China's trade measurement uses quality to settle. Tianchen vPro's mass flow meter can directly measure the quality of fluids, and it occupies a great advantage in foreign trade. Especially for the oil product metering work, it is necessary to grasp the performance of the mass flow meter and the method of daily use. In the later use process, the mass flow meter can be used to the maximum.
First, the working principle of Coriolis mass flow meter
Coriolis mass flow meter measuring part mainly consists of elastic measuring tube, excitation unit, vibration pick-up unit, closed-loop self-excitation amplification unit and other components. The working principle is: the elastic excitation unit maintains the sensitive structure mainly consisting of the elastic measuring tube in the resonance state, the measuring tube performs the “bending main vibrationâ€, and the “Coriolis effect†occurs when the mass flow flows through the vibrating measuring tube. Based on the above "main vibration", the measuring tube produces a "twisted secondary vibration" directly related to the "mass flow" flowing through it; by measuring the "composite vibration" of the measuring tube, the mass flow rate of the fluid can be directly obtained. Obviously, the Coriolis mass flow meter can work stably and reliably. The key is to ensure that it is in an ideal vibration state, that is, to suppress or minimize its influence in practical applications.
Second, the main factors affecting the measurement performance and solutions
2.1 Installation Stress
During mass flow meter installation, if the sensor's sensor flange is not aligned with the central axis of the pipe (ie, the sensor flange is not parallel to the pipe flange) or the pipe temperature changes, the stress generated by the pipe will create pressure. The torsional and tensile forces act on the measuring tube of the mass flow meter, causing asymmetry or deformation of the detection probe, which leads to zero drift and measurement errors.
Solution:
(1) The flowmeter is strictly adhered to when it is installed.
(2) After installation of the flowmeter, call out “zero adjustment menu†and record the factory default zero value. After zero adjustment, observe the zero value at this time. If the difference between the two values ​​is large (the two values ​​must be guaranteed in one The order of magnitude) indicates that the installation is stressful and should be reinstalled.
2.2 Environmental Vibration and Electromagnetic Interference
When the mass flowmeter works normally, the measuring tube is in a vibrating state and is very sensitive to external vibrations. If there are other vibration sources on or near the same support platform, the vibration frequency of the vibration source will interact with the vibration frequency of the measuring tube of the mass flow meter, causing abnormal flow and zero drift of the flowmeter, causing measurement error, and in severe cases, Can cause the flow meter to can't work; At the same time, because the sensor makes the measuring tube vibrate through the excitation coil, if the flowmeter has a large magnetic field interference nearby, it will cause great influence on the measurement result.
Solution: With the continuous improvement of the manufacturing process and technology of mass flowmeters, such as the application of DSP digital signal processing technology and MVD technology by companies such as Micro Motion and Beijing Tianchen Borui, compared with the previous analog equipment, the front-end The digital processing greatly reduces the signal noise and optimizes the measurement signal. When the instrument is selected, the flow meter with the above function should be considered as limited as possible. However, this does not eliminate the interference fundamentally. Therefore, the mass flowmeter should be kept away from devices that generate large magnetic fields such as large transformers and motors when it is designed and installed to prevent interference with the excitation magnetic field. When vibration interference cannot be avoided, isolation measures such as connecting with a flexible tube between the vibrating tubes and isolating the vibration are used to isolate the flowmeter from the sources of vibration interference.
2.3 Measuring the Influence of Medium Pressure
When the operating pressure and the test pressure differ greatly, the change in the pressure of the measuring medium will affect the degree of tightening of the measuring tube and the degree of the Bouton effect, destroying the symmetry of the measuring tube, and thus causing the measurement sensitivity of the sensor flow and density to change. Accuracy measurement can not be ignored.
Solution: We can eliminate or reduce this effect by performing pressure compensation and zero pressure adjustment on the mass flow meter. There are two methods of pressure compensation configuration:
(1) If the operating pressure is a known fixed value, an external pressure value can be entered on the mass flow transmitter to compensate.
(2) If the operating pressure changes significantly, mass flow meter transmitter configuration can be performed to poll an external pressure measurement device, and the external pressure measurement device can obtain real-time dynamic pressure value for compensation. Note: The flow verification pressure must be provided when configuring pressure compensation.
2.4 Two-phase flow problem
Because current flow meter manufacturing technology can only accurately measure single-phase flow, in the actual measurement process, when the working conditions change, the liquid medium will vaporize, forming two-phase flow, thus affecting the normal measurement.
Solution: Improve the working conditions of the fluid medium so that the air bubbles in the process fluid are distributed as evenly as possible to meet the requirements of the meter's normal measurement. The specific measures are as follows:
(1) Straight pipe laying. The vortex created by the elbow in the pipe will cause the air bubbles to enter the sensor tube unevenly, causing measurement errors.
