeters with a wide range of flow rates. In the operating instructions, the flow rate of the upper flow rate is very high. For example, the liquid is increased to 7 to 10m/s (generally 6m/s); the gas is increased to 50~. 75m/s (usually 40~50) m/s); in fact, such a high flow rate is not available. In fact, the key to wide range is to have a lower limit flow rate to meet the measurement needs. Therefore, a wide-range flow meter with a lower limit flow rate is more practical. (7) Pressure loss Pressure loss generally refers to the flow sensor's stationary or moving detection element set in the flow channel or changing the flow direction, resulting in unrecoverable pressure loss that varies with flow, sometimes reaching several tens of thousands. Pa. Therefore, the flow meter should be selected based on the pumping capacity of the piping system and the inlet pressure of the flowmeter to determine the allowable pressure loss of the maximum flow. Improper selection can limit the flow of fluid and produce excessive pressure loss and affect the flow efficiency. Some liquids (high vapor pressure hydrocarbon fluids) should also be aware that excessive pressure drops may cause cavitation and vaporization of the liquid phase, reducing measurement accuracy and even damaging the flow meter. For example, for a water flow meter with a pipe diameter greater than 500 mm, the pumping cost should be taken into account if the energy loss caused by the pressure loss is too large. According to relevant reports, the pressure loss of large flowmeters for measurement over the years often exceeds the purchase cost of low-pressure loss and expensive flowmeters. (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; 4 flow. Some flow meters output analog (current or voltage), and some output pulse. The analog output is generally considered to be suitable for process control and is more suitable for connection with a control valve or other control circuit unit; the pulse output is more suitable for total and highly accurate flow measurement. Long-range signal transmission pulse output has higher transmission accuracy than analog output. The mode and amplitude 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 transmission systems. (9) Response time The fluctuating flow application should pay attention to the response of the flow meter to the flow step change. Some applications require the flowmeter output to follow fluid flow changes, while others require a slower response output to obtain a composite average. Instantaneous response is often expressed as a time constant or response frequency, with the former being from a few milliseconds to a few seconds and the latter being hundreds of Hz or less. The use of display instruments can considerably increase the response time. It is generally believed that the asymmetry of the dynamic response when the flow rate is increased or decreased will accelerate the increase of the flow measurement error. Second, the fluid characteristics In the flow measurement due to a variety of flowmeter will always be affected by one or several parameters in the fluid properties, so the fluid properties will largely affect the flowmeter selection. Therefore, the selected measurement method and flowmeter must not only adapt to the properties of the fluid to be measured, but also take into account the influence of a change in one parameter of fluid properties on the other. For example, the effect of temperature changes on the viscosity of a liquid. Common fluid properties are density, viscosity, vapor pressure, and other parameters. These parameters can generally be found in the manual, evaluating the fluid parameters and the adaptability of the selected flowmeter under conditions of use. However, some physical properties cannot be found. Such as corrosive, scaling, plugging, phase change and miscibility. (1) Fluid temperature and pressure Careful analysis of the working pressure and temperature of the fluid in the flowmeter, especially when measuring the gas temperature and pressure changes resulting in excessive density changes, may require changing the selected measurement method. For example, when temperature and pressure affect the performance of flow measurement accuracy, temperature or pressure correction is required. 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 temperatures and pressures must be known exactly. Careful selection should be made when the temperature and pressure vary greatly. 