Main issues of gas level flow measurement and flow meter

1. Reliability
　　
　　Reliability includes instrument quality and maintainability. Flow meters are field instruments. The detection parts are in direct contact with the measured medium and face harsh working conditions. It is unrealistic to require the instrument to be 100% reliable. However, if it is easy to repair and the maintenance cost is not high in the event of a failure, it should be said that it is also an aspect of the reliability of the instrument. Characteristics of flow meter operation:

　 　The instrument should be able to withstand
the influence of chemical corrosion, scaling, abrasion, clogging, phase change, temperature resistance, pressure resistance, etc. of the measured medium; 　　 　　
　 　because the instrument and the pipeline are connected as one with flanges, it is sometimes very difficult to disassemble, repair and replace, especially for
　 　high-temperature high-pressure and large-diameter pipelines, which makes periodic inspection very difficult;
　 　for continuous production processes, disassembly during flow stop is not allowed, and the detection parts cannot be disassembled for maintenance when they fail,
　　 so how to deal with it is a thorny problem;
　 　due to backward equipment technology and poor management in China, fluid media are generally dirtier than those abroad, such as natural gas,
　　 coal gas , water, etc., which puts higher requirements on the performance of flow meters.
　　 The following methods can be used to improve the reliability of flow meters:
　 　improve the quality of instruments;
　 　change the structural form, such as using a continuous flow plug-in structure, or find ways to improve the measurement system, such as
　　 multi-tube parallel pipelines for easy cleaning and replacement;
　 　strengthen on-site maintenance management.

2. Accuracy
　
　　The repeatability of an instrument is a characteristic of the instrument itself, while accuracy is an external characteristic. For a flow meter to have high accuracy, it must first have high repeatability, and then be calibrated with a high-accuracy value transfer system to obtain a high-accuracy instrument coefficient (or outflow coefficient).

　　For the accuracy of flow meter, we should pay attention to the characteristics of this instrument. FCKinghom, a famous British flow expert, said it well: flow meter is one of the few instruments that is much more difficult to use than to manufacture. In the laboratory, it can achieve extremely high accuracy, but in the field of use, once the conditions change, everything will be in vain.

　　The accuracy listed in the instrument manufacturer's product manual refers to the accuracy of laboratory calibration, which is called the basic error. The instrument will produce additional errors when working on site due to the different use conditions and laboratory working conditions. The accuracy on site is the synthesis of the basic error and the additional error. The synthesis is not necessarily a simple algebraic sum, but depends on the specific situation. Therefore, the error estimation of field instruments is a complex task. Only those who are familiar with the characteristics of the instrument and the object being measured and who have mastered the error theory can make a correct estimate.

　The accuracy of the flow meter involves the knowledge of flow value transmission. Here is a brief introduction:

　　Flow is a derived quantity that does not have a physical standard in nature. It is derived from basic quantities (length, mass, time and temperature) under specific conditions. The physical standard of the value (called the original standard) is actually a flow standard device. The basic quantities are combined into derived quantities on the device, and then the value is transferred to one or a group of flow meters, which is called the working benchmark or transfer standard. The transfer standard (the carrier of the value) is used to transfer the flow value to the next level standard (also the flow standard device). With the help of the transfer standard, the flow values ​​of the whole country are unified (consistent). The unification of flow values ​​between countries is achieved by comparing international devices.

　　Among all kinds of detection parameter value transmission systems, the flow value transmission system is the most difficult to establish because the flow value has the following characteristics:

　　(1) Flow is a derived quantity that does not have a physical standard in nature and must be synthesized from basic quantities (length, mass, time, temperature, etc.) under specific conditions;

　　(2) Flow rate is a dynamic quantity. It is a physical quantity that only exists when the fluid is moving. Therefore, it is not only a static combination of basic quantities. Due to its dynamic nature, the flow rate value is affected by many complex factors, such as the interaction between microscopic molecules in the fluid, macroscopic turbulence, vortex motion, etc. In a specific pipeline, it is also restricted by boundary conditions (pipe wall).

　　(3) The flow value needs to be reflected by the physical changes of the fluid medium. Therefore, the medium used for calibration is preferably the working medium. However, there are tens of millions of media, and it is impossible to follow this principle. We have to use a simulated medium, and then transfer the flow value to the working medium through medium conversion.

　　(4) Fluid media in different working states exhibit different physical properties, so the flow value must take this factor into account when in different working states;

　　(5) The accuracy difference between the flow value benchmark and the working instrument cannot be too large (for example, the current benchmark is 10-4, while the working instrument can reach 10-3). The difference in their order of magnitude is not as large as that of the basic quantity or other derived quantities. The error of the standard when transferring the value cannot be ignored. When calibrating the flow meter, the error estimation is more complicated.

　　(6) The value transferred from the reference to the working instrument becomes increasingly complex due to the difficulty in maintaining the reference working conditions, and the difficulty of error estimation gradually increases;

　　(7) The accuracy of flow measurement is not high (the current highest accuracy is less than 10-4) because of its derived dynamic and comprehensive nature.