Flare systems are critical safety components in oil and gas operations, designed to safely dispose of excess hydrocarbons during routine processes, startups, shutdowns, or emergencies. While their purpose is straightforward, designing a flare system involves navigating complex technical, safety, and environmental challenges. A well-designed flare system not only ensures safe operations but also minimizes environmental impact and adheres to regulatory requirements.
In this blog, we’ll explore the key design considerations for flare systems in the oil and gas industry.
1. Flare System Objectives
Before diving into the design process, it’s crucial to define the objectives of the flare system. These typically include:
• Safely combusting excess gases.
• Managing process upsets and emergencies.
• Ensuring environmental compliance by minimizing emissions.
• Preventing overpressure in equipment.
2. Type of Flare System
The selection of the flare system type is one of the first steps in the design process. Factors such as facility size, location, and operational requirements determine the choice. Common types include:
• Elevated Flares: Positioned high above the ground to disperse heat and emissions safely. These are common in onshore facilities.
• Ground Flares: Used where height restrictions apply, equipped with enclosures to minimize noise and light pollution.
• Offshore Flares: Designed for space-constrained offshore platforms, often with enhanced structural stability to withstand marine conditions.
• Flare Gas Recovery Systems (FGRS): Capture and reuse flare gases, reducing waste and emissions.
3. Flare Capacity and Flow Rates
The flare system must be designed to handle the maximum possible flow rate of gases during normal and emergency scenarios. Key considerations include:
• Steady-State Operations: Typical flow rates during normal operations.
• Emergency Scenarios: Surge flow rates during equipment failures or process upsets.
• Pressure and Temperature: Ensuring the system can withstand high pressures and temperatures.
4. Design Requirements for a Flare System
Flare systems provide for the safe disposal of gaseous wastes. Depending on local environmental constraints these systems can be used for:
• Extensive venting during start-up or shutdown
• Venting of excess process gas
• Handling emergency releases from safety valves, blowdown and venting systems
• Causes of emergency relief are many and can include fire, blocked outlets, utility failures (steam, electricity, instrument air, cooling medium etc.) abnormal heat input, chemical reaction and so on
5. Flaring Scenarios
Fire relief
• Example – PSV to protect separator in the event of a pool fire under the vessel
Start-up flaring
• Example – flaring of separator gas prior to compressor start
Emergency depressurisation / blowdown
• Example – Requirement to depressurise system due to confirmed fire
Manual venting
• Example – final depressurisation of vessel prior to purging and entry
6. Early Stage Design Considerations
The requirement for typical greenfield projects is to have a preliminary plot plan and to achieve a Class 4 (+/– 30%) estimate. At this stage, the estimate of flare system capacity and configuration is based on data from similar past projects and any local regulatory requirements. High-level flare system configuration decisions should be taken at this stage. Typically, such decisions include:
• Ground vs. elevated flare
• Segregation and number of flares [e.g., high pressure, low pressure, acid gas flare, low temperature]
• Sparing and maintenance requirements
• Mounting flares on a common derrick vs. a mix of derrick and guy-wired flares, etc.
7. Typical Flare System Design Considerations:
• Flow Rate
• Temperature
• Header sizing
• Back pressure
• Smokeless operation
• Flash back protection
• Ignition system
• Fuel gas system
• Location
• Unignited gas dispersion
• Environment
8. Flow Rate and Header Sizing
Primary considerations for calculating the size of the flare pipework include:
• Relief and blowdown valve flowrates
• Flare tip back pressure
• Allowable velocity of gas in network
• Set pressure of relief device
• Higher set pressures = increased allowable back pressures and reduced line sizes
• Calculated back pressure will determine the type of relief device selected
9. Header Design Considerations
The architecture of the flare network will depend on the layout of the plant
• System architecture will affect system back pressure and relief device selection
• To minimise noise and vibration, gas velocities in flare headers should not exceed 0.5 Mach
• Similarly relief valve tail pipes should not exceed 0.7 Mach
• Sub headers for different areas i.e. compression area, separation area, gas treatment area
• Purge gas injection required at each header end to ensure no air can enter the system through the flare tip
10. Flare Tip Design
The flare tip is a critical component that affects the combustion efficiency and emissions of the flare system. Key considerations include:
• Flame Stability: Ensuring the flame remains stable under varying wind and gas flow conditions.
• Smokeless Combustion: Using steam or air-assist systems to reduce smoke and unburned hydrocarbons.
• Durability: Selecting materials that can withstand high temperatures and corrosive gases.
11. Heat Radiation and Dispersion
Flare systems generate significant heat and radiant energy, which can pose safety risks to personnel and equipment. The design must account for:
• Radiation Zones: Calculating safe distances to minimize heat exposure.
• Wind Effects: Considering wind direction and speed to ensure safe dispersion of combustion products.
• Shielding: Implementing physical barriers or water curtains to protect nearby equipment.
12. Environmental Considerations
Environmental regulations play a significant role in flare system design. Key considerations include:
• Emission Limits: Ensuring compliance with limits on carbon dioxide (CO₂), methane (CH₄), and other pollutants.
• Flare Gas Recovery: Incorporating systems to capture and reuse flare gases, reducing environmental impact.
• Noise and Light Pollution: Minimizing the impact on surrounding communities through sound-dampening and light-reduction measures.
13. Material Selection
Flare systems operate under extreme conditions, requiring materials that can withstand:
• High Temperatures: Resistant to thermal fatigue and creep.
• Corrosive Environments: Durable against sour gas and other corrosive substances.
• Mechanical Stresses: Able to handle vibrations and structural loads.
Common materials include stainless steel, Inconel, and other high-performance alloys.
14. Safety and Reliability
Safety is paramount in flare system design. Key considerations include:
• Redundancy: Incorporating backup systems, such as multiple pilot ignitors.
• Flame Detection: Installing reliable sensors to monitor flame presence and ensure continuous operation.
• Pressure Relief: Designing the system to handle sudden pressure surges without failure.
• Maintenance Access: Providing safe and easy access for inspections and repairs.
15. Compliance with Standards and Codes
Flare system design must adhere to industry standards and regulations, including:
• API Standards: Such as API 521 (Pressure-Relieving and Depressuring Systems) and API 537 (Flare Details for General Refinery and Petrochemical Service).
• Environmental Regulations: Specific to the region or country, such as EPA guidelines in the United States.
• Safety Codes: Ensuring compliance with safety codes to protect personnel and equipment.
16. Monitoring and Control Systems
Modern flare systems often include advanced monitoring and control systems to optimize performance and enhance safety. Features include:
• Real-Time Monitoring: Tracking flow rates, temperatures, and emissions.
• Automation: Adjusting steam or air-assist systems for smokeless combustion.
• Emergency Shutdown Systems: Ensuring safe operation during critical events.
Conclusion
Designing a flare system for the oil and gas industry requires a balance of safety, efficiency, and environmental compliance. By considering factors such as flare capacity, material selection, heat dispersion, and regulatory requirements, engineers can create systems that meet operational needs while minimizing risks and environmental impact.
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