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 the working condition or the temperature and pressure under the standard condition. (2) Density of fluid For liquids, the density of liquid is relatively constant in most applications. Unless the temperature changes greatly and causes large changes, density correction is generally not required. In gas applications, the range and linearity of the flowmeter depend on the gas density. Generally, it is necessary to know the values ​​under standard conditions and operating conditions for selection. There are also conversions of the flow state values ​​to some accepted reference values, which are commonly used in oil storage and transportation. Low-density gases can be difficult for some measurement methods, especially those that use gas momentum to drive the detection sensor (such as turbine flow meters). (3) Viscosity The viscosity of various liquids varies greatly, and there are significant changes due to temperature changes. The gas is different, and the difference in viscosity between various gases is relatively small, and its value is generally low. It does not change significantly due to changes in temperature and pressure. Because the viscosity of the liquid is much higher than the gas. For example, at 20°C and 100kPa, the dynamic viscosity of water is Pa·s, while the dynamic viscosity of air is Pa·s, so the liquid must consider the influence of viscosity, and the viscosity of gas is not as important as the liquid. Viscosity has different effects on various types of flowmeters. For example, for electromagnetic flowmeters, ultrasonic flowmeters, and Coriolis-type mass flowmeters, the flow values ​​are in a wide range of viscosity and can be considered to be independent of the liquid viscosity. Volume flowmeter error characteristics and viscosity related, may be slightly affected; and floater flowmeter, turbine flowmeter and vortex flowmeter, when the viscosity exceeds a certain value so that it can not be used. Some flowmeter characteristics are described using the pipeline Reynolds number as a parameter, and the pipeline Reynolds number is a function of the fluid viscosity, density, and the flow rate of the pipeline. Therefore, the viscosity has an effect on the characteristics of the meter. Viscosity is also a parameter that discriminates Newtonian fluids or non-Newtonian fluids. Most flow measurement methods and flowmeters are only suitable for Newtonian fluids. All gases are Newtonian fluids. Most liquids and liquids containing small amounts of spherical particles are also Newtonian fluids. Only applicable to Newtonian fluid measurement methods and flowmeters, if applied to non-Newtonian fluids will have an impact on the measurement. Therefore, Newtonian fluid is an important condition for the normal use of fluid flow measurement. The effect of viscosity on the range of different types of flowmeters varies. Generally, the volumetric flowmeter increases in viscosity and expands in range. Turbine flowmeters and vortex flowmeters, on the other hand, have increased viscosity and reduced range. Therefore, when evaluating the adaptability of a flow meter, it is necessary to grasp the temperature-viscosity characteristics of the liquid. Some non-Newtonian fluids (such as drilling mud, pulp, chocolate, and paint) have complex flow states and are not easy to judge. They must be careful when choosing a flow meter. (4) Chemical corrosion and scaling 1 Chemical corrosion problems Chemical corrosion of fluids can sometimes be a deciding factor in our selection of measurement methods and the use of flow meters. For example, some fluids can corrode the flowmeter in contact with parts, deposit fouling or deposit crystals on the surface, and produce electrolytic chemistry on the surface of metal parts. These phenomena can degrade the performance and lifetime of the flowmeter. Therefore, in order to solve the problems of chemical corrosion and fouling, the manufacturer has taken many methods, such as selecting anti-corrosion materials or adopting anti-corrosion measures on the structure of the flow meter. For example, the throttling device orifice plate is made of ceramic material, and the metal float flow rate The meter is lined with a corrosion-resistant engineering plastic. However, for more complicated flowmeters, such as positive displacement flowmeters and turbine flowmeters, corrosive fluids cannot be measured. Some flow meters are corrosion-resistant or easy-to-corrosion-resistant in principle. Ultrasonic flowmeter transducer probe installed on the outer wall of the pipe is not in contact with the measured fluid, and is essentially anti-corrosion. Electromagnetic flow meters only measure the lining of the tube and a pair of electrodes with simple shapes are in contact with the liquid. In recent years, some designs have electrodes that do not come into contact with the liquid. This is also an anti-corrosion measure. 