Steam Boiler Damage

[forensiclab]

Dear All,

I have got a problem with my fire tube boiler. The boiler has three passes in which the hot gasses pass through. The first pass is where the fuel oil burner is mounted and is also referred to as the furnace chamber. This chamber has a diameter of 1254mm, length 4453mm and a plate thickness of 22mm. The plate was of mild steel with a yield strength of 355 MPa. The boiler operates at a pressure of 17 bar. The boiler was shutdown for the annual inspection. It was found to be in good condition visually. A pressure test was conducted hydrostatically. The hydrostatic pressure was to be conducted at a maximum of 18 bar. The boiler was capable of being pressure tested to 27 bar safely.

A pressure test was conducted by blinding all flanges of the boiler and then utilizing a pump that had a maximum pressure of 18 bar to pressure the boiler with water. As the pressure test was conducted, there was a strange sound from the furnace chamber. Upon checking it was found that the a section of the circumferential chamber had slightly caved in at the 3 and 9 o'clock positions. The pressure at the time was only 8 bar, well below the operating pressure of the boiler.

We are at a loss as to what has happened to our boiler. Could anyone help us out here. Has anyone else encountered a similar situation with their boiler. Any input is greatly appreciated.


Thanks in advance.


Anand

 

[Keith53]

Now that I've seen the picture of the seal welded tubes/sheet, and understand that you yourself did not see the furnace BEFORE the hydro, and understand that the tube/sheet welds were performed just prior to hydro......I suspect boiler was subjected to overfiring and/or suffers from scale buildup where deformation occured while in operation and under pressure.

What does water side look like?

 

[chicopee]

One last note in regard to an above reponder alluding to low water condition. I believe had that happened several top row tubes would have overheated, being exposed to steam instead of water, and ultimately leak at their tube ends, however, in your case the leaking tube ends are randomly spread out throughout the tube sheets which is an indication of overfiring eventhough the tubes were submerged in water.

 

[forensiclab]

Thank you so much for your thoughts.

The water side of the furnace chamber is scaled. Usually if there is scale, you would find evidence of long term overheating when we examine the microstructure of the boiler steel. We do not find this, which suggest that the overheating had only recently occurred.

Yes, there is no evidence of a low water level condition having occurred. None of the second and third pass tubes are distorted. Though there is scale on the second pass tubes.

 

[Keith53]

As you can see, you must prevent scale, ESPECIALLY in firetube boilers. I've seen many where the furnace collapsed and/or rear tube sheets cracked/bulged and tubes cracked at tube-to-sheet joint.....You were very lucky in that the furnace only deformed and didn't fail under pressure.

The scale can be chemically removed and furnace replaced, perhaps even jacked back into shape, but plan to replace this boiler soon. Strong acid cleaning ALWAYS leads to corrosion of tube-to-sheet joints due to concentration of acids that get trapped in the rolled joints and cannot be neutralized.

 

[4Pipes]

How did you calculate 126 bar g? The fire chamber only has 3 ribs. At 20 mm thick and 1254 dia, I suspect it would be very easy to over heat the 12 o'clock position. Highest heat input and poorest surface heat transfert coefficient on the water side. Scale and debris collecting at this point might not help. My guess is that it would not have much spare external pressure capacity to cope with the geometric and apparant out of roundness. Material variations caused by uneven temperature through thickness would create the same effect as out of roundness.

 

[eliebl]

At first glance it appears that this was a case of overpressure. The fire tube is external pressure design. It there was a slight out of round then all bets are off. External pressure relies heavily on stability and not so much on strength.

I once chased down how the code rules were developed for external pressure and the theoretical safety factor on external pressure is 10 to 1. In actuality, due to manufacturing variability and material properties, the experimental safety factor for external pressure design is around 3 to 1.