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Introduction to the selection, installation, testing and maintenance of mass flow meters

Selection of mass flow meter
The volume of a fluid is a function of fluid temperature and pressure and is a dependent variable, and the mass of a fluid is an amount that does not change with time, space temperature, or pressure. As mentioned above, the flow measurement values of commonly used flow meters, such as orifice flowmeters, turbine flowmeters, vortex flowmeters, electromagnetic flowmeters, rotameters, ultrasonic flowmeters, and elliptical gearmeters are fluid. Volume flow. The amount of fluid involved in scientific research, production process control, quality management, economic accounting, and custody transfer is generally of mass. The use of the above flow meter to measure only the volumetric flow of the fluid often fails to meet the requirements of the people, and it is often necessary to find a mass flow rate of the fluid. Previously, only after measuring the temperature, pressure, density and volume of the fluid, the quality of the fluid was obtained indirectly by means of correction, conversion and compensation. This kind of measurement method has many intermediate links, and the accuracy of mass flow measurement is difficult to be guaranteed and improved. With the development of modern science and technology, some measurement methods and devices for directly measuring mass flow have emerged, which has promoted the progress of flow measurement technology.
Should pay attention to the main parameters
Mass flow accuracy: ±0.002 × flow ± zero drift
Density measurement accuracy: ±0.003g/cm3
Density measurement range: 0.5~1.5g/cm3
Temperature measurement accuracy: ±1 °C

sensor
Ambient temperature: -40~60°C
Medium temperature: -50~200°C
Type of explosion protection: iBIIBT3
Associated equipment: matching transmitter

Transmitter
Working temperature: 0~60°C
Relative humidity: 95% or less
Power source: 220±10% VAC, 50Hz or 24±5% VDC, 40W

When the fluid is at rest, the thermocouples placed symmetrically across the heat source indicate equal temperatures. The commonly used bypass tube mass flowmeter divides the pipe into a small number of small pipes, and then one of the pipes is led out, and the sensor is placed outside the bypass pipe. The measuring component is separated from the medium, but the response is The speed and accuracy are slightly lower. Through the advancement of processing technology, the accuracy and repeatability of the flowmeter have been greatly improved.
Two-hole differential pressure mass flowmeter. Install two identical orifice plates at conduits A, B. Two equal flow pumps are provided in the split line that produce a constant volume flow q in the opposite direction. The pressure difference between the two orifice plates is ΔP=P1-P3=4KρQq, which is proportional to ρ and Q. Where K is the constant coefficient, ρ is the density, Q is the pipe volume flow, and ρQ is the mass flow.
Double impeller mass flow meter. Two impellers with different blade angles are coaxially mounted in the housing, and the middle is integrally connected by a spring. The difference between the torques received by the two wheels causes the spring to twist the angle α. α is proportional to the mass flow M and the angular frequency &owega;, ie α ∝ M & owega;. The M value can be measured by measuring the time required for the angular displacement α. The method of measurement is to mount two electromagnetic detectors on the housing and start counting when the first turbine generates a pulse. When the second turbine produces a pulse, the count is stopped. The time t is measured based on the standard frequency of the counter, and the M value is obtained.
Indirect Mass Flow Meters Indirect mass flow meters are available in three main types: a combination of a speed flow meter and a densitometer, a combination of a throttle (or target) flow meter and a volumetric flow meter, and a throttling (or target) The flow meter is combined with a densitometer.
The mass flow measuring instrument with micro-computer is also called mass flow computer, and it is a versatile new flow measuring instrument. It can input the density, flow (turbine flow meter, throttle flow meter), specific gravity, temperature, pressure, heat and other signals of the fluid.
The mass flow meter should be tested and maintained during the work process. In the process of using Coriolis mass flowmeter, there will be zero drift. Learn to analyze the cause of zero drift and find a solution.
Mass flow meter installation

1. The installation location should be chosen to avoid electromagnetic interference. The installation position of the sensor and transmitter and the cable laying should be as far away as possible from equipment that is prone to strong electromagnetic fields, such as high-power motors, transformers, and inverter equipment.

