November 2024
Environment and Safety
Safe design and operation of burn pit flare systems
This article will mainly focus on horizontal flare systems and grade-level burn pits. Design considerations, operational challenges and several lessons learned will be discussed.
In oil and gas fields, including refining and petrochemicals facilities, flare and relief systems are considered the last line of defense to accommodate all operating scenarios safely. Flare and relief systems are usually designed for overpressure scenarios that cannot be eliminated using instrumentation or operational procedures. Therefore, the flare and relief system's design is critical to the facility's safety. Elevated flare systems are the most common systems applied and in service, compared to horizontal grade-level burn pit flare systems. This has led to a lack of knowledge in industry about the horizontal grade-level burn pit flare system. This article will mainly focus on the horizontal flare systems and grade-level burn pits. Design considerations, operational challenges and several lessons learned will be discussed.
Burn pits are located at grade level, where fluids enter through a horizontal tip and are ignited and flared as they enter the pit. Usually, a burn pit flares two-phase streams that cannot be separated easily with knock-out (KO) drums or other means. Most burn pit flare systems are designed to handle some amount of liquids. Alternatively, burn pits are used in remote areas in intermittent operation where elevated flares could be difficult to control due to the lack of proper infrastructure to knock out and handle associated liquids in the stream. Burn pits are considered to be a cost-effective solution for pipeline maintenance and other planned activities. Burn pit flares do not require a structure like elevated flares and are more cost-effective. The trade-off is the need for a sterile zone, an area around a flare system where personnel cannot enter without following health and safety procedures. Burn pits are attractive for locations with a large amount of space—an additional benefit is that all components are at grade level and cranes or lifts are unnecessary for maintenance (FIG. 1).
FIG. 1. A typical burn pit design.
Types of burn pits operation. There are two types of burn pit operation: those in continuous (emergency) operation, and those in maintenance (intermittent) operation.
The design considerations for a continuous (emergency) burn pit should match the same requirements of elevated flare systems in terms of continuous sweep gas on the main header and fuel gas through pilot lines. Also, this kind of operation will require dedicated main and back-up ignition and detection systems for each pilot to ensure redundancy. This is critical to maintain the operation and avoid the potential of a hazardous flame-out scenario.
Conversely, maintenance (intermittent) operation burn pits are widely used to perform planned maintenance activities for pipelines. Such burn pits are used by the author's company, which has installed hundreds of maintenance burn pits. The sources to these flares are intermittent, occasional or rarely used. Such prolonged non-use of these flare systems can lead to deterioration of the components that could affect reliability and operational safety; therefore, the authors’ company's operation has developed stringent conditions and procedures to govern the proper maintenance of equipment and risk of such operation.
Typically, maintenance burn pits are fully manually operated during depressuring activities. Furthermore, all sources that could be diverted to the burn pit for depressuring activities are positively isolated with proper isolation valves and blinds to avoid any potential of passing the fluid to the atmosphere. In other words, maintenance burn pit systems are not serving a live operation, such as vents, pressure relive valves (PZVs), emergency isolation valves (ZVs), level control valves (LCVs), pressure control valves (PCVs), motor operated valves (MOVs), instrumented or physically operated valves that could be operated remotely without manual operation by personnel. These maintenance burn pit systems are out of service and idle between applications when maintenance is required on the pipeline.
DESIGN CONSIDERATIONS
Several aspects should be considered for the proper design of burn pit systems to ensure they meet all operational requirements.
Mode of operation. The mode of operation or the type of burn pit operation should be determined and clearly defined by the system supplier. In summary, the burn pit is designed for emergency (continuous) or maintenance (intermittent) operation.
Sizing scenario (flare and relief/blowdown analysis). The burn pit header and tip sizing should accommodate the anticipated full facility relief scenario, which could be multiple or single scenarios in parallel. To complete this, a detailed analysis of the burn pit network should be performed, beginning by identifying each source in the burn pit network.
