The principle of flowmeter selection:
The principle of choosing a flowmeter is first to have a deep understanding of the structural principles and fluid characteristics of various flowmeters, and at the same time, choose according to the specific conditions of the site and the surrounding environmental conditions. Economic factors must also be taken into account. In general, you should choose from the following five aspects:
① Performance requirements of the flowmeter;
② fluid characteristics;
③ Installation requirements;
④ Environmental conditions;
⑤ The price of the flow meter.
1. Performance requirements of the flowmeter
The performance aspects of the flowmeter mainly include: measured flow (instantaneous flow) or total amount (cumulative flow); accuracy requirements; repeatability; linearity; flow range and range; pressure loss; output signal characteristics and flowmeter response time Wait.
(1) Measure flow or total amount
There are two types of flow measurement, namely instantaneous flow and cumulative flow. For example, the crude oil in the pipeline of the sub-transportation station belongs to the custody transfer or the petrochemical pipeline for continuous proportional production or process control of the production process, etc. The total amount needs to be measured, sometimes supplemented by the instantaneous flow observation. In some workplaces, flow control requires instantaneous flow measurement. Therefore, the choice should be made according to the needs of on-site measurement. Some flowmeters such as positive displacement flowmeters, turbine flowmeters, etc., the measurement principle is to directly obtain the total amount by mechanical counting or pulse frequency output, which has high accuracy and is suitable for measuring the total amount, if equipped with a corresponding signaling device Flow can also be output. Electromagnetic flowmeters, ultrasonic flowmeters, etc. deduce the flow rate by measuring the flow rate of the fluid, with fast response, suitable for process control, and the total amount can also be obtained with the accumulation function.
(2) Accuracy
The accuracy level of the flowmeter is specified within a certain flow range. If it is used under a certain condition or within a relatively narrow flow range, for example, it only changes within a small range, then its measurement accuracy will be reduced. higher than the specified accuracy class. If a turbine flowmeter is used to measure oil in barrels and distribution, when the valve is fully opened, the flow rate is basically constant, and its accuracy may be improved from 0.5 to 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 the accuracy measurement should be considered. Generally, the flowmeter is used in the above cases, and the accuracy level is required to be 0.2. In such workplaces, measurement standard equipment (such as volume tubes) is generally equipped on site to conduct online detection of the flowmeters used. In recent years, due to the increasing tension of crude oil and the high requirements of various units for crude oil measurement, the implementation of coefficient handover for crude oil measurement is proposed, that is, in addition to the periodic inspection of the flowmeter every six months, the two parties to the trade handover negotiate every 1 month or 2. The flowmeter is verified monthly to determine the flow coefficient, and the data is calculated according to the data measured by the flowmeter and the flow coefficient of the flowmeter for handover to improve the accuracy of the flowmeter, also known as zero-error handover.
The accuracy level is generally determined according to the allowable error of the flowmeter. It is given in the flowmeter instructions provided by each manufacturer. It is important to note whether the percentage of error refers to relative error or citation error. The relative error is the percentage of the measured value, usually expressed as "% R". The reference error refers to the upper limit of the measurement or the percentage of the range, commonly used as "% FS". Many manufacturer's instructions do not indicate this. For example, float flowmeters generally use reference errors, and some models of electromagnetic flowmeters also use reference errors.
If the flowmeter is not simply measuring the total amount, but is used in the flow control system, the accuracy of the detection flowmeter should be determined under the control accuracy requirements of the entire system. Because the whole system not only has the error of flow detection, but also includes the error and various influencing factors of signal transmission, control adjustment, operation execution and so on. For example, if there is a hysteresis difference of about 2% in the operating system, it is uneconomical and unreasonable to determine an excessively high accuracy (above level 0.5) for the measuring instrument used. As far as the instrument itself is concerned, the accuracy between the sensor and the secondary instrument should also be properly matched. For example, the designed error of the average velocity tube without actual calibration is between ±2.5% and ±4%, with 0.2% A differential pressure gauge with a high accuracy of ~0.5% is of little significance.
Another problem is that the accuracy level specified for the flowmeter in the verification procedures or the manufacturer's manual refers to the allowable error of the flowmeter. However, due to the influence of changes in environmental conditions, fluid flow conditions and dynamic conditions when the flowmeter is used in the field, some additional errors will occur. Therefore, the flowmeter used in the field should be a combination of the allowable error and additional error of the instrument itself. This problem must be fully considered. Sometimes the error within the range of the use environment on site may exceed the allowable error of the flowmeter.
(3) Repeatability
Repeatability is determined by the principle of the flowmeter 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 regulations, not only the accuracy level is specified for the flowmeter, but also the repeatability is specified. /3～1/5.