2 Scaling Fouling or precipitation crystallization on the flowmeter cavity and flow sensor will reduce the clearance of the moving parts in the flowmeter and reduce the sensitivity or measurement performance of sensitive components in the flowmeter. For example, in the application of ultrasonic flowmeter fouling layer will hinder the ultrasonic emission. In the electromagnetic flowmeter application, a non-conducting scale layer insulates the electrode surface, making the flowmeter inoperable. Therefore, some flowmeters are often used in the outside of the flow sensor to prevent precipitation and crystallization or install device descaler. The result of chemical corrosion and fouling 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, users are advised to pay attention to this problem. For example, the pipeline used for many years should be cleaned and descaled. Corrosion and scaling affect the change in flow measurement varies depending on the type of flow meter. The following uses ultrasonic flowmeters and electromagnetic flowmeters as an example to illustrate the effect of pipeline fouling. For example, a pipe with an inner diameter of 50mm, with scaling or deposition of 0.1 to 0.2mm on the inner wall, will reduce the area of ​​the measurement pipeline by 0.4% to 0.6%. The resulting error will be an insignificant deviation for flowmeters of class 0.5 to 1.0. (5) Compression factor The gas compression coefficient z is a ratio of the actual specific volume to the "ideal specific volume" of a gas of a certain mass under the same temperature and pressure. In general, for the ideal gas z = 0; the actual gas z may be greater than 1 or less than 1. The value of z deviating from 1 indicates the degree to which the actual gas is biased toward the ideal gas. The z value of the gas compression factor depends on the type or composition, temperature, and pressure. Therefore, the gas measurement must obtain the fluid density under working conditions through the compression factor. If the fluid is fixed, the density is calculated by the temperature, pressure, and compression factor. If the fluid is multi-component (such as the measurement of natural gas) and works close to (or in) the supercritical area, it is necessary to measure the density online with an online densitometer. Third, the installation of the flow meter 1, the installation should pay attention to the problem of the installation of different principles of flow meter requirements are not the same. For some flowmeters, such as differential pressure flowmeters and velocity flowmeters, regulations require that a certain length or a long straight pipe section be provided upstream and downstream of the flowmeter to ensure that fluid flow at the inlet end of the flowmeter is fully developed. . Other flow meters, such as positive displacement flowmeters, float flowmeters, etc., do not require or require low straight pipe lengths. Some flowmeters are subject to certain errors due to the influence of the installation. For example, Coriolis mass flowmeters can cause great errors due to the influence of mounting stress. Retrospective flow meter problems in use may not necessarily be due to the flowmeter itself, many of the conditions are due to improper installation. The common problems are the following: 1 Reverse the flow of the inlet face of the differential pressure flowmeter orifice plate; 2 The flow sensor is installed in the place where the flow velocity profile is poor; 3 There is an undesirable pressure pipe connected to the differential pressure device The existing phase; 4 The flowmeter is installed in a hazardous or inaccessible place; 5 The flowmeter is installed in the wrong direction; 6 The flowmeter or electric signal transmission line is placed under a strong electromagnetic field;7 The flowmeter that is susceptible to vibration is installed in the On vibrating pipes; 8 Lack of necessary protective fittings. 2. Installation conditions The flow meter should pay attention to the adaptability and requirements of the installation conditions in use, mainly from the following aspects, such as the installation direction of the flowmeter, the flow direction of the fluid, the configuration of the upper and lower pipelines, the position of the valve, and the protection Accessories, pulsating flow effects, vibrations, electrical interference, and flow meter maintenance. 1 On-site Pipeline Wiring Pay attention to the installation direction of the flowmeter when piping on-site. Because the mounting direction of the flowmeter is generally divided into vertical installation mode and horizontal installation mode, there are differences in flow measurement performance between these two installation modes. For example, the downward flow of the fluid causes the flowmeter sensor to exert additional force and affect the performance of the flowmeter, resulting in a decrease in the linearity and repeatability of the flowmeter. The installation direction of the flowmeter also depends on the physical properties of the fluid. For example, the horizontal pipeline may precipitate solid particles. Therefore, it is preferable to measure the flowmeter with this status and install it in a vertical pipeline. 2 The flow direction of the fluid is similar to the installation direction of the flowmeter. Because some flowmeters can only work in one direction, reverse flow can damage the flowmeter. When using similar flowmeters, it is also considered that reverse flow may occur when there is no operation, and it is necessary to take measures such as installing a check valve to protect the flowmeter. Even if the flow meter can be used in both directions, the measurement performance between the forward and reverse directions may be somewhat different and should be used in accordance with the requirements specified by the manufacturer. 