athomas236
Mechanical
- Jul 1, 2002
- 607
I have been asked to advise on the the possible reasons and solutions for furnace tube cracking problem.
The sliding pressure boiler burns a severely slagging coal and supplies main steam to a 420MWe turbine at 3755psi/1055F. Although the turbine MCR is 420MWe it has an overload capability of 475MWe but typically runs at 460MWe during the day and reduces load to about 200MWe overnight to shed the slag off the furnace walls. The thickness of slag is typically 12 inches.
The furnace is spirally wound up to the furnace nose level where they are connected to transition headers after which the tubes are vertical.
The two tube failures have occurred on the one side wall after 30,000hours of operation. The failures are characterised by cracks starting from the outside of the tube and propagating inwards towards the bore. Furnace inspections have identified an area 4metres wide by 2metres high that is showing surface cracking. The cracks run at 90degrees to tube centre line.
The furnace has three rows of four burners in the front wall and two rows of four burners in the rear wall. The cracks have occurred just above the second row of burners.
The metallurgical report attibutes the cracks to thermal fatigue as a result of slagging/deslagging with evidence of overheating and oxide build up on the bore. The oxide is typically 65microns with a max of 85microns.
The overheating seems to occur overnight when tubes that are designed for 800F experience temperatures upto 950F.
Now my questions.
1. Why do tube temperatures increase when the load is reduced when I would normally expect them to decrease. There is a theory that the heat stored in the slag is being transferred to the steam when the load is reduced causing increased temperatures.
2. What is causing the cracks. To cause cracks at 90degrees to the tube centre line I would expect high longitudinal thermal stresses but cannot understand what is causing the longitudinal stresses to be greater than the circumferential.
Regards,
athomas236
The sliding pressure boiler burns a severely slagging coal and supplies main steam to a 420MWe turbine at 3755psi/1055F. Although the turbine MCR is 420MWe it has an overload capability of 475MWe but typically runs at 460MWe during the day and reduces load to about 200MWe overnight to shed the slag off the furnace walls. The thickness of slag is typically 12 inches.
The furnace is spirally wound up to the furnace nose level where they are connected to transition headers after which the tubes are vertical.
The two tube failures have occurred on the one side wall after 30,000hours of operation. The failures are characterised by cracks starting from the outside of the tube and propagating inwards towards the bore. Furnace inspections have identified an area 4metres wide by 2metres high that is showing surface cracking. The cracks run at 90degrees to tube centre line.
The furnace has three rows of four burners in the front wall and two rows of four burners in the rear wall. The cracks have occurred just above the second row of burners.
The metallurgical report attibutes the cracks to thermal fatigue as a result of slagging/deslagging with evidence of overheating and oxide build up on the bore. The oxide is typically 65microns with a max of 85microns.
The overheating seems to occur overnight when tubes that are designed for 800F experience temperatures upto 950F.
Now my questions.
1. Why do tube temperatures increase when the load is reduced when I would normally expect them to decrease. There is a theory that the heat stored in the slag is being transferred to the steam when the load is reduced causing increased temperatures.
2. What is causing the cracks. To cause cracks at 90degrees to the tube centre line I would expect high longitudinal thermal stresses but cannot understand what is causing the longitudinal stresses to be greater than the circumferential.
Regards,
athomas236
