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