Fire Checks
Fire Checks are necessary line restriction on all burner installations where oxygen is being used, or where the possibility of a flashback or backfire is considered a hazard. Their purpose is to prevent flame from traveling beyond a predetermined point in the mixture line.
Fire checks do not prevent flashback, nor do they stop the resulting concussion. They simply retard combustion and reduce flame speed to a level where the burning mixer is cooled, then quenched but the exhaust velocity of the mixture.
Principles of operation are simple: the mixture is fed into the fire check and must pass through four coils of corrugated stainless steel ribbon before being returned the line. The total area of the openings in each coil through which the mixture passes is slightly larger than the total area of the outlet. This provides a free flow of mixture with a minimum of pressure drop during operation. If you look into a fire check by holding it up to a light source, you will see light passing through it, but will notice that you do not have a direct line of sight.
When a flashback occurs, the flame burns at an explosive rate through the mixture line unobstructed until it reaches the fire check. At this point, the flame enters the cavity of the fire check and experiences a pressure drop as it comes into contact with the first could of stainless steel ribbon. The flame front is forced to burn through the grating formed by the coils, and this action retards the flame speed of the burning mixture. This reoccurs at each successive coil. A considerable amount of heat is absorbed by the coils which cool the fire. Their subsequent expansion serves to further restrict the passage of flame through the fire check until the flame speed is exceeded by the exhaust velocity of mixture and fire is extinguished.
Proper selection of a fire check is based upon the mixer size. A fire check should be of equal size to the outlet of your mixer. Do not install any valves, gas cocks, or any other restrictions other than the fire check between the mixers and burners.
A fire check can be installed from either direction. Maintenance involves inspection following any major backfire to be sure that the coils have not been damaged, which would restrict the normal flow of mixture to the burners. Here, again, you simply hold the fire check up to a light source and view the condition of the coils and the passage of light. The coils closest to the burner will naturally be the ones to inspect, and in severe enough cases the stainless steel will actually be burned beyond recognition and the fire check should be replaced. Continued operation of fire checks damaged to this degree will result in serious safety hazard.
Minor flashbacks do not as a rule fuse the coils. In these cases you simply turn off your supply lines, feeding the problem zone, then relight the system as you do at the start up. If you are unable to return to normal operation, the fire check should be inspected.
Most capacity charts in use today are based on a 2-1/2" W.C. pressure drop. Fire checks operate at higher through-put capacities, understanding that the pressure drop will be higher. The use of oversized fire checks, or dual fire checks is not recommended .
Should you have any questions or concerns, do not hesitate to contact AGF Burner, Inc.
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