Considerations When Specifying Multipoint Ground Flares

Paper from the AFRC 2015 conference titled Consideration When Specifying Multipoint Ground Flares

Multipoint Ground Flares (MPGF) have many advantages when compared to other flare technologies. These flares can have very large capacities and operate with very high destruction efficiency. Along with these advantages these flares also have unique design considerations. This paper details the additional considerations and information specified by the purchaser in packages when procuring new MPGF. Purchasers need to select the flare configuration, including 1st stage flare type. The specification includes details on the possible range of the flare gas operational envelope. The availability and type of flare gas enrichment must be noted. Required environmental benefits (efficiency) and community benefits (flame visibility, noise) are also specified. Flare mechanical details are detailed, including requested redundancy in pilots, pilot fuel, and flare protective and control system requirements. Lastly the purchaser may include engineering QC activities such as CFD modeling of flare performance and testing at the supplier's facility to confirm stability and smokeless performance. These additional details are in addition to the items historically included in low pressure flare specifications.

AFRC PAPER NUMBER: CONSIDERATIONS WHEN SPECIFYING MULTIPOINT GROUND FLARES Ian M. Fischer Fired Equipment Engineer, Discipline Technology Lead ExxonMobil Research and Engineering Prepared for Presentation at the 2015 AFRC Meeting Salt Lake City, UT September 9, 2015 CONSIDERATIONS WHEN SPECIFYING MULTIPOINT GROUND FLARES Ian M. Fischer Fired Equipment Engineer, Discipline Technology Lead ExxonMobil Research and Engineering Abstract: Multipoint Ground Flares (MPGF) have many advantages when compared to other flare technologies. These flares can have very large capacities and operate with very high destruction efficiency. Along with these advantages these flares also have unique design considerations. This paper details the additional considerations and information specified by the purchaser in packages when procuring new MPGF. Purchasers need to select the flare configuration, including 1st stage flare type. The specification includes details on the possible range of the flare gas operational envelope. The availability and type of flare gas enrichment must be noted. Required environmental benefits (efficiency) and community benefits (flame visibility, noise) are also specified. Flare mechanical details are detailed, including requested redundancy in pilots, pilot fuel, and flare protective and control system requirements. Lastly the purchaser may include engineering QC activities such as CFD modeling of flare performance and testing at the supplier's facility to confirm stability and smokeless performance. These additional details are in addition to the items historically included in low pressure flare specifications. Introduction: Multipoint Ground Flares (MPGF) have many advantages when compared to other flare technologies. These flares can have very large capacities and operate with high destruction efficiency. Along with these advantages these flares also have unique design considerations. This paper details the additional considerations and information specified by the purchaser in packages when procuring new MPGF. Drivers for Detailed MPGF Specification: MPGF can have very large flaring capacities with burner arrays including tremendous scale up capability. Burners are aligned in stages with an instrumented control system bringing stages on line when the capacity is required, and removing them from service when it isn't. The stages are made up of rows of burners in a field enclosed by a fence. These burners are designed such that they light and cross light for all potential release materials. The flare system, including upstream safety valves, needs to be designed to accommodate the high flare system backpressure that the burners and instrumented control system create. The fence provides protection from the radiation and heat generated within the MPGF. It can also be designed to block a significant portion of the light coming off of the flare. This reduced light is helpful in reducing the visibility of the flare. If this feature is desired, the burners and fence must be designed to achieve the specified flame height. MPGF can be designed to be 100% smokeless over the entire operating range. This can be significantly different than conventional low pressure flares where smokeless capacity may only be 20 or 25 percent of the flare ultimate capacity. The pressure assisted burners within a MPGF do not require the assist medium (steam or air) that low pressure flares require. There can also be a savings in flare system construction costs as the flare gas is under higher pressure and flare headers can be smaller in size. Environmental regulations may be a hurdle as pressure assisted MPGF exit velocities are outside current regulations in some locations. MPGF operate under higher flare gas pressures than traditional elevated flares, some up to 30 psi. This gas supply pressure allows for high exit velocity, stable combustion, and high efficiency. In some locations velocity limiting regulations have been based on other flare technologies or testing completed only at low velocities. This presentation will cover some of the major items that should be covered in MPGF specifications. The first element of a successful MPGF installation is the full specification of operating requirements by the Purchaser. Flare Staging Configuration: The first step in the specification of a flare is determination of the staging system. Flares often experience a very wide range of rates and compositions and can have turndowns of more than 100:1. MPGF are inherently staged flares by their nature. Burners are aligned in stages with an instrumented control system bringing burners on-line when the capacity is required, and removing them from service when it isn't. The stages are made up of rows of burners in a field enclosed by a fence. This system provides the burners with the pressure required for sonic or near sonic flare gas exit velocities and high environmental performance. This inherent staging capability allows MPGF to operate efficiently at low rates. Many applications, however, have a small gas flows during some periods- flows lower than the pressure assisted stages of the MPGF can turn down. Often these low rate streams are due to analyzers, purges, etc. If normal flows are below the rate at which the MPGF pressure assisted first stage can operate, then the first stage will cycle or come into and out of service as required. With the first stage closed the flare header will pressure up as there is no outlet. Upon reaching the first stage set pressure the first stage will open and efficiently and safely burn the flare gasses. As the flaring rate will now be above the rate at which the gasses are entering the system, the flare header pressure will drop until below the flare destaging pressure at which point the 1st stage will turn off. This cycle will continue in applications where base flare rates are low. To prevent this cycling most applications include a lower pressure, lower rate, assisted flare. There are several options available for this portion of the flare, often referred to as the "Low Pressure Stage" (LP). One option is to include a low pressure assisted burner array within the MPGF flare field. This array (often a single runner) can be assisted with air or steam to achieve smokeless performance at low pressures. Another option includes the use of an elevated, low pressure flare located outside