3 Flowmeter upstream and downstream straight pipe sections Because the flowmeter will be affected by the pipe inlet flow state, pipe fittings will also introduce flow disturbances. Flow disturbances generally have vortex and flow profile distortions. Vortex presence is generally due to two or two More than one space (stereoscopic) bend caused. Distortion of the velocity profile is usually made up of partial obstructions (such as valves) or elbows. These effects need to be improved with an appropriate length of upstream straight pipe sections or installation of flow regulators. In addition to considering the effects of flow meter attachments, it may also be necessary to consider the effects of the upstream piping assembly because they may produce different sources of disturbances, so it is important to pull the distance between disturbance sources as much as possible to reduce their impact. For example, like a partially bent valve followed by a partially open valve. Downstream of the flow meter also requires a straight section to reduce downstream flow effects. For volumetric flowmeters and Coriolis mass flowmeters, it is unlikely that they will be affected by asymmetrical flow profiles; Turbine flowmeters should minimize vortices when they are used; electromagnetic and differential pressure flowmeters should limit vortices to small values. Within range. Cavitation and condensation are caused by irrational piping arrangements and avoid sharp changes in pipe diameter and direction. Poor piping can also produce pulsation. 4 Pipe Diameter and Pipe Vibration Some types of flowmeters do not have a wide range of pipe diameters, so being too large or too small will limit the choice of flow meter type. To measure the flow rate at low flow rate or high flow rate, you can select the flowmeter pipe diameter that is different from the pipe diameter. You can use different diameter pipe connection to make the flowmeter operate within the specified range. If the flow rate exceeds the range and the flow rate is too low, the increase in flowmeter error will not work. If the flow rate is too high, the flowmeter error may increase. At the same time, the flow sensor may overspeed or pressure drop too much and damage the use of the flowmeter. Some flowmeters such as vortex flowmeters and Coriolis mass flowmeters for piezoelectric sensing devices are sensitive to mechanical vibration and are prone to interference with pipeline vibrations. Attention should be paid to the design of the front and rear pipelines of the flowmeter. For the elimination of pulsation effects using a pulse eliminator, it is also important to keep all installed flow meters away from sources of vibration or pulsation. 5 Valve installation position The control valve and isolation valve are installed in the pipeline where the flowmeter is installed. In order to avoid flow velocity disturbances and cavitation caused by the valve and affect the flowmeter measurement, the general control valve should be installed downstream of the flowmeter. Installing downstream of the flowmeter also increases the backpressure of the flowmeter, making it easier to reduce the possibility of cavitation inside the flowmeter. The purpose of isolation valve installation is to isolate the flowmeter from the pipeline for easy maintenance. The upstream valve should be a sufficient distance from the flowmeter. When the flowmeter is operating, the upstream valve should be fully open to avoid disturbances such as distorted flow distribution. 6 Protective fittings The installation of protective fittings is a protective measure to ensure the normal operation of the flowmeter. For example, positive displacement flowmeters and turbine flowmeters generally require installation of filters and other necessary equipment upstream. The installation of all these devices must be done without affecting the use of the flowmeter. 7 Electrical Connections and Electromagnetic Interference Most current flow measurement systems, whether they are the flow meter itself or its accessories, have electronic equipment, so the power supply used must be compatible with the flow meter. When the output level of the flow meter is low, a preamplifier that is suitable for the environment should be used. The output signals of some types of flowmeters are easily interfered by high-power switchgears, which can cause the output pulse of the flowmeter to fluctuate and affect the performance of the flowmeters. For example, signal cables should be kept away from power cables and power sources as much as possible to reduce electromagnetic interference and radio frequency interference. influences. 