Repeatability is generally defined as the consistency of multiple measurements in the same direction for a certain flow value within a short period of time under the condition that environmental conditions and medium parameters remain unchanged. However, in practical applications, the repeatability of the flowmeter is often affected by changes in the fluid viscosity and density parameters. Sometimes the changes in these parameters have not reached the level that requires special correction, which may be mistaken for poor repeatability of the flowmeter. . In view of this situation, a flowmeter that is not sensitive to changes in this parameter should be selected. For example, the rotameter is easily affected by the fluid density. Small-diameter flowmeters are not only affected by the fluid density, but may also be affected by the fluid viscosity; if the turbine flowmeter is used in a high viscosity range, the viscosity is affected; some have not been corrected. Processed ultrasonic flow meters are affected by fluid temperature and more. This effect may be more pronounced if the output of the flowmeter is non-linear.
(4) Linearity
The output of the flowmeter mainly has two kinds of linear and nonlinear square root. Generally speaking, the nonlinear error of the flowmeter is not listed separately, but is included in the error of the flowmeter. For a flowmeter with a generally wide flow range, the output signal is pulsed and used for total accumulation, linearity is an important technical indicator. If a single meter coefficient is used within its flow range, when the linearity is poor, will reduce the accuracy of the flowmeter. For example, a turbine flowmeter adopts a meter coefficient in the flow range of 10:1, and its accuracy will be lower when the linearity is poor. With the development of computer technology, its flow range can be divided into segments and fitted by the square method. The flow-meter coefficient curve corrects the flowmeter, thereby improving the accuracy of the flowmeter and extending the flow range.
(5) Upper limit flow and flow range
The upper flow is also known as the full-scale flow or flow of the flowmeter. When we choose the diameter of the flowmeter, it should be configured according to the flow range used by the pipeline to be tested and the upper and lower flow rates of the selected flowmeter. It cannot be simply matched according to the diameter of the pipeline.
Generally speaking, the design pipeline fluid flow rate is determined according to the economic flow rate. If the selection is too low, the diameter of the pipe will be thick, and the investment will be large; if the selection is too high, the transmission power will be large and the operating cost will be increased. For example, the economic flow rate of low-viscosity liquids such as water is 1.5-3m/s, and the high-viscosity liquids are 0.2-1m/s. The flow rate of the upper flow rate 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, and the installation is more convenient. If they are not the same, there will not be too much difference. Generally, the specifications of the upper and lower adjacent gears can be connected by reducing pipes.
In the selection of flowmeters, attention should be paid to different types of flowmeters, whose upper limit flow rate or upper limit flow rate is greatly different due to the limitation of the measurement principle and structure of their respective flowmeters. Taking a liquid flowmeter as an example, the flow rate of the upper limit flow is generally between 0.5 and 1.5m/s for a glass float flowmeter, between 2.5 and 3.5m/s for a positive displacement flowmeter, and between 5.5 and 3.5m/s for a vortex flowmeter. Between 7.5m/s, the electromagnetic flowmeter is between 1 and 7m/s, or even between 0.5 and 10m/s.
The upper limit flow rate of the liquid also needs to consider that the cavitation phenomenon cannot be generated because the flow rate is too high. The location of the cavitation phenomenon is generally the position of the flow rate and static pressure. In order to prevent the formation of cavitation, it is often necessary to control the back pressure of the flowmeter ( 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 rotameter. Once the differential pressure flowmeter, such as the orifice plate of the throttling device, has been designed and determined, its lower limit flow rate cannot be changed, and the upper limit flow rate change can be changed by adjusting the differential pressure transmitter or replacing the differential pressure transmitter. For example, for some models of electromagnetic flowmeters or ultrasonic flowmeters, some users can reset the upper limit of the flow by themselves.
(6) Range degree
The range degree is the ratio of the upper limit flow rate and the lower limit flow rate of the flowmeter. The larger the value, the wider the flow range. Linear meters have a wide range, generally 1:10. The range of nonlinear flowmeters is only 1:3. For flow meters generally used for process control or custody transfer accounting, if a wide flow range is required, do not choose a flow meter with a small range.
At present, in order to promote the wide flow range of their flowmeters, some manufacturers have increased the flow rate of the upper limit flow rate very high in the instruction manual, for example, the liquid is increased to 7-10m/s (usually 6m/s); the gas is increased to 50- 75m/s (usually 40~50)m/s); in fact, such a high flow rate is unusable. In fact, the key to a wide range is to have a lower lower limit flow rate to meet the measurement needs. Therefore, a wide-range flowmeter with a low lower limit flow rate is more practical.
