The Turndown Dilemma

February 20, 2020 at 3:30pm - Flares for Regulatory Compliance Track

4C HSE Conference February 19-22, 2020 in Austin, TX

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Clayton A. Francis - Application Manager - Zeeco

Nothing in your plant must turn down like a flare. Many types of equipment (variable frequency drives, process heaters, etc.) can perform at turndown rates 10:1, sophisticated electronic transmitters might reach 1000:1, yet a large refinery flare can have a turndown of 100,000:1. These extremely low flows are not optimal for the flare, and it is difficult to reach precise control in these low ranges. Even with the required precision in the new rules, most steam flow and flare flow measurements have issues accurately reflecting the minimum rates – creating errors in the program.

The new operating rules, first made for the refinery sector but now quickly promulgating throughout the petrochemicals and gas industries, came about largely due to this turndown dilemma. Significant flare rates are luminous and robust in their combustion. However, the normal, day-to-day rates risk dilution from the smokeless injection utility. This unintentional venting is the most well-known and well-understood consequence of vast flare turndown.

For steam flares, it is largely the air transported to the combustion zone that creates smokeless flaring, not the steam itself. The ratio of air entrained by steam is predicated on the velocity of the steam injection. Therefore, the steam performance at turndown (i.e., normal, day-to-day operation) is non-linear, and published ratios can allow smoking. Having implemented very sophisticated flare flow measurement and flare gas speciation technologies, several operators have been surprised their newly commissioned control programs allowed smoking at low rates and required manual intervention. At minimum, more complex injection calculations are required to capture the non-linear portion of the steam control curve.

For air flares, the most common VFD application is insufficient to reach the net heating value dilution parameter (NHVdil) without supplemental gas. While motor and drive combinations are commonly capable of turning down to 3 hertz, many sites lack the line rectification or power infrastructure to permit blowers to operate so low. Operators program clamps to prevent blowers from operating any slower than 20 hertz but doing so dilutes the combustion zone and invokes enrichment requirements. A combination of controls approaches, such as inlet vane dampers in series with a VFD-controlled vane axial blower, can alleviate smoking and diluting issues.

For both steam and air flares, it is possible to suppress the flame inside the flare barrel with the over application of steam or air. The great imbalance between the smokeless medium injection rate and flare purge gas creates a slight increase in ambient pressure above the tip. This effectively caps combustion inside the tip where the gases coke and exit as smoke. It is counter-intuitive, but flares can smoke from both the over-application and the under-application of air. It is important to distinguish between the two in order to respond appropriately.

Combustion zone net heating value (NHVcz) is a necessary quantitative tool to ensure healthy combustion; however, it is an indirect approach that assesses flame quality by measuring flows and calculating smokeless utility rates. There are direct, qualitative approaches to understanding and controlling the combustion zone that can be useful in overcoming the turndown gap. The simplicity of direct control has value in addressing many of the shortcomings above. 

Contact: Clayton Francis

Applications Manager

Phone: 918-258-8551 [email protected]

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