Waste Heat Boiler Major Leak

[APIPapi]

We had a major leak in our waste heat boiler causing a major unscheduled outage recently. After opening up the single pass fire tube boiler, we found out that numerous leaks were present along the tube-to-tube sheet interface where the tubes were rolled/welded. All of them were on the hot side of the boiler. The tubes and tube sheets are only 4 years old, so we are wondering why it failed so early and so catastrophically.

1) Exterior of boiler tubes have a good amount of scale that flakes off very easily after touching it. Boiler water chemistry looks good.
2) Tubesheet is not deformed/distorted.
3) About half of the tube-to-tube sheet welds show indications after PT.
4) Due to frequent power outages (at least once a month), we are forced to valve in city water to the waste heat boiler to maintain adequate water level since our furnace stays hot days after burners are off.
5) There are no signs of any cracked tubes/tubesheet. No pitting due to dissolved gases in water noted.
6) We have reason to believe that we had a minor tube leak a few weeks prior to our major failure causing us to shut down.

Do these symptoms appear to be thermal fatigue related?

 

[LittleInch]

Pictures?

Drawings or sketches?

Difficult to visualise this.

What's being fired?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[weldstan]

Is this in a desulfuring unit? There were many early failures in such boilers in the '70s. What's the temp in the inlet? What are the fluids?

 

[GD2]

Op,

Looks like you have fire tube boiler where waste hot gas passes through the tubes.
From your description,the tubes were rolled and also welded at ends.
Leak at the hot ends tubes surely implies that not only the tube-to-tubesheet joints have weakened but the welds have also failed.
One possible cause could be the appreciable thermal expansion difference of the tube and the tubesheet materials at the hot end where the expansion effect will be the highest.
Is it possible to send the design conditions with tube and tubesheet materials of construction?

GDD
Canada

 

[APIPapi]

It’s a horizontal firetube boiler that uses waste heat flue gas coming off our ~2300F large glass furnace. Tubes were rolled and welded. We found out recently that most of the leaking tube edges/welds have eroded away.. most likely from steam erosion. Therefore, these cannot be rerolled. Tubes/tubesheet are all still within mill tolerance and no deflection noted.

 

[Trestala]

Welds have eroded away? Were those tubes really welded properly in the tubesheet? Do you have fabrication reports?

 

[weldstan]

The welds should have been partial penetration into the tubesheet plus fillet weld reinforcement. at least 2 passes, preferably 3. There also appears to be lack of weld fusion. Was there an engineering spec for this boiler?

 

[APIPapi]

The welds are not true seal welds. They are only used to transfer heat off the tip of the tubes so they don't overheat and crack. With that being said, we are probably going to bevel the tube sheet and welded per ASME Sect I PFT-12.1 (g). Repair history shows us we are replacing these on a 4-7 year frequency. I am starting to lean in the direction of particulates in the air (mostly unmelted sand that makes it's way over to the boiler) eroded the weld and worked their way in between the tube/tubesheet and eventually created a leak.

 

[georgeverghese]

That may be likely : erosion from high velocity hard sand particles entrained in the hot gas. Would have been better if there were some cyclone tubes (to knock out these sand particles) installed before this hot gas gets in to the boiler tubes.
Is there an expansion joint on the shellside to accommodate the differential expansion between the hot tube bank and the relatively cooler shellside ?

 

[Christine74]

Is there some reason that tube ferrules were not installed into the tube ends on the inlet tubesheet?


-Christine