(7) Pressure loss
Pressure loss generally means that the flow sensor produces an irrecoverable pressure loss that varies with the flow due to static or active detection elements set in the flow channel or changes in the flow direction, and its value can sometimes reach tens of kilopascals. Therefore, the flowmeter should be selected according to the allowable pressure loss of the flow rate determined by the pumping capacity of the pipeline system and the inlet pressure of the flowmeter. Improper selection will limit the fluid flow and cause excessive pressure loss and affect the flow efficiency. Some liquids (high vapor pressure hydrocarbon liquids) should also be aware that excessive pressure drop may cause cavitation and vaporization of the liquid phase, reducing measurement accuracy or even damaging the flowmeter. For example, a flowmeter for water delivery with a pipe diameter greater than 500mm should consider the increased pumping cost caused by excessive energy loss caused by pressure loss. According to relevant reports, the pumping cost of a flowmeter with a larger pressure loss for measurement often exceeds the purchase cost of a flowmeter with a low pressure loss and a higher price.
(8) Output signal characteristics
The output and display volume of the flowmeter can be divided into:
① Flow (volume flow or mass flow); ② Total; ③ Average flow rate; ④ Point flow rate. Some flow meters output analog quantities (current or voltage), while others output pulse quantities. Analog output is generally considered to be suitable for process control, and is more suitable for connection with control loop units such as regulating valves; pulse output is more suitable for total and high-accuracy flow measurement. Long-distance signal transmission pulse output has higher transmission accuracy than analog output. The mode and amplitude of the output signal should also have the ability to adapt to other equipment, such as control interfaces, data processors, alarm devices, open circuit protection circuits and data transmission systems.
(9) Response time
When applied to pulsating flow applications, attention should be paid to the flowmeter's response to a flow step change. Some applications require the flowmeter output to follow the fluid flow, while others require a slower response output to obtain a composite average. Transient responses are often expressed in terms of time constants or response frequencies, the former ranging from a few milliseconds to a few seconds, and the latter below hundreds of Hz. The use of a display instrument may considerably extend the response time. It is generally believed that the asymmetry of the dynamic response of the flowmeter when the flow rate increases or decreases will accelerate the increase in flow measurement error.
2. Fluid characteristics
In flow measurement, various flowmeters are always affected by one or several parameters in the physical properties of the fluid, so the physical properties of the fluid will largely affect the selection of the flowmeter. Therefore, the selected measurement method and flowmeter should not only adapt to the properties of the fluid to be measured, but also consider the influence of a change in the physical properties of the fluid on another parameter during the measurement process. For example, the effect of temperature changes on the viscosity of liquids.
Common fluid properties are density, viscosity, vapor pressure and other parameters. These parameters can generally be found in the manual to evaluate the adaptability of various parameters of the fluid and the selection of flowmeters under the conditions of use. But there are also some properties that cannot be found. Such as corrosion, scaling, plugging, phase transition and miscible state.
(1) The temperature and pressure of the fluid
Carefully analyze the working pressure and temperature of the fluid in the flowmeter, especially when measuring the gas, the temperature and pressure changes cause excessive density changes, and the selected measurement method may be changed. For example, when temperature and pressure affect performance such as flow measurement accuracy, temperature or pressure corrections should be made. In addition, the structural strength design and material of the flowmeter housing also depend on the temperature and pressure of the fluid. Therefore, the values and values of temperature and pressure must be known exactly. Careful selection should be made when the temperature and pressure fluctuate greatly.
It should also be noted that when measuring the gas, it is necessary to confirm that its volume flow value is the temperature and pressure under the working condition or the temperature and pressure under the standard state.
(2) Density of the fluid
For liquids, the density is relatively constant in most applications, unless there is a large change in temperature, generally no density correction is 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 working conditions for selection. There is also the conversion of the value of the flow state to some recognized reference value, which is widely used in petroleum storage and transportation. Low-density gases can be difficult for some measurement methods, especially instruments that use the momentum of the gas to push the detection sensor (such as turbine flow meters).
(3) Viscosity
The viscosity of various liquids varies widely and varies significantly with temperature changes. The gas is different, the viscosity difference between various gases is small, and its value is generally lower. And will not change significantly due to temperature and pressure changes. Because the viscosity of liquid is much higher than that of 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 influence of viscosity must be considered for liquids, while the viscosity of gases is not as important as liquids.
The influence of viscosity on various types of flowmeters is different. For example, the flow value of electromagnetic flowmeters, ultrasonic flowmeters and Coriolis mass flowmeters is within a wide range of viscosity, which can be considered unaffected by liquid viscosity. ; The error characteristics of positive displacement flowmeters are related to viscosity and may be slightly affected; while rotameters, turbine flowmeters and vortex flowmeters have a greater impact when the viscosity exceeds a certain value and cannot be used.
The characteristics of some flowmeters are described by the pipe Reynolds number as a parameter, and the pipe Reynolds number is a function of fluid viscosity, density, and pipe velocity. Therefore, the viscosity still has an influence on the characteristics of the instrument.
