economizer tubes

[rivcitymw]

Why would the outside economizer tubes be welded to the boiler membrane? The boiler is used to burn hog fuel and provide steam for a 20 MW turbine/generator.

 

[metengr]

Can you provide more information on your boiler design? Typically, membrane seal welds are used to provide for a gas tight seal (to prevent the egress of flue gas or the ingress of air).

 

[rivcitymw]

The boiler is a B&W design, 330,000 lbs/hr. The tubes have been welded to the inside of the membrane at two places. The two locations are also where the buckstays are located on the outside of the boiler. The outside tubes have been developing leaks over the years and I was wondering if the attachment of the tubes to the membrane would cause this. The boiler burns hog fuel and there have been issues with the combustion. The pressure inside the boiler fluctuates from positive to negative quite a bit so there is a lot of movement of the boiler walls.

 

[metengr]

rivcitymw;
Yes, the leaks could be caused by fatigue damage in service. Are the cracks occurring external to the boiler tube or internal? The reason for my question is that the cracks could be caused by corrosion fatigue along the ID surface of the tube beneath the welded attachment. I have seen this numerous times in waterwall tube circuits. The cracks could also be caused by mechanical fatigue crack propagation because the external fillet weld acts as stress risers along the OD surface of the tube.

You need to confirm the damage mechanism. To do this, grind the crack. If the crack increases in length or results in more spider web cracking, it is probably corrosion fatigue that initiated from the tube ID surface. If the crack disappears from grinding on the OD side of the tube, it is most likely mechanical fatigue damage from the seal weld attachment.

 

[rivcitymw]

The cracks increase in length when we ground it out. The only tubes that leak or develop leaks are the ones that have been welded to the membrane. (The welds on the tubes are only about an inch or so in length)

 

[metengr]

Ah, ha... this sure sounds like corrosion fatigue emanating from the tube ID surface. You need to keep the fillet welds off the tube crowns. I suspect you have other damaged economizer tubes.