Performance factors to consider when selecting flow meters

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Performance factors to consider when selecting flow meters Abstract: This article details the nine main performance factors that need to be taken into account when choosing a flow meter after the measurement method has been determined. It also discusses how different measurement subjects require different emphasis on meter performance depending on their respective measurement objectives. Keywords flow meter ； Measurement method ； Performance requirements ； Accuracy ； Pressure loss ； Once the measurement method is selected, the key factors to consider when choosing a flow meter in terms of performance include: whether it is for measuring instantaneous flow or total volume, accuracy, repeatability, linearity, maximum flow rate and flow range, span, pressure loss, characteristics of the output signal, and response time. Different measurement objects have various purposes, and there are also different emphasis points regarding the performance of the instruments. For example, business accounting and storage and transportation require a high level of accuracy ； Continuous measurement process control usually only requires good reliability and repeatability; sometimes a wide measurement range is also needed, while measurement accuracy is given less importance ； For batch mixing production, good accuracy is desired, and so on. Therefore, after determining the measurement method, we need to conduct a detailed analysis of the various performance requirements for the instruments based on the specific object being measured, and on this basis select the appropriate instruments. 1 Analysis of performance factors 1.1 Instantaneous flow rate or total volume measured The purposes of using a measuring device are of two types: measuring flow rate and determining total volume. Places where continuous batching production in pipelines or process control is carried out mainly measure instantaneous flow rate ； In applications such as the mass production of filling containers, commercial accounting, and storage, transportation, and distribution, it is generally sufficient to measure the total volume, with occasional use of instantaneous flow rates as well. Two different requirements result in different focuses when choosing a measurement method. Some instruments, such as positive displacement flowmeters and turbine flowmeters, rely on mechanical principles or pulse frequency output in their measurement mechanism, allowing for the direct determination of the total volume measured; as a result, they offer high accuracy and are suitable for measuring totals. If it is necessary to obtain the instantaneous flow rate, corresponding signaling devices must be used ； Meters such as electromagnetic flowmeters and ultrasonic flowmeters derive the flow rate by measuring the fluid velocity; they have a fast response time and are suitable for process control. However, with an integrated accumulation function, it is also possible to obtain the total volume flow. 1.2 Accuracy: What is the required overall measurement accuracy for the instrument? Is it to be used at a specific flow rate, or within a certain range of flow rates? Within what measurement range is the above accuracy maintained? How long can the accuracy of the selected instrument be maintained? Is it easy to re-verify? Should (or can) the accuracy of the instruments be verified on-site online? All these issues must be carefully considered. When it is not used merely for measuring total quantities but in flow control systems, the accuracy of the measuring instrument must be determined based on the overall accuracy requirements of the system. This is because the entire system involves not only errors in flow measurement but also errors in signal transmission, control adjustment, and operation execution, as well as various other influencing factors. For example, there is often a backlash of around 2% in the operation execution stage; therefore, it is unreasonable and uneconomical to require overly high accuracy from the measuring instrument (such as 0.5 grade). As for the flow meter itself, the accuracy between the sensing element (or sensor) and the conversion/display instrument also needs to be properly determined. For differential pressure devices such as average velocity tubes, wedge tubes, and elbow tubes that have not been actually calibrated, their errors range from 0.25% FS to 5% FS; in such cases, it makes no sense to use a high-precision differential pressure gauge with them. The accuracy class specified in the flow meter standards applies over a relatively wide range of flow rates. If the operating conditions involve a specific flow rate or a very narrow range of flow rates, such as when using a turbine flow meter to measure oil in barrels with the valve fully open and the flow rate remaining essentially constant or varying only within a very small range, the measurement accuracy can be higher than the specified value. If calibration is performed specifically at this measurement point, the accuracy can be improved, for example from 0.5 grade to 0.25 grade or higher. When high accuracy is required for tasks such as business accounting, storage and transportation, and material balance, it is also necessary to consider key factors such as the durability of accuracy, the ease of re-calibration, and whether online calibration is possible. When comparing the performance specifications of instruments from different manufacturers, it is important to note whether the percentage error refers to the reference error (the percentage of the upper measurement limit or full scale, commonly denoted as %FS) or the relative error (the percentage of the actual measured value, commonly denoted as %R). Typically, the sample or instruction manual only indicates the error percentage, without specifying whether it is %FS or %R; since in the past, the error in the instantaneous flow rate of flow meters was usually expressed as %FS, this is what is meant here. 　 