Viscosity is also a parameter to distinguish Newtonian or non-Newtonian fluids, and most flow measurement methods and flowmeters are only suitable for Newtonian fluids. All gases are Newtonian fluids. Most liquids, as well as liquids containing a small number of spherical particles, are also Newtonian fluids. Measurement methods and flowmeters that are only applicable to Newtonian fluids will affect the measurement when applied to non-Newtonian fluids. Therefore, Newtonian fluid is an important condition for the normal use of fluid flow measurement.
The influence of viscosity on the range of different types of flowmeters is different. Generally, the viscosity of positive displacement flowmeters increases and the range expands. The turbine flowmeter and vortex flowmeter are the opposite, the viscosity increases and the range decreases. Therefore, the temperature-viscosity characteristics of the liquid should be grasped when evaluating the suitability of the flowmeter.
Some non-Newtonian fluids (such as drilling mud, pulp, chocolate, and paint) have complex flow states, and it is difficult to judge their properties. Care must be taken when choosing a flowmeter.
(4) Chemical corrosion and scaling
① Chemical corrosion problems
The problem of chemical corrosion of the fluid can sometimes be the deciding factor in our choice of measurement method and the use of flow meters. For example, some fluids will corrode the contact parts of the flowmeter, fouling or depositing crystals on the surface, and electrolytic chemistry on the surface of metal parts, which will reduce the performance and service life of the flowmeter. Therefore, in order to solve the problem of chemical corrosion and scaling, manufacturers have 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 flow rate of the metal float is The gauge is lined with corrosion-resistant engineering plastics. However, for flowmeters with more complex structures, such as positive displacement flowmeters and turbine flowmeters, it is impossible to measure corrosive fluids. Some flowmeters have corrosion resistance or are easy to take corrosion resistance measures from the principle structure. The transducer probe of the ultrasonic flowmeter is installed on the outer wall of the pipeline and is not in contact with the measured fluid, which is essentially anti-corrosion. The electromagnetic flowmeter only has a measuring tube lining and a pair of electrodes with a simple shape in contact with the liquid. In recent years, some designs do not contact the electrodes with the liquid, which is also an anti-corrosion measure.
② Scaling
Due to scaling or crystallization on the flowmeter cavity and flow sensor, the clearance of moving parts in the flowmeter will be reduced, and the sensitivity or measurement performance of the sensitive elements in the flowmeter will be reduced. For example, on ultrasonic flowmeter applications, a fouling layer can hinder ultrasonic emission. In electromagnetic flowmeter applications, a non-conductive scaling layer insulates the electrode surfaces and renders the flowmeter inoperable. Therefore, some flowmeters often use heating outside the flow sensor to prevent precipitation of crystallization or install a descaling device.
The result of chemical corrosion and scaling is to change the roughness of the inner wall of the test pipe, and the roughness will affect the flow rate distribution of the fluid. Therefore, it is recommended that users should pay attention to this problem. For example, pipes that have been used for many years should be cleaned and descaled.
Corrosion and fouling affect flow measurement changes that vary by flowmeter type. The following takes ultrasonic flowmeter and electromagnetic flowmeter as examples to illustrate the results due to the effect of pipeline scaling. For example, for a pipeline with an inner diameter of 50mm, the inner wall scaling or deposition of 0.1-0.2mm will reduce the area of the measuring pipeline by 0.4%-0.6% , the resulting error will be a deviation that cannot be ignored for a flowmeter of class 0.5 to 1.0.
(5) Compression factor
The gas compression coefficient z is the ratio of the actual specific volume to the "volume" of a certain mass of gas at the same temperature and pressure. In general, for gas z=0; the actual gas z may be greater than 1 or less than 1. The magnitude of the deviation of z from 1 indicates the degree to which the actual gas deviates from the gas. The gas compressibility z value depends on the species or composition, temperature, pressure. Therefore, the gas measurement must obtain the fluid density in the working state through the compressibility coefficient. Density is calculated from temperature, pressure and compressibility for fluids with fixed components. If the fluid is multi-component (such as the metering of natural gas) and works near (or in) the supercritical region, an online density meter is required to measure the density online.
3. Installation of flowmeter
1. Matters needing attention during installation
Installation problems have different requirements for flow meters of different principles. For some flowmeters, such as differential pressure flowmeters and velocity flowmeters, according to regulations, a certain length or a long straight pipe section should be equipped upstream and downstream of the flowmeter to ensure that the fluid flow before the inlet end of the flowmeter is fully developed. . While other flowmeters, such as positive displacement flowmeters, float flowmeters, etc., have no or lower requirements on the length of straight pipe sections.
