Purpose of flare system: The primary function of a flare is to use combustion to convert flammable, toxic, or corrosive vapors to less objectionable compounds like CO2.
Why not cold vent instead of flaring? Methane is roughly 30 times more potent as a heat-trapping gas than CO2. Hence cold venting of HC gases is not allowed as per pollution control board directives.
- API 521 : Pressure-relieving and Depressuring systems
- DEP 18.104.22.168 : Flare and vent systems (amendments to API 521)
- API537 : Flare details for refinery and petrochemical service
- DEP 22.214.171.124 : Flare details (amendments to API 537)
Types of Flares (Fig. 1):
- Elevated flares commonly used in oil and gas industry. Most economical.
- Enclosed flares when visible flame not acceptable. Also used for offshore facilities. Advantage : Low noise and radiation levels; Disadvantage : Poor dispersion of gases during flameout condition (flare needs to be tripped on gas detection)
- Ground flares for liquid or two phase relief flaring. Advantages : Low radiation, low noise; Disadvantage : accumulation of vapor cloud, high initial cost.
Types of Elevated Flares (Fig. 2):
- Self supported stacks: Simplest and most economical design; Stack height up to 100 ft overall height; As the flare height and/or wind loading increases, the diameter and wall thickness required become very large and expensive.
- Guy wire supported stacks: Most economical design in the 100- to 350-ft height range.Normally, sets of 3 wires are anchored 120 degrees apart at various elevations.
- Derrick supported stacks: The most feasible design for stack heights above 350 ft. Derrick supports can be fabricated from pipe (most common), angle iron, solid rods, or a combination of these materials. They sometimes are chosen over guy-wire-supported stacks when a limited footprint is desired.
Non Assisted / Assisted Flares:
- Non-assisted flares are the flares which does not use any assist media and typically used for hydrocarbon or vapour streams that do not cause smoking (i.e. For clean burning gases like methane, hydrogen, carbon monoxide, ammonia, hydrogen sulphide) or when smoke is not a consideration.
- Incomplete combustion of heavy HC gases produces Carbon monoxide, which is the main component to create smoke. For flaring heavy gases, smokeless operation can be achieved by assist media such as steam, air or gas which improves the mixing of flare gas with air.
- Steam assisted flares (Fig. 3) for smokeless operation. Steam increases momentum of flare gas which enhances fuel air mixing leading to complete combustion. Also water-gas shift reaction converts CO to CO2
CO + H2O ⇌ CO2 + H2
- Air-assisted flares (Fig. 3) are used where smokeless burning is required. It is used when steam is not available or where low-pressure air delivery offers a lower cost. (only fraction of requirement of air is mixed with flare gas to promote momentum which effectively entrains additional combustion air from surrounding).
Flare load estimation (Fig. 4):
- Fire zone : Wetted areas within a 300 m2 (3200 ft2) plot area shall be considered when a system’s relief loads are calculated.
- Flare gas flow rate: Tip diameter is decided based on design flowrate.
- Mach number in Stack : 0.5
- Mach number in tip : 0.5 to 0.8 (depends on allowable pressure drop)
- Lower gas velocity (Fig. 5): When gas flow is so low that the local gas velocity is less than flame velocity, air entrains in to the flare tip leading to burn back / flash back. At very low gas velocities, flame can travel back through the mixture (flash back) into piping and KOD.
- Higher gas velocity (Fig. 5): When gas flow is higher than design capacity, then the local gas velocity becomes higher than the flame velocity leading to detached flame or flameout (higher velocity leads to turbulence , which in turn reduces HC component concentration below LFL)
- Flame velocity:The burning velocity or flame speed is the velocity at which a flame front moves through the un-burnt gas/air mixture. This flame speed varies with the air/gas mixture ratio and the chemical make-up of the gas.
- Purge gas requirement:To avoid air ingress down the flare stack purge gas is injected in flare header. Injection rate should be controlled by a fixed orifice, rotameter or other device that ensures the supply remains constant and is not subject to instrument malfunction or maladjustment. Refer Part 2 of this article for further details…