the flare fence. This flare can be steam assisted or air assisted as well. The next step in the specification of a MPGF is to define the required performance criterion. Purchasers may consider specifications that may have community benefits (lower visibility, less noise, etc.), or environmental benefits (destruction efficiency requirements). Specification should also include plot space availability. Flare Burner Design and Layout/ Flare Gas Composition: Low pressure flares, the most common type used historically at refineries and chemical plants, are very tolerant of varying flare gas composition. For these flares, streams defining flare design tended to include the highest relieving capacity and the smokeless design capacity. These cases have been typically included in the flare specification data sheet. The pressure assisted burners used in MPGF require additional specification of flare gas composition due to their design incorporating individual burners. These burners have an operating window, and are designed for the range provided in the specification. Robust gas specification is needed to provide stable combustion, crosslighting, and smoke free performance across the operating envelope. It is generally not practical to specify the full myriad of streams that can be sent to the flare on the data sheet; however, Purchasers must, at a minimum, bracket the possible range of streams. In addition to streams that historically defined the flare size, this also includes: • • • • The highest smoking potential stream. This may be the highest molecular weight (MW) or highly olefinic/aromatic streams. This stream often drives the burner size and arrangement; The lowest heating value stream when due to inerts (Low heating value streams due to hydrogen can be excellent fuels.); Highest air demand stream; and Molecular weight extremes. Often each of these cases is associated with different rates. For example the molecular weight extremes are often associated with specific scenarios and are limited to the approximate rates included in those scenarios. Including the rate associated with each of the design cases above allows optimization of the flare design, possibly including different burners on different stages. Flare Capacity: MPGF equipment may be less tolerant to flaring above specified design rates. The flare specification needs to include all design defining capacity cases and the flare operating requirements for each. These design case flow may include the traditional sizing basis for flares often associated with loss of major utility, power or cooling water, trips of major machinery, or large safety relief scenarios. In these cases the purchaser needs to specify: • • • • the required smokeless performance; design allowable flare system back pressure; whether flames must be visually contained within radiation fence; and that the mechanical integrity shall be maintained after the flaring event. There is often another case to consider in the flare design. This case, the remote case, is the design case coupled with unrelated but event worsening factor. It can include the failure of a high availability protective system to function correctly or an incorrect human response. In this rare scenario it may not be necessary to hold all the design requirements listed above, the purchaser may carefully consider the potential impacts of: • • • • smoking; extended length flames; higher backpressure; or allow minor impact on mechanical integrity that does not prevent use of flare. The purchaser's decision will depend on probability of remote event and evaluation of consequences. Additional Considerations: It is common for flare systems to include flare gas enrichment if it is possible that the flare gas composition can be outside the flares capability. The relatively high target heating value for these flares (~800 btu/scf, but is design/application specific) requires consideration of possible flare gas composition and selection of enrichment gas. Enrichment gasses with higher heating values than methane may be required. At most facilities, nearly any composition is possible at low rates. This impacts the selection of LP stage size and enrichment capacity. Heating value requirements will be different for the LP and pressure assisted stages, typically being much lower for the LP stage. The purchaser should carefully evaluate the number of pilots, their location, and their fuel basis. The pilots confirm that piloted burners are always lit. When necessary, multiple pilots and redundant fuels can be specified to provide pilots with high availability. As the flare gas exits near grade within the MPGF, the potential impact of an unignited release should be evaluated. This is a very unlikely event, but it has different impacts than an elevated flare. The burners inherently entrain significant air, resulting in high dilution of the flare gas. Purchasers can review the potential impacts of heavy molecular weight releases, low heating value releases, or releases with toxics present when determining flare specifications. Computational Fluid Dynamics (CFD) can be a useful tool in this analysis. While the flare is designed in accordance with the purchaser's requirements, it is recommended that flare testing be completed as a confirmation. While full scale testing isn't practical, testing can be performed at the supplier's facility. This testing is completed with enough burners to confirm burner stability, smokeless performance, and crosslighting. Purchasers must also work with suppliers to confirm that mechanical facilities are protected from excessive radiation or convective heat. Heat can be a concern for instrumentation within the flare fence or mounted on it, and for mechanical components. Additional mechanical features that are detailed on the purchase specification include: • • • • • • • • • Design temperatures, maximum radiation, and equipment protection requirements for the flare, nearby equipment, and locations where personnel may be present; Mechanical, instrument materials, shielding; Instrumented systems, including flare protective and control system (flare logic controller criticality must be defined, e.g. SIL rating, and the supplier must be defined) Staging and bypass valve technology specification; Flare visual monitoring requirements, including engineering for hot operating conditions if located physically near flare; Environmental requirements; Design wind velocity conditions for mechanical components; Pilot design wind and rain requirements; Design wind for crosslighting (low and high conditions); • Construction plan for clean construction, including removal of materials after the flare knock out drum that could get clogged in the burners. Lastly, there is additional considerations for designs that are outside the supplier's experience base. First of kind features should be identified and evaluated for their value vs. the risk that they don't function reliably. It should be understood whether these features are driven by the specification or vendor wishes, and if they provide cost savings or functionality improvements. Moving outside vendor experience requires additional scrutiny and risk management. Conclusion: MPGF have many advantages when compared to other flare technologies. This paper detailed the additional information specified by the purchaser in packages to procure new MPGF. ©2015 ExxonMobil With regard to ExxonMobil, to the extent the user is entitled to disclose and distribute this document, the user may forward, distribute, and/or photocopy this copyrighted document only if unaltered and complete, including all of its headers, footers, disclaimers, and other information. 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