8 Pulsation and unsteady flow In addition to the use of pulsation eliminators, it has been noted that the effect on pulsating flow should also be taken to keep all mounted flow meters away from the pulsation source. The most common sources of pulsation are hydrodynamic oscillations such as fixed displacement pumps, reciprocating compressors, oscillating valves or regulators, and vortex columns. In general, differential pressure flowmeters have pulsating flow errors. Turbine flowmeters and vortex flowmeters also produce pulsating flow errors. Unsteady flow refers to flow that changes over 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 separately handle the pulsation effects of the flow sensor and the secondary display instrument. Where the flow sensor is installed away from the pulsating source, a low-pass filter such as a gas-filled buffer (for liquids) or a choke (for gas) can also be installed in the piping system to reduce the pulsation. For secondary display instruments, the flowmeters with good response characteristics (such as electromagnetic flowmeters and ultrasonic flowmeters) can be used to increase the damping, and the pulsation parameters can be used to estimate the additional error of pulsation. Fourth, the environmental conditions in the process of selecting the flowmeter should not ignore the surrounding conditions and related changes, such as ambient temperature, humidity, safety and electrical interference, etc., 1 ambient temperature, ambient temperature changes will affect the flow meter electronic parts and flow Sensor section. For example, temperature changes can affect the size of the sensor, change the fluid density and viscosity through heat transfer through the flow meter housing. When the ambient temperature affects the display meter electronics, the component parameters will be changed. The flow sensor and the secondary display instrument should be installed in different places, like the secondary display instrument should be installed in the control room to ensure that the electronic components are not affected by temperature. It should be said that when the influence of ambient temperature is used to estimate the total uncertainty of flow measurement, the impact should not be one of the major influences of uncertainty. 2 Ambient humidity in the environment is also one of the issues affecting the use of the flowmeter. For example, high humidity can accelerate atmospheric corrosion and electrolytic corrosion and reduce electrical insulation. Low humidity can cause static electricity. Sharp changes in ambient temperature or medium temperature can cause humidity problems, such as surface condensation. 3 Safety The flow meter should be selected in accordance with the relevant regulations and standards to suit the use in explosive hazardous environments. The requirements of the site are in accordance with the explosion protection standards. 4 Electrical disturbances Power cables, motors, and electrical switches all generate electromagnetic interference. If no measures are taken, this can be a cause of errors in the flow measurement. Fifth, the economic considerations 1, from the economic considerations to purchase the cost of flow meter When purchasing the flow meter should compare the impact of different types of flow meter on the economics of the entire measurement system. For example, a flowmeter with a smaller range than a flowmeter with a wider range requires more than one flowmeter to be connected in parallel and multiple lines in the same measurement range. Therefore, in addition to the flowmeter, many additional auxiliary devices such as valves and pipeline accessories need to be added. Wait. Although on the surface the flow meter costs less, other costs increase, making it uneconomical to calculate. For example, the installation of orifice meter plus differential pressure meter is relatively cheap, but the measurement loop including the fixed attachment of the orifice may cost more than the cost of the basic part 2. When installing the flow meter, the installation cost must not only consider the flow meter. In addition to the purchase fee, other costs such as attachment purchase costs, installation and commissioning fees, maintenance and periodic inspection fees, operating expenses, and spare parts fees must also be considered. For example, many flow meters should be equipped with a relatively long upstream straight pipe section to ensure its measurement performance. Therefore, proper installation requires additional piping or bypass piping for regular maintenance. Therefore, the installation fee should be considered in a reasonable number of ways, for example, it should also include the auxiliary costs such as stop valves and filters required for operation. 