Methods for improving the accuracy of mass flowmeters

Coriolis mass flowmeter is a kind of mechanical vibration type multi-parameter intelligent flow measuring instrument, it can directly measure the mass flow, density, temperature of the fluid at the same time, according to the mass flow and density parameters measured above can directly derive the volume flow of the fluid.

1. Working principle of Coriolis mass flowmeter

Coriolis mass flowmeter uses the phase difference of vibration frequency of the two halves of the measuring tube (also known as the vibration tube) to measure the mass flow rate, and uses the functional relationship between the resonant frequency of the measuring tube and the density of the measured medium in the tube to obtain the density. Coriolis mass flowmeter can also obtain volume flow qv from two basic parameters: mass flow qm and density ρ.

2. Factors affecting the practical accuracy of Coriolis mass flowmeter

The Coriolis mass flowmeter is an electromagnetic driven mechanical vibration type flow meter. The material forming the measuring tube of the flowmeter is affected by the pressure and temperature of the process medium. The flow measuring pipe (also known as flow vibration pipe) installed in the pipeline will be affected by the mechanical vibration, mechanical stress, mechanical torque of the factory pipeline and so on. In addition, the error characteristics of the Coriolis mass flowmeter itself show a "horn" rather than a flat straight line at different flow rates. In view of the above reasons, measures to improve the practical accuracy of Coriolis mass flowmeter are:

1) According to the actual process parameters of the Coriolis mass flowmeter, the "personalized standard" of the flowmeter is carried out, so as to realize the actual measurement error of the Coriolis mass flowmeter (difference with the standard value) tends to zero within a certain range of the actual use flow point;

2) Make the installation link of Coriolis mass flowmeter well, reduce or eliminate the influence of mechanical stress, mechanical torque and mechanical vibration on the use of the flowmeter;

3) Considering the influence of process pressure and temperature on the measuring accuracy of Coriolis mass flowmeter;

4) Put the Coriolis mass flowmeter into operation.

3, the realization of Coriolis mass flowmeter practical accuracy scheme

3.1 Implementation of "personalized standard" scheme to achieve flow metering near zero error, error formula of Coriolis mass flowmeter: where: E allowed -- allowable error of flowmeter; E0 -- accuracy class of flowmeter; qz -- zero drift value of the flowmeter; q measurement - Current measurement of the flowmeter.

According to the error characteristics of Coriolis mass flowmeter, combined with the actual use process, Coriolis mass flowmeter only works in a relatively small flow range, so we can try to make the measuring error of the flowmeter in a certain range of the actual use flow point (usually the range is controlled as ±20% of the actual use flow point) tend to zero. To this end, the liquid flow standard device can be used to set the flow detection point according to the national mass flowmeter verification regulations (JJG1038-2008), and the actual use of the flow point within the range of ±20% of the point calibration (at least three points: 80%Q practical, Q practical, 120%Q practical), to achieve the "personalized standard" of the mass flowmeter, its main purpose is not only to make its standard results in line with the requirements of the mass flowmeter verification regulations, but also to make the mass flowmeter in the actual use of the flow point within the range of ±20%, the sampling standard of three points of the measurement error tends to zero. In particular, it is necessary to make the measurement error (the difference between the value indicated by the flowmeter and the standard value of the standard device) of the actual use of flow point zero, only in this way, the mass flowmeter "personalized standard" work is completely completed.

According to the above assumptions, "personalized standard" is carried out for a mass flowmeter. It is known from the factory that the actual flow point of the flowmeter is 21.7t/h, and the large flow is: 43.6t/h, according to the requirements of the national mass flowmeter verification regulations (JJG1038-2008) verification, verification results at 21.7t/h indicating error is -0.153%.

Judging from the above data, the measuring accuracy of the mass flowmeter is not very high at the actual use point of 21.7t/h flow rate. Therefore, according to the above data, the flowmeter is adjusted, the flow calibration coefficient is increased by 0.153%, and then the flowmeter is standardized according to the "personalized standard" method. The indicating error of the re-standard result at 21.7t/h is 0.002%.

Through the above practice, the effective implementation of the Coriolis mass flowmeter "personalized standard", so as to achieve the flow measurement near zero error. This practical technique is worth spreading.

