The use of flare stacks to burn off harmful emissions is common in hydrocarbon processing. Flare stacks are among the most vital safety mechanisms in industrial plants; for example, they are used in natural gas processing, petrochemicals, chemical production, and refineries.
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Burning away flammable gases released by pressure relief valves both supports the disposal of excess gases and avoids potentially dangerous unplanned pressure build-ups.
Burning the vent gas is generally considered to be much more environmentally responsible than releasing this directly into the atmosphere. This is typically done by balancing the mix of hydrocarbons with air and steam in order to ensure a clean flame without harmful emissions.
This process is also useful in supporting planned combustion over short periods, for example, as part of plant start-ups and shutdowns.
It is also important to monitor oxygen levels in the flare, as well as at the header before reaching the flare. This is key to ensuring safety and maintaining efficiency while also complying with environmental targets.
The Importance of Accurate, Continuous O2 Measurement
Accurate O2 monitoring at the burner prevents the buildup of a potentially explosive mixture of flammable gas and O2, eliminating the risk of an ignition triggering a flashback of vented gases into the plant.
The safe level for O2 in vented hydrocarbons is between 0 % and 1 %, meaning that an alarm will typically be triggered at O2 levels higher than 1 %. It is important that waste gases meeting at the header en route to the flare stack do not increase O2 levels above 1 %.
Preventing an explosive gas mixture in both scenarios requires the use of an accurate, rapid-response measurement system capable of detecting low levels of O2, essential to plant safety.
O2 Measurement Solutions
The measurement of O2 levels in hazardous situations, for example, the flare stack and headers, has historically involved the use of paramagnetic technology, generally by fitting extractive analyzers with a paramagnetic cell.
Oxygen is highly paramagnetic (meaning it is attracted to a magnetic field), while other gases are only weakly affected by magnetic fields. It is, therefore, possible to measure this attraction in order to calculate O2 concentration.
Paramagnetic analyzers require a sampling system, but they provide a rapid response to changing O2 levels. Background hydrocarbons vary considerably however, and this can lead to minor errors on the paramagnetic analyzer. A Tunable Diode Laser (TDL) measurement is recommended in these cases, offering improved safety and more accurate measurements.
This may involve replacing the paramagnetic measurement with an extractive measurement or conducting it as an in-situ measurement, offering a path-average measurement with an extremely rapid response.
It is also important to note that the pressure at the vent header is generally dictated by the process pressures connected to it. This means that the sample will be considered pressurized.
TDL O2 analyzers from Servomex can accommodate process pressures up to 2 bara (29 psia), while the company’s paramagnetic process analyzers can be used with vent pressures up to 3.1 bara (45 psia). Both of these measurement techniques offer advantages and disadvantages.
Evolving Accuracy in Measurement
Conventional paramagnetic, electrochemical, and zirconium oxide instruments can effectively deliver many measurements, but TDL technology offers considerable advantages in terms of reliability, response speed, and overall cost-of-ownership.
TDL analyzers offer notable benefits when used for combustion control, helping balance the levels of excess oxygen and combustibles, such as carbon monoxide, in fired heaters. These benefits include efficiency, reliability, cost of ownership, and improved safety.
Zirconium oxide sensors provide accurate, continuous O2 measurements, and some manufacturers offer an integrated O2 and combustibles sensor as part of the same analysis solution. The widespread use of zirconium oxide cell technologies has been impacted by the introduction of TDL technologies, which offer significant advantages.
Studies have shown that O2 measurements for both zirconium oxide and TDL analyzers are relatively similar, with both technologies generally working well. The TDL measurement for CO has a faster response time than alternative methods, such as thick-film calorimetry.
TDL analyzers also differ from other analytics solutions in that they measure along the process, delivering a path-average measurement that more rapidly highlights changes in gas concentration along the length of the process, rather than at a single point.
TDL analysis is highly responsive, quickly responding to changes in the concentration of the measured gas. It is also highly specific to the gas it has been configured to detect, offering a highly accurate and reliable reading.
Understanding Tunable Diode Laser Technology
Tunable Diode Laser technology leverages a single-line, monochromatic spectroscopy technique that offers a range of measurement advantages, including continuous, fast, and in-situ measurements, highly stable calibration, and the avoidance of cross-interference from other gases.
TDL analysis systems comprise a laser light source, an optically accessible absorbing medium, transmitting optics, receiving optics, and detectors.
Signal information is contained in the gas absorption line shape, which is acquired by scanning the laser wavelength across the absorption line. This approach reduces the measured signal intensity, which is then detected by a photodiode before being used to determine gas concentration, among other properties.
Servomex’s TDL analyzer range employs a second-harmonic detection (2f) modulation technique that provides improved accuracy, sensitivity, and measurement reliability. This is especially effective in low-concentration measurements, where gas-molecule absorption lines tend to be weak and may be close to other potentially interfering components.
Choosing the right absorption line is, therefore, essential. The analyzer’s sensitivity yields a detection limit of 0.02 % for O2, with a measurement update rate of 5 readings per second.
Acknowledgments
Produced from materials originally authored by Servomex Group Limited.
This information has been sourced, reviewed and adapted from materials provided by Servomex.
For more information on this source, please visit Servomex.