Tube to tube sheet welding failure

[ak1965]

We have a Low pressure steam boiler. It operates at around 45 kg/cm.2. Its a kettle type heat exchanger with straight tubes. Recently leakage was observed in the boiler.Inspection reavaled, that 06 tubes failed & many places tube to tube sheet weld leak was observed . The tubes are SA179 tubes. Steam is on shell side & tubes carrying the hot gases. It is importnat to note that All leaks were obserbed on the inlet side lower half (inlet tube sheet temp. is approx 570 Deg. C whereas outlet temp. is around 250 deg c). What could be the reason of this failure ? Inlet side shell support legs are fixed type where as outlet side shell support legs are floating. Upon inspection, it was observed that the tubes are bent along the length. This observation was made while peeping through the tubes.

Necessary repairs have been carried out.

WIll any one share his view about the problem & share the experience.

 

[metengr]

ak1965;
How are the tubes installed into the tubesheet? Rolled and seal welded or strength welded?

Can you explain the type of tube to tubesheet leak - crack or separation or other?

 

[ak1965]

Thanks Metengr

The tubes are installed horizontally with a strength weld & the defects were porosities ( in some tubes big poros )

 

[metengr]

ak1965;
This sounds like it was poor workmanship of the tube to tubesheet welds from day 1. How long has this low pressure steam boiler been in service? Is this the first time these weld failures have occurred?

{Sorry for my repeated questions, but sometimes it provides valuable input to others and myself to help you}.

 

[DG72]

Without knowing more details, this sounds like a strong candidate for a thermal expansion failure. The thermal gradient is large and he noted that tubes are bent (in addition to the tube to tubesheet being broken). I'm curious why poor workmanship would be mentioned, when nothing seems to point to this.

 

[TBP]

Were the defects actually porosity in the welds, or possibly oxygen pitting due to inadequate water treatment?

 

[metengr]

I'm curious why poor workmanship would be mentioned, when nothing seems to point to this.

OP

weld & the defects were porosities ( in some tubes big poros )

That is why

 

[ak1965]

The exchanger has been in service for last 09 years & similar defects were observed in last turn around too. Point of curiosity is the defects are being obeserved in lower portion of tube sheet on inlet side. If it had been a poor workmanship it should have been all around. More of it the tubes are also bent from the central axis. I think this might be contributing soem how in the failure as the tubes are also failing in adition to the tube to tube sheet joints. of course burried defects in the weld might be a catalyst to the failure. Hope all in the thread would agree to it to some extent.

Any new idea please ??

 

[metengr]

ak1965;
Can you post any pictures of the tube to tubesheet failures or provide a better desription other than pores? To a metallurgist, when I hear pores I immediately think of porosity which translates back to poor workmanship. By the way, depending on the welder or welders you could have defects confined to a particular location on the tubesheet, so don't necessarily rule it out.

I only suggested workmanship as a possibility. Without seeing the failure first-hand or reviewing pictures, it certainly could be thermal/mechanical stresses that are attempting to overload the weld joint.

 

[ak1965]

I am in a process to find out the possible restrictions in expansion of shell. will let know the forum if any thing abnormal is observed.

 

[themal1]

To me, bending the tubes is an over heat on that side of the boiler which caused the tubes to leake around the tube sheet. This is due to poor water circulation around these tubes due to scale deposit in the water side plus the burner flame could be impinging towards that side of the boiler.

 

[DG72]

Sorry metengr, I didn't see his follow-up e-mail where he mentioned porosities. I agree that weld porosities would indicate poor workmanship of the tube-to-tubesheet weld; still, the bent tubes, etc is indicative of differential thermal expansion problems.

 

[0707]

I suppose that the tube bending is the result of heat exchanger poor design. Normally kettle type exchanger has a floating head for tube bundle expansion. If the tubes cannot expand the tube welds into tube sheets will be stressed and weld failures will occur. If I were you I will revue exchanger design and strength welds procedures.

luis marques

 

[stanweld]

ak1965,
How many cycles has the unit seen. Cylic service at the high temperature gradients can easily lead to leakage at the tube-to-tubesheet joints, especially at pore sites in single pass welds. Were the tube-to-tubesheet weld 2-passes or one? Porosity is also often seen at the tie-in locations; is porosity throughout the weld or localized?

 

[LSUME]

Rookie question.
Would it be possible that the level control system malfunctioned causing the shell side fluid to evaporate completely while allowing hot tube side gas to flow?

 

[MJCronin]

ak1965....

Consider posting this question on a more boiler oriented forum:

http://www.steamforum.com

Let us know about you final decision....

-MJC

 

[ak1965]

I agree with LSUME,

We have found some indicative history of boiler running with very low water levels in the shell. So as, I am convinced the bent tubes are the result of execssive heating of tubes due to water starvation, This would have caused loading on tube tube sheet joints also, casuing TTS weld failure at many places.

Thank you forum for the support & share