The analysis should consider various engineering aspects such as condensation, flashing and other applicable parameters. Determining loads, type of fluid, composition, temperature, pressure, flow regime, Joule-Thomson effect (especially in manual depressuring scenarios) and others is critical to ensure a proper burn pit design. The design includes the depressuring pipeline design from the depressuring valve to the burn pit tip.
The sizing should include worst-case scenarios of vapor and liquid releases. In addition to defining the maximum flowrate for the system (which sets the diameter of the flare tip and flare header), the flowrate and duration of any long-term cases should be defined (including liquid flowrate) so the burn pit area and depth can be properly sized.
Hydraulic analysis. Detailed hydraulics analysis should be performed using a flare net or other tools to assess the overall hydraulics in the network. Therefore, the header sizing and design should be validated through the model. For each scenario, hydrate formation, freezing possibilities, two-phase flow and the potential of vibration [including water hammering, flow-induced vibration (FIV) and stress analysis] should be identified and assessed in such an analysis.
Hydraulics analysis with a flare analyzer or other alternative can provide a screening of stress analysis. However, detailed analysis should be conducted by the engineering consultant, where deemed applicable, to determine the required civil and piping work to accommodate the process design basis that will result from relief and hydraulics analysis.
Metallurgy. Material selection is a critical aspect of burn pit design, and should consider the worst-case scenario of the flow, composition, velocity, high and low temperature, pressure, corrosion, type of fluid, duration of anticipated reliefs and other applicable parameters.
Low-temperature conditions can be estimated using appropriate tools, such as a blowdown tool. The worst-case scenario can vary for each parameter, so the selection should encompass all the controlling scenarios, including the sizing scenarios for the burn pit or other system components.
Part of the hydraulics and metallurgy analysis should consider whether header segregation is required (i.e., multiple separate burn pit headers and tips). This is necessary if there is any potential of mixing wet streams and cold streams that could promote the freezing and plugging of relief lines. Due to temperature, pressure and corrosion concerns, segregation could be required on another basis.
Typical construction materials for the upper half of the flare tip are American Iron and Steel Institute (AISI) Grade 310 stainless steel. Stainless steel, such as AISI 310, offers the best performance for heat resistance at a reasonable cost. Components within the berm/refractory can be carbon steel. Low-temperature steels like AISI 333 and 304 can handle the extremely low temperatures required for cryogenic service. Pilot materials will be a mixture of stainless steel and carbon steel.
Radiation and dispersion analysis. The maximum instantaneous relief load typically dictates flare and relief system radiation analysis.
As part of the radiation analysis, maximum flare radiation intensities for personnel exposure should be determined. For burn pit flares, the last 100 m (328 ft) of flare gas supply headers and auxiliary piping are typically buried underground or protected by radiation shields as per structural analysis (SA) practices. In addition, ignition panels and flame monitoring instrumentation should be protected with a radiation shield and located at a safe distance from the burn pit. Burn pits contain a fence that restricts personnel access and is located at the boundary of the allowable limit of personnel radiation exposure as per SA standards and the American Petroleum Institute (API)-521 (TABLE 1).1
Dispersion analysis is required to determine the level of toxic gas content in the atmosphere at grade caused by unignited relieving liquid/gas. A proper subject matter expert (SME) should review the analysis from a process, environmental and safety perspective. Dispersion analysis is more critical for a burn pit than an elevated flare because the vapor exit point is at ground level. Additionally, the burn pit could become a source of ignition for any adjacent fuel sources.
In a dispersion analysis for flame-out scenarios, the design rate in the dispersion analysis may not be the same as in the flare-sizing case. Dispersion studies can determine the high toxicity and flammability of specific components in the flared gas. To account for system dynamics accurately, the SA requires a dispersion analysis based on several relief load scenarios: 100%, 75%, 50% and 25% of the identified process cases. Note that for applications where the dispersion of hazardous materials is critical, a burn pit flare is probably not the best choice.2
Location. The selection of the burn pit location should consider the prevailing wind in two aspects: proximity to the facility and orientation of the flare tip direction. First, for the optimum location of new burn pits, normal wind direction (wind rose) must be considered. Burn pit flares should be located in the prevailing crosswind direction from the facilities to prevent the flare from igniting any gases accidentally released from the plant.
