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same boiler other problem

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Posted by Raymond on March 25, 2004 at 08:27:26:

Same boiler other problem.
After the problem with the nuisance trips due to high frequency radio signals and the vibration problem witch is still not really solved, our two pas, fire tube, 11barg, 30tons/hour, super heated HDS/Saacke steam boiler, for the a second time, has cracks in the welds of the super heater outlet collector.
The super heater is a counter current finned pipe heat exchanger placed in a vertical flue gas duct. The saturated steam flowing upwards from the HDS fire tube boiler making a 90 bend before, axial, entering the inlet collector of super heater.
10 x 3 rows of U-tubes are connecting the inlet (one up two in the side) collector with the underlying outlet collector(two in sides one under) The collectors and the U- bends are outside the flue gas duct.
The steam leaving the outlet collector makes a 90 turn and runs to the stop valve through a horizontally line with a 1/1000 slope.
The in and outlet collector have a blind flange on the opposite side from inlet of inlet collector and outlet of outlet collector.
Welds are leaking of three tube rows near to the blind flange end at the bottom part of the outlet collector. Also the welding neck flange with blind flange but this time the crack is in the weld. So the cracks in the welds are occurring only in the outlet collector of the super heater opposite to the outlet of the outlet collector.
The welds where 100 % controlled but not heat released. Collector material astm a106ngrb; welding neck flange material 300lb sch 40 a105; tubes and bends material st35.8/1 din 17175
Is there some one out there who can help me with an explanation?

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