Same boiler other problem 1

[ww2]

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, 235°C 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?

 

[metengr]

ww2;
This sounds like it could be thermal fatigue cracking related to past cycling problems. If the SH tubes are fillet welded to the collection header, it is possible that after a trip, condensate forms on the SH tube ID or outlet header pipe ID surface (horizontal run) and collects back into the collection header resulting in thermal shock when you re-start the boiler. Also, during lay up of the boiler, you could be having corrosion damage from the inside surface where condensate is trapped. These would be more pin-hole type leaks.

What is the method of SH tube attachment to the collection header?

Have you performed any visual internal inspections of the collection header ID surface using a boreoscope or by some other means?

Can you explain your statement regarding ...the welds are 100% controlled but not heat released.

 

[ww2]

Thanks a lot metengr
The SH tubes are welded to the surface of the collector ¦ The previous connection to the former collector was with a sockolet. These welds had the same problem in the same area (I don't understand wy this is explicitly in that area). But I agree the cause of both failures could well have something to do with the plentiful shut downs (may 100).
The first examination of the inside of the collector showed a magnetite layer. A more thorough control will be performed after accessibility is improved .
Welds where x rayed but not thermal stress released
Raymond

 

[unclesyd]

Have you ever flooded the superheater with treated BFW? If so what is your BFW treatment?

I just looked at some superheater tube failures on a much larger boiler where they had lost level control and flooded the superheater tubes on several occasions. The leaks were all in the return bends and weld area and were caused by a chelating boiler treatment chemical.

 

[metengr]

ww2;
If possible, you might want to consider installing several thermocouples on the SH collection header at the location where problems persist? I would be interested to see if you have any significant temperature excursions during start-up and after shut down conditions.

 

[ww2]

Thank you Unclesyd

The super heater starts to fill after every trip (maybe a 100 since august 2003). Even when the stop valve remains open due to the pressure dropping in the boiler consequently closure of the checkvelve. The water quality therefore is steam condensate.
The boiler has a trisodium phosphate water treatment program.
The difference between the upper(inlet) and lower(outlet) collector is that the lower collector must have been filled more frequently
raymond

 

[unclesyd]

I would try to get a leak or leaking area cut out for metallurgical examination as soon as possible. There are a couple of possibilities for the root cause, but a little more information is needed.

From what I understand you are able to weld repair the area.
Is this correct?
Does the leak return to the same place?
Are you completely grinding out the leaking area prior to welding?
Can you have someone use dye penetrant on the ground surface?

Hate to ask so many questions but such information will help everyone in the solving of you problem with leaks.

 

[ww2]

Onclesyd

The outlet flange has the maximum mass flow while on the other hand the flange with the blind has the minimum mass flow, in fact zero. In operation the wall temperature of the outlet header will be different along its length maximum on the outlet flange, minimum on the blind flange.
On the other hand, on low ore very little steam production there will be no flow through the pipes starting from the nearest to the blind flange. The wall temperature, due to conduction, will be the highest in that area giving the biggest delta T’s during trips. Therefore the area to be most susceptible to fatigue cracking?
Steam production have rarely ore never been above 50% and very often at its minimum say 10% of its maximum
What do you think?

 

[ww2]

Unclesyd,
I agree with you completely concerning the metallurgical examination. We are in the process of cutting the welding neck flange (opposite the outlet) for examination by one of our universities. This is the weld where the crack was in the weld, not on a border thick/thin; weld/pipe material.
On previous occasion we replaced the collector but yes, we are able to weld repair. But whatever action it will depend on the results of the metallurgical examination.
The leak returned to the same location.
Yes we would
Yes we have an inspection team who have experience with dye penetrant.
And don’t bother about the questions seeing the results I’ll most happily will answer them

 

[metengr]

ww2;
It might be beneficial to get the boiler OEM involved to determine if you need to install a low point drain on the collection header or modify your boiler shut down cycle. If you continue to fill the collection header with condensate, you will not eliminate the source of thermal fatigue - leaks will continue and you will sustain more damage, possibly to the collection header ID surface.

