Olavo Cunha Leite
T Thermal Inc.
Conshohocken, Pa.
A properly designed flare works as an emissions control system with greater than 98% combustion efficiency.
The appropriate use of steam, natural gas, and air assisted flare tips can result in smokeless combustion.
Ground flares are used when it is necessary to conceal the flame, otherwise the elevated flare is commonly chosen because it handles larger flow releases more economically.
Background
Flaring has become more complicated than just lighting up waste gas. Companies are increasingly concerned about efficiency.
In addition, U.S. Occupational Safety & Health Administration (OSHA) and U.S. Environmental Protection Agency (EPA) have become more active, resulting in tighter regulations on both safety and emissions control. These regulations have resulted in higher levels of concern and involvement in safety and emissions matters, not to mention smoke, noise, glare, and odor.
This first of two articles on flare design and components looks at elevated flares, flare tips, incinerator type flares, flare pilots, and gas seals. Part 2 will examine knockout drums, liquid seal drums, ignition systems, ground flares, vapor recovery systems, and flare noise.
Elevated flares
The major components of an elevated flare system are the flare stack, flare tip, pilot, gas seal, liquid seal, knockout drum, and ignition system (Fig. 1).
The stack height is generally based on the heat intensity radiated by the flame. Other factors like wind, dispersion, and noise also can affect stack dimensions.
Heat radiation, K, is defined as shown in Equation 1 (see equations and nomenclature boxes).
Total heat release is defined in Equation 2. For unknown mixtures of hydrocarbons (HC), the low heat value can be calculated by Equation 3.
The distance between the flame midpoint and measurement point (D) is defined in Equation 4. At the stack base, D is defined as in Equation 5. The flame midpoint coordinates, Xc and Yc, are commonly determined using API Recommended Practice (RP) 521 methods.
Experimental data indicate that the fraction of heat radiated (F) increases toward a limit. If liquid droplets are present, the value of F should be somewhat higher than the following data:
- Hydrogen, F = 0.15
- Natural gas, F = 0.19
- Butane, F = 0.29.
(These values are for near ideal combustion conditions. They result in reasonable but conservative stack heights. Efficient combustion is not expected at peak flaring rates.)
Some expressions for F are designed to fit the experimental data, namely those shown in Equations 6 and 7.
The fraction of heat intensity transmitted is defined in Equation 8. This equation corrects the effect of atmospheric absorption for distances, D = 100 500 ft.
Stack height, H, is defined in Equation 9. If K is given at the stack base, then D can be calculated as in Equation 5, and H as in Equation 10.
Exposure to heat radiation over a period of time affects both people and equipment. With a heat intensity of 2,000 BTU/hr sq ft, the pain threshold is reached in about 8 sec and blistering occurs in 20 sec (Table 1).
In emergency releases, the total exposure period is about 8 10 sec, 3 5 sec being the reaction time plus 5 sec for escaping to a safe area. Appropriate clothing can extend the permissible exposure time and heat intensity.
The equipment exposure to heat should be evaluated to prevent damages to heat sensitive materials, flammable vapor areas, and electrical equipment.
Solar radiation may be a factor but it has a minor effect when added to flare radiation.
The effect of heat radiation on the equipment results in an increase in temperature with exposure time. The higher the radiant heat, the higher the temperature. The equilibrium temperature is reached in greater than 40 min.
Assuming a view factor (a factor dependent on the incidence angle of the heat flux) of 0.5 and no cooling effect on the steel surface, the equilibrium temperature is related to heat intensity by a fitting equation, Equation 11. If the tank has liquid or vapor flowing through it, the surface temperature may be lower because of the cooling effects.
Most flares are elevated to decrease heat radiation and to improve dispersion of combustion products. Usually, the maximum heat intensity allowed on equipment is 3,000 BTU/hr sq ft.
The amount of heat absorbed by a vessel exposed to an open fire is affected, according to API RP 521, by size, type of installation, and environment (Equation 12).