If the burn pit location is placed downwind from the facility, a risk assessment study evaluating a hazard and flammability analysis for gases released from the process and ignited by the flare should be conducted in line with spacing requirements or, if necessary, by the process safety team. Evaluating the possible relocation of the burn pits or avoiding preforming the controlled depressuring in high wind time is recommended.
The second aspect is to avoid the prevailing wind blowing directly into the outlet of the burn pit tip (headwind: 180°). Headwind is considered the main root cause of burn pit tip/pilot failures due to promoting internal burning and flare tip impingement (FIGS. 2–6).
FIG. 2. Burn pit tip/pilot failures.
If the burn pit must be located where a headwind into the outlet of the flare tip exists, increasing the velocity to the tip with higher rates of purge/fuel gas during high wind is also recommended. That condition should be governed by the operating procedures established for the burn pit systems.
Smokeless operation. The tip vendor should provide the optimum design to meet the highest practical smokeless capacity. For the existing facilities, the enhancement of smokeless operation could be evaluated; however, any modification should be assessed from all aspects, such as back pressure and radiation dispersion.
Furthermore, sonic tips, air assist and other types of designs can address the need for smokeless operation. Burn pit flares are often utility tips, meaning they will not flare appreciable amounts of gas without smoking. Methods of smoke suppression are steam, air and pressure assist. Steam-assisted flare tips operate by injecting a high velocity of steam around the tip. The steam injection's momentum will pull air around it into the combustion zone. Steam assist typically provides the highest smoke suppression at the lowest cost.
Air-assisted flares operate similarly by using a blower to force low-pressure air into the combustion zone. Alternatively, some flare tips can be designed to use high-pressure air injected into a similar manifold. The final method for smoke suppression is to use the available pressure of the gas to allow a high exit velocity. As mentioned above, this can be at sonic velocities. The flare will typically burn without smoking if the exit velocity is sonic.
Additionally, the pressure assist tips can have a fixed area where smoking may occur at low flowrates when the pressure drops below 10 psig. It should be noted that sonic flare tip design would require a higher backpressure on all upstream equipment, so this should be evaluated in the relief system analysis to determine if it is acceptable. The variable area pressure-assisted flare tip is a technological improvement to the fixed pressure-assisted flare tip. The variable area tip operates with a mechanical spring to open or close the exit area to maintain sonic velocities at all times, eliminating smoking at lower gas flowrates.
Ignition and detection systems. Continuously operating flare systems, including burn pits, must be equipped with continuously burning pilots that should be equipped with a primary and back-up ignition system.
Normally, various ignition system types, such as high-energy ignition (HEI) and high-voltage ignition, will be assigned as the primary system, while back-up systems include the flame front generator (FFG) ignition system. This is also applicable to the detection system, where each pilot should be equipped with two independent detection systems—thermocouples are typically the primary system and other systems such as acoustic, ionization or infrared technologies should be used as back-up monitoring systems.
Continuous pilots are applicable when a flare system is mandatory for all uncontrolled relief conditions. Relief valves, process vents and automated valves are routed to continuous flare systems. The requirement for permanent ignition systems and continuous pilot operation is not mandatory for the hundreds of flare systems used by the authors’ company for maintenance (intermittent) use. Many intermittent flare systems are utilized for planned and scheduled maintenance activities, and different requirements can be applied.
Installing a complete pilot ignition system with a back-up system can be costly, considering these systems' rare usage. Additionally, the remote location of these sites can make regular maintenance of components difficult, resulting in component deterioration and causing reliability issues when the system is needed.
Therefore, it is recommended to utilize a mobile ignition system that should be maintained and tested frequently to ensure the system is ready for use at any time. Other alternatives, such as flare guns, are not recommended due to safety concerns and operating limitations (e.g., obtaining certifications). Pilot detection systems typically utilize thermocouples, though other detection systems can sometimes be used to monitor the pilots and/or the flare flame.