3, operating costs Flowmeter operating costs are mainly energy consumption at work, including the internal power consumption of the electric meter or the air source energy consumption of the pneumatic instrument and the energy consumed to drive the fluid through the meter during the measurement process, that is to overcome the meter due to measurement Pressure loss pumping energy costs. For example, differential pressure produced by differential pressure flowmeters, a large part of which cannot be recovered, positive displacement flowmeters and turbine flowmeters also have considerable resistance. Only the full-channel, unobstructed electromagnetic flowmeters and ultrasonic flowmeters basically have zero cost. The plug-in type flowmeters, because they are used for small diameters, have a small pressure loss, which can also be ignored. It is estimated that the 100-mm diameter differential pressure orifice meter 1 pump energy consumption and flow meter purchase fee is equivalent, if you use electromagnetic flowmeter, the purchase fee is only equivalent to more than 4 years differential pressure orifice meter The energy consumption fee. It is envisaged that the larger pumping energy costs will account for more of the share. It is generally believed that flowmeters exceeding 5000mm should use low-pressure loss and no-loss flowmeters as much as possible. For example, in the water supply project, the traditional differential pressure flowmeter rarely uses an orifice plate and a low pressure loss venturi tube, and now it is updated to an electromagnetic flowmeter and an ultrasonic flowmeter. Pumping energy consumption costs are calculated by the following formula: Annual pump energy consumption = yuan liquid: k Gas: k type, - power loss, k, set pump (or compressor) group efficiency is 80%; - year Hours of operation, h; - Electricity price, yuan k / h; - Unrecoverable pressure loss, Pa; - Liquid flow, m/h; - Standard state gas flow, m/min; - Liquid relative density; - Gas temperature, K; - Gas absolute pressure, Pa. 4. The cost of testing and testing shall be determined according to the verification period of the flow meter. The detection of crude oil or refined oil, which is generally used for trade settlement, is often set on site by a standard volumetric tube to perform on-line verification of the flowmeter. 5, maintenance costs and spare parts costs and other maintenance costs for the flowmeter to maintain the cost of normal operation of the measurement system, including maintenance and spare parts costs. Flowmeters with moving parts require more maintenance work, such as the frequent exchange of wear-resistant bearings, shafts, runners, transmission gears, etc.; flowmeters without moving parts also need to be inspected, such as the most common use of geometric measurements to check the hole Plate flow meter. Spare parts costs increase as the performance of the flowmeter increases. When selecting a flow meter, consideration should be given to increasing the purchase cost of the spare parts at the same time. In particular, flow meters imported from abroad may sometimes replace the entire flow meter due to the difficulty of wearing spare parts. Sixth, the measurement method and the choice of flow meter The last few sections are all about the problem of general flow meter selection. In this section, for example, the measurement of the flow rate of the slurry, the large liquid flow, and the flow rate of the steam flow meter are selected. 1. Selection of slurry flow measurement From the flow meter selection list, the available flowmeters for particle-containing fiber slurry can be found out: Differential pressure flowmeters include elbows, wedge-type tubes, electromagnetic flowmeters, and more. Puller ultrasonic flowmeter, vortex flowmeter, target flowmeter, Coriolis mass flowmeter, etc. According to the current situation of the use of domestic flowmeters and the measurement performance of various flowmeters, electromagnetic flowmeters are preferred for measuring the flow rate of the slurry unless the measured slurry is non-conductive or contains ferromagnetic particles, and the measurement piping system is not allowed to cut off. Connect the flow sensor before selecting another flow meter. It has been reported that many years of experience in the measurement of pulverized coal as high as 65% of the coal-water slurry flow rate suggests that the electromagnetic flowmeter is still the best. Differential pressure flowmeters can be used to measure differential pressure sensors in slurries except for elbows, wedges, and loops. When there are few solid phases, circular orifices, eccentric orifices, and Venturis can also be used for measurement. . The Doppler ultrasonic flowmeter can measure the ultrasonic transducer (probe) outside the tube without cutting the tube, but the measurement accuracy is not high. Vortex flowmeters can only measure a small amount of powdered solids, and the solid content or fiber-like noise can be used. The target flowmeter is used for the flow of heavy oil or residual oil containing pulverized coal, and is a strain-type target flowmeter. Coriolis mass flowmeters have been used in slurry measurement in foreign countries. Generally, they are suitable for their straight-tube measuring tubes, but their domestic application experience is limited. 