This article discusses applications of computer simulations for restoring aging boiler vessels. Companies are using finite element analysis (FEA) to predict the intensity and distribution of these stresses, and hence the distortion magnitude and the residual stress state produced by the welding operation, so engineers can evaluate the structural integrity that a repaired component will have. The analysis can be applied to boiler tubes, pressure vessels, and chemical storage tanks. A particularly challenging application for weld-overlay FEA arises with the more complicated shapes found in boiler-tube waterwall panels, which consist of a row of hollow tubes connected by a membrane, a continuous layer of steel that connects the tubes. Such waterwalls are typically arranged to form a square box around a heat source that causes water in the tubes to boil and generate steam, which is then carried off to operate a turbine. Software from Algor can be used with linear and nonlinear material models to determine the stresses that the waterwall's distortion would place on the supporting buckstay structure.
After Many Years of service, the walls of pressure vessels and storage tanks can become thin due to corrosion or erosion. When this happens, welding an overlay of more resistant metal on the inside or outside of the container is a consideration. Then the welder restores the tank to get a few more years of service at a fraction of the replacement cost.
But the welding operation itself will introduce thermal stresses that can pose a hazard to the structural integrity of the tank. In extreme cases, the thermal stresses of welding can cause the walls of the vessel to buckle.
According to Tony Scandroli, project manager at Welding Services Inc. of Norcross, Ga., his company uses finite element analysis to predict the intensity and distribution of these stresses, and hence the distortion magnitude and the residual stress state produced by the welding operation, so engineers can evaluate the structural integrity that a repaired component will have. The analysis can be applied to boiler tubes, pressure vessels, and chemical storage tanks. He notes that "the FEA methodology, which we have validated in a laboratory mockup, eliminates unexpected surprises." Scandroli uses software from Algor Inc. of Pittsburgh, notably its new Acccupak/VE Mechanical Event Simulation software.
Over time, material builds up on the inner surface of tanks, harboring corrosion, or the walls of vessels are eaten away until they become dangerously thin. Welding a new layer of metal to the tank wall, either on the inside or the outside, is often the most economical solution, as new tanks can cost several million dollars. The problem is that the intense heat of the welding torches, as much as 3,100 degrees Farenheit, can cause the already fragile fabric of the tank walls to deform and even buckle, thus bringing about the very result the operation was designed to prevent.
The traditional answer to this dilemma was to perform tests on small-scale mockups prior to firing up the welding torches. This method, however, did not allow engineers to quantify the resulting distortion with any exactitude. There also have been attempts to perform computer simulations to better comprehend the thermal stresses and consequent distortions. One problem, though, according to Scandroli, is that many investigators, rather than focusing on structural integrity, have concentrated on the weld zone, without reference to the rest of a large and heavy structure. Scandroli cited distortion magnitude, buckling, and phase transitions as oth er areas that often had been ignored in the past.
Scandroli said that the weld process inherently weakens the vessel wall so the risk of buckling goes up. This is more likely to occur when weld overlays are manually applied, given the high heat inputs and start-stop residual stresses involved. He added that his firm uses automatic equipment that minimizes the stress inputs.
The event simulation software makes it possible to do two-dimensional analysis, that is, analysis involving time integration. This lets engineers perform time-dependent analysis using elatoplastic elements and heat transfer analysis to develop a temperature history and establish temperature gradients. These thermal loads are used to create a residual stress model. From this it is possible to determine an applied weld load, indicating the internal tensile or compressive forces brought about by the welding process.
The data for the weld loads are input into a three-dimensional model and subjected to a large incremental analysis, which can predict the amount of distortion to be expected. The process is intended to reveal incompatible thermal strains th at can lead to plastic strains. These can cause the material to undergo plastic upsetting, that is, growth or shrinkage, which can result in permanent deformation or warping, microcracks, or even buckling.
The software's ability to follow the situation in time allows it to monitor the heat source as it travels. The weld area heats up rapidly, taking one to two seconds, and then, once the heat source—the welding torch-passes on, the weld area cools down quickly, too, as the heat soaks into the base metal. The nonlinear dynamic stress analysis incorporates elastoplastic material behavior to pinpoint the incompatible thermal stresses as internal forces create distortion.
