Composition: Infer Molecular Weight (MW) from Speed of Sound (SoS)

Do I know the gas composition? > Composition: Infer Molecular Weight (MW) from Speed of Sound (SoS)


To allow the combustion and destruction efficiencies of a flare system to be estimated, the molecular weight of the flare gas can be helpful.  This is normally derived from the composition of the flare gas.

If taking a sample for laboratory analysis or on-line compositional analysis is not possible, then, if a gas ultrasonic meter is installed on the flare system and the speed of sound is available as an output, then the molecular weight of the gas can be derived as a direct relationship to the SoS.

How it Works

To derive MW from the SoS, we need to understand the relationship of SoS in an ideal gas, this is as follows:


This calculation can then be entered into either the flare meter controller/ flow computer or within the DCS / ICSS and the corresponding MW can be utilised to enable the calculation of the combustion and destruction efficiency.

Utilisation of the inference of MW from SoS is not a common application but can be used when no sampling or analysis facilities are available on the flare systems.


  • No Equipment costs

  • Minimal Maintenance costs

  • On-line continuous calculation


  • An assumption on the adiabatic constant must be made, based on the likely composition of the flare gas

  • Low Accuracy

  • High Uncertainty

Case study

No case study available at this time.

On-line analysis

On-line analytical methods involve extracting a sample from a flare system via an automatic sampling system and analysing it continuously with a field installed analyser system.

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