2. The mass flow sensor should be installed so that the sensor flow direction indicator is consistent with the fluid flow direction. And point the arrow at the same direction as the flow configured inside the transmitter. (Note: The mass flow meter can be used in both directions. If the installation direction is opposite to the actual flow direction, modify the flow direction configuration in the transmitter.)

2. The mass flow meter is a flow meter measured according to the vibration principle of the measuring tube. Therefore, the sensor should be installed with strong support at the near flange of the process pipe on both sides (about 2 to 10 times the pipe diameter) to avoid the instrument and The associated piping produces vibration. If strong pipe vibration is unavoidable, it is recommended to use a flexible pipe to isolate the piping system from the instrument sensor.

4. Because the mass flowmeter works according to the Coriolis force principle, in order to avoid the influence of gravity on the measurement accuracy. Whether the measuring tube of the mass flow meter is installed up or down, the measuring tube should be kept perpendicular or horizontal to the ground plane.

5. The flange faces of the joints should be parallel to each other during installation. The center of the two flanges should be on the same axis to avoid additional stress. It is strictly forbidden to straighten the upstream and downstream process pipes with the sensor, otherwise it will affect the measurement or even damage the sensor.

6. On the upper and lower pipelines of the sensor, it is recommended to install the shut-off valve and bypass to facilitate zero adjustment, routine maintenance and to ensure that the sensor can be in full state when it is not working. Flow control is performed using a regulating valve downstream of the flow meter.

7, the sensor installation direction
Make sure the measured media is completely filled with sensors. Do not collect gas for liquids. For gas or liquid accumulation, it is easy to empty the viscous, dirty, high-condensation medium (can also be equipped with low-point valves on both sides as needed).

8. Newly installed pipes or pipes that have not been used for a long time and have just been activated. The entire pipe must be purged (especially the upstream pipe) before the mass flowmeter is installed to ensure that no weld slag or debris remains in the pipe.
Mass flow meter detection and maintenance
1, zero check (zero calibration)
Zero drift is a problem often encountered in the actual operation of Coriolis mass flowmeters: there are many factors that cause zero drift, such as the installation stress of the sensor, the structural asymmetry of the measuring tube, and the change of the physical properties of the measured fluid. Especially in small flow measurement, the effect of zero drift on measurement accuracy is more serious. Therefore, it is necessary to perform zero check and adjustment on a regular basis.
The zero check should be carried out at least three months. When the production permits, the time interval of the zero check should be appropriately shortened for the Coriolis mass flowmeter installed at the important monitoring point. Whether the zero calibration is completed is the key to directly affecting whether the flowmeter is operating within the accuracy range.
2. Inspection of working parameters
During the use of the flowmeter, it should always pay attention to whether the set working parameters have changed, whether the displayed flow, density, and temperature values are normal. If there is a large difference between the actual and the actual, the method described in the instruction manual can be used. Perform zero flow calibration again. If the above work still does not feel normal after completion, you should check whether the working parameters set inside the transmitter are correct.
3. Observe the flowmeter's fault indication regularly.
Depending on the model, specifications, and manufacturer of the flowmeter, the fault display method and contents vary. For different fault alarm indications, check the product instruction manual to determine the cause of the fault and handle it.
4, regular comprehensive inspection and maintenance
For the flowmeter in use, it should be thoroughly inspected regularly: from the appearance of the sensor, the degree of installation, the vibration of the process pipeline, the indication of the transmitter and the display instrument, etc., and the problems should be dealt with in a timely manner.
5, create a flowmeter file
In order to ensure long-term reliable operation of the flowmeter, the user should establish a flowmeter file, record each inspection, maintenance and verification in detail and load it into the file for better maintenance and use in the future.
6. Maintenance for easy scaling fluid measurement
The flow meter measures the fouling (such as scaling and waxing) fluid. It should always check the operation of the device. When the flowmeter is found to be working abnormally or the deviation is large, the possibility of scaling in the sensor should be considered first. The sensor should be used. Remove it and use appropriate methods such as purging or cleaning.
7, regular calibration
Regular calibration according to the application of the flowmeter and according to the requirements of the standard system.
The mass flow meter features high precision and high stability. In order to maintain these characteristics of the mass flow meter, it is necessary to check and maintain the mass flow meter from time to time. Only when the mass flow meter is in good condition can you get accurate measurement and fully exert the effect of the mass flow meter.

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