Some flowmeters have certain errors due to the influence of installation. For example, Coriolis mass flowmeters will bring great errors to use due to the influence of installation stress. Problems in the use of retrospective flowmeters may not necessarily be due to the problems of the flowmeter itself, and many situations 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 a place with poor flow velocity distribution profile;
③ Undesirable phases exist in the impulse pipe connected to the differential pressure device;
④ The flowmeter is installed in a harmful environment or in an inaccessible place;
⑤ The flow direction of the flowmeter is installed incorrectly;
⑥ The flowmeter or electrical signal transmission line is placed under a strong electromagnetic field;
⑦ Install the flowmeter susceptible to vibration interference on the pipeline with vibration;
⑧ Lack of necessary protective accessories.
2. Installation conditions
When using the flowmeter, attention should be paid to the adaptability and requirements of the installation conditions, mainly from the following aspects, such as the installation direction of the flowmeter, the flow direction of the fluid, the configuration of upstream and downstream pipelines, valve positions, protective accessories, pulsating flow influence, vibration, electrical disturbances and maintenance of flowmeters, etc.
① On-site piping wiring
Pay attention to the installation direction of the flowmeter when wiring the pipeline on site. Since the installation direction of the flowmeter is generally divided into a vertical installation method and a horizontal installation method, there are differences in the flow measurement performance for these two installation methods. For example, the vertical downward flow of the fluid will cause additional force to the flowmeter sensor, which will 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, the horizontal pipeline may precipitate solid particles, so the flowmeter measuring this state is installed in the vertical pipeline.
② Flow direction of fluid
This problem is similar to the installation direction of the flowmeter. Since some flowmeters can only work in one direction, reverse flow will damage the flowmeter. The use of similar flowmeters also considers the possibility of reverse flow in the event of inactivity, which necessitates measures such as installing check valves to protect the flowmeter. Even a flowmeter that can be used in both directions may have some differences in measurement performance between forward and reverse, and should be used as specified by the manufacturer.
③ The upstream and downstream straight pipe sections of the flowmeter
Since the flowmeter will be affected by the flow state of the pipeline inlet, the pipeline fittings will also introduce flow disturbance. The flow disturbance generally includes vortex and flow velocity distribution profile distortion. The existence of vortex is generally due to the presence of two or more space (stereo) elbows caused by. Distortion of the velocity profile is usually caused by local obstructions in pipe fittings (eg valves) or elbows. These effects need to be ameliorated with upstream straight runs of appropriate lengths or the installation of flow conditioners. In addition to considering the influence of the flowmeter connection fittings, you may also consider the influence of the combination of upstream pipe fittings, because they may generate different disturbance sources, so be sure to keep the distance between the disturbance sources as far as possible to reduce their influence. For example, a partially open valve follows immediately after a single bend.
A straight pipe section is also required downstream of the flowmeter to reduce downstream flow effects.
For volumetric flowmeters and Coriolis mass flowmeters, they are not affected by asymmetric flow profiles; turbine flowmeters should be used to minimize vortex; electromagnetic flowmeters and differential pressure flowmeters should limit the vortex to a very small within the range.
Cavitation and condensation are caused by unreasonable pipe arrangement, avoiding sharp changes in pipe diameter and direction. Poor piping layout can also create pulsation.
④ Pipe diameter and pipe vibration
Some types of flowmeters do not have a wide range of pipe diameters, so too large or too small will limit the choice of flowmeter varieties. To measure the flow rate of low flow rate or high flow rate, you can choose a flowmeter with a different diameter from the pipe diameter, and you can use a reducer to connect to make the flowmeter run within the specified range. If the flow rate exceeds the range, if the flow rate is too low, the error of the flowmeter will increase, and the flowmeter error may increase.
Some flowmeters, such as vortex flowmeters and Coriolis mass flowmeters of piezoelectric detectors, are sensitive to mechanical vibrations and are easily disturbed by pipeline vibrations. Attention should be paid to the design of support on the pipelines before and after the flowmeter. In addition to the use of pulsation eliminators to eliminate the effects of pulsation, attention should also be paid to keeping all installed flowmeters away from sources of vibration or pulsation.
⑤ Installation position of valve
The 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 and affect the flowmeter measurement, the general control valve should be installed downstream of the flowmeter, and the control valve should be installed in the flowmeter. The back pressure of the flowmeter can also be increased downstream to reduce the possibility of cavitation inside the flowmeter.
The purpose of the isolation valve is to isolate the flowmeter from the fluid in the line for easy maintenance. The upstream valve should be far enough away from the flowmeter. When the flowmeter is running, the upstream valve should be fully opened to avoid disturbances such as flow rate 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 flowmeters and turbine flowmeters, some necessary equipment such as filters generally need to be installed upstream. All these equipments must be installed so as not to affect the use of the flowmeter.