2. The choice of large flow measurement for closed pipeline liquids The large flow rate mentioned here does not refer to the "relatively large flow rate" when the flow velocity of a certain pipe diameter is high, but to the flow of an absolute value. Since the flow rate of the liquid in the pipeline has a certain range, the commonly used economic flow rate of the low-viscosity liquid is 1~3m/s. Therefore, the “large flow” measurement here refers to the measurement of large flow. In general, the flowmeters with diameters below DN300 are called small and medium diameter flowmeters, those with diameters above DN300~DN400 are called large diameter flowmeters, and those with diameters above DN1200 are called extra large diameter flowmeters. In general, the measurement of liquid flow in extra large pipe diameters is mainly water, and there are petroleum products other than water. In general, there are differential pressure flowmeters, electromagnetic flowmeters, ultrasonic flowmeters, and plug-in flowmeters for large diameter flowmeters, and positive displacement flowmeters and turbine flowmeters for DN300~DN500. (1) Installation conditions Installation conditions are mainly based on whether the measurement method can allow the flow of the pipe to be cut off, the operation is suspended, whether it is permissible to make a hole in the pipe, and whether to allow the pipe flow to be installed on the flow sensor. If you are allowed to cut off the flow and install the flow sensor, you can choose the electromagnetic flowmeter, ultrasonic flowmeter with measuring tube segment, positive displacement flowmeter and turbine flowmeter. If it is permissible to drill a hole in the pipe, an external transducer ultrasonic flowmeter and plug-in flowmeter can be selected. If the above requirements are not allowed, you can only choose the external clamp transducer ultrasonic flowmeter. (2) Measurement Accuracy Requirements For custody transfer requirements, high-accuracy, non-conductive fluids can be selected with ultrasonic flow meters, multi-channel ultrasonic flow meters, positive displacement flow meters, and turbine flow meters with non-conductive liquids. Is a conductive liquid can also choose electromagnetic flowmeter. For the control ratio, the differential pressure type venturi and the external clamp transducer ultrasonic flowmeter with lower measurement accuracy are required. Optional plug-in flowmeter with low measurement accuracy requirements. (3) Pressure loss (pumping energy cost) The pumping energy consumption cost of large flow measurement occupies a considerable proportion of the flow measurement operating cost, and the pressure loss and the (pumping energy consumption cost), for example, the larger is the differential pressure. Venturi tube, positive displacement flowmeter and turbine flowmeter. The smaller ones are plug-in type flow meters, and there is no pressure loss for the electromagnetic flowmeter. 3. Selection of steam flow measurement The steam flow measurement is divided into two types from the measurement technology. One is superheated steam and saturated steam (dryness x = 0.9 or higher), and the other is low-saturated saturated steam. The former can be treated as a single-phase fluid while the latter is a two-phase flow. Since all current flowmeters are only suitable for single-phase fluids, deep-saturated steam needs further research. (1) flow rate measurement of superheated steam and high-saturated saturated steam Flowmeters commonly used are: throttling differential pressure flowmeters, which are still the main instruments for measuring steam flow, and have technically Experience development,. For example, throttling device, differential pressure transmitter, and three-valve unit are integrated into an integrated throttling flowmeter. The throttling flowmeter solves the defect that differential pressure signal lines are prone to failure. There is also the use of a throttling throttling device, which uses a standard nozzle instead of a standard orifice because the nozzle has an outflow coefficient that is comparable to that of the orifice plate. It does not change the outflow coefficient due to blunt acute edges, and the pressure loss is lower than that of the orifice plate. Generally, the pressure loss is about 30% to 50% of the orifice plate at the same flow rate and value. Vortex flowmeters measure medium temperatures, ie, below 200°C, and should be said to have matured when applied to steam, which is a type of flowmeter commonly used for steam measurement. However, it must be noted that low-dryness media will cause its meter factor to deviate from the detection value and increase the measurement error. Even flow tube flowmeters, split-rotor flowmeters can be used for internal management distributions where accuracy is not as critical, because they are cheaper, simpler, and suitable for measuring small to medium-volume steam. For target flowmeters, domestic electric and pneumatic target flow transmitters were developed in the 70s of the last century. They are instrumentation instruments for electric and pneumatic unit instrument clusters. Since the force converter directly used the force balance mechanism of the differential pressure transmitter at the time, it brought many deficiencies caused by the force balance mechanism itself. For example, low measurement