Discussion on the measurement methods and related selection considerations of industrial automation flow meters

1.2 Preliminary measurement and issuance

After determining that the flow meter must be installed through the above analysis, it is necessary to have a more detailed understanding of the various conditions and usage requirements of the flow meter, combine the characteristics and types of the relevant fluids, and mark the measurement schemes that cannot and are not suitable in the "preliminary selection table", and use the elimination method for further in-depth analysis and consideration.

1.3 Analyze the indicators of the preliminary selected instruments

Based on the various schemes determined by the preliminary selection, all technical data, selection manuals and samples related to the preliminary selected instruments are collected from the manufacturers of the preliminary selected instruments, and the various specifications and performances of the instruments are deeply understood; then the five factors (i.e. performance requirements, fluid characteristics, installation site, environmental conditions and economic costs, etc.) are considered respectively, and corresponding questions are raised for possible situations, and they are analyzed one by one through list comparison. When considering each factor, the order is usually based on the measurement purpose and measurement focus, usually starting with "performance requirements and instrument specifications", and then considering other factors. For example, if "economic factors" are the main factors for the user, then consider other factors other than "performance requirements and instrument specifications"; in addition, combined with specific circumstances, other migration factors are often considered.

2. Discussion on flow meter measurement methods and selection considerations

2.1 Ten considerations for performance requirements and instrument specifications

After determining the measurement method, the following aspects need to be considered for the performance requirements of the selected flow meter:

First, is it to measure flow or total volume? The measurement of flow and the measurement of total volume are the two main purposes of the object measurement. For example, for continuous proportioning production or process control in pipelines, the main purpose of measurement is instantaneous flow, while for commercial accounting and storage and transportation distribution, the main purpose of measurement is total volume.

Second, accuracy; usually the accuracy level locked by the flow meter specification can be applied to a relatively wide flow range;

Third, repeatability; an important indicator in process control applications is repeatability, which is determined by the manufacturing quality and principle of the instrument itself;

Fourth, linearity; linearity and square root nonlinearity, which are the two main outputs of flow meters. For most flow meters, nonlinear error is regarded as a component of basic error, rather than listed as a separate indicator;

Fifth, the upper limit flow rate and flow range; the so-called upper limit flow rate is the full-scale flow rate usually mentioned. When choosing the caliber of the flow meter, you cannot simply match it according to the diameter of the pipeline. You should choose it according to the upper and lower limit flow rates and flow range of the selected instrument.

Sixth, range; the ratio of the upper and lower flow limits is the range, and its value is positively correlated with the flow range. Usually, linear instruments have a larger range, while nonlinear instruments have a smaller range.

Seventh, pressure loss. For most flow sensors, when the flow direction changes or a static sensor is placed in its flow channel, there will be a pressure loss that varies with the flow and cannot be recovered.

Eighth, output signal characteristics; the output signal plays a decisive role in the selection of the instrument;

Ninth, response time; if the flow meter is used in a pulsating flow environment, the response time of the flow step change should be tested;

Tenth, maintainability: Since it is troublesome and time-consuming to remove and maintain the installed flow meter when a fault occurs, the maintainability factor must be carefully considered.

2.2 Considerations of fluid properties

After the preliminary measurement methods and other preliminary plans are determined according to the fluid type, in order to make the obtained fluid characteristics (parameters and properties) as detailed as possible, it is also necessary to investigate and visit the relevant process flow departments so that the adaptability of the selected plan can be further verified; for the fluid category, there are many fluid characteristics that need to be considered, which can be summarized as follows: 1) temperature; 2) pressure; 3) density; 4) viscosity; 5) lubricity; 6) corrosiveness; 7) abrasiveness, etc.; in addition, for individual places, relevant hygiene requirements must also be taken into consideration; for individual measurement methods, special parameters related to fluid characteristics must also be considered, such as the application of electromagnetic flowmeters, the conductivity of the relevant liquid must be understood. Various parameters related to fluid properties will have different effects on various types of flow meters. Therefore, the selection of the flow meter to be selected is largely dominated by the fluid properties. Therefore, for the selected flow meter or measurement method, it is necessary not only to adapt to the properties of the measured fluid, but also to adapt to the impact caused by the change of a certain parameter of the fluid property during the measurement process. The various parameters of the fluid under the conditions of use and the technical specifications and adaptability of the selected instrument can usually be found in the evaluation manual; for those fluids whose exact composition is not well understood, the relevant users must take the initiative to consult the manufacturer to clarify whether the selected instrument can be used normally.

2.3 Installation considerations

2.4 Environmental considerations

During the modeling process, the expected changes in the relevant instruments and the surrounding conditions must be fully considered. The specific analysis is as follows: 1) Ambient temperature; changes in the ambient temperature will have a greater impact on the electronic components of the instrument and some flow detection parts of the instrument; if the flow characteristics are affected by changes in ambient temperature, some pipelines will need to be wrapped with a thermal layer; 2) Ambient humidity; if the ambient humidity increases, the electrical insulation will be reduced, and atmospheric corrosion and electrolytic corrosion will be accelerated; if the ambient humidity decreases, static electricity will be easily generated; therefore, for problems caused by humidity, the user should anticipate the possible range of changes and verify whether unnecessary problems will occur in the operation of the selected instrument; 3) Safety; when selecting and installing instruments, safety rules must be considered, such as whether the installation environment poses an explosive threat; 4) Electrical interference; Electrical electromagnetic interference is usually generated in power cables, electrical switches and motors, which is also a source of measurement errors.

3. Conclusion

In short, the installation of flow meters and the selection of measurement methods must attract sufficient attention from relevant departments and users. On the basis of fully analyzing and deeply comparing various types of flow meters and measurement methods, five major factors such as instrument performance and fluid characteristics must be fully considered. This will promote the correct installation of flow meters and the correct selection of measurement methods.