⑦ Electrical connection and electromagnetic interference
At present, most flow measurement systems, whether it is the flowmeter itself or its accessories, have electronic equipment, so the power supply used must be matched with the flowmeter. When the output level of the flowmeter is low, a preamplifier suitable for the environment should be used. The output signal of some types of flowmeters is easily interfered by high-power switching devices, which makes the output pulses of the flowmeter fluctuate and affects the performance of the flowmeter. For example, the signal cable should be as far away from the power cable and power source as possible to reduce electromagnetic interference and radio frequency interference. influences.
⑧ Pulsating flow and unsteady flow
In addition to the use of pulsation eliminators, attention should be paid to keeping all installed flowmeters away from pulsation sources. Common sources of pulsation include fixed displacement pumps, reciprocating compressors, oscillating valves or regulators, vortex trains and other hydraulic oscillations. Generally, differential pressure flowmeters have pulsating flow errors, and turbine flowmeters and vortex flowmeters also have pulsating flow errors. Unsteady flow is a flow that varies with time and slow pulsation is a special case of unsteady flow. Such as slow pulsations caused by the operation of an oversized control valve.
The flow meter can handle the pulsation effects of the flow sensor and the secondary display instrument separately. Install the flow sensor away from the source of pulsation, or install a low-pass filter such as a snubber (for liquids) or choke (for gas) in the piping system to reduce the degree of pulsation. The secondary display instrument can choose a flowmeter with good response characteristics (such as electromagnetic flowmeter, ultrasonic flowmeter) to increase the damping, and measure the pulsation parameters to estimate the additional error of the pulsation.
4. Environmental condition requirements
In the process of selecting flowmeters, surrounding conditions and related changes, such as ambient temperature, humidity, safety and electrical interference, should not be ignored.
① Ambient temperature
Ambient temperature changes can affect the electronic part of the flowmeter and the flow sensor part. For example, temperature changes can affect changes in sensor size, heat transfer through the flowmeter housing, changes in fluid density and viscosity, etc. When the ambient temperature affects the electronic components of the display instrument, the component parameters will be changed. The flow sensor and the secondary display instrument should be installed in different places, such as 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 the influence of ambient temperature should not be one of the main influences of uncertainty when estimating the total uncertainty of flow measurement.
② Ambient humidity
The atmospheric humidity in the environment is also one of the problems affecting the use of the 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 or medium temperature can cause humidity problems such as condensation on the surface.
③ Security
The flowmeter should be selected in accordance with relevant specifications and standards to be suitable for use in explosive hazardous environments, and the site should be required in accordance with explosion-proof standards.
④Electrical interference
Power cables, motors, and electrical switches all generate electromagnetic interference, which can cause errors in flow measurement if no measures are taken.
5. Economic considerations
1. Consider the cost of purchasing a flowmeter from an economic perspective
When purchasing a flowmeter, the economic impact of different types of flowmeters on the overall measurement system should be compared. For example, a flowmeter with a smaller range than a flowmeter with a wider range needs to be covered by multiple flowmeters in parallel and multiple pipelines under the same measurement range. Therefore, in addition to the flowmeter, many auxiliary equipment, such as valves and pipeline accessories, need to be added. Wait. Although the cost of the flow meter is reduced on the surface, other costs are increased, which is not cost-effective to calculate. For example, the cost of installing an orifice flowmeter plus a differential pressure gauge is relatively cheap, but the cost of composing the measurement loop, including the fixed accessories of the orifice plate, may exceed the cost of the basic parts.
2. Installation cost
When purchasing a flowmeter, not only the purchase cost of the flowmeter, but also other costs, such as accessory purchase cost, installation and commissioning cost, maintenance and regular inspection cost, operating cost and spare parts cost should be considered.
For example, many flow meters should be equipped with relatively long upstream straight pipe sections to ensure their measurement performance. Proper installation therefore requires additional piping arrangements or bypass piping for regular maintenance. Therefore, the installation fee should be considered reasonably in many aspects, such as the stop valve, filter and other auxiliary costs required for operation.
3. Operating costs
The operating cost of the flowmeter is mainly 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 and the energy consumed to push the fluid through the instrument during the measurement process, that is, the pump that overcomes the pressure loss caused by the instrument due to measurement. Shipping costs etc. For example, a large part of the differential pressure generated by differential pressure flowmeters cannot be recovered, and positive displacement flowmeters and turbine flowmeters also have considerable resistance. Only full-channel, unobstructed electromagnetic flowmeters and ultrasonic flowmeters basically have zero cost, and insertion flowmeters have a small blockage ratio for large pipe diameters, and their pressure loss can be ignored.
It is estimated that the one-year pumping energy consumption of a differential pressure orifice flowmeter with a diameter of 100mm is equivalent to the purchase cost of the flowmeter. If an electromagnetic flowmeter is replaced, the purchase cost is only equivalent to four years. of energy consumption. It is envisaged that the pumping energy consumption of the larger diameter pipe will be more expensive. It is generally believed that the flowmeter with a low pressure loss and no pressure loss should be used as much as possible for the flowmeter exceeding 5000mm. For example, traditional differential pressure flowmeters used in water supply projects rarely use orifice plates and use Venturi tubes with low pressure loss. Now they are updated to electromagnetic flowmeters and ultrasonic flowmeters.