 

[unclesyd]

I concur with metengr that you have to keep the SH dry and avoid concentrating any BFW. Being able to weld repair eliminates one problem and as stated tends toward some type of fatigue with an assist from the TSP water treatment. I don't think that you have enough cycles for thermal fatigue.
I'm not familiar with this boiler and the location of the SH is a little puzzling. The SH collector/header should have a low point drain as stated. The next outage you should install a dutchman (a disk with a drain) at the blind flange. I wouldn't weld a drain on the collector/header until more information is gained from the metallurgical examination. One thing I would look for is corrosion fatigue. This would normally be in the form of small round bottom pits with cracks going in all directions.
Another thing that I have a problem with how your are able to run the boiler at the capacities you state. I haven't seen a boiler with a turndown of that magnitude and still be able maintain metal temperatures.

 

[ww2]

Thank you guys for all your professional advice. I think we all agree that the way of operation, low loads, many trips, with the consequence of temperature changes and maybe the effect of water treatment could lead to thermal or corrosion fatigue. Also that we have to wait for the metallurgical examination by the expert to be conclusive.
The already installed drain should be enlarged and get a condensate trap with bypass.
thanks again
Raymond

 

[unclesyd]

Be careful with a trap on the superheater side of the boiler. Make sure you get recognized trap installation expert help. Unless the proper trap and installation is used you could get a continuously blowing trap.

I would also work on the boiler controls in an effort to prevent the events you describe from happening. The BFW control system sounds like it need a thorough review.

We have 2 boilers that have tremendous thermal mass and if there is an unplanned outage it takes about 4 hours to take the boiler off line. During this time it is still under process control, mainly the steam drum level. The headers all have non-return valves and automatic valves that will take the boiler off line. There is an operator at the boiler to ensure that things are happening in the proper order. His main job is watch the level in the steam drum to prevent drying out or flooding the SH section.

Please let us know what the lab results are. This will be a benefit to everyone as a similar situation will come up again and it might allow someone to prevent a problem before it does damage as in your case. You probably have a lot of people looking at their boilers.
Please post any additional questions you might have.
Waiting to hear from you and good luck with the resolution of your problem..

 

[ww2]

I keep you guys posted
tanks again!
raymond

 

[rmw]

ww2,

I think there is a clue for us in a statement you made where you said that the boiler is rarely operated at more than 50 percent of rated output, and normally around 10 percent.

Is the superheated steam temperature you gave us a rated SH steam temp, or a measured temperature.

Steam, while being superheated, believe it or not, actually cools the tubing. What is the measured steam temperature, and the measured flue gas inlet/outlet to the superheater??

rmw

 

[ww2]

Steam inlet to the SH 195°C outlet 229°C
Velocity at 3 ton/hour 4,4 m/sec; at 30 ton/hour 41,5 m/sec. Pressure differential at 3 ton/hour: 0,000 bar; at 30 ton/hour 0,035 bar
Fleugas inlet 333 °C, outlet 248 °C
raymond

 

[ww2]

Epilogue

505/04/2004 we received the report of the metallurgical examination which was carried out on our request on the dead end flange of the damaged SH outlet collector of the boiler.
The conclusion of the report inputs the damage mainly to thermal fatigue and some intergranulare fragilisation of the weld (the full report can be send personaly to those who are interrested). To be able to exclude the hydrogen embitterment option, as a cause for those little weld fissures, further investigation and examination of the other cracked welds is needed The execution of this investigation will depend on the type of repair we will have to chose: renewal of the collector ore repair
This conclusion agrees with the hypothesis of the cause, namely the numerous thermal shocks where brought about by the number of nuisance trips (4 trips a day was not unusual). Most of the time on a boiler at very low load because of lack of confidence of it by the operator Consequently, because of the low loads, with preferential streams and dry tube loops in the SH coil.
In the design of the new SH collector we will take in to account the concerning recommendation namely, annealing treatment of the welds and the tie-in of a 1” nozzle, for draining facility, near the dead-end flange of the collector.
The main reason: low loads and nuisance trip where already, but to late, tackled
raymond

 

[ww2]

Epilogue

Sorry guy's, embitterment should read embrittlement.
raymond

 

[rmw]

Actually, as many problems as you have presented to us about this boiler, you may have gotten it right the first time. Sounds like embitterment to me. HA!

rmw