Flaring is a combustion control process for gases (such as flammable gases and hydrocarbons) in which the gases are piped to a remote, usually elevated, location and burned in an open flame in the open air using a specially designed burner tip (often flare stack tip), auxiliary fuel and steam or air to promote mixing for nearly complete destruction. Completeness of combustion in a flare is governed by flame temperature, residence time in the combustion zone, turbulent mixing of components to complete the oxidation reaction, and available oxygen for free radical formation. Elevating the flare can prevent potentially dangerous conditions at ground level where the open flame (i.e., an ignition source) is located near a process unit or operating facilities. Further, the products of combustion can be dispersed above working areas to reduce the effects of heat, smoke, noise and objectionable odors.
A flare is a critical mechanical piece of equipment intended for the safe, reliable and efficient discharge and combustion of gases and materials from pressure-relieving and vapor-depressurizing systems. Since it is critical to the safety of an operating plant, a flare should be continuously available with high reliability and capable of its intended performance through all operating plant emergency conditions, including a site-wide general power failure or whole plant emergency trip. The flare and related mechanical components should be designed to operate and properly perform for the specified service conditions for an overhaul-to-overhaul period, say five to seven years, without the need for an outage of the operating facility. If a flare fails (for instance, because of a tip failure), whole systems that rely on it for protection should usually be tripped for some days (or even a week or more) for the flare repair, which can result in considerable financial losses.
In most flares, combustion occurs by means of a diffusion flame. A diffusion flame is one in which air diffuses across the boundary of the fuel and combustion product stream toward the center of the fuel flow, forming the envelope of a combustible gas mixture around a core of fuel gas. On ignition, this mixture establishes a stable flame zone around the gas core above the burner tip. This inner gas core is heated by diffusion of hot combustion products from the flame zone. Cracking can occur with the formation of small, hot particles of carbon that give the flame its characteristic luminosity. If oxygen is deficient and if the carbon particles are cooled to below their ignition temperature, smoking occurs. In large diffusion flames, combustion product vortices can form around burning portions of the gas and shut off the supply of oxygen. This localized instability causes flame flickering, which can be accompanied by soot formation. As in all combustion processes, an adequate air supply and good mixing are required to complete combustion and minimize smoke. The various flare designs differ primarily in their accomplishment of mixing.
Flares can be used to control waste gas flows, and they can handle fluctuations in gas concentration, flow rate, heating value and inert content. Flaring is appropriate for continuous, batch and variable flow vent and relief stream applications. The majority of plants have existing flare systems that are designed to relieve emergency upsets that require the release of large volumes of gas. These large-diameter flares designed to handle emergency releases can also be used to control vent streams from various process operations.
Consideration of vent and relief stream flow rates and available pressures should be given for a flare design. Normally, emergency relief flare systems are operated at a small percentage of total capacity (total capacity is supposed to be for total plant shutdown) and at negligible pressure. To consider the effect of controlling an additional vent stream, the maximum gas velocity, system pressure and ground-level heat radiation during an emergency release should be evaluated.
The whole flare system should be evaluated if the gas stream pressure is sufficient to overcome the flare system pressure, since it is not economical nor technically possible to provide gas mover systems (compressors or blowers) to a flare system. Other considerations should also be respected such as maximum gas velocity limits or groundlevel heat radiation limits for the flare system and flare stack location, height and details. A flare system should be optimized while considering effects and factors such as pressures, flows, hydraulic effects, flare stack height, radiation limits, temperature limits, material selection for all components and optimal cost of the whole system. To ensure an adequate air supply and good mixing, some flare systems inject steam into the combustion zone to promote turbulence for mixing and to induce air into the flame.
Operation & design
The flammability limits of flared gases influence ignition stability and flame extinction. The “stoichiometric” composition is a chemically correct mix of air and flammable gas capable of perfect combustion with no unused fuel or air. The flammability limits are defined as the stoichiometric composition limits (maximum and minimum) of an oxygenfuel mixture that will burn indefinitely at given conditions of temperature and pressure without further ignition. In other words, gases should be within their flammability limits to burn. When flammability limits are narrow, the interior of the flame may have insufficient air for the mixture to burn. Gas fuels with wide limits of flammability are therefore easier to combust.
For most vent streams, the heating value also affects flame stability, emissions and flame structure. A lower heating value produces a cooler flame that does not favor combustion kinetics and is more easily extinguished. The lower flame temperature also reduces buoyant forces, which reduces mixing. The density of the vent stream affects the structure and stability of the flame through the effect on buoyancy and mixing. Gas velocities in many flare systems are extremely low; therefore, most of the flame structure is developed through buoyant forces as a result of combustion.
Lighter gases tend to burn better. In addition to burner tip design, the density directly affects the minimum purge gas required to prevent flashback, with lighter gases requiring more purge. Poor mixing at the flare tip is the primary cause of flare smoking when burning a given material or gas. Streams with high carbon-to-hydrogen mole ratio (say greater than 0.34) have a greater tendency to smoke and require better mixing for smokeless flaring. For this reason, if steam injection is required and selected, one generic steam-to-gas (gas being flared) ratio is not necessarily appropriate for all vent and relief streams. The required steam rate is dependent on the carbon-to-hydrogen ratio of the gas being flared. A high ratio requires more steam to prevent a smoking flare.
Vent and relief streams are sent from the facility release point to the flare location through the gas collection flare header. The flare piping is designed to minimize pressure drop. Ducting is not used because it is more prone to air leaks. Valves should be kept to an absolute minimum — usually, no valve is allowed expect one or two check valves in a whole flare system. Piping layout is designed to avoid any potential dead legs and liquid traps. The piping is equipped for purging so that explosive mixtures do not occur in the flare system on startup or during operation. An important consideration in the material selection of flare and piping design is the possibility of low temperatures as the result of relief and blowdown of high-pressure gases. For instance, as a rough indication, in a unit with gas pressure above 90 Barg, it could be expected that some low temperatures be achieved in case of blowdown or relief. Different scenarios should be established and simulated to find the lowest possible temperature. High-pressure gas inventories, if assumed at an ambient low temperature (say between 2°C and 10°C), can produce low temperatures of around -30°C to -50°C in flare piping. A flare system design temperature of -50°C or -60°C is not unusual for a high-pressure facility.
Liquids that may be in the vent stream gas or that may condense out in the collection header and transfer lines are removed by a “knockout drum.” The knockout drum is typically either a horizontal or vertical vessel located close to the base of the flare stack. Some traditional flare designs used a vertical vessel located inside the base of the flare stack as a knockout drum. Liquid in the flare stream can extinguish the flame or cause irregular combustion and smoking. In addition, flaring liquids can generate a spray of burning materials that could reach ground level and create a safety hazard. For a flare system designed to handle emergency process upsets, a knockout drum should be sized for worst-case conditions (for instance, total plant shutdown), and it is usually quite large.
In next month’s February issue, Part 2 of this article will discuss design considerations in more detail.
Amin Almasi is a senior rotating machinery and equipment consultant. He is a chartered professional engineer of Engineers Australia and IMechE. Almasi is an active member of Engineers Australia, IMechE, ASME and SPE and has authored more than 100 papers and articles dealing with rotating equipment, condition monitoring, offshore, subsea and reliability.