The wetted area is the vessel outer surface where the liquid contacts the inside wall. If the unwetted surfaces of a vessel are exposed to a high rate of heat absorption, this may, in time, cause a metal temperature high enough to reduce its safe strength significantly.
Vessels with nonfireproof insulation should be assumed to be bare.
Flare tips
Flare tips may be mounted on guy supported stacks, derrick supported stacks, or self supporting stacks. The guy supported stack requires a space with a radius close to half the stack height to connect the guy anchors. These systems have been supplied with overall heights as high as 550 ft.
Guyed stacks generally are less expensive. They cannot be used to flare gases at temperatures much different from ambient temperature because the thermal expansion and contraction differences between the stack and guy wire change the guy wire tension. This leads to high stresses and structural instability.
The derrick supported stack is ideal for tall stacks with reduced ground clearance. These systems are the most expensive type to erect.
They allow for different expansion rates between stack, piping, and derrick. Derrick flares have been built to heights of 350 ft.
The self supporting stack is the most economical and easiest to erect for short flare stacks because it requires less space for installation. The bottom sections are larger than the top, which produces a practical height limit of 200 feet. Internals are usually added at the base section, thus incorporating a vertical knockout drum or a liquid seal drum.
Stacks generally are designed and fabricated in accordance with American National Standards Institute (ANSI), Uniform Building Codes (UBC), American Institute of Steel Construction (AISC), and American Society of Mechanical Engineers (ASME) codes.
Stacks are shipped with weld prepared or flanged ends and the majority of components outfitted. If applicable, caged ladders, intermediate rest platforms, and a 360 top platform are supplied and designed to meet OSHA requirements.
Nonsmokeless tip
The nonsmokeless flare tip should include a flame retention device and heavy duty pilots, allowing the flare to operate at high exit velocities without flame lift off. Specially designed flame retention devices built into the flare tip and the correct positioning of reliable pilots achieve flare stabilization (Fig. 2).
The addition of aerodynamically designed wind deflectors eliminates the local wind vortexes responsible for sucking the flame down the leeward side of the flare. The deflectors also extend the operational life of the flare tip, thermocouple cables, ladder, etc.
Flare tip diameter is generally based on a velocity of up to 0.5 Mach for peak, short term, infrequent flow, and a maximum of 0.2 Mach for normal and continuous flow conditions. Sonic velocity is defined as in Equation 13, and exit velocity, in Equation 14.
Volumetric flow, in actual cfs (acfs), is calculated as shown in Equation 15. Tip diameter, d, is calculated as in Equation 16. Use the larger d (using 0.5 Mach vs. 0.2 Mach) and round up to the next larger size, after checking both continuous flaring conditions and intermittent emergency flaring conditions.
For continuous, nonemergency flaring, EPA requires low heat value (Hv) and exit velocity limitations to ensure a combustion efficiency of 98% or greater. Nonsmokeless flares must be designed and operated with exit gas velocity as shown in Equations 17 19.
Steam assisted tip
The steam assisted flare tip (ring type) is widely used for smoke suppression where heavy saturates and unsaturates may be flared. The carbon/hydrogen weight ratio of the gas is related to its propensity to smoke.
Fuels with C/H 35% tend to smoke (or with volumetric ratio H/C < 25%). Steam is used to entrain air in the flame, thus lowering the proportion of steam to HC for smokeless combustion.
The principal feature of the steam assisted flare tip is an upper pipe ring, or steam manifold, with steam injector nozzles mounted on individual risers placed equi distant around the tip. The nozzles are designed to entrain primary air and provide turbulence and mixing in the combustion zone. For proper combustion of the gas/air steam mixture, the nozzles should be close to the gas discharge, thereby resulting in a smokeless flame of low luminosity.
When fitted with a center steam nozzle, this type of tip can be used over a wide range of flaring rates (Fig. 3). The central nozzle injects steam alone, also providing turbulence and good mixing with the waste gas.
Smoke is generated during combustion of hydrocarbons only when the mixture is fuel rich, either overall or locally. Smoke formation can be reduced by reactions that consume hydrogen.