MOBILE IGNITION SYSTEMS
Supplying intermittent operation burn pit flare ignition systems in a portable or mobile arrangement presents numerous advantages. The assembly enables ignition capabilities, fuel gas and power to be accessible in remote locations of a facility where a burn pit system is generally located.
These mobile ignition systems would typically consist of an electrical control panel powered by a battery that is charged by solar panels, propane cylinders for fuel gas, a piping spool for fuel gas regulation, nitrogen cylinders for purging prior to use, and the necessary conduit/hose and electrical wire required for junction box connections. The solar panels are the sole source of power, charging the batteries to allow operation without any incoming fixed electrical power.
The system can be utilized as needed for the burn pit system and stored elsewhere when not in use. This also allows for easy transport of the ignition system around the plant and intermittent use of the ignition system in line with the intermittent or non-frequent burn pit operations. It also allows more frequent and thorough ignition system maintenance to ensure it is in proper working order when needed.
System purge requirements. Safety during flare operation requires eliminating the potential for air/gas mixture in the pilot or main header of burn pit systems to avoid internal burning or flashback.
For continuously operated burn pit flare systems, a purge is required prior to ignition, during operation and before shutdown. The continuous sweep gas is necessary to avoid liquid accumulation. Therefore, with proper sloping, sweep gas will ensure the sweeping of liquid toward the burn pit tip to prevent any internal burning in the tip or corrosion problems in the pipelines. This can be achieved by sweeping the header with fuel gas or nitrogen toward the end of the burning tip. The recommended velocity to the burn pit tip should reach a minimum of 1 ft/sec velocity in the flare header.
For the maintenance burn pits, a purge of the pilot and main flare header should occur prior to the release of waste gas into the system, before the ignition system is engaged, continuously during the maintenance activity, and prior to the final shutdown of the equipment. Sweep gas is not required in this type of burn pit as long as depressuring activities are ongoing. Operating a maintenance burn pit that has been isolated for a long time without proper purging before operation presents significant risk. Air could be trapped in the lines: with fuel or flare gas introduction, this could lead to an internal explosion.
Therefore, purging the burn pit system with at least 20 volumes of nitrogen (N2) is required. Following the use of the maintenance burn pit, a N2 purge should be applied on the main burning header to ensure no leftover hydrocarbon remains in the flare header to escape to the atmosphere and to avoid air/fuel mixture (stagnation). This step should also be part of a detailed shutdown procedure. In summary, a detailed pre-startup checklist, and commissioning and shutdown procedures should be developed by the vendor and operating facilities.
A complete N2 purge should be applied on the pilot gas lines to avoid a gas/air-trapped mixture that could be ignited, resulting in a flame flashback toward the fuel gas source. Therefore, replacing the fuel gas with a N2 purge is required before the shutdown of the burn pit. The burn pit and pilot headers should be ignited only after the system purge and shutdown are complete.
Sloping. Sloping requirements for the burn pit inlet header should follow industrial best practices and hydraulics analysis results to ensure that liquids flow to the burn pit and are not trapped in low points in the flare head. Further sloping requirements should be advised by the burn pit tip designer.
Another sloping aspect is also required for the ground in the pit area to allow the liquid to flow away from the tip and avoid the restriction from the liquid to the tip. The ground near the flare tip discharge point should be covered with a refractory to avoid erosion and the formation of a liquid pool under the tip. This refractory on the burn pit floor/ground should be a high-temperature fire brick, typically with a 6:1 sloping ratio (horizontal:vertical slope). This enables any liquids to drain towards the center of the burn pit instead of gathering and burning close to the burn pit flare tip(s).
The designer should address pit design, hold-up capacity, pit sloping, and tip and pilot system elevation. It is recommended that the design of the burn pit and associated equipment and auxiliaries should be done by a well-known flare vendor; however, the plant contractor must confirm the soil type utilized for the burn pit/berm as the slope is heavily dependent on this information. The overall size of the burn pit is designed based on the amount of vapor flow or liquid hold-up required in the pit and the expected flame length and shape coming from the burn pit flare tips with respect to the prevailing wind direction.
