Flare Gas Meter Verification: Zero-Point Calibration and In-Situ Verification

If flare gas meters are not properly and regularly calibrated, their accuracy is likely to deteriorate. Additionally, operators often require annual verification to satisfy legal and regulatory requirements.

Zero-point calibration of flare gas meters during installation, as well as at regular intervals during the meter’s lifetime, is crucial to ensure consistent reliability of measurement.

This article describes the different types of calibration available for your Fluenta ultrasonic flare gas meters.

Multi-Point and Zero-Point Calibration

Fluenta offers two types of calibration, zero-point calibration and multi-point calibration.

Multi-point calibration uses a flow rig to compare meter readings against an extremely accurate master meter. This master meter provides the standard and our system is calibrated against this at multiple points of flow to ensure the required accuracy is met across the range of measurement.

By contrast, zero-point calibration does not involve flow. This type of calibration is performed at production, at the point of commissioning, and at regular intervals in the meter’s lifetime. Instead of the system being calibrated at multiple points, the system is evaluated in a no-flow environment to establish a zero-point reading. If the meter has strayed from this point, then it may be giving incorrect readings, for example showing flow when there is none.

Meter Verification

There are two options for meter verification, in-situ verification, and zero-point calibration described above.

These options differ in their levels of thoroughness and are applicable in different scenarios.

In order to understand the differences, it is helpful to first identify the required factors and zero-point calibration process.

The Four Key Factors  

There are four key factors required to perform a zero-point calibration. These are:

  1. Zero air flow
  2. An accurate tip-to-tip distance
  3. Known velocity of sound of the gas (VoS)
  4. Known pressure and temperature readings

During a zero-point calibration, a Fluenta engineer removes the transducers from the pipe, and puts them in a ‘zero box’. This box provides a sealed environment, ensuring there is no air flow. The engineer uses this alongside the tip-to-tip distance, VoS, and pressure and temperature readings. This process rigorously verifies that the transducers are working correctly. If for some reason they read other than ‘0’, the engineer will input an offset to correct the error.

As well as calibration, the process also allows for an important visual inspection. Occasionally a meter may read incorrectly due to a build-up of debris on the sensors. This is especially true if the sensors intrude into the pipe, as this leaves them particularly vulnerable to fouling. This will impact measurement, however, is challenging to diagnose without removing the sensors from the system.

In-Situ Verification

Meter verification whilst the transducers remain in the pipeline is also an option. However, this will not be as thorough as a zero-point calibration due to reasons relating to the four key factors previously outlined.

  • Zero air flow – transducers are incredibly sensitive, and even wind blowing over the top of the flare stack can impact calibration. Without removing the transducers and inserting them into a zero-box, in which there is no flow at all, it’s difficult to understand if they are truly reading correctly. This is especially important when looking for accurate measurement at low flow rates.
  • Velocity of sound – if the transducers remain in the pipe, then it won’t just be air passing through. To thoroughly calibrate a system, an accurate gas composition is required at the time of measurement. This can be obtained, however requires extra equipment to be installed such as a gas chromatograph. However, this only takes a snapshot, and not a live reading. If the gas composition changes during calibration the result will be invalid.

It should also be noted that the transducers are also not visually inspected as they remain in the pipe and debris in the process may adversely affect the results.

The Health Check

With Fluenta’s UFM Manager software, operators have access to remote in-situ verification in the form of a health check. The health check should also be used as the first step in diagnosing any issue should you have reason to believe the meter is reading incorrectly. For a more in-depth verification process, a zero-point calibration should be performed.

For more information on this subject and guidance on the most appropriate verification and calibration for your process, please refer to your Fluenta certified service engineer or contact us at support@fluenta.com.

If you aren’t currently a customer and want to learn more about how we can help you, please email sales@fluenta.com.

December 9, 2021 | News

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