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FLARE OPERATIONS
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The most common method of Flare and Flare Pilot Flame ignition is with the use of a The principle of Flame Front generation is simple. Flows of both Fuel Gas and Air are mixed together in a flammable ratio at a location which is easily accessible to
operating personnel, usually fairly close to the bottom of an elevated flare. The mixture is piped to the top of the flare, where it emerges from the pipe close to the
pilot or main gas flow. The pipe carrying the mixture is called the flame front line or flame front tube. The entire pipe has to be filled with the flammable mixture from bottom to top. The mixture is ignited, within the pipe, at the lower end, using a spark plug or similar device in the ignition panel. The flame gradually "eats" its way through the mixture (propagates) until it gets to the top, after some time, and ignites the pilot gas or main flame. The speed of flame propagation depends on the fuel gas being used and the specific mixture with air which has been created in the pipe. Typically, flame speeds, for the sort of mixture commonly used, are about 0.5 - 1 fps , but the actual case will be a little more complicated because the speed is enhanced by a continuing forward velocity of the mixture in the pipe and, in addition, the burned mixture leaves behind a lot of hot CO2, H2O and N2 which also pushes the flame forward. The result is as though there is a "ball" of flame rolling along the line at something between 10 and 70 fps (usually). Flame front generators are available in a variety of different configurations depending on manufacturer. Most use a 1" diameter flame front tube although 3/4", 2" and 3" are also available. Design and sizing of the flame front system depends largely on the available pressures for the Fuel Gas and the Ignition Air supplies, and on the distance which it is
desired to send the "ball" of flame. Just as with any piping system, the quantity and velocity of the mixture in the flame front line are related to the line diameter and available driving pressure. The distances over which the flame "ball" can travel are, similarly, related to the available pressures and line size. Forced draft, pressurized systems are more amenable to longer distances than natural draft, venturi systems which cannot generate a great driving pressure. During its passage along the pipe, the flame gives up some heat to the pipe wall and this also tends to create limitations on the flame run distances. Typical operational distances are shown below:- Flame Front generation is a well-used system and has the advantage that all the working parts are at the bottom of the flare where they are accessible for maintenance and repair without shutting down the flare. However, there are some drawbacks and these have lead to a reputation for unreliability in Flame Front systems. Most of the problems are easily soluble however if addressed with knowledge of what is to be achieved. I have posted a page of generic instructions for flame front generators and another with tips for debugging problems with flame front generation which should help.
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