During the years, there have been opposite opinions about the use of slag retainers in the Roof & Shell panels. Even though the retainers help to retain the slag, when you weld them to the pipe there is a grain growth in the steel microstructure that weakens the pipe’s strength in the weld heat affected area; normally the