1. Reliability Reliability includes meter quality and maintainability. The flow meter is a field meter. The test piece is in direct contact with the measured medium. Under severe working conditions, it is unrealistic to require the meter to be 100% reliable. If it is convenient to repair when a failure occurs, the maintenance cost is not large, it should be said that it is also an aspect of the reliability of the instrument. Features of flow meter work:
The instrument must be able to withstand chemical corrosion, scaling, abrasion, blockage, phase change, temperature resistance, pressure resistance,
Due to the flange connection of the instrument and the pipeline as a whole, sometimes it is very difficult to disassemble, repair and replace it, especially the high-temperature and high-pressure large-caliber pipeline, which causes great difficulties in the periodic inspection;
For the continuous production process, it is not allowed to dismantle in the middle of the flow, and the detection part fails to be dismantled for maintenance. How to deal with it is a difficult problem;
Due to the backward equipment technology and poor management in China, the fluid medium is generally more dirty than foreign countries, such as natural gas, gas, water, etc. This puts higher requirements on the performance of the flowmeter. The following methods can be used to improve the reliability of the flow meter:
Improve instrument quality;
Change the structural form. If a continuous flow type plug-in structure is used, you can also find a way on the measurement system, such as multiple pipes in parallel to facilitate cleaning and replacement;
Strengthen on-site maintenance management.
2. Accuracy meter repeatability is a characteristic of the meter itself, while accuracy is an additional characteristic. A flow meter has high accuracy, it must first have high repeatability, and then use a high-accuracy measurement value transfer system to perform calibration to obtain a high-accuracy meter coefficient (or outflow coefficient).
For the accuracy of the flow meter , we must pay attention to the characteristics of this meter. The famous British flow expert FCKinghom said it well: The flow meter is one of the few meters that is much more difficult to use than manufacturing. In the laboratory, it can get extremely high accuracy. Degrees, but at the site of use, once conditions change, everything is in vain.
The accuracy listed in the product manual of the instrument manufacturer refers to the accuracy of the laboratory calibration. It is called the basic error. Due to the difference between the operating conditions of the instrument and the laboratory working conditions, additional errors will occur. The accuracy of the field is the basic error. Composition with additional errors, the composition is not necessarily a simple algebraic sum, it depends on the specific situation. Therefore, field instrument error estimation is a complicated task. Only those who are familiar with the characteristics of the instrument and the measured object and master the error theory can make a correct estimation.
The accuracy of the flow meter involves the knowledge of the flow value transmission. Here is a brief introduction:
Flow is a derived quantity in which there is no 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. On the device, each basic quantity is integrated into the derived quantity, and then the quantity value is transferred to a set or a group of flow meters . It is called the working standard or transfer standard, and the transfer standard (the carrier of the value) is used to the next level standard. (Also a standard device for flow). Utilize transmission standards to unify (unify) the volume of traffic across the country. The unity of international traffic volume is achieved by international device comparison.
Among various types of detection parameter value transmission systems, the flow value transmission system is a kind of more difficult to establish, because the flow value has the following characteristics:
(1) The flow is the derived amount in which there is no physical standard in nature, and it needs to be synthesized from basic quantities (length, mass, time, temperature, etc.) under specific conditions;
(2) Flow is a dynamic quantity. It is a physical quantity that actually exists only when the fluid is in motion, so it is not only a static combination of basic quantities, but also because of its dynamic nature, the flow quantity is affected by many complex factors. For example, the interaction between microscopic molecules in the fluid, macroscopic turbulence, vortex motion, etc., are also restricted by the boundary conditions (tube walls) in the specific pipeline.
(3) The amount of flow needs to be reflected by the physical change of the fluid medium. Therefore, it is best to use a medium for verification. However, there are tens of millions of mediums. It is impossible to follow this principle. Transfer the flow value to the working medium through medium conversion;
(4) The fluid media present in different working states exhibit different physical properties, so the influence of this factor must be considered in different working states;
(5) The difference between the accuracy of the flow volume value benchmark and the working instrument cannot be too large (for example, the current benchmark is 10-4, and the working instrument has 10-3), their order of magnitude difference is not like the basic quantity or other derived quantities So large, the standard error in the transmission of the value cannot generally be ignored, and the error estimation is more complicated when calibrating the flowmeter;
(6) The transmission of the value from the reference to the working instrument is difficult to maintain due to the reference working conditions, the impact is becoming more complex, and the difficulty of error estimation is gradually increasing;
(7) The accuracy of the flow measurement value is not high (currently the highest accuracy is not high 10-4) because of the dynamic and comprehensive nature of its derivation.