4. Testing fee
The testing fee shall be determined according to the verification period of the flowmeter. For the detection of crude oil or refined oil generally used for trade settlement, a standard volume tube is often set up on site to perform online verification of the flowmeter.
5. Maintenance costs and spare parts costs, etc.
The maintenance cost is the cost required to keep the measurement system working normally after the flowmeter is put into use, mainly including maintenance and spare parts cost. Flowmeters with moving parts need more maintenance work, such as regular replacement of wear-resistant bearings, shafts, runners, transmission gears, etc.; flowmeters without moving parts also need to be inspected, such as the ordinary geometric measurement method to check the orifice plate flowmeter.
Spare parts costs will increase as the performance of the flowmeter improves. When selecting a flowmeter, consideration should be given to increasing the purchase cost of spare parts, especially the flowmeter imported from abroad, and sometimes the entire flowmeter is often replaced due to the difficulty of wearing spare parts.
6. Selection of measurement methods and flowmeters
The previous sections are all about the selection of general flowmeters. This section takes the selection of flowmeters for measuring slurry flow, large liquid flow and steam flow as an example.
1. Selection of slurry flow measurement
From the flowmeter selection list, the optional flowmeters that can be used for particle fiber slurry include: differential pressure flowmeters include elbows, wedge-shaped tubes, electromagnetic flowmeters, Doppler ultrasonic flowmeters, 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 the first choice for measuring slurry flow, unless the measured slurry is non-conductive or contains ferromagnetic particles, and the measurement pipeline system is not allowed to be cut off to Only when the flow sensor is connected, other flow meters are selected. According to reports, many years of application experience in measuring the flow rate of coal-water slurry with a pulverized coal content of up to 65% are considered to be better than electromagnetic flowmeters.
Differential pressure flowmeters can be used to measure slurry. In addition to elbows, wedge-shaped tubes and annular tubes, the differential pressure sensor can also be used for circular orifice plates and eccentric orifice plates when the solid phase is small. Venturi tubes are also used for measurement. .
Doppler ultrasonic flowmeter can be measured without cutting off the pipe and clamping an ultrasonic transducer (probe) outside the pipe, but the measurement accuracy is not high.
Vortex flowmeter can only measure solids containing a small amount of powder, and the solid content is large or fibrous will cause noise and cannot be used.
The target flowmeter is used for liquid flow such as heavy oil or residual oil containing pulverized coal, and the strain target flowmeter is used.
Coriolis mass flowmeters have experience in measuring slurry in foreign countries, and generally their straight-tube measuring tubes are suitable, but there is not much domestic application experience.
2. Selection for large flow measurement of liquid in closed pipelines
The large flow mentioned here does not refer to the "relatively large flow" when the flow velocity of a certain pipe diameter is high, but the large flow of the absolute value of the flow. Since the flow velocity of the liquid transported by the pipeline has a certain range, the economical flow velocity of the low viscosity liquid is usually 1~3m/s. Therefore, the "large flow" measurement mentioned here refers to the measurement of the large pipeline flow.
Generally speaking, the flowmeter with pipe diameter below DN300 is called small and medium pipe diameter flowmeter, the one above DN300~ DN400 is called large pipe diameter flowmeter, and the one above DN1200 is called extra large pipe diameter flowmeter. Usually, the liquid flow measurement of extra-large diameter pipes is mainly water, and in addition to water, there are petroleum products. Generally, large-diameter flowmeters include differential pressure flowmeters, electromagnetic flowmeters, ultrasonic flowmeters and insertion flowmeters. There are also positive displacement flowmeters and turbine flowmeters for DN300~DN500.
(1) Installation conditions
The installation conditions are mainly based on whether the measurement method can allow the pipe flow to be cut off and the operation to be suspended, whether it is allowed to drill holes on the pipe, and whether it is allowed to cut off the pipe flow to install the flow sensor.
If the flow sensor is allowed to cut off the pipe flow, electromagnetic flowmeters, ultrasonic flowmeters with measuring pipe sections, positive displacement flowmeters and turbine flowmeters can be selected.
Extrapolation transducer ultrasonic flowmeters and insertion flowmeters can be selected if drilling holes in the pipeline are allowed.
If the above requirements are not allowed, you can only choose an ultrasonic flowmeter with an external clip-on transducer.
(2) Measurement accuracy requirements
For custody transfer requiring high measurement accuracy and non-conductive liquids, ultrasonic flowmeters with measuring pipe sections, multi-channel ultrasonic flowmeters, positive displacement flowmeters and turbine flowmeters can be selected, and electromagnetic flowmeters can also be selected for conductive liquids flowmeter.