3.2 Starting from the installation link, eliminate the influence of mechanical factors on the use of flowmeter. Coriolis mass flowmeter is a kind of mechanical vibration flow meter. In order to ensure the high accuracy of the flowmeter in actual use, we should try to eliminate the influence of vibration, stress, torsion and other mechanical factors on the use of Coriolis mass flowmeter.

3.2.1 Solution to eliminate mechanical vibration Mechanical vibration exists objectively in the actual use of Coriolis mass flowmeter. Since it is a mechanical vibration type flow measuring instrument, there is whether the vibration system of the flow meter itself will be affected by the external vibration system, whether there is a way to make the vibration system of the flow meter itself not transferred to the outside world, and at the same time not to let the vibration of the external vibration system to the flow meter vibration system, try to make the vibration of the two vibration systems isolated from each other, Independent of each other, the effect of mechanical vibration on measurement can be eliminated.

Although the manufacturer of Coriolis mass flowmeters has taken steps to reduce the impact of mechanical stress and torque on the mass flowmeter in practical applications, "stress-free" installation is a prerequisite for the installation of Coriolis mass flowmeter sensors, and is what distinguishes them from other flow sensor installations.

Only the installation of "no stress and no torque" can ensure the accuracy and reliability of the actual application process of Corriolis mass flowmeter. Through practice, an "integrated installation" scheme is proposed: make the sensor coaxial with the process pipeline, and the installation size of the sensor flange is consistent with the reserved size of the process pipeline, and apply force evenly and symmetrically to the connector during installation (torque wrench can be used if conditions permit), so that there is no torque force between the flow sensor and the front and rear connecting pipes.

In practical practice, the following measures can be adopted: using the accurate dimension of the process connector from surface to surface, firstly align and connect the sensor to be installed with the configured upper and downstream pipelines (each with an appropriate length) in a coaxial manner, and at the same time, pay attention to maintain the uniform force of the connector, and then install the upper and downstream globe valves and their external pipes (each with a small section).

These are pre-connected as a whole, then the whole is hoisted and preset on the support column, and the pipe clamp is tightened, and finally welded with the process main line. Pay attention to spot welding alignment in the process of welding, so that the uniform force.

Practice has proved that this is an effective "stress-free" installation scheme. The Coriolis mass flowmeter installed according to this scheme also has obvious benefits during weekly inspection and disassembly. The scheme can eliminate the influence of mechanical stress and torque well, and ensure the accuracy and reliability of the actual measurement results of Coriolis mass flowmeter.

3.3 The influence of process pressure and temperature on measurement accuracy should be considered. When the Coriolis mass flowmeter is calibrated out of the factory, the pressure and temperature of the calibrated medium cannot be consistent with that of the actual used medium. How to eliminate the influence of pressure and temperature on the measurement accuracy? In the case of low accuracy requirements, you can refer to the pressure and temperature correction coefficient provided by the manufacturer to the user (referred to as the coefficient correction method).

In the case of high accuracy requirement, "field process pressure simulation standard" of the mass flowmeter is carried out by "flow standard device that can simulate field working pressure (simulated pressure range: 0.3~1.5MPa)" in the metering center station of Yangzi Petrochemical Company. At present, the temperature of Coriolis mass flowmeters from Emerson, E+H and other companies has little influence on the measurement accuracy, which can be ignored. The effect of actual process pressure on measurement is mainly considered.

3.4 Put the Coriolis mass flowmeter into operation. The "personalized accuracy" and "accurate simulation of field pressure" of Coriolis mass flowmeter are actually consistent with the conditions of the actual application occasions. The ultimate purpose is to make the mass flowmeter suitable for the actual situation of the measured object, so as to ensure the accurate and reliable measurement results. Starting from the installation link, it is to let the mass flowmeter have a stable foundation, have a good "living environment", so as to try to eliminate the mechanical vibration, mechanical stress, mechanical torque and other influences on the use of Coriolis mass flowmeter. After completing the standard and installation of mass flowmeter, how to verify whether the installation meets the requirements and how to ensure the high accuracy of actual use, it is particularly important to put Coriolis mass flowmeter into operation. The specific contents of operation include: check the verification or standard certificate, confirm the configuration of the flowmeter, confirm the electrical connection, power supply, grounding of the flowmeter, confirm whether the installation is standard, and carry out "zero check".