Factors that impact the accuracy of mass flow meters saw by thermal dispersion; the flow profile and its conditioning PART II

We continue with the second part of the series of articles about our Sierra mass flow meters.


Calibration in factory.

When measuring a gas with mass flow meters by thermal dispersion, it is important that the manufacturer knows the process conditions of the application that the end user requires for that meter. In this way, the manufacturer can calibrate the instrument according to the application. Without doing those previous steps, inaccuracies in the flow velocity may occur.

During the calibration, in the flow bank, by the manufacturer, a specific amount of gas flowing through the sensor is measured. This process is repeated several times throughout the operating range to determine the relationship between the mass flow rate and the signal for the gas and the sensor subjected to calibration.

Points to consider:

1.- Size of the pipeline

check the size of the pipe at the installation site, the thickness of the pipe and the maximum gas velocity. This information must be transmitted to the manufacturer for optimal calibration.

2.- Composition of the gas

It is essential to calibrate the meter with the gas that the end user will use since each gas has different thermal properties. In addition, any change in the composition of the gas will generate incorrect results. While some gases are pure, such as nitrogen or argon, others, such as biogas, consist of a mixture of gases. In the latter case, it is important to have both the Methane and CO2 content. This information must also be transmitted to the manufacturer for a proper calibration.

3.- Wet gas

If there is moisture or particles in the gas, the installation must be modified correctly to prevent any condensate from reaching the sensor. The primary principle of mass flow meters by thermal dispersion involves the transfer of heat caused by the gas flow. The moisture condensed in the gas that comes into contact with the heated sensor rapidly increases the heat transfer, and the flow meter responds with a peak, giving an inaccurate measurement of the flow rate.

Following the next steps can help solve problems with wet gases when measured with Sierra mass flow meters by thermal dispersion:

  • a) Tilt the sensor 45 degrees into the pipe so that if condensations occur on the pipe wall, the gravity itself will move the condensation away from the sensor.
  • b) When a condensate separation tank is used, the condensate accumulates in the bottom separated from the gas. In this case, the gas flows through the container reducing the speed and where it makes a change of direction. Gravity causes condensates to fall while the gas rises and rises, eliminating moisture from the gas.
  • c) In some cases, a heated tape may be applied to the outer surface of the pipe to prevent condensation.

4.- Straight sections previous

Location of the meter installation is crucial. The disturbances adversely affect the flow profile of the gas flow. For more previous disturbances, a pipeline with more straight sections will be needed to achieve the required flow profile.

5.- Depth of insertion

The sensor of the mass flowmeter sensor should be in the center of the pipe because the higher flow velocity will be detected as far as possible from the walls of the pipe. There are at least two methods for inserting the probe in the center of the pipe.

The flowmeter sensor should be installed in the center of the pipe.

Contact us for more information at MATELCO, S.A tel., or send us a mail from the contact section.

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