For the control ratio, the differential pressure Venturi tube and the external clamping transducer ultrasonic flowmeter can be selected with lower measurement accuracy requirements. Optional insertion flowmeter with low measurement accuracy requirements.
(3) Pressure loss (pumping energy cost)
The pumping energy cost of large flow measurement accounts for a considerable proportion of the flow measurement operating cost, pressure loss and (pumping energy cost) such as differential pressure Venturi, positive displacement flowmeter and turbine flow count. The smaller is the insertion flowmeter, and the one without pressure loss is the electromagnetic flowmeter.
3. Selection of steam flow measurement
The steam flow measurement is divided into two categories in terms of measurement technology, one is superheated steam and saturated steam with high dryness (dryness x = 0.9 or more), and the other is saturated steam with low dryness. The former category can be treated as a single-phase fluid, while the latter category is a two-phase flow. Since all the current flowmeters are only suitable for single-phase fluids, the low dryness saturated steam needs to be further studied.
(1) Flow measurement of superheated steam and high dryness saturated steam
The commonly used flowmeters are: throttling differential pressure flowmeter, which is still the main instrument for measuring steam flow. For example, the throttling device, the differential pressure transmitter and the three-valve group are integrated into an integrated throttling flowmeter. The throttling flowmeter solves the shortcoming of the differential pressure signal tube failure. There are also planting throttling parts, and standard nozzles are used instead of standard orifice plates. Because nozzles are compared with orifice plates, the outflow coefficient of nozzles is stable, and the outflow coefficient will not change due to the blunt edge of the sharp angle. The pressure loss is also lower than that of the orifice plate. , generally at the same flow rate and value, the pressure loss is about 30% to 50% of the orifice plate.
The vortex flowmeter measures medium temperature, that is, below 200 °C. It should be said that the application of steam has become mature. It is a type of flowmeter commonly used in steam measurement at present. However, it must be noted that the medium with low dryness will make the instrument coefficient deviate from the detection value and increase the measurement error.
The uniform velocity tube flowmeter and the shunt rotor flowmeter can still be used in the internal management distribution where the accuracy requirements are not too high, because the use is relatively cheap and simple, and it is suitable for the measurement of small and medium 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. Since the force converter directly used the force balance mechanism of the differential pressure transmitter at that time, it brought many deficiencies caused by the force balance mechanism itself. For example, the measurement accuracy is low, the zero point drift, the reliability of the lever mechanism, and the poor stability. Therefore, the original JJG 461-1986 "target flow transmitter" regulations were formulated in 1986, which has been 25 years old. Because electric and pneumatic target flow transmitters are basically no longer produced and used. The original regulations are no longer suitable for use, so a new
Target Flowmeter Protocol.
The structure of the target flowmeter is composed 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 read the display or output the standard signal. The force sensor is composed of a cylindrical elastic body and a force strain gauge, and can be either internal or external. When the elastic body deforms under the action of force, it breaks the balance of the bridge composed of force strain gauges, producing an electrical signal that is squared with the flow rate.
Its working principle is to set a target plate perpendicular to the direction of the flow beam in a straight pipe section of constant cross-section. When the fluid passes around the target plate, the target plate is subjected to thrust, and the magnitude of the thrust is proportional to the kinetic energy of the fluid and the area of the target plate. proportional. Within a certain range of Reynolds numbers, the flow through the flowmeter is proportional to the force on the target plate. The force on the target plate is detected by the force sensor.
Taking a circular target plate as an example, the basic formula for flow calculation is:
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The force on the target plate is converted into a current signal (4-20) mA or an air pressure signal (20-100kPa) through the force converter, and the relationship between the output signal and the flow rate can be determined according to the above formula.
Because the new strain-type target flowmeter has a new structure and measurement principle, it has a relatively superior application prospect in steam measurement, and is suitable for the measurement of small and medium 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, but in the process of long-distance transportation, due to many factors such as poor heat preservation or unbalanced intermittent steam use, the dryness is constantly increasing. drop, and even become a wet steam with a high water content, that is, a two-phase fluid of gas and water. The flow characteristics of two-phase fluids are fundamentally different from those of single-phase flow. Flowmeter meter coefficients or outflow coefficients measured in single-phase flow cannot be used for two-phase flow measurements. For example, the outflow coefficient in the two-phase flow test of the orifice flowmeter must be corrected for dryness. Therefore, in the flow measurement of low dryness saturated steam, the dryness parameter is a parameter that must be measured. It is a pity that there is no mature dryness meter yet. In addition, the dryness correction of the meter coefficients of other types of flowmeters has not been studied in depth. Only by solving this problem can the flow rate of low dryness saturated steam be measured.