Flare

The objective of a flare is to burn the combustible, corrosive or toxic gas to produce combustion products which are neither toxic nor combustible. The system is designed to destroy uncontrolled or unburned vapors produced during process upsets and emergency releases. In such instances, pressure relief devices (such as pressure relief valves or emergency depressuring valves) opened and discharge vapors into the flare system. See the Figure.

A flare knock-out drum is often used upstream of the flare. It is usually a horizontal vessel and its function is to receive the discharge from the unit header(s) in order to remove liquids that condense in the pressure relief system.

To prevent flashback of the flare system into the plant pressure relief system, a liquid seal and purge gas system with is installed between the flare K.O. drum and the flare stack. See the Figure.

Key design factors to ensure flare safety and performance include:

• Smokeless operation
• Flame stability
• Flare size and capacity
• Thermal radiation
• Noise level
• Reliable pilot and ignition system
• Flashback protection

The major safety issues are the latter two items. Flares are provided with continuously operating pilot(s) to prevent flameout and ensure that all releases to the flare are burned. Thermocouples are usually mounted on the flare pilots to provide pilot monitoring and alarm on loss of pilot flame. An additional means of ignition, e.g. a piccolo tube should be provided, independent of power supplies. Flare header systems should be provided with an inert gas purge sufficient to provide a positive gas flow up the stack to prevent back diffusion of air.

Steam can be provided to achieve smokeless flaring of day-to-day discharges of purge gas or vent gas from balancing line, relief valve leakage, and discharges that are encountered during unit start-up and shutdown. Steam is injected into the vapour at the flare tip, either manually or automatically.

A 2-flare system can be employed: an operating flare and an emergency flare. The operating flare handles the burning of day-to-day releases and the emergency flare handles the infrequent emergency releases.

See also the discussion on using HIPS for flare header protection.

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