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DKL Engineering, Inc.
Maintenance and Inspection - Converter
September 10, 2003
Corrosion and Scaling
Most maintenance and inspection of a converter can only be done during an extended shutdown where there is enough time to purge and cool down the plant to the point where the vessel can be opened up for entry.
Deteriorated, improperly installed, and/or insufficient thickness of thermal insulation and cladding could cause uneven temperatures across the converter. The may cause unexpected thermal stresses in the converter that may leak to damage.
The surface of insulation must be kept thoroughly sealed to prevent infiltration of water or circulation of air behind the insulation. This may lead to high rates of external corrosion which will lead to premature failure of the vessel.
An effective way of monitoring the effectiveness of thermal insulation is to take thermal images of the vessel during operation. These images will reveal ‘hot spots’ which indicate area where the insulation has failed. These areas must be repaired as soon as possible.
Whenever maintenance is done on the converter that required that insulation and cladding be removed, the insulation and cladding must be reinstalled to ‘as-new’ condition prior to restarting the plant.
Gas can escape a converter when a mechanical failure of a part of the converter results in a leak to the environment. Mechanical failures are generally a result of thermal stresses on the converter that cause welds or joints to fail. All gas leaks should be repaired as soon as possible.
Locating the exact point of the gas leak will be a problem since the gas may have travel under the insulation and cladding prior to emerging at the ‘leak point’. Insulation and cladding must be removed until the source of the leak is found. The most likely location of the leaks is in and around gas nozzles which are areas under a lot of stress due to thermal stresses and forces from the ducting and nozzles.
Repair of the leak must be done during a plant shutdown since it will not be possible to weld in the area due to the escaping gas. Prior to repairing the leak, the cause of the leak should be investigated to determine the probable cause(s). The repair should not only seal the leak but also address the cause so the leak will not re-occur.
After any maintenance inside the converter all dirt, dust, scale, etc. must be swept up and vacuumed from the inside of the converter. Any loose material inside the converter may be picked up by the gas and carried into the catalyst bed where it will contribute to premature pressure increases across the catalyst.
Catalyst support screens must also be clean and be free of dirt and scale.
There have been a few instances of excessive scaling of 304 SS reported in metallurgical plant operations. The exact mechanism of scale formation is not known but it is suspected that high operating temperature, high sulphur dioxide concentrations, present of nitric oxides play some role in the scaling phenomenon.
During the annual shutdown, the converter should be inspected for excessive scaling and corrosion. Thickness measurements should be taken at key locations on the shell, floor, roof and division plates so the progress of any scaling and corrosion can be monitored.
Any scale found should be sampled and analysed to determine the type of scaling or corrosion that is occurring.