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

 

[metengr]

Did you properly vent the firetube boiler before applying hydrostatic pressure??? Another possibility is the plate was thinned from corrosion and it simply deformed under lower pressure.

 

[macmet]

Can you provide some more information about your boiler? Is it a Firebox type or Scotch Marine, etc.

Do you have any pictures?

 

[forensiclab]

Thank you for the response.
The boiler is a Babcock Robrey 3 pass wetback fire tube boiler. It has a capacity of 15000 PPH, design pressure 262 PSIG and working pressure 250 PSIG. The hydrostatic pressure is 393 PSIG (27 bar).
We have done a thickness test of the plate and the thickness is fine in the order of 19-20mm (original thickness:22mm). There is no significant thinning due to corrosion. A hardness test was conducted at the bulged area and it was found to be fine as well.

The inward bulging was only at the furnace chamber. Attached is a photo showing the bulging at the furnace chamber.

Thank you.

 

[forensiclab]

More photos

 

[forensiclab]

photos

 

[macmet]

The bulges you see at 3 and 9 positions, is it only in one section of furnace chamber or is all along the length?

 

[marty007]

Could you confirm that water level was maintained during the shutdown?

If it had been drained prematurely, a temperature differential could have taken place and caused some thermal damage. One place to check are the tubesheets, look for any warping there?

 

[forensiclab]

Thank you for the input. The bulging is throughout at those positions but there is one area where the extent of bulging is more obvious.

It can be confirmed that there was no drainage of water to the boiler that could have caused overheating. There is no warping to the tubesheets. The second and third pass tubes are also fine (not distorted). Damage was confined only to the furnace chamber.

 

[metengr]

I still think you had a problem with improper venting prior to fill and conducting your hydrostatic test.

 

[forensiclab]

When you say improper venting do you mean that there might have been air pockets trapped on the water side of the boiler? On the fire side, the manhole covers were all opened during the hydrostatic test so that leaks could be detected.

For years the hydrostatic test was done in this manner without incident. Cant seem to find an explanation of what went wrong this time.

 

[metengr]

When you say improper venting do you mean that there might have been air pockets trapped on the water side of the boiler?

Yes.

 

[Keith53]

I'm a boiler inspector......This had NOTHING to do with trapped air.....For hydro test, we make certain to remove all air for safety reasons, not to prevent damage to boiler.

Air is compressable, therefore in the event of vessel failure while testing, it would escape explosively. The same vessel full of water would fail at same pressure with much less violence and destruction.

The damage to this boiler was caused by overpressure during testing....The same exact damage would have occured had the boiler been tested with air to same pressure. I am certain the boiler was overpressurived while being tested......Owner admits it had no damage before testing and it normally operated at his target test pressure of 18 bar, but failed at only 8 bar. I suggest owner obtain all new test gages and test rig, then have it calibrated each time before used for testing.

I'm also a former (20 year) nuclear submarine mechanic where I conducted hyro testing on my systems and equipment on a daily basis.

 

[chicopee]

How cold was your water used to conduct your hydrostatic test to determine if brittle fracture amy have happened. Secondly, tell us the reason for conducting the hydrostatic test since this would not be done unless repairs were made. Thirdly, you may need a metallurgist to determine if the plain furnace was overheated or there was substandard metal to fabricate the furnace. Did the failures at 3 and 9 oclock positions taake place along circumferential weld? also do an examination if possible of the waterside of the furnace where the failures occured for any unusal contaminants such as fuel oil or sediment buildup that may have overheated the furnace in the areas of failure.

 

[forensiclab]

Hi Keith,

Thank you for your insights. It was very useful. The pressure gauge used to conduct the hydrostatic test was calibrated before the test was conducted and checked again after the incident. The gauge was reported to be in good condition. My initial thoughts were overpressure but the pump that was claimed to have been used was rated for only 18 bar which is the normal working pressure of the boiler.

I did a calculation to determine whats the maximum pressure that is required to cause plastic deformation of the furnace chamber. I get a figure of about 126 bar. The calculation was done based on the following parameters. The yield strength of the furnace material is 355 N/mm2. The outer diameter of the furnace chamber is 1254mm. The plate thickness of the chamber is 22mm. The length of the chamber is 4453mm.

126 bar is very high and for the furnace to fail at only 8 bar seems impossible.


Hi Chicopee,

There has been no fracture of the furnace chamber. The temperature of the water used for the hydrostatic pressure was in the region of 28 degress centigrade. The hydrostatic test was done every 18 months to ensure that the boiler met the requirements of the local authority (machinery department). No repairs were conducted on the furnace chamber before the test. The bulging was not at a weld.

A sample from the bulged area was obtained and metallurgically examined. The steel was typical boiler steel with no evidence of long term overheating. A hardness test was also conducted and the readings were all normal.

 

[macmet]

I am not familiar with these types of boilers, so I am curious if there is a baffle or a structural component at the 3 and 9 positions? Is there water below this level?

 

[chicopee]

Obviously, something unusal happened and it appears that the possible causes were eliminated. Have you notified your boiler and machinery insurance company for their opinion. The decision will be to either repair the furnace which will probably be more expensive than buying a new or second hand boiler. Plain furnaces are alright, however, I prefer corrugated furnaces for fire tube boilers.

 

[chicopee]

I noted in one of the JPG pictures, several tubes were seal welded. The randon seal welding on the tube ends indicate to me that the boiler burner may have been overfiring and exceeding the steaming rate of this boiler. Did that take place and if so then that could explain the damage to the furnace. You should know that the rule of thumb for firetube boiler steaming rates is between 3.5 and 7 lbs of steam per hour per square foot of heating surface; some F.T. boilers may go up to 10 lbs/hr/sq.ft..

 

[forensiclab]

Thanks again for the responses.

Macmet,
There is no structural components at the 3 and 9 positions. These positions are immersed in water so does the entire furnace chamber.

Chicopee,

You are right the cost of repair is more or less equivalent to a new boiler, so a decision has been made to purchase a new unit instead of repair. But I am still curious to know what happened to this boiler.

I should have mentioned this earlier, this boiler is a backup unit and would only be fired at full capacity if there is a breakdown of the main boiler. While on standby, it is fired with low flame and kept at a relatively low pressure (hot standby). The possibility of it being overfired seems slim but not impossible. During this shutdown, several tubes of the boiler on the second pass were noted to have cracks at the tube/tubeplate joint. This was repaired by welding at the tube ends and that is what you see in the photo.



I have thought of another theory which you may correct if it does not make sense. I think the bulging in the boiler may have already existed in the furnace chamber even before the hydrostatic test. Probably my boiler operator is not telling me the whole truth. This may have gone unnoticed during the visual inspection as the bulging was slight.

There may have been flame impingement from the burner (not angled correctly) as it may be firing towards this area where there was bulging causing this area to be hotter than normal. This may have occurred only recently and thats why we dont have degradation of the steel as a result of long term overheating. When this area was running hotter than normal, the steel would temporarily weaken at this elevated temperature, the steam pressure acting on the water side of the boiler pushed this area inward resulting in the bulging observed.

Does that make sense to all of you.

 

[chicopee]

Check the burner high firing rate and calculate its btu/hr output, then with an estimated efficiency at 70%, convert the results to a steaming rate under steady state conditions. To do that you'll need the steam table, condensate return flow rate and temperature. Compare the calculated steaming rate to the manufacturer's data plate.