Posted by Rod Nissen on December 01, 2001 at 08:00:33:
In Reply to: CBLE 800hp Tube End Splitting posted by J Mihalov on November 14, 2001 at 06:30:16:
I haven't had any exposure to the newer CBs, but extensive experience on the old ones. However the problem you have is possibly applicable to a number of firetube boilers types.
Is it natural gas fired? If so this is more critical, because there is less heat transfer in the furnace, as opposed to oil firing. This is because of the more radiant flame of the oil. Hence the gases are hotter as they enter the tubes. On the welded tubes it is good practice to remove any sharp edges at the tube mouth by rounding them off with a grinder. The sharp edges will "catch" the heat and locally overheat the metal, which will generate cracking.
Another good idea(especially if you suspect overfiring) is to fit a thermocouple through the rear door into the gas flow in way of the tube bank entry(either side of the furnace will do. The temperature here (we call this area the reversal chamber in Australia)must never exceed 950deg C, or 1740deg F. This can be linked to an alarm.
When we converted old boilers from oil to NG we used to recommend a derate of 20%, unless this temp was monitored. By the way this temp can be exceeded at lower firing rates if the burner isn't in tune and the excess O2 is too low.
Be aware that if your waterside is in bad shape, with scale, it will aggravate the situation as the heat transfer to the water is impaired and the metal temp increases.
Finally raising steam too quick is a no no. The old CBs with their low slung furnace were bad for this (poor natural circulation)as it was possible to be steaming out the top, with the water under the furnace stone cold. This caused uneven stresses on the structure and severe hinging stress on the furnace welds, again causing cracks. Hopefully this isn't the case with the new models.
Hope this helps,
Rod.
Hope this helps,
Rod.