Case
Case
Since flow is a dynamic quantity, the measuring instrument itself is affected by many factors, such as: pipeline, diameter size, shape, boundary conditions, physical properties of the medium (temperature, pressure, density, viscosity, dirtiness, corrosiveness, etc.), fluid flow state (turbulence state, velocity distribution, etc.) and the influence of installation conditions and levels.
How to make a reasonable selection based on factors such as flow rate, flow pattern, installation requirements, environmental conditions, and the economy is the prerequisite and foundation for the good application of flow meters.
In general, the principle of selecting a flow meter is to first have a deep understanding of the structural principles and fluid characteristics of various flow meters. At the same time, the selection must be based on the specific conditions of the site and the surrounding environmental conditions. Economic factors must also be taken into consideration. Generally speaking, you should mainly choose from the following four aspects:
① Performance requirements of the flow meter;
② Fluid characteristics;
③ Installation requirements;
④ Environmental conditions.
The performance requirements to consider when selecting an instrument include: instantaneous flow or total flow (accumulated flow), accuracy, repeatability, linearity, flow range and range, pressure loss, output signal characteristics and response time, etc. Different measurement objects have their own measurement purposes and have different emphases in instrument performance.
There are two types of purposes for using object measurement, namely measuring flow and measuring total quantity. The places where pipelines are used for continuous proportioning production or process control mainly measure instantaneous flow; most places where the mass production of filling containers, commercial accounting, storage, and transportation distribution, and other places only need to obtain the total amount or supplement it with flow. Two different functional requirements have different priorities in selecting measurement methods.
Some instruments, such as volumetric flow meters, turbine flow meters, etc., use mechanical technology or pulse frequency output to directly obtain the total amount. Therefore, they have high accuracy and are suitable for measuring the total amount.
Electromagnetic flowmeters, ultrasonic flowmeters, throttling flowmeters, and other instruments in principle derive the flow rate by measuring the fluid flow rate. They have fast response and are suitable for process control. However, the total amount can also be obtained after being equipped with an accumulation function.
Repeatability is an important indicator in process control applications and is determined by the principle and manufacturing quality of the instrument itself. In addition to repeatability, accuracy is also related to the value calibration system. Strictly speaking, repeatability refers to the consistency of multiple measurements of a certain flow value in the same direction within a period of time when environmental conditions, medium parameters, etc. remain unchanged. However, in actual applications, the excellent repeatability of the instrument is interfered by many factors, including changes in fluid viscosity, density, etc. However, these changing factors have not yet reached the point where special detection and correction are required. These effects are often mistaken for poor repeatability of the instrument.
For example, float flowmeters are affected by fluid density, and small-diameter instruments are also affected by viscosity; turbine flowmeters are affected by viscosity when used in high viscosity ranges; some uncorrected ultrasonic flowmeters are affected by fluid temperature on the speed of sound, etc. If the instrument output characteristics are nonlinear, this effect will be more prominent.
The upper limit flow is also called full flow. The caliber of the flow meter should be selected based on the flow range used by the pipeline under test and the upper and lower flow limits of the selected instrument, rather than simply based on the diameter of the pipeline. Although the maximum flow rate of the fluid in the designed pipeline is usually determined based on the economic flow rate. Because if the flow rate is too low, the pipe diameter will require a large investment; if it is too high, the transmission power will be high and operating costs will increase.
However, the upper limit flow rates (or upper limit flow rates) of different types of instruments with the same caliber are constrained by their respective working principles and structures and are very different. Taking liquid as an example, the upper limit flow rate of the glass tube float flowmeter is the lowest, between 0.5-1.5m/s, the volumetric flowmeter is between 1.5-2.5ｍ/s, and the vortex flowmeter has the highest flow rate of 5.5-7m. /s, the electromagnetic flowmeter is between 1-7m/s (or even 0.5-10m/s).
The range is the ratio of the upper limit flow and the lower limit flow of the flow meter. The larger the value, the wider the flow range. Linear meters have a wider range, generally 1:10. The range of a nonlinear flowmeter is as small as 1:3. Flow meters are 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, to promote the wide flow range of their flow meters, some manufacturers have raised the upper limit flow rate very high in the instruction manual, for example, for liquids, it has been raised to 7 to 10 m/s (usually 6 m/s); for gases, it has been raised to 50 to 50 m/s. 75m/s (usually 40~50) m/s); in fact, such a high flow rate is not used. 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 lower limit flow rate is more practical.
Pressure loss generally refers to the irrecoverable pressure loss that changes with the flow rate due to the static or movable detection element installed in the flow channel or the change of flow direction in the flow sensor, and its value can sometimes reach tens of kilopascals. Therefore, the flowmeter should be selected based on the pumping capacity of the pipeline system and the inlet pressure of the flowmeter to determine the allowable pressure loss of the maximum flow rate. Improper selection will restrict the fluid flow and cause excessive pressure loss, which will affect the circulation efficiency.
