A flare stack is a specialized safety device used across numerous industrial sectors, including petroleum refineries, chemical plants, and oil and gas extraction sites. This tall, vertical structure is engineered for the controlled burning of excess or waste gases that cannot be processed or stored. By combusting these flammable hydrocarbons, the flare stack converts them into less hazardous compounds before release. It functions as a last line of defense for plant safety.
The Physical Structure and Components
The visible part of the system is the stack, a tall tower that raises the combustion point high above the facility to minimize heat radiation on the ground. At the top is the flare tip, where the waste gas is mixed with air and ignited. The system begins downstream from the industrial process with a network of piping, known as the flare header, which collects the excess gases.
Before the gases travel up the stack, they pass through a knockout drum, a vessel designed to separate liquids, such as oil or water, from the gas stream. Removing these liquids prevents dangerous fireballs and ensures stable combustion at the tip. A small, continuous pilot light is located at the flare tip, serving as the constant ignition source ready to burn the waste gas the moment it arrives.
Why Flaring Is Operationally Necessary
The primary reason for flaring is the immediate and safe relief of pressure within industrial equipment, a mandatory safety requirement. If pressure relief valves open due to an unplanned surge or equipment failure, the flare system provides a safe route to vent and combust the gas, preventing catastrophic explosions. Pressure relief is also necessary during routine operational events, not just emergencies.
Flaring is frequently used during plant startup and shutdown procedures when process streams are unstable or cannot be managed. Facilities may also flare off-spec gas during a process upset when the gas composition is unsuitable for processing or sale. Flaring is a controlled method of dealing with excess flammable gas that cannot be captured, stored, or processed, making it an operational necessity to maintain facility integrity.
The Mechanics of Controlled Combustion
The process involves routing the collected waste gas through the knockout drum and up the stack to the flare tip. Once the gas reaches the tip, the continuously burning pilot light provides the reliable ignition needed for combustion. This controlled reaction is safer than simply releasing raw, unburned hydrocarbons.
To achieve efficient and smokeless combustion, particularly for gases with higher carbon content, assist mechanisms are often employed.
- Steam-assisted flares inject steam into the gas stream at the tip to create turbulence, promoting a better mix of gas and air before ignition.
- Air-assisted flares use blowers to inject combustion air directly into the waste gas, which increases combustion efficiency and eliminates black smoke production.
- Elevated flares, the most common type, combust the gas high above the ground.
- Ground flares burn the gas near the ground inside an enclosure to shield the flame and noise from view.
Environmental Impact and Regulatory Oversight
While combustion is a safety measure, it still results in the release of greenhouse gases and other pollutants. The main environmental benefit is the conversion of methane—a potent greenhouse gas—into the less potent carbon dioxide and water vapor. This combustion is a trade-off: it prevents the release of highly warming methane but still contributes to atmospheric carbon loading.
The combustion process is not always 100% efficient, and an incomplete burn can release pollutants like uncombusted methane, volatile organic compounds, and black carbon (soot). Black carbon is a health concern, which is why assist mechanisms are used to achieve high combustion efficiency, often aiming for 98% or higher. Regulatory bodies mandate the monitoring and minimization of flaring activities, requiring companies to adopt best practices to improve efficiency and reduce the volume of gas flared.