It is widely believed that steam separates the hydrogen molecules, minimizes polymerization, forms oxygen compounds that burn at a reduced temperature, avoids cracking, and reduces smoke generation. Another theory says steam reacts with carbon forming carbon monoxide, carbon dioxide, and hydrogen. This mechanism is thought to remove the carbon before it cools and forms smoke.
A steam assisted, ring type flare tip should be equipped with a flame retainer and heavy duty pilots to prevent flame lift off at high velocities. To avoid damage to the steam ring and nozzle risers by flame lick down, a shield should be positioned so as not to generate low pressure areas around the flare tip. It is these low pressure areas that cause the flame to roll over and touch the outside of the flare tip (i.e., lick down) in high winds.
Flare tip diameter is generally designed for an exit velocity of up to 0.2 Mach for the smokeless condition, and 0.5 Mach for emergency conditions. The diameter of the flare also depends on the available pressure drop.
For continuous, nonemergency releases, EPA has established the maximum exit gas velocity to be identical to the nonsmokeless flares (0.5 Mach). The heat content must be larger than 300 BTU/scf, which is the minimum value allowed to be flared on an assisted smokeless flare.
Mixing head tip
The steam assisted, mixing head flare tip is designed for smokeless operation with high efficiency combustion. Its principal feature is a converging/diverging body and shroud assembly (Fig. 4). The flare gas flows from the stack into a center body, from which it emerges through slots in radial spokes.
A steam injection ring induces primary air through the bottom of the shroud into waste gas above the spokes, providing turbulence, good mixing, and proper combustion of the waste gas/steam air mixture. Also, an additional center steam nozzle can be provided below the inner steam ring to inject steam alone to provide turbulence and mixing.
The advantages of this design are:
- Effective use of steam to induce air inside the shroud
- Large area for mixing waste gas with air and steam
- Shroud provides acoustic shielding for steam nozzles
- Large capacity for smokeless operation
- Inner steam ring not susceptible to flame lick down and heat in the absence of steam flow
- Flare gas and steam air are well mixed at low flaring rates, even with strong winds
- Longer life because of normal operating temperatures are far below design maxima (also, there is no need for refractory materials)
- Center steam nozzles give good performance at low flaring rates.
Based on the above and on recent studies, this flare tip is expected to achieve a combustion efficiency well above 98% in smokeless design operation.
Coanda effect
The application of the Coanda effect in the design of flare tips has resulted in more efficient combustion with lower radiation levels and shorter flame lengths.
Henri Coanda early this century noticed the tendency of a fluid jet discharging from a nozzle or slot to adhere to an adjacent surface, entraining the surrounding fluid. This fluid dynamic mechanism was applied to flares, resulting in several designs of burner tips and steam injectors. These designs include the external Coanda design (also known as the Indair flare), the Injector rod, and the "Flarejector."
Indair flare
This design is based on a tulip form with a radial outward slot at the bottom for the waste gas exit. The pressure energy in the high pressure gas is used to induce up to 20 times its own volume of air.
Fig. 5 illustrates the Coanda principle, as applied in the Indair flare tip. The waste gas, instead of continuing on its horizontal path, adheres to the tulip type Coanda profile, producing a smokeless, low radiation flame just above the maximum diameter of the tulip.
The flame is initiated equally around the flare. This gives it excellent wind stability because at least half of the flame is sheltered from the winds.
The flame propagates from the outside, always keeping a layer of gas protecting the tulip body against extremely high temperatures. Because of this protection, the flare tip can be fabricated of stainless steel alloys without refractory protection.
Indair flares operate at elevated pressures (minimum, 10 psig) to achieve smokeless combustion. They can reach turndown ratios above 3:1. Gas with as much as 25 wt % liquid can be burned without the carry over of liquid droplets.
The flow rate through this flare is a function of the gas pressure, assuming a constant annular slot area and constant gas properties. The flame is highly aerated, radiating extremely low heat (on average, the fraction of heat radiated is 0.1).
The flame is shorter than with conventional flares, and it has a high directional stability.