It should also be noted that the accuracy specified in the manufacturer’s product manual refers to the basic error; in actual operating conditions, factors such as changes in power and fluid conditions will result in additional errors. The on-site accuracy should be the sum of the basic error and the additional error caused by influencing factors; if these influencing factors are significant, the additional error can far exceed the basic error. 1.3 Repeatability x Repeatability is an important indicator in process control applications; it is determined by the principles of the instrument itself and its manufacturing quality. Accuracy, on the other hand, depends not only on repeatability but also on the value calibration system. Strictly speaking, repeatability refers to the consistency of making multiple measurements in the same direction of a certain flow rate over a period of time, under conditions where environmental factors and medium parameters remain unchanged. However, in practical applications, the excellent repeatability of instruments is disrupted by various factors such as changes in fluid viscosity and density. Yet these changing factors do not yet require special testing or correction, and these effects are often mistaken for poor repeatability of the instruments. Therefore, in locations that require high repeatability due to changing conditions (i.e., usage conditions with parameters that can vary), instruments that are highly sensitive should not be chosen. For example, float flowmeters are affected by the density of the fluid, while instruments with small diameters are also influenced by viscosity ； The effect of viscosity when using turbine flowmeters in high-viscosity ranges ； Some unmodified ultrasonic flowmeters are affected by factors such as fluid temperature on the sound speed. This effect is more pronounced if the instrument’s output characteristics are nonlinear. 1.4 Linearity: The output of flow meters is mainly characterized by linear and square-root nonlinearities. For most flow meters, the nonlinear error is not listed as a separate parameter but is included in the basic error. However, for instruments that use pulse outputs over a wide flow range for total volume measurement, linearity is an important parameter; it enables the use of the same instrument constant across the entire flow range. A poor level of linearity reduces the accuracy of the instrument. With the development of microprocessor technology, signal adaptation techniques are employed to correct the non-linearity of instrument systems, thereby improving instrument accuracy and expanding the flow range. When it is necessary to adjust pipeline flow rates, add flows together, or for a calorimeter to multiply the temperature difference by the flow rate, instruments with linear output should be selected, as this simplifies the calculation process. 1.5 Maximum flow rate and flow range The maximum flow rate is also known as the full-scale flow rate. The diameter of the flow meter to be selected should be chosen based on the flow range of the pipeline in question, as well as the upper and lower flow limits of the meter chosen, rather than simply according to the diameter of the pipeline. Although typically, the maximum flow velocity of the fluid in a pipeline is determined based on economic considerations. Because the flow rate selection is too low, the pipe diameter is large, resulting in high investment costs ； If the value is set too high, the transmission power increases, raising operating costs. For example, the optimal flow rate for low-viscosity liquids such as water is 1.5 m/s to 3 m/s, while for high-viscosity liquids it is 0.2 m/s to 1 m/s. For most flow meters, the flow rate at their maximum flow capacity is close to or slightly higher than the optimal flow rate for the pipeline; therefore, it is common to choose a meter with the same diameter as that of the pipe, which makes installation easier. If they are not the same, the difference will not be significant; generally, for adjacent specification grades, a reducer can be used for connection. However, the upper flow rate (or upper flow velocity) of gauges of the same caliber but different types varies significantly due to the constraints imposed by their respective working principles and structures. Taking liquids as an example, the flow velocity at the upper limit of flow rate is lowest for glass tube float flow meters, ranging from 0.5 m/s to 1.5 m/s; for positive displacement flow meters it ranges from 2.5 m/s to 2.5 m/s. Vortex flow meters have a higher value, ranging from 5.5 m/s to 7 m/s, while electromagnetic flow meters range from 1 m/s to 7 m/s (or even from 0.5 m/s to 10 m/s). The selection of the upper flow velocity for liquids also needs to take into account the avoidance of cavitation. For some meters, the upper flow limit value cannot be changed after ordering, such as positive displacement flowmeters and float-type flowmeters ； Once the design of the orifice flow meter is finalized, the lower flow limit cannot be changed; however, the upper flow limit can be adjusted by modifying the range of the differential pressure transmitter (or by replacing it with another differential pressure transmitter) ； Some instruments allow the user to reset the flow rate limit on their own, without undergoing any actual calibration, such as certain models of electromagnetic flowmeters and ultrasonic flowmeters. 