If the simulation shows that significant weakening or even buckling of the structure is a real possibility, there are basically two sorts of remedial options available. One is to preheat the vessel wall, thus lessening the temperature difference between the heat source and the surface. However, this solution is impractical over large areas. The other option is to add stiffeners. Metal plates might be tack-welded to the opposite side, say, or I-beams attached, depending on the circumstances.
Recently, the Exxon Bay town Refinery in Bay town, Texas, hired Welding Services to overlay the outside of a long-out-of-use refinery tower shell. Scandroli noted that certain areas of the vessel had very thin walls due to corrosion, but the analysis showed that enough of the wall was left to make buckling unlikely. Tom Hildenbrand, quality support group supervisor with the inspection section of the refinery, confirmed that the operation was a success.
A particularly challenging application for weld-overlay FEA arises with the more complicated shapes found in boiler-tube waterwall panels, which consist of a row of hollow tubes connected by a membrane, a continuous layer of steel that connects the tubes. Such waterwalls are typically arranged to fonn a square box around a heat source that causes water in the tubes to boil and generate steam, which is then carried off to operate a turbine.
These surfaces are exposed to a highly corrosive environment on the fire side. It eventually may become necessary to weld on a layer of corrosion-resistant cladding, typically stainless steel or Inconel 625.
While most waterwalls are arranged in a boxlike configuration, a few are built as a sort of square spiral. Banks of membrane-clad pipes, 40 to 60 feet in length, recline at an angle of about 11 degrees above the horizontal, with their bases toward the fireball. The banks of panels rise in a steady spiral-"like a square spring," as Scandroli put it—with 90-degree angles at the turns. Scandroli performed an analysis on one such installation in preparation for a weld overlay at a midwestern utility's plant.
A series of buckstays help keep the walls of such structures in place. To keep these walls of tubes in place, plates are attached to the cool side of the waterwalls. A U-shaped bracket on the back of each plate holds a retainer pin, which is free to move in any direction within the bounds of the bracket. The pin has square lugs attached at the end that fit into a large vertical I-beam. This allows the waterwalls room to expand.
A particular concern was that the welding process might cause the buckstays to loosen or become detached, as has happened in the past. Scandroli performed a separate analysis specifically to determine the stress distribution around the buckstays.
"Once the actual distortion solution for the waterwall was arrived at," said Scandroli, " those displacement magnitudes and the reactions at the buckstays were input into a separate model focused on the buckstay. The 3-D distortion model has boundary elements at the buckstay location.
"I determined the displacement magnitude, which is equivalent to the distortion, and what kind and magnitude of forces and moments were acting on the buckstays," he explained. "Those were wrung out in 3-D analyses. These values were input into the 3-D model of the buckstay, and we used the event simulation software to determine stress levels and distortion in the buckstay."
Scandroli always looks for the worst-case scenario. In the case of the waterwall structure, that was represented by the pin in the buckstay shearing off when the welding heat causes the metal in the structure to expand unevenly. The most likely failure mode, on the other hand, would be the U-shaped bracket tearing off the plate, or the lugs tearing off from the I-beam. Any of these outcomes would entail downtime for repairs.
Scandroli said distortion is cumulative. The larger the area to be welded, the more distortion. " If we were to weld twice as large an area, the stress on the buckstays would go up, and you could have permanent deformation." T hen, for instance, it might be desirable to reinforce the buckstay pins. However, this would raise the residual stress in the panel, and the consequences of that might be even less desirable.
So do we bust the buckstays or not? Scandroli said this question shows the importance of this type of analysis. "It can help the customer make a decision on wh ether to mitigate distortion and have other possible consequences, or just live with the risk of breaking the buckstays."
In this instance, the utility was spared making this decision, because analysis determined that no deformation under stress would exceed the material's yield point. Scandroli concluded there was no risk of failure, and in fact, he said, the welding proceeded without a hitch.