The hazards of fired heater flooding

Heater flooding is a serious and potentially dangerous condition which must be dealt with properly and quickly.

John Zink Hamworthy Combustion
DOUG BASQUEZ, Consultant

Viewed : 11516

Article Summary

Heater flooding incidents continue to happen. This has resulted in increased requests for the John Zink Institute to include fired heater flooding in its training classes. There is also increased interest in burner management systems (BMS) which are designed, among many other things, to help prevent such incidents. Currently, most fired heaters do not have a BMS and are manually operated. An informal industry survey has shown that about three quarters of fired heater incidents occur at start-up which is generally considered to be the most dangerous time in fired heater operation. For burners that must be manually lit at start-up, this exposes the operators to potential hazards including heater flooding. Since flooding incidents continue to occur, more training and information are needed to prevent them.

According to API 535, fired heater flooding is defined in section 14.2 as “operation with insufficient combustion air, resulting in unburned fuel or combustibles in the firebox and/or flue gas”. It further notes that the temperature typically decreases when a heater is flooded because not all of the fuel is combusted. Fuel flow is normally controlled based on the process fluid outlet temperature. If that temperature begins to drop, the control system calls for more fuel. In a flooded heater, this exacerbates the problem. If this continues, the heater can become bogged, where the flames are extinguished due to lack of oxygen because they exceed the upper flammability limit for the fuel. This is very dangerous if the heater is above the autoignition temperature because any sudden introduction of oxygen can lead to a re-ignition of the fuel, potentially leading to an incident.

Fired heater flooding is simply too much fuel going to the heater for the available combustion air. It is possible for the excess oxygen sensor, which should be located at the bridgewall of the heater, to indicate sufficient O2 levels and yet the burners can be unstable for lack of air because of too much tramp air leakage into the heater. That tramp air is not going through the burners which are designed to have all the combustion air come through them. Depending on the tramp air source, the flames may be very long. There could even be afterburning in the convection section; depending on where the fuel finds the rest of the air it needs to complete combustion.

There are two basic causes for heater flooding: too much fuel flow to the heater or too little air flow through the burners. Too much fuel flow could occur if, for example, the process feed rate increases beyond the design rate which would cause the fuel flow to increase to try to keep up with the increased feed rate. Too much fuel flow could also occur if there is a sudden change in fuel composition where significantly more air is required for complete combustion. For example, if the fuel suddenly contains components with a much higher heating value (Btu/ft3), much more air would be needed, assuming there is no change in the fuel pressure. A related problem is a sudden loss of hydrogen in the fuel which requires less air for combustion compared to other fuels, or switching to natural gas/methane which requires more air than many other hydrocarbons. Liquid fuel carryover in a gaseous fuel could send significantly more fuel to the burners than desired. The fuel control system would eventually correct this situation by reducing the fuel flow rate to maintain a certain heat rate. However, the initial surge of higher heating value fuel could put a heater into a flooded condition. Assuming the process fluid is some type of hydrocarbon, a tube failure leading to process fluid leaking into the heater (see Figure 1) could also cause a heater to flood due to the addition of flammable process fluid. Fuel control valve problems could cause flooding. This could be a valve that is stuck open or one that opens too rapidly before the combustion air has a chance to catch up to the fuel flow.

Another possible cause could be improper settings on the burner dampers/registers so that insufficient air is flowing through the burners. A related problem is something blocking the air flow to the burners, such as a plastic bag sucked into a burner air inlet. While that would not likely be a significant problem on a large heater with many burners, it could be a serious problem on a smaller heater with few burners. For a forced or induced draft heater, the combustion air fan or induced draft fan, respectively, could be improperly set. A related problem is inadequate draft at the burners which could be caused by an improperly set stack damper or a plugged convection section.

Significant changes in ambient conditions could also cause a heater to run out of air if no adjustments are made to compensate for the changes. When the air temperature increases, the humidity increases, or the barometric pressure decreases, there is less oxygen per cubic foot of air. Higher temperature and lower pressure reduce the air density, while higher humidity causes water to displace some oxygen. Since process burners are volume flow devices, the air flow to the burners would need to be increased to compensate for reduced oxygen per unit volume of air.

One possible indication of a flooded heater is firebox pressure pulsations sometimes referred to as huffing or panting. This may be caused by lack of combustion air to the burners which causes them to become unstable because they are searching for air to complete combustion. The flames may bounce up and down or pulse. This is caused by rapidly changing combustion dynamics where the flames first lift off while searching for air and then burn back toward the burner when they find some air. However, because that reburn air is not coming through the burners, the flames again lift, searching for air, and the cycle is repeated. These pulsing flames are usually at a low enough frequency they can be seen with the naked eye. The pulsations may also cause the draft gauge to swing back and forth. Very severe pulsing of large burners can cause the heater walls to flex and damage brick and castable refractory because of the vibrations. Refractory falling from the roof can damage and block burners (see Figure 2). Severe pulsations can also cause sight ports and explosion doors to flap open and close.

If the flooding is severe enough, some flames may be extinguished. While burners that have flamed out may no longer have attached flames, there may be flames elsewhere in the heater such as higher in the radiant section or afterburning in the convection section. The flooding could be severe enough that all burners have flamed out. There may or may not be flames elsewhere in the heater depending on how fuel rich the conditions are and the amount of tramp air leaking into the heater.

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