accuracy, zero drift, reliability of the lever mechanism, poor stability, etc. Therefore, the original JJG 461-1986 "Target Flow Transmitter" was established in 1986, has been 25 years. Due to the fact that electric and pneumatic target flow transmitters are no longer produced and used. The original procedure was not suitable for use, so the new target flowmeter procedure was revised. The structure of the target flowmeter consists of a measuring tube, a target plate, a force sensor, and a signal processing unit. The force sensor is a strain gauge type sensor, and the signal processing display can directly display or output a standard signal on the spot. The force sensor is composed of a barrel type elastic body and a force strain gauge, and may be of two types, namely an inner mount type and an outer type mount type. When the elastic body is deformed under the force, it breaks the balance of the bridge formed by the force strain gauges and generates an electrical signal that is in quadratic relation to the flow. The working principle is to set a target plate perpendicular to the flow beam direction in a constant section straight pipe section. When the fluid passes around the target plate, the target plate is subjected to the thrust force. The magnitude of the thrust force is equal to the kinetic energy of the fluid and the area of ​​the target plate. Proportional. In a certain Reynolds number range, the flow through the flowmeter is directly proportional to the force on the target plate. The force on the target is detected by the force sensor. Taking a circular target as an example, the basic formula for flow calculation is: where - mass flow (kg/s); - volumetric flow (m/s); - flow coefficient (pure number); - flow Beam expansion coefficient (pure number). For incompressible fluid = 1, for compressible fluid <1; - measuring tube inner diameter (m); - target diameter (m); - fluid density (kg/m); - target force (N). The target force transducer is converted into a current signal (4-20 mA) or a pressure signal (20-100 kPa). The relationship between the output signal and the flow rate can be determined according to the above formula. Due to the new structure and measurement principle of the new strain gauge flowmeter, it has a superior use prospect in steam measurement and is suitable for the measurement of medium and small flow steam. (2) Flow rate measurement of low-saturated saturated steam The saturated steam produced by a typical industrial boiler is saturated steam (above 0.95) saturated at the outlet, but during long-distance transportation, heat insulation is poor or intermittent steam is used. Many factors, such as the occurrence of unbalanced conditions, have caused the continuous decline of the dryness, and even become the wet steam with high moisture content, that is, it has become a gas and water two-phase fluid. The flow characteristics of these two-phase fluids are essentially different from single-phase flow.在单相流中检测的流量计仪表系数或流出系数是不能用于两相流测量的。比如对孔板流量计进行的两相流试验中的流出系数必须进行干度修正。因此，在低干度饱和蒸汽的流量测量中，干度参数是必须测量的一个参数。遗憾的是目前还没有成熟的干度计出现。另外其他各类流量计的仪表系数的干度修正都尚未进行深入研究。只有解决这个问题，才能测量低干度饱和蒸汽的流量。表1 流量计流体特性和工艺过程条件选型一览表流体特性和工艺过程条件液体气体符号说明：流体特性液体工艺过程条件★ 最适用清洁赃污含颗粒纤维腐蚀性浆腐蚀性粘性非牛顿流体液液混合液气混合高温低温小流量大流量脉动流一般小流量大流量腐蚀性高温蒸汽● 通常适用◆在一定条件下适用× 不适用名称差压式孔板★★××●★◆★●★★●★◆★●●●★★喷嘴★◆××●●◆★●★◆×★◆★●●●★★文丘里管★●××●●◆★●★◆×★◆★●●●★★弯管★●●●●×◆★●◆◆◆★◆★××●◆◆楔型管★★★★●★◆★●●●××◆××××××均速管★×××●××★●★★×★◆★×★●★★浮子式玻璃管★×××●●×××××★××★★×●××金属锥管★×××●●×××●×★◆◆★★◆●×★容积式椭圆齿轮★××××●×●×◆×★××××××××腰 ★××××●×●×◆××★×★×××××刮板★××××● ●××××××××××××膜式××××××××××××××★★××××涡轮式★××××◆●●×★×★××★×××××电磁式★★★★★★★★◆×◆★★★××××××振荡式涡街★●●×◆××●×★★×××★×★×★★旋进旋涡★×××××××××××××★×××××超声式多普勒法★★★★★×●★◆××●××××××××传播时间法★××××◆◆●×◆◆×★★◆×★◆×★靶式★★×××◆◆★●◆×××◆××××××热式×××××××××★××★×××××××科式力质量式★★●××★★●◆◆◆××◆◆××◆××插入式（涡轮）（涡街）（电磁）★——×———————×★—★×★×××1、—：取决于测量头的类型；2、用于赃污的孔板为圆缺孔板；3、用于粘性的孔板为四分之一圆孔板和锥型入口孔板。表2 流量计测量性能、安装条件选型一览表符号说明：测量性能安装条件★ 最适用准确度最低雷诺数范围度压力损失输出特性高准确度流量适用性高准确度总量适用性公称通径范围（mm）传感器安装方位和流动方向上游直管段长度要求● 通常适用◆在一定条件下适用× 不适用名称差压式孔板中2×10小中～大SR◆×50～1000任意短～长喷嘴中1×10小小～中SR◆×50～500任意短～长文丘里管中7.5×10小小SR◆×50～1200(1400)任意短～中弯管低1×10小小SR××＞25任意短～中楔型管中～低5×10小中SR××25～300任意短～中均速管低10小小SR××＞25任意短～中浮子式玻璃锥管低～中10中中L××1.5～100垂直从上到下无金属锥管中10中中L◆×10～150无容积式椭圆齿轮中～高10中大L×★ 6～250水平或垂直无腰轮中～高10中大L×★15～500无刮板中～高10中液大气小L×★25～300无膜式中2.5×10大小SR×●15～100水平无涡轮式中～高10低～中中L★★10～500水平短～中电磁式中～高中～大无L★★6～3000任意无～中流体振荡(旋涡)式涡街中无限制小～大小～中L◆×50～任意中～长旋进旋涡中2×10中～大中L◆×50～150任意很短超声式多普勒法低5×10中～大无L××＞100(25)任意短～长传播时间法中5×10小～中无L◆◆＞25任意短～中靶式低～中2×10小中L◆×15～200任意短～中热式中10中小××4～30任意无～中科式力质量式高无数据中～大中-很大L★●6～15水平或任意无插入式（涡轮）（涡街）（电磁）低无数据取决于测量头小L××＞100取决于测量头中～长

Circal Arc Rigde Cap Roll Forming Machine is a kind of Ridge Cap Forming Machine. The rigde cap is alike arc-shaped. It can be used for glazed tile roofing and normal roofing. Glazed ridge cap making machine makes the ridge cap for the glazed roofing. It is matched with the glazed roofing making machinewe will give you bigger discounts, if you buy it with the roofing roll forming machine.

Circal Arc Rigde Cap Roll Forming Machine

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