For some liquids (high vapor pressure hydrocarbon liquids), it should also be noted that excessive pressure drop may cause cavitation and liquid phase vaporization, reduce measurement accuracy and even damage the flow meter. For example, for flowmeters used for water delivery with pipe diameters greater than 500mm, the increased pumping costs due to excessive energy loss caused by pressure loss should be considered. According to relevant reports, the pumping costs paid for measurement by flow meters with large pressure losses in recent years often exceed the purchase cost of more expensive flow meters with low-pressure losses.
The output and display volume of the flow meter can be divided into:
① Flow (volume flow or mass flow);
② Total amount;
③ Average flow velocity;
④ Point flow rate.
Some flowmeters output analog quantities (current or voltage), while others output pulse quantities. Analog output is generally considered suitable for process control and is more suitable for coupling with control loop units such as regulating valves; pulse output is more suitable for total volume 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 be compatible with other equipment, such as control interfaces, data processors, alarm devices, circuit breaker protection circuits, and data transmission systems.
When there is a huge gap between the actual working conditions and the design selection or the instrument fails, attention should be paid to whether there are means for on-site repair and correction, because once the flow instrument is installed and removed for maintenance, it will be troublesome and time-consuming.
The best performance in this regard is the differential pressure measurement method, because the components in contact with the fluid are maintenance-free fixed parts, and the electrical components used for measurement are detachable and adjustable universal differential pressure transmitters.
Therefore, the differential pressure measurement method has the highest normal operation rate. According to statistics, the differential pressure throttling measurement method accounts for more than 45% of all measurement methods in the world.
The operating temperature and pressure of the fluid must be defined. Especially when measuring gases, temperature, and pressure cause excessive density changes, which may require changing the selected measurement method. If changes in temperature or pressure cause large changes in flow characteristics and affect measurement performance, temperature and/or pressure corrections must be made.
In most liquid applications, the liquid density is relatively stable. Unless the density changes significantly, correction is generally not required. In gas applications, the range and linearity of some instruments depend on density. Low-density gases present difficulties with certain measurement methods, such as instruments that use the momentum of the gas to push a detection element (such as a turbine) into operation.
Some instrument properties vary with Reynolds number, which is related to viscosity. When evaluating instrument suitability, it is necessary to understand the temperature-viscosity characteristics of the liquid. Unlike liquids, the viscosity of gases does not change significantly due to changes in temperature and pressure. Its value is generally low. The viscosity difference of various gases except hydrogen is small.
So the exact viscosity of a gas is not as important as it is for a liquid. The influence of viscosity on the range of different types of flow instruments has different trends. For example, for most positive displacement instruments, the viscosity increases in the range, while for turbine and vortex types, the opposite is true, and the viscosity increases in the range. Lubricity is a physical property that is difficult to evaluate. Lubricity is very important for instruments with movable measuring elements. Poor lubricity will shorten the life of the bearings, and the bearing conditions will affect the operating performance and range of the instrument.
The problem of chemical corrosion of fluids sometimes becomes a deciding factor in our choice of measurement method and use of flow meters. For example, certain fluids will corrode the parts in contact with the flow meter, scale or precipitate crystals on the surface, and produce electrolytic chemical effects on the surface of metal parts. The occurrence of these phenomena will reduce the performance and service life of the flow meter.
Therefore, to solve the problem of chemical corrosion and scaling, manufacturers have adopted many methods, such as using anti-corrosion materials or taking anti-corrosion measures on the structure of the flow meter. For example, the orifice plate of the throttling device is made of ceramic materials, and the metal float flow meter is lined with corrosion-resistant engineering plastics. However, flowmeters with more complex structures, such as volumetric flowmeters and turbine flowmeters, cannot measure corrosive fluids.
Some flowmeters are corrosion-resistant from the principle structure or can easily take corrosion-resistant measures. The transducer probe of the ultrasonic flow meter is installed on the outer wall of the pipeline does not come into contact with the fluid being measured, and is essentially anti-corrosion.
Due to scaling or crystallization on the flow meter cavity and flow sensor, the gaps between the moving parts in the flow meter will be reduced, and the sensitivity or measurement performance of the sensitive components in the flow meter will be reduced. For example, in ultrasonic flowmeter applications, the scale layer will hinder the emission of ultrasonic waves. In electromagnetic flowmeter applications, the non-conductive scale layer insulates the electrode surface and makes the flowmeter unable to operate. Therefore, some flow meters often use external heating of the flow sensor to prevent precipitation and crystallization or install a descaler.
The result of chemical corrosion and scaling is to change the roughness of the inner wall of the test pipeline, and the roughness will affect the flow velocity distribution of the fluid. Therefore, it is recommended that users pay attention to this issue. For example, pipelines that have been used for many years should be cleaned and descaled.
Fluid corrosion of instrument contacts, surface scaling or precipitation of crystals will reduce performance and lifespan. Instrument manufacturers often provide modified products for this purpose, such as developing anti-corrosion types, adding thermal insulation sleeves to prevent precipitation and crystallization, and installing descalers and other preventive measures.
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