Refractory walls for concrete bunkers. While some burn pit applications are buried underground, other burn pits are installed in concrete and refractory-lined bunkers either above or below grade. The installation method varies across different SA operations and applications.
Refractory-lined concrete bunkers are encompassed by a concrete bunker enclosure with a refractory-lined wall. The refractory that lines the concrete encompasses the front-firing wall and should be composed of high-temperature fire brick, ceramic fiber insulation and a 94% cast alumina refractory tile block. Thicknesses of all refractory components may vary depending on the specific plant or project parameters. The burn pit flare tip and pilots are horizontally installed in the center of the tile block, with the exit of the flare tip being flush with the external facing surface of the tile.
Due to the high temperature of the refractory material on the front-firing wall, two additional factors must be taken into design consideration: some form of inclination of the front-firing wall towards the burn pit flare header, and the ceramic fiber expansion joints on the front-firing wall.
At some point above the refractory tile, the front-firing wall must incline backward towards the burn pit flare header. This incline may vary in angle and should be confirmed by the flare vendor. This wall incline minimizes the flame impingement that the front-firing wall could experience with the upward movement of the flame exiting the burn pit flare tip.
The ceramic fiber expansion joints are installed within the fire brick on the front-firing wall to allow for thermal expansion and allowable movement within the refractory wall. An experienced flare designer should set the thickness and spacing of these systems.
Takeaway. Burn pit systems are critical systems that must be designed, maintained and operated safely. Due to the proximity to grade-level personnel, burn pit systems present unique challenges and risks compared to elevated flare systems. Proper design of burn pit systems must consider various important factors. Additionally, the type of operation for the burn pit will also greatly influence the design and scope requirements. An appropriately designed burn pit will provide a safe and reliable means to burn gases and liquids, enabling facilities to keep operations running smoothly.
LITERATURE CITED
1 API Standard 521, “Pressure-relieving and depressuring systems,” June 2020.
2 ANSI/API Standard 537, “Flare details for petroleum, petrochemical and natural gas industries,” March 2017.
The Authors
ABDULAZIZ H. AL-TIJANI is an Engineering Consultant at Saudi Aramco's Process and Control Systems Department in Dhahran, Saudi Arabia. Al-Tijani supports company operations and project design, mainly in flare and relief systems and flare gas recovery applications. He supports Saudi Aramco and joint venture oil and gas operational facilities, pipelines, process simulations and various phases of projects. He earned a BS degree in chemical engineering from King Fahd University of Petroleum and Minerals (KFUPM) and an MS degree in oil and gas surface facilities from KFUPM (in partnership with IFP).
MOHAMMAD AL-MUBAYEDH is a process engineer at Saudi Aramco's Process and Control Systems Department within the Flare and Relief Systems Group. He has more than 6 yr of experience with Saudi Aramco, including company operations and project support. Al-Mubayedh has led the commissioning and process stabilization of a flare system in one of the company's gas operation facilities. He has also conducted several FGRS feasibility studies to identify reliable FGRS applications. Al-Mubayedh earned a BS degree in chemical engineering from Oregon State University in 2017.
Abdullmajeed Al-Sanad is a Process Engineer at Saudi Aramco. He works with the Process and Control Systems Department within the Flare and Relief Systems Group. He has 8 yr of experience. Al-Sanad has been involved in major tasks at the company, such as NGL recovery plant commissioning and Saudi Aramco’s decarbonization strategy development. He has also led several milestones in the company’s flaring minimization programs. He earned a BS degree in chemical engineering from King Fahd University of Petroleum and Minerals (KFUPM).
GREG SEEFELDT is the Global Director of Flare Systems at Zeeco, Inc. During his 20-yr career at Zeeco, Seefeldt has held various technical and leadership positions involving flare systems, flare gas recovery systems and vapor recovery systems. He graduated from Oklahoma State University with a BS degree in mechanical engineering and is a registered professional engineer in Oklahoma (U.S.).
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