Flare Operation with Low MFR

Flare Operation with Low MFR Abstract:

By:  Alejandra Garcia, Marcus Hatch and Denise Karstensen

Flares are process safety systems utilized at many manufacturing sites. “The primary function of a flare is to use combustion to convert flammable, toxic or corrosive vapors to less objectionable compounds.” (API 521 paragraph 6.4.1).  A secondary function of a flare is as an air pollution control device.  The long standing assumption was that if the flare was operated in accordance with 40 CFR 60.18 (>300 Btu/scf, smokeless, max exit velocity), it was achieving acceptable combustion efficiency.  The primary focus has been to prevent smoke, which is usually suppressed with steam.  The 2010 TCEQ Flare Study, in which USEPA participated, provided evidence that over steaming a flare educts excess air into the combustion zone that can impact combustion efficiency.  The 2010 TCEQ Flare Study also indicated that there was little correlation between destruction efficiency and smoking; some of the highest destruction efficiencies were found with smoking flares.  Nevertheless, minimizing smoke is one of the explicit requirements of 40 CFR 60.18.

With USEPA's national enforcement initiative on air toxics focusing on flare operation, flare operators should assess whether additional parameters need to be addressed. EPA's enforcement office takes the position that operating in compliance with 40 CFR 60.18 may not be sufficient to meet >98% destruction efficiency if the steam to flare gas ratio is excessive.  While olefins plant flares are designed for high flow rates associated with process upsets, they typically operate at vent gas flow rates of <1% of design capacity while maintaining steam  assist rates intended to protect the integrity of the flare tip and prevent smoking.   These conditions may result in “steam to vent gas ratios” higher than identified in the 2012 EPA guidance on “Parameters for Properly Designed and Operated Flares”.  The challenge is in maintaining appropriate combustion efficiencies with high turndown, low flow hydrocarbon rates.   

A project was installed to improve measurement and control of the steam to hydrocarbon ratio to the flare combustion zone. Equipment design challenges included accurate steam flow measurement and control, as well as accurate vent gas flow and composition measurement. Plants in colder climates pose the additional challenge of getting good steam quality to the tip to mitigate tip icing.  This paper provides the improvements implemented and the results achieved.