It is essential that troubleshooting be done in a systematic, well-organised fashion. Effective and safe troubleshooting involves four basic steps:
- Recognising 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.
Flame impingement on process tubes
Since the majority of process heaters have a feed that is a hydrocarbon, flame impingement can cause serious problems. Visual observation of the burners may show that the flames are contacting the tubes. In some cases a gradual increase in the tube metal temperature (TMT) could also indicate possible flame impingement. The operators should make it a point to look into each of their fired heaters at least once a shift to check for any problems with the flame patterns.
Effect on operations
The reason that tubes do not overheat inside a furnace is because of the cooling effect of the fluid inside the tubes. This is why many heaters have carbon steel tubes. Once a tube starts to overheat there is a gradual buildup of carbon on the inside of the tube. This layer of carbon acts to insulate the tube from the cooling effects of the process flow. This in turn makes the tube hotter. As the carbon continues to build the flow area of the tube is reduced. If allowed to continue the carbon will choke the tube completely which can result in a tube rupture.
Hot spots will normally develop in progressive stages. When the flames contact the tube surface there is a cooling effect on the flame. This results in ash being laid down on the tube. This build up will lead to scale on the tubes as the outer layer of the tube starts to burn away.
Let’s look at the various stages during this process:
- Dark areas first start to appear from the carbon coating on the side of the tubes facing the burners.
- Silver or light grey spots form within the dark areas. This is caused by the carbon being burned off.
- These light grey spots will enlarge and cover more area.
- As the coking continues red spots will begin to appear in the grey areas of the tubes. In some cases the tube will take on a “mirror” finish that looks almost like a chromed piece of pipe.
- The tube will eventually start to bulge and then develop “pin hole” leaks. At this point the tube is ready to rupture and immediate action must be taken.
Corrective / preventive actions
The most important thing is to keep the flames off of the tubes! If flame impingement is noticed, the first step should be to adjust the burner causing the impingement to get the flame off the tube.
- Check the burner air register to confirm that it is open. Then look at the gas tips and determine if there is any plugging that would cause the flame to impinge on the tube. If there is plugging, then remove the tips and clean them. Be sure the gas tips are properly orientated by looking at the burner drawing.
- Confirm that the excess oxygen and draft requirements are per the heater design specifications.
If the heater cannot be shut down there are three options:
- Take the burner out of service or reduce the firing rate by manually closing the block valve.
- Increase the excess air to help cool the firebox.
- Increase the process flow to the overheated pass.
There are other options such as wrapping the tubes or clamping them with the heater in service. These options would have to be considered extreme risk situations and approved by the safety department.
Read the article online at: https://www.hydrocarbonengineering.com/gas-processing/27052019/diagnosing-and-solving-problems-with-burners-on-heaters/
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