Out-of round boiler tubes

Date 09-13-2001 19:43

Has anyone run across this? We are currently doing a shutdown on a Black Liquor Recovery Boiler. We are replacing the entire floor and 6, 10 tube waterwall panels, 8' in length. The wall sections were cut about 1/2" below the previous weld line. There is 3/16" difference in ID dimensions on some of the worst tubes , as measured in the membrane to membrane direction, and the hot to cold side of the tubes, respectively. The larger dimension is in the membrane to membrane direction. We got a rolling machine to "round" the tubes to minimize the hi-lo issues.
Question is, how did the tubes get so out of round? I've personally never seen tubes this out of round before.
Our engineer says the condition of the tubes is due to the amount of cladding done in previous years. But upon closer examination of the membrane welds, they were welded with a 1 pass showing ( it's anybody's guess whats underneath the cover pass ), vertical down bead, completely covering the 1" membrane and touching the wall of the composite tubes. The welds look like Stainless FCAW, but it's hard to say with complete certainty.
Dale Simonds

By G.S.Crisi (****)

Date 09-15-2001 00:22

This is only a guess and I won't swear it's true.
You say that the longer diameter, measured on the MEMBRANE TO MEMBRANE direction is about 3/16" larger than the shorter diameter, measured on the HOT TO COLD side direction.
This is a clue leading to the possible reason of the problem.
Sometime in the past the boiler was shut down and allowed to cool with all of the valves closed, especially the vent valve. When the temperature arrived to 212ºF (100ºC), i.e., the condensing point, all the steam contained into the boiler condensed suddenly, thus creating a vacuum. Most pressure containing parts or vessels are designed to withstand internal pressure but not EXTERNAL one. When subjected to external pressure they tend to flatten, and this is what happened to your tubes.
What you should have done, after the boiler was shut down, was to wait until the temperature reached a point closely above condensing (say 220ºF) and then open the vent valve (or valves). Some steam would have gone away making some noise, but right after, when condensing took place, air would have got into the boiler not allowing vacuum to build up.
Why is "membrane to membrane" the longer diameter of your flattened tubes? Because the membranes are a reinforcement that prevents deformation. The shortest diameter is the "hot to cold side" one, which is the weakest.
Again, this is only a guess. Investigate your boiler history and find out whether it's true or not.
Giovanni S. Crisi
Sao Paulo - Brazil

By dasimonds (**)

Date 09-15-2001 09:05

I understand the principle your describing, and I can vividly remember my high school shop teacher demonstrating what 14.7 psi (atmospheric air pressure at sea level) will do to a 1 gallon metal can, containing boiling water, when it is removed from the heat, capped, and allowed to cool. And I'd bet I'm not the only one out there that has seen what happens to a tank that is drained without a vent open.
Let me add some more information.
The bottom cut line for the new waterwall panels is 1/2" below the old weld line. The top cut line is located 10' up, past the transition point from composite tubes to completely carbon boiler tubes, part of the original tube sheet. None of the carbon tubes are distorted or flattened. None of the composite tubes on the cut line for the new floor tubes are distorted.
When we started preping the tubes on the old weld line with a millhog ( air powered beveling tool ), the bit in the millhog never touched the cold side face of the tube ( a flat spot ). Thats when we noticed the distortion. Only the cold side of the tube is distorted.
The membrane welds on the hot side of the boiler are small, 1 pass verticle down seal welds, as opposed to the cold side, which are, in some cases, over an inch in width.
Wouldn't a large 1 pass membrane weld put a lot of heat and stress on both tubes touching it, causing the tube wall next to the membrane to bend towards the membrane being welded as the weld cooled? Wouldn't going across the cold side of the boiler welding membrane with the high heat input 1 pass welds have the effect of trying to pull the tube towards the membrane on each side of it, giving it a flat spot?
The questions I have about the vacuum effect are these;
Why would the vacuum have a localized effect?
Why only around the old welds?
Why only the cold side?
I will inquire as to how the boiler was shut down.
Dale Simonds

By - Date 09-16-2001 20:11

I would agree that the large fillet weld you described could lead to a distortion that pulls the tube oval. The only issue that I would have with this theory is that you described "flat spots". My experience is not that you would get flat spots, merely that the tubes would become oval. This oval should also be seen on the hot side of the tube, although it would be less because of the strengthening effect of the cladding.

Another possibility that you could look into is that the flat spots were created by efforts to remedy distortion during the original fabrication. This could be distortion created by the large fillet welds, or the cladding on the hot side of the tubes. Possibly, the panels were passed through rolls to flatten them. The weakest parts would then experience a "flat spot". As the tubes had extra strength on the clad side, the major problem would be seen on the cold side.

Just another possibility?

Regards
Niekie

By pipewelder_1999 (****)

Date 09-15-2001 11:50

I have experienced out of round wall tubes before. These were sa 210 A1 wall tube at a BLRB in Georgia. The out of roundness was < 1/8" but it did cause one to have to pay attention when milling the upper tube.

Gerald Austin
http://www.weldinginspectionsvcs.com/