(2) Increase the flow rate. Increasing the flow rate is to make the bubbles in the two-phase flow pass through the measuring tube at the same speed as the measuring tube, so that the bubble buoyancy and the low-viscosity fluid effect can be counteracted (the bubbles are not easily dispersed in the low-viscosity fluid and easily aggregate into the atmosphere) When using a Micro Motion flowmeter, it is recommended that the flow rate is not less than 1/5 of full scale.
(3) Choose to install in the vertical pipe, up flow. At low flow rates, the air bubbles will collect in the upper half of the measuring tube; after the laying of the vertical tube, the bubble buoyancy and the flowing medium can easily discharge the air bubbles evenly.
(4) The use of a rectifier to assist in the distribution of air bubbles in the fluid is better with the use of an air eliminator. 2.5 Measuring the Influence of Medium Density and Viscosity
Measuring changes in the density of the medium directly affects the flow measurement system, thereby changing the balance of the flow sensor and causing a zero offset; and the viscosity of the medium will change the damping characteristics of the system, leading to a zero offset. Solution: Use as much as possible when measuring single or several media with little difference in density.
2.6 Measurement tube erosion
In the use of the mass flowmeter, due to the effects of fluid corrosion, external stress, and foreign matter entering the human body, erosion of the measuring tube is directly caused and the performance of the measuring tube is affected, resulting in inaccurate measurements. Solution: The front end of the flowmeter is recommended to install a corresponding filter to prevent foreign matter from entering; minimize the installation stress during installation.
Third, daily use and maintenance
Through the above analysis, through our continuous practice and exploration, we have formed a relatively complete method for the operation, monitoring, maintenance and troubleshooting of mass flow meters, and achieved good results. The following is introduced to everyone.
3.1 Establishment of equipment files
The construction of the mass flow meter is indispensable, which can facilitate the daily maintenance and management of the mass flow meter. The equipment file should include the installation location of the mass flowmeter, the name of the measurement medium, the working condition parameters of the medium, the diameter of the pipeline, the internal setting parameters of the mass flowmeter, the normal flow, the maximum flow, etc., as well as each verification as far as possible. The certificate of conformity and daily inspection and maintenance records.
3.2 Zero Adjustment
Before the mass flowmeter is put into operation, the flowmeter must be zero-pressed and the steps are as follows:
(1) After the flow meter is installed, the power meter is allowed to warm up for about 30 minutes. During this period, the flow meter internal parameters can be checked, the output loop can be checked, and the data acquisition of the commissioning network can be performed.
(2) Pipeline operation: Turn on the front and back of the valve in order, close the bypass valve, make the measuring medium flow normally in the flowmeter, wait until the flow is stable, cut off the valve behind the watch and ensure no leakage, at this time it is necessary to Ensure that the mass flow meter is filled with media.
(3) Observe whether the "live zero point" of the flowmeter is stable, and after the "live zero point" is stable and the small signal cutoff value is set to zero, the zero point adjustment can be performed.
(4) After the zero adjustment is completed, open the valve behind the watch and resume the normal operation of the piping.
3.3 Implementation of mass flow meter network data monitoring Sinopec's internal metering data collection network uses distributed acquisition workstations and server/client data sharing structures. Data acquisition is divided into three levels: instrumentation, data acquisition, and data management. The network monitors the operating status of field instruments in real time. Once abnormalities occur, they can be discovered within the first time and can be analyzed and processed in a timely manner.
3.4 Zero tracking
Zero stability is one of the important technical indicators of the flowmeter, so pay close attention to zero changes in daily maintenance.
(1) It should normally be checked once every quarter;
(2) In the daily use process, if there is a big adjustment in the process (media change, pressure change, temperature change, etc.), the zero point change should be checked immediately. If the manufacturer's limit is exceeded, the zero point should be recommissioned.
3.5 General Procedures for Troubleshooting Mass Flow Meters
(1) Check whether the power supply and signal wiring are correct;
(2) Check whether the process variables and internal parameter settings are correct;
(3) Initially determine the cause of the fault through the transmitter status indicator, status alarm, and self-diagnosis results;
(4) Check the sensor coil, RTD and core processor resistance;
(5) Check whether there is vibration or electromagnetic field interference near the flow meter installation site;
(6) Check and determine whether the physical state of the measuring medium has changed, such as density, viscosity, slug flow, bubbles, impurities, etc.;
(7) Turn on the bypass, cut off the flowmeter path, remove the flowmeter, check whether the flowmeter is blocked, damaged, and whether the pipeline is unblocked;
(8) Recalibrate the flowmeter and determine if it is normal. If the above steps still cannot solve the problem, contact the manufacturer.
Fourth, the conclusion
In the actual use of the mass flowmeter, objective factors often affect the mass flowmeter's measurement results. We must take corresponding measures to reduce this effect and make the mass flowmeter more accurate. Only by mastering the performance of the mass flow meter can one understand how to use it.
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