The flare is a last line of defense in the safe emergency release system in a refinery or chemical plant. It uses to dispose of purged and wasted products from refineries, unrecoverable gases emerging with oil from oil wells, vented gases from blast furnaces, unused gases from coke ovens, and gaseous water from chemical industries. Flares are also used for burning waste gases from sewage digesters process, coal gasification, rocket engine testing, and nuclear power plants with sodium, water heat exchangers, heavy water plants, and ammonia fertilizer plants.
The flare provides a means of safe disposal of the vapor streams from its facilities, by burning them under controlled conditions such that adjacent equipment or personnel are not exposed to hazards, and at the same time obeying the environmental regulation of pollution control and public relations requirements.
The chemical process used for flaring is a high temperature oxidation reaction to burn combustible components, mostly hydrocarbons, or waste gases from industrial operations.
In combustion, the gaseous hydrocarbon (natural gas, propane, ethylene, propylene, butadiene, butane and etc) reacts with atmospheric oxygen to form carbon dioxide (CO2) and water. Several byproducts formed will be carbon monoxide, hydrogen and others dependent upon what is being burned. Efficiency of hydrocarbon conversion is generally over 98%.
In industrial, the most common utilized flare systems are elevated flares and ground flares.
Selection of the type of flare is influenced by several factors, such as availability of space; the characteristics of the flare gas (composition, quantity and pressure); economics; investment and operating costs; public relations and regulation.
I) Elevated Flare
Elevated flare (refer below Figure) is the most commonly used type in refineries and chemical plants. Have larger capacities than ground flares. The waste gas stream is fed through a stack from 32ft to over 320ft tall and is combusted at the tip of the stack.
The elevated flare, can be steam assisted, air assisted or non-assisted.
Elevated can utilize steam injection / air injection to made smokeless burning and with low luminosity up to about 20% of maximum flaring load.
The disadvantage of steam injection / air injection is it introduces a source of noise and cause noise pollution. If adequately elevated, this type of flare has the best dispersion characteristics for malodorous and toxic combustion products.
Capital costs are relatively high, and an appreciable plant area may be rendered unavailable for plant equipment, because of radiant heat considerations.
II) Ground Flare
A ground flare is where the combustion takes place at ground level. It varies in complexity, and may consist either of conventional flare burners discharging horizontally with no enclosure or of multiple burners in refractory-lined steel enclosures. The type, which has been used almost exclusively, is the multi-jet flare (enclosed type).
Compare to elevated flare, ground flare can achieved smokeless operation as well, but with essentially no noise or luminosity problems, provided that the design gas rate to the flare is not exceeded. However, it have poor dispersion of combustion product because it stack is near to ground, this may result in severe air pollution or hazard if the combustion products are toxic or in the event of flame-out. Capital, operating and maintenance requirements cost are high.
Because of poor dispersion, multi-jet flare is suitable for "clean burning" gases when noise and visual pollution factors are critical. Generally, it is not practical to install multi-jet flares large enough to burn the maximum release load, because the usual arrangement of multi-jet flare system is a combination with an elevated over-capacity flare.
Whether you are looking for a Flare system, KASRAVAND designs and builds the system as a complete package with particular emphasis on:
- Gas collection header and piping for collecting gases from processing units,
- A knockout drum to remove and store condensable and entrained liquids,
- A proprietary seal, water seal, or purge gas supply to prevent flash-back
- A single or multiple burner unit and a flare stack,
- Gas pilots and an ignitor to ignite the mixture of waste gas and air and
- A provision for external momentum force (steam injection or forced air) for smokeless flaring.
Is very important for the flare designer to understand several factors which can affect his flaring system design, the major factors influencing flare system design are:
When design the flare system, several important parameters have to be consider, there are flare head design, flare exit velocity, VOC heating value, and whether the flame is assisted by steam or air.
Besides that, the design should be based on consideration bellow as well,
1. Flare Spacing, Location, and Height
- Radiant heat
- Burning liquid fall out
- Pollution limitations
2. Flare Capacity and Sizing
Flare design capacity is design to handle largest vapor release from pressure relief valve, vapor blow down and other emergency system
3. Flashback Seals -flashback protection, which prevents a flame front from traveling back to the upstream piping and equipment. Sizing of flare systems is a function of maximum allowable back pressure on pressure relief valves and other sources of release into the emergency systems