1.6 Range Ratio The range ratio is the ratio of the upper flow rate to the lower flow rate; a higher value indicates a wider flow rate range. Linear instruments have a large range, typically 10:1 ； Non-linear meters are smaller, usually only 3:1, and are sufficient for flow measurement in general process control as well as for measuring total quantities in commercial accounting. However, some commercial accounting applications require instruments with a wide measurement range. For example, in the case of water consumption measurement for public facilities, where usage levels vary significantly between day and night as well as between winter and summer, a wide measurement range is necessary; in such situations, venturi differential pressure meters are clearly not suitable. In recent years, there have been certain advancements in expanding the range of differential pressure instruments, primarily through measures taken in the area of differential pressure transmitters and the use of microcomputer technology; a ratio of 10:1 can be achieved. However, such instruments are much more expensive. For certain models of electromagnetic flowmeters, users can adjust the upper flow limit themselves; the adjustable range for this limit (the ratio between the maximum and minimum upper limits) can be as high as 10. The expansion range of such meters (taking into account this adjustable limit) can be as much as (50–200):1. Some other models of these meters even have a function for automatically switching the upper flow limit value. Some manufacturers, in order to indicate a wide range of products, state very high maximum flow rates; for example, 7 m/s to 10 m/s for liquids, and 50 m/s to 75 m/s for gases. In reality, such high flow rates are generally not necessary, and what’s important is whether the minimum flow rate meets the requirements for measurement. Generally, a wide range is required, but it is preferable to have a lower lower limit flow rate. 1.7 Pressure loss: Except for unobstructed flow sensors, most flow sensors either require a change in the flow direction or involve the use of stationary or movable sensing elements within the flow channel, thereby generating a pressure loss that varies with the flow rate and can sometimes reach several tens of kPa. The allowable pressure loss at maximum flow rate should be determined based on conditions such as the pumping capacity of the piping system and the inlet pressure of the instrument, and the instrument should be selected accordingly. Excessive pressure losses resulting from improper selection often affect process efficiency. In piping systems for certain liquids, it is also important to be aware that excessive pressure losses can cause phenomena such as cavitation, which can reduce measurement accuracy or even damage the instruments. Experience shows that for instruments with high pressure losses, the annual pumping costs associated with measurement often exceed the costs of purchasing instruments with lower pressure losses, even though those latter instruments are more expensive. 1.8 Output signal characteristics: The output signal often determines the choice of instruments. The signal output characteristics of flow meters can be summarized as: flow rate (volumetric flow rate or mass flow rate), total volume, average flow velocity, and local flow velocity. Some instruments output analog quantities in the form of current (or voltage), while others output pulse quantities. Analog output is generally considered suitable for process control, as it can be easily integrated with control loops such as control valves ； Pulse output is suitable for measuring total volume and flow rate with high precision. Pulse output for long-distance signal transmission provides higher transmission accuracy compared to analog output. The method and amplitude of the output signal should also have the capability to adapt to other devices, such as control interfaces, data recorders, alarm devices, circuit protection circuits, and data transmission systems. 1.9 Response time: When applying this in pulsating flow conditions, attention should be paid to the instrument’s response to sudden changes in flow. Some application areas require the instrument output to follow changes in flow, while others only need an output with a slower response in order to achieve an overall average effect. Transient response is often expressed in terms of time constants or response frequencies; the former ranges from a few milliseconds to several seconds, while the latter is below a few hundred hertz. The use of display instruments can significantly prolong the response time. Generally, it is believed that an asymmetric dynamic response of the instrument’s flow rate to increases and decreases increases measurement errors. 2 Conclusion As enterprises fully enter the market, their production and operation management will become more sophisticated, making flow measurement increasingly important. To make the most of flow meters, strict requirements exist for their selection: first, the measurement method must be determined, then the performance requirements analyzed, and only after that can a specific meter be chosen. Therefore, automation instrumentation professionals, having a certain level of professional knowledge, need to understand the various performance requirements of flow measurement instruments and select the appropriate ones based on the specific object being measured. Only in this way can the requirements of enterprises be met, allowing flow measurement to play an important role.