Source: China Instrument Network
Core hint: the verification of measuring instruments, which way to check the flow meter depends on the measurement system's requirements for measurement uncertainty, the type of flow meter being inspected, the purpose, the verification conditions available, and the verification costs.
The verification of measuring instruments usually adopts two methods: solid flow verification and dry type verification. The way to check the flow meter depends on the requirements of the measurement system for the uncertainty of the measurement, the type of flow meter being inspected, the purpose, the verification conditions possessed, and the cost required for the verification. In general, flow meters used for liquid measurement(such as crude oil and water meters) basically use real flow verification, while the vast majority of flow meters for gas measurement(such as differential pressure flow meters for natural gas trade settlement) use dry verification methods. Only a handful of critical flow nozzles are used for online stream verification or offline verification.
Selection of several verification methods
Validation of flow measuring instruments usually adopts both solid flow verification and dry type verification. The way to check the flow meter depends on the requirements of the measurement system for the uncertainty of the measurement, the type of flow meter being inspected, the purpose, the verification conditions possessed, and the cost required for the verification. In general, flow meters used for liquid measurement(such as crude oil and water meters) basically use real flow verification, while the vast majority of flow meters for gas measurement(such as differential pressure flow meters for natural gas trade settlement) use dry verification methods. Only a handful of critical flow nozzles are used for online stream verification or offline verification.
The difference of several verification methods
1, the difference in the conclusion of the test
The total uncertainty of the flow measurement of the meter is synthesized according to the error processing method based on the measurement results and uncertainty of the relevant parameters. The uncertainty range of the flow meter is determined indirectly with a certain degree of confidence. It can not give a specific error value. It is usually premised on a large amount of rich experimental data and standardized technical requirements, maintaining the experimental and consistent characteristics of measurement. For example, the standard orifice plate throttle device, critical fluvial churchy nozzle and so on have been quite mature dry verification technology. Taking the orifice flowmeter as an example, the outflow coefficient formula is based on extremely rich and sufficient experimental data. The error range of the outflow coefficient given on the standard is: not more than 0.6 %. The uncertainty range can only be given with a certain confidence degree when synthesizing the uncertainty of orifice flow measurement.
The real flow verification, especially the online real flow verification, is most consistent with the accuracy, consistency, traceability, and experimental measurement characteristics. It can achieve true flow meter calibration or assignment, and can ensure the continuous and closed of the value transfer or traceability. Offline verification gives the error value or flowmeter coefficient of the flow meter under the verification conditions. However, because the actual operating conditions and installation conditions are different from the verification conditions, the relevant physical parameters of the medium and even the medium itself are different. In fact, this verification is not a true calibration or assignment. Strictly speaking, the off-line verification results of the flow meters can only explain the measurement characteristics under the verification conditions. Most of the actual field environmental conditions, installation conditions and operating conditions of the instruments are very different from the verification conditions. This will bring additional errors to the flow meter, and the amount of additional errors is always subjective to a certain degree of experience. Therefore, offline verification does not require high flow measurement results, or even if there are additional errors, it can meet the expected measurement requirements. It is a simple and easy choice.
2, the degree of modification of physical parameters is different
Almost all flow measuring instruments are affected by the physical parameters of the medium being measured, but the degree of influence is not the same. For physical parameters that can express their influence on the flow measurement results in explicit functions, as long as the actual values of these parameters are known, they can be corrected, such as the relative density of natural gas, the compression factor, and the isotropy index of the orifice flowmeter. The impact of measurement. However, for most flow measuring instruments, the influence of physical parameters on their measurement performance is difficult to accurately express by mathematical formulas. For example, in liquid measurement, volumetric flow meters and velocity flow meters are very sensitive to changes in liquid viscosity., Especially at the low viscosity and the lower limit of the instrument measurement range, there is no universal viscosity correction formula. In the measurement of natural gas flow, the change in natural gas density has a significant impact on the velocities such as turbines and vortex streets. If considering whether the results of the flow meter using air as a medium under low pressure can be directly used for natural gas under high pressure, Online real flow verification becomes the only choice to completely eliminate physical parameters, because dry type verification and offline verification can not eliminate the impact of physical parameters on the above flow measuring instruments.
3, the degree of modification of the impact of operating conditions is different
The operating conditions or operating conditions of the flow meter directly affect its measurement performance. The most direct effect of the operating pressure or temperature change on the flow meter is the change of its measurement cavity, and its indirect influence is the viscosity and density of the measured medium. Indirect effects can be considered when correcting physical parameters. Due to the complexity of the structure and shape of the flow meter and the discrete nature of the assembly, it is almost impossible to use the calculation method to accurately correct the cavity with changes in operating conditions. It is also impossible to fit empirical formulas that meet the accuracy requirements for all flowmeters based on test data.
For volumetric flowmeters, changes in the volume of the measurement cavity will lead to changes in the volume used as the measurement base, and will cause changes in the internal leakage volume, which will affect the flow measurement results. For the speed flowmeter, the change of operating conditions will cause the change of its circulation area, which will lead to the change of instrument coefficient. If the operating conditions of the flow meter during verification can be the same or close to those of the actual use, the offline verification can meet the requirements. However, due to the complexity and variability of the operating conditions, offline verification often can not restore the actual operating conditions. Only online verification can solve the problem of high accuracy flow measurement, otherwise consider additional errors.
In summary, for flow measuring instruments, dry verification applies a very limited number of instruments, and it can only determine that the uncertainty range of flow measurement can not directly calibrate or assign the flow meter. Offline verification is suitable for situations where the accuracy of traffic is not high. It reflects the dynamic characteristics of traffic traceability, but it is difficult to trace the source or close the transmission chain. The online real flow verification best meets the dynamic traceability characteristics of the flow value. It fully considers many effects such as physical parameters, operating conditions, environmental conditions, and installation conditions, and ensures that the verification conditions are fully consistent with the actual use conditions. The measuring instrument for trade settlement, Both buyers and sellers hope to use online streaming verification to achieve true measurement fairness.