Radiant Wall Burners / Flame Patterns
The most common use of Radiant Wall burners is for cracking heaters in Ethylene Plants. These burners are pre-mix gas only and usually fire in the horizontal position. The name “Radiant Wall” comes from the configuration of the heater. There would be dozens, or even hundreds, of burners mounted in the wall of the heater. The idea is that the entire wall of the heater would be heated evenly and “radiate” heat to the process tubes in the center of the heater. The radiated heat is accomplished by the number of burners and the shape of the flame. These burners have a radial flame shape that “hugs” the wall and is very efficient in transferring heat from the combustion reaction to the hot surface of the heater wall.
Effect on Operations
The two most common problems with radiant wall burners are “Flashback” and “Lift-Off.”
Flashback is a condition where the flame is burning inside the gas tip or mixer of the burner. Operating at this condition can ruin the burner and reduce the heat to the process.
Lift-Off occurs when the flame is detached from the gas tip. This can cause instability in the flame and possible impingement on the process tubes. Once the flame has lifted from the gas tip, there is normally pulsation as it tries to re-attach to the gas tip. This causes a “huffing” noise and can lead to the flame going out.
Corrective / Preventive Actions
Flashback is generally due to the amount of hydrogen in the fuel. When the concentration of hydrogen increases above 30%, by volume, then the chance of flashback is possible. The reason that hydrogen presents a problem is due to the flame speed. Hydrogen burns more rapidly than any other fuel. When the air/fuel mixture velocity in the venturi is too low then the flame can flashback, the flashback will generally produce a very loud noise, like a rifle shot, when it occurs.
Several designs and operational adjustments can be used to overcome Flashback:
- The gas tip can be designed with a reduced flow area to increase the exit velocity. This may decrease the capacity of the burner when firing natural gas.
- The orifice spuds can be reduced in size to increase the required fuel pressure. This increases the amount of air entrained in the venturi, which results in higher exit velocities at the gas tip.
- Although flashback is a function of flame speed and mixture velocity, in some cases simply reducing the amount of primary air will resolve the issue. By reducing the air contained in the mixture, there is less oxygen to promote flashback. Since flashback can only occur in pre-mix burners, reducing the primary air shifts the burner more to the “raw” gas type of design.
- Another easy fix could be to take burners out of service if the heater is at low rates. This will increase the fuel pressure on the operating burners and entrain more air.
Flame Lift-Off can also be addressed with design and operational adjustments:
- The most frequent cause of the flame lifting off is the position of the gas tip in the burner tile. The burner drawing will show the required dimension to locate the gas tip. This will be noted as a dimension from the face of the tile to the back of the slots or in some cases to the end of the gas tip. This dimension is critical for proper burner operation.
- Primary air can have a significant impact on flame stability. Too much primary air can blow the flame off of the gas tip. This frequently happens at start-up in a cold firebox. During light-off, the primary air may need to be closed, and the secondary air open to establishing the flame and heat the tile. A hot tile surface is a key factor in keeping the flame attached to the gas tip and on the wall.
- Natural gas can be a challenge on many types of burners, including pre-mix radiant wall designs. Even though natural gas burns cleanly, it has a relatively high ignition temperature, and the molecular structure resists dissociation. In some cases, the addition of a stability ring placed on the gas tip will help to stabilize the flame when burning natural gas.
- Fouling of the gas tips, orifice spuds and mixer’s is a very common problem in ethylene facilities. Most of the plugging is due to the presence of “green oil” in the fuel gas. This oil comes from the gas compressors that are part of the fuel system. A regular maintenance program will help to keep the burners clean and working properly.
Effective and Safe Troubleshooting.
Frequently, the operator of the heater must be trained to use knowledge of the equipment and process unit to make adjustments that bring operations back to the required capacity desired by plant management.
It is essential that troubleshooting be done in a systematic, well-organized fashion. Effective and safe troubleshooting involves four basic steps:
- Recognizing the problem
- Observing indications of the problem
- Identifying solutions for the problem
- Taking corrective action
When a problem is noted, it is necessary to evaluate its likely effect on the process or product being produced. Some solutions may require the heater to be shut down for the problem to be resolved.
Once a cause has been determined, standard procedures should be followed to solve the problem. All personnel involved should be aware of the problem, the planned corrective actions, the ways that safety is addressed, the expected results, and the proper action to take should the problem worsen or not be solved.
Did you know that the first burner sold by John Zink was a pre-mix design for residential home heating? Properly trained personnel could reduce operational costs. Check out the John Zink Institute for the latest training in process burners and heaters.
