recurrent steam boiler fire tube failure

[hicbee]

Hie colleagues

I am a member of the regulating authority in our state.
Transferred to a new region recently, I had to deal with a composite boiler that has had recurrent tube cracking problems every year since 1999. Every annual inspection of the boiler has seen at least 10 tubes needing replacement due to leaking and cracked tube ends on the combustion chamber side.
I have begun researching on what the possible causes of this are.
The vessel has a working pressure of 32 bar.

Any leads to the most likely causes of this unusual problem?

 

[metengr]

hicbee;
Cannot open the file attachment, and not enough information in your post. You need the following for any help;

1. Tube Material
2. Type of previous inspections
3. Description of failure - axial rupture, circumferential crack, or other.......

 

[GenB]

you mention firetube boiler at 32 bar,
can you provide brand, design-3pass, 4pass.
cracks can be stress due to overfire.water condition meaning hydrogen
attack.

 

[hicbee]

Further to my question yesterday, some eng-tips members asked for further details.
The boiler is a composite type with afire tube design at the front and water tube design on the rear end. The section that is giving problems is the fire tube designed section on the combustion chamber side.
Cracks are normally linear from tube end into tube plate;length of cracks from 2mm to 12mm. Other cracks were multiple hairline.
Another report on failures talks of linear fissures 2-8mm in length on fire side.
Failure occurs on the primary first pass.
Tube material is fully killed cold drawn low alloy steel seamless tubes.( Given on one paper as being according to BS 3059 Part 1 ER/HFS 360). The individual chemical components are there but are too bulky to list them here.

Worrying though that design pressure is the same as working pressure at 32bar. These should not be the same.
I hope I have now given enough information.

 

[metengr]

hicbee;
By your description this sounds like creep rupture failures in the tubes - axial cracks of various lengths. It would appear the boiler must be re-tubed with either high alloy tubing to provide increased strength at temperature or it is possible you have some type of flow problems where the tubing is exposed to long term overheating.

 

[blacksmith37]

Thermal fatigue ; try sacrificial loose liners in the problem zone, Inconel 600,601, or "poor boy" Incoloy 800. Actually for a trial, 304 SS would work. Standard for oil country fire tube "heater/treaters" for 50 yr.

 

[chicopee]

Are the tube ends flared or beaded?

 

[hicbee]

Hi Chicopee.
I got your question. The tubes are in the combustion chamber and thus seal welded. Procedure here is to expand them slightly, then seal weld and grind off the tube ends flush with the welds.
Thanks guys for the assistance so far. Will appreciate it if more keeps flowing in. I will consider your contributions especially the possible change of tube material.

 

[Weldmedic]

Just a thought here... Your description of the seal weld followed by grinding flush with the weld. It would seem that perhaps there may be instances where the is excess removal of the fillet weld attaching the tube to tube sheet. Perhaps a better method would be to flush grind or machine the tube ends first then seal weld. I think this would produce a more consistant surface and eliminate problems associated with overginding/or improper grinding such as excessive feed or tool pressure.

Weldmedic

 

[hicbee]

Hie Weldmedic
That is quite interesting.We have tried to be carefull but there is always a case where you realise you have ground too far inwards. Point taken.

 

[hicbee]

Hie colleagues! I am trying to determine whether use of fire tubes with welded seams is permissible in today's boiler construction ir in re-tubing work.

 

[metengr]

hicbee;
Yes, electric resistance welded tubes are permitted with limitations under PG-9.5 READ Section I, see PFT-14 General, 14.1 and 14.2.

 

[stanweld]

Are the failures limited to the old tubes or both new and old tubes? How long has the boiler beeen in service? It may be judicious to replace all of the old tubes if cracking is limited to the old tubes rather than replacing only those leaking tubes.