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Combustion Efficiency and the Flare Monitor Output:

The Williamson model FM Flare Monitor is used to control the combustion efficiency of a flare flame and to prevent smoking conditions from occurring. It provides an output signal over a scale of 0 to 2000. When the flame is smoking, the output signal is above 1300. When the signal is below 700, then a low combustion efficiency is indicated. A signal between 700 and 1300 indicates a high combustion efficiency and a non-smoking flame.

The combustion efficiency of a flame determines what percent of the vent gas fuel is incinerated and what percent is vented to the atmosphere. The principle of operation for the Williamson model FM is to monitor the balance of hot carbon-based compounds to hot oxygen-based compounds within the flame. Smoking occurs when there is not enough oxygen to react with the carbon and instead of burning, the carbon particles clump together. When the flame is burning with a high combustion efficiency, then the levels of carbon, oxygen, and hydrogen are all in balance and a ratio near 1.000 (an output value of 1000 +/- 300 = 700 to 1300) is produced. A low combustion efficiency results when the air-to-fuel level becomes too lean (an output value lower than 700). It is not uncommon for flares, particularly when operating under a high turn-down (low vent gas flow) condition and with a ring of steam diluting the fuel concentration, to be operating at a combustion efficiency lower than the ideal range, and readings below 700 are common.

Earlier generations of smokeless flare monitors were designed to monitor the presence or absence of smoke particles. These devices were commonly referred to as monitoring the “opacity” of the flame, but this obsolete approach is flawed for five reasons:

  1. The first flaw is that an opacity-based flare monitor does not respond until after a flame is smoking.
  2. The second flaw is that the alarm set point of an opacity-based flare monitor varies with the type of fuel being burned.
  3. The third flaw is that an opacity-based flare monitor is sensitive to the size of the flame.
  4. The fourth flaw is that the opacity-based flare monitors cannot tolerate high-turn-down (low vent gas flow) conditions.
  5. The fifth flaw is that opacity-based flare monitors produce a false reading when steam is within the optical path.

The Williamson model FM resolves all of these problematic issues: The output signal is independent of the size or position of the flame. It works well under high-turn-down conditions. The alarm set point is the same for all hydrocarbon-based fuels. It is not affected by steam. And, finally, the output signal is continuously proportional to the combustion efficiency of the flame, permitting the flame to be controlled to ideal operating conditions.

The modern approach used by the Williamson model FM does produce an output signal proportional to the opacity of the flame once the flame begins to smoke. Therefore the Williamson model FM meets existing standards and specifications based on the earlier technology. As previously stated, an output signal of 1300 indicates a smoking condition. As the flame becomes more opaque, then the signal increases above the 1300 level.

Soot Formation and Opacity:

Flare-stack operators and monitors will occasionally refer to a parameter they call “Opacity”. The “opacity” of a flame has long been used to characterize the extent to which a flame is forming smoke. There are two common opacity scales. One is the percent opacity, and the other is the Ringelmann Scale.

The most typical way to measure the opacity of a flame is by a visual comparison to an image printed on a card. The user compares the “color” of the smoke to the color on the chart. The number shown is the Ringelmann number. The percent shown is the opacity value.

The Williamson model FM is not designed to provide a measure of opacity, but rather it is designed to assure a high combustion efficiency and to prevent soot formation. However, there is a strong correlation between the sensor reading and the formation of soot. The following is a best-guess for the correlation between the Ringelmann and Opacity values for the measured reading from the Williamson model FM. There is, however, one important caveat: solar reflections off soot particles can cause readings above 1300 to be artificially high. The sensitivity to solar reflections should be seriously considered if applying the model FM to a flare that is expected to regularly produce smoke.


Based upon our personal observations:

Reading below 1300: No soot is being formed. Ringelmann Scale = 0, Opacity = 0% (These are firm values)

Reading between 1300 and 1600: Ringelmann Scale = 1, Opacity = 10-20%

Reading between 1600 and 1990: Ringelmann Scale = 2, Opacity = 20-40%

Reading above 1990: Ringelmann Scale = 3 or more, Opacity = 40% or more.

To learn more about the Williamson flare products download our Flare Monitor Datasheet or click below to connect with a Williamson Expert who will review your application in detail with you.

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