Flare stack

(CC) Photo: Ari Frede
An industrial flare stack in a petroleum refinery.

A flare stack or gas flare is a tall vertical vent pipe used in petroleum refineries, petrochemical plants, chemical plants and natural gas processing plants for burning off flammable gas released by pressure relief valves during unplanned over-pressuring of plant equipment.

When crude oil is extracted and produced from oil wells, natural gas associated with the oil is produced to the surface as well. In areas of the world lacking pipelines and other infrastructure, this associated gas is usually flared as waste or unusable gas. The flaring of associated gas may occur at the top of a vertical flare stack (as in the adjacent photograph) or it may occur in a ground-level flare in an earthen pit.

The purpose of industrial plant flare stacks

In industrial plants such as petroleum refineries, petrochemical or other chemical plants, and natural gas processing plants, the main purpose of a flare stack is that of safety by protecting pressure vessels or pipes from over-pressuring due to unplanned upsets.

Whenever any plant equipment items are over-pressured, pressure relief valves (in most cases, required by law) automatically release gas and sometimes liquids as well. The released gases and/or liquids are routed to a flare stack and burned as the they exit the flare stack.

How industrial plant flare stacks work

Whenever plant equipment items are over-pressured, the pressure relief valves provided as essential safety devices on the equipment automatically release gases and sometimes liquids as well. Those pressure relief valves are required by industrial design codes and standards as well as by law.

The released gases and liquids are routed through large piping systems called flare headers to a flare stack. The released gases are burned as they exit the flare stacks. The size and brightness of the resulting flame depends upon the flammable material's flow rate in terms of joules per hour (or btu per hour).

Most flares have a vapor-liquid separator upstream of the flare to remove any large amounts of liquid that may accompany the relieved gases.

Steam is very often injected into the flame to reduce the formation of black smoke. In order to keep the flare system functional, a small amount of gas is continuously burned, like a pilot light, so that the system is always ready for its primary purpose as an over-pressure safety system.

Environmental impacts of flaring associated gas from oil drilling sites

(CC) Photo: Environmental Rights Action, Friends of the Earth, Nigeria
Ground-flaring of associated gas from an oil well site in Nigeria.

The flaring and venting of associated gas from oil drilling sites is a significant source of carbon dioxide (CO₂) emissions. The amount of such emissions has declined from a peak of about 110×10⁶ metric tons in the 1970s to 40×10⁶ metric tons in 2005, a decrease of about 64 percent.

The World Bank estimates that 150 billion cubic meters of raw natural gas are flared. This flaring is highly concentrated: 10 countries account for 75% of emissions, and twenty for 90%. The largest flaring operations occur in the Niger Delta region of Nigeria. The leading contributors to gas flaring are (in declining order): Nigeria, Russia, Iran, Iraq, Angola, Qatar, Algeria, Venezuela, Equatorial Guinea and Indonesia.