"FS" flare tip
The FS flare tip is a steam, smokeless flare tip (Fig. 6). The steam induces atmospheric air into the flare tip to achieve clean combustion without smoke. Steam is introduced using several "Flarejector" units, which entrain the air into the center of the frustrum cone section of the flare tip body (Fig. 7).
The FS tip uses steam efficiently because of the confined mixing of waste gas with steam and induced air before combustion begins. Opening of the mixing section may generate low flame stability at low operating rates and high winds.
The Coanda ejectors are mounted on the body cone section (Fig. 6). They are quieter and more efficient than the typical steam ring designs. The steam is evenly supplied by using a steam chest built around the upper conical mixing chamber. The chest is refractory lined on the inside to protect it in the event of burn back (i.e., the burning of gases inside the flare tip).
Injector rod
Steam injection is effected by this type of Coanda injector. They are straight tubes with elongated slots in their walls through which steam is discharged tangentially. The gas jet adheres to the rod, inducing the atmospheric air into the combustion zone (Fig. 8).
Several rods are placed around the flare exit, forming an intensive mixing chamber immediately prior to ignition. This results in smokeless combustion and increased flame stability.
Injector rods require low steam rates, reducing the steam jet noise and decreasing low frequency combustion noise, as a result of improved flame stability. The acoustic efficiency is improved because of the high perimeter to area ratio.
Incinerator-type flares
Fired flares are used when higher energy is required to ensure better efficiency of waste gas combustion. A gas injection ring producing a crown of fire at the flare burner tip is also used to increase combustion efficiency.
When the waste gas heat value is greater than 115 BTU/scf, it burns without adding natural gas or incinerating the exit flow with a fire ring.
Low-BTU gas flares
Generally, the fire ring design produces a 3 4 ft high flame around the flare tip, heating and incinerating the waste gas. This process is ideal with mixtures containing hydrogen, carbon monoxide, etc., which impart good flammability to the waste mixture. This design reduces natural gas consumption tremendously, but it is not very efficient.
The pilots need to be redundants with an automatic ignition system, always ready to ignite the waste gas. Some gas mixtures containing pollutants such as sulfur require a net heat value higher than 115 BTU/scf to improve combustion efficiency and provide environmental dispersion.
For continuous, nonemergency flaring, EPA dictated low heat value (Hv) and exit velocity limitations to ensure a combustion efficiency of at least 98%. The minimum allowable Hv of flared gas is 200 BTU/scf. If a gas has an Hv of less than 200 BTU/scf, it must be enriched with support fuel to attain that value.
Waste gases with Hv less than 450 BTU/scf do not smoke in nonassisted flares. Nonsmokeless and steam assisted flares have the same exit velocity limitations when the heat content is larger than the 300 BTU/scf minimum value allowed to be flared using a steam assisted flare tip. In the case of air assisted flares, the minimum Hv is also 300 BTU/scf.
Ammonia flares
Ammonia has a net heat value of 365 BTU/scf well above the minimum required to ensure combustion. Without assist gas, the combustion efficiency of ammonia is about 92%. This generates ammonia odor because of ammonia's low ol factory threshold of 20 ppm.
Ammonia burns with low efficiency, even when using low exit velocity to prevent flame extinction. It can also produce a white plume in high humidity conditions, because of the quenching effect of nitrogen in the combustion.
Sometimes steam and gas injection are used in ammonia flares to ensure smokeless operation and complete combustion of the ammonia. The amount of assist natural gas needs to be controlled because it increases NOx emissions, which cannot exceed the acceptable limits.
The pilots need to be redundants with automatic ignition system always ready to ignite the flare ammonia.
Air-assisted flare tip
Air assisted flare tips have been widely used for smokeless combustion in applications where heavy saturates and unsaturates are flared. It incorporates a mixing head to provide extra mixing and turbulence, which results in a vertical, shorter flame with low heat radiation.
Primary air is supplied by a low pressure fan axial or centrifugal. The fan provides turbulence to mix the gas and air and ensures entrainment of secondary air to achieve smokeless combustion.
