Principle and Installation of Mass Flow Meter

 The working principle of mass flow meter

The Coriolis mass flowmeter uses the principle of modulation of the fluid mass flow rate on the vibration of the vibrating tube. The sensor consists of a vibrating tube (measuring tube), a signal detector, a vibration driver, a supporting structure and a shell. The transmitter is based on a microprocessor, which provides driving force to the sensor and converts the signal measured by the sensor into a mass flow signal. It also has the function of compensating and correcting the mass flow and density measurement according to temperature parameters. The output current signal or frequency signal can communicate with DCS according to a certain protocol.

Mass flow meter installation

1) The mass flowmeter works based on the principle of vibration, and in the process of its work, electromagnetic technology is used for vibration excitation, torsion angle detection and detection signal processing, so the mass flowmeter installation position cannot be large. The vibration source of this type cannot be installed near transformers, motors and other equipment that generate strong magnetic fields to prevent external interference from affecting its normal operation.

2) There is no requirement for straight pipe section in the installation of mass flowmeter, but stress should be avoided when connecting the sensor and pipe. If the concentricity between the sensor and the pipeline centerline is deviated during the installation of the mass flow meter, it will cause stress in the vibrating tube, which will cause pressure, tension, or shear force to affect the alignment of the pipeline and cause the asymmetry of the detection probe , Resulting in zero point changes and affecting measurement accuracy. If the concentricity deviation between the pipeline and the sensor is too large, the zero point may not be adjusted. Therefore, reinforcement measures should be taken to stabilize the pipeline near the instrument. When installing the flow sensor, a soft connection method should be adopted as much as possible, and the flange connection screws should be evenly and symmetrically tightened. In order to reduce the influence of residual stress, the flow sensor must be zero-point calibrated after installation. In order to facilitate the installation and zero adjustment of the mass flowmeter, stop valves should be installed at the front and back of the mass flowmeter.

3) The connection cable between the sensor and the transmitter should use the manufacturer's special cable; the transmitter receives low-level signals, so the cable should not be too long. When installing the transmitter, make sure that the partition used to separate the intrinsically safe sensor cable from the power supply and output cable is installed in place; ensure that the sensor junction box and the transmitter housing are sealed to avoid short-circuits that may cause measurement errors or flowmeter failures.

4) Choose different sensor installation orientations according to the different properties of the fluid, and always keep the fluid in the sensor flow tube full. The U-shaped tube of the mass flow meter for gas feed measurement faces upwards to avoid accumulation of condensate in the measuring tube; the U-shaped tube of the mass flow meter for liquid feed measurement faces downwards to avoid gas accumulation in the measuring tube.

The above information is provided by mass flowmeter manufacturer.

 

Electromagnetic Flowmeter Installation Recommendations(Part 2)

6. Installation plus bypass pipe

The pipeline where the flowmeter is installed should have a valve that can cut off the medium in the pipeline to facilitate daily maintenance and maintenance. When the process does not allow flow interruption. A bypass pipe can be added to the pipe where the flowmeter is installed.

7. Install according to the calibration direction

When the flowmeter sensor has a clear flow mark, it should be installed in the calibration direction.

8. Cable entry, cable and conduit

The inlet of the flowmeter, the cable and the threading pipe should be bent downwards and open at the lowest point to prevent water from entering and causing a short circuit.

9. Welding flange

When installing the flowmeter, the flange should be welded first, and then the flowmeter should be installed. It is not possible to weld the flange directly with the flowmeter sensor to avoid the welding slag burning the lining.

10. Distance between converter and sensor

The signal wire between the converter and the sensor cannot be longer than 50m, and the signal wire must be sheathed with galvanized pipe.

11. Grounding measures of the sensor

The flow signal generated by the sensor is very small, only a few millivolts at full scale, so the sensor should be well grounded. The grounding requirements of electromagnetic flowmeters have two aspects:

From the analysis of the working principle of the electromagnetic flowmeter and the loop of the flow induction signal current, the ground terminal of the sensor and the converter must be at the same potential as the measured medium.

Ground, take the earth as zero potential to reduce external interference.

Under normal circumstances, process pipelines are all metal pipes, which are grounded. This requirement is easy to meet. However, in the case of large external electromagnetic field interference, the electromagnetic flowmeter should be equipped with a separate grounding device. The grounding wire should be a multi-strand copper wire with a cross-section greater than 5mm2. The grounding wire of the sensor must not be connected to the common ground wire of the motor or other equipment. To avoid the influence of leakage current. The grounding resistance should be less than 10Ω.

The above information is provided by flow instrument factory.

Electromagnetic Flowmeter Installation Recommendations(Part 1)

1. Ensure that the full pipe is installed

The measuring tube of the electromagnetic flowmeter must be full, and there must be no part full. If the pipeline is not full or the outlet is vented, the sensor should be installed on a siphon.

Avoid installation upstream of the downward pipeline

Avoid the highest point of the pipeline, which tends to accumulate bubbles

2. Horizontal and vertical installation

The sensor can be installed horizontally and vertically, but it should be ensured to avoid the influence of deposits and bubbles on the measuring electrode, and the electrode axis should be kept horizontal. When installing on a horizontal pipeline, the two measuring electrodes should not be directly above and directly below the pipeline. When installed vertically, the fluid should flow from bottom to top.

The inlet straight pipe should be larger than 5D, and the outlet straight pipe should be larger than 2D. Those with specific installation requirements should be installed according to the electromagnetic flowmeter manual. The plug-in inlet straight pipe section should be ≥ 20 D, and the outlet straight pipe section ≥ 7D (D is the nominal diameter of the sensor).

3. Installation when the outlet is empty

When the outlet is in the empty state, the sensor should not be installed in the empty place of the pipeline, but should be installed in a lower place. When the sensor is installed below the pipe, it should be ensured that the sensor is filled with liquid and there is no empty pipe state.

4. Series installation and parallel installation

If several sensors need to be connected in series on the same pipe in sequence, the distance between each sensor should be at least the length of 2 sensors.

If two or more sensors are installed parallel to each other, the distance between the sensors must be greater than 1m.

5. Installation between elbows, valves and pumps

To ensure the stability of the measurement, straight pipe sections should be set before and after the sensor, the length of which is given in the figure below. If this is not possible, a current regulator should be used or the cross-sectional area of the measuring point should be reduced.

The above information is provided by flow instrument manufacturer.