This flare tip has a flame retention device, allowing the flare to operate at high exit velocities without flame lift off. Wind deflectors are not required because wind has little influence on the vertical flame.
These flares generally are furnished with a two speed fan and a pressure switch in the flare header. These devices save energy during flaring at a fraction of the smokeless design rate. The flare will smoke when flaring is greater than smokeless design rates.
Air assisted flares are ideal for applications in remote areas where steam is not available, and also can be used in offshore locations. There are several advantages to this type of flare, including lower maintenance costs, extended flare tip life, and elimination of steam lines and controls.
Flare tip diameter is generally based on a maximum velocity of 0.2 Mach for the smokeless condition, with a 0.5 Mach maximum for the emergency intermittent conditions. Use the greater diameter of the two.
The air stream duct is designed for a velocity of up to 120 fps, depending on the available pressure drop. For continuous, nonemergency releases, EPA has established the maximum exit gas velocity to ensure a combustion efficiency higher than 98% (Equation 20).
Flare pilots
Heavy duty flare tip pilots are designed for continuous operation and flame stability, regardless of wind conditions. They offer maximum reliability with efficient operation.
Each pilot is fed by its own separate natural draft inspirator, positioned at the flare tip base for maximum performance and reliability. Fuel gas consumption is about 75,000 BTU/hr/pilot. Pilot burners are made out of high nickel alloy to ensure long operational life.
The pilots are connected to a fuel gas distribution manifold by a single inlet connection. A flame front splitter manifold can be included to reduce the ignition piping between the flare tip and the ignition panel to a single pipe.
Type K thermocouples can be incorporated on the pilots and wired to a flame failure panel for continuous monitoring and alarm upon loss of pilot flame.
Gas seals
Gas seals are used on elevated flares to prevent the entry of air, which, when mixed with waste gas, can become explosive. The purge gas maintains an HC rich atmosphere in the flare stack, and the gas seal reduces the amount of gas consumed for this purpose.
There are two main types of gas seals: molecular and integral fluidic seals.
The molecular type seal forces the purge gas to make two "U turns," forming a seal because of the different molecular weights. This type of seal requires a purge flow of natural gas with a velocity of less than 0.01 fps to keep the oxygen concentration below the seal at less than 1%, with winds up to 20 mph.
The molecular seal is placed just below the flare tip. It works on the principle of purge gas buoyancy, creating a zone with greater than atmospheric pressure. If the gas in the stack is lighter than air, however, the pressure at the bottom of the stack can be lower than atmospheric. The purge gas flow must counteract this situation.
The integral fluidic type seal is located within the flare tip body. It provides low flow resistance in one direction and high resistance in the other. To maintain the seal under the same conditions, it requires a natural gas flow velocity of 0.03 0.04 fps to keep the oxygen concentration below the seal at 6%.
This design has a low, yet sufficient, pressure drop, and is compact and light. It can be installed in flare tips without increasing structural loads.
Purge rates normally quoted prevent the ingress of air only. If purge rates are very low, however, the flame begins to burn back into the tip. If this continues for long periods, the life of a tip is severely reduced. Purge rates should be sufficiently high to prevent burn back.
If nitrogen is used as purge gas, the volumetric flow will be 75% of the purge rate with methane. Also, burning back will not occur, thus increasing the operational life of the flare tip.
When the flare system is filled with high temperature gases and flaring is interrupted, the gas will cool and shrink, giving place to the same volume of air. To prevent this, a purge rate must be introduced to compensate for the shrinkage volume of hot gas. This purge rate is also a function of the time for cooling, generally 15 20 min.
Sometimes, with high temperature flow conditions, shrinking rate becomes the governing factor until the system cools down. Also, when a buoyant condition of gases lighter than air is present, purge volume needs to be added to replace at least an equal volume of buoyant light gas, thereby avoiding air entering the system.
When the size of the flare tip is 36 in. or larger, the use of a molecular type seal is recommended.
Copyright 1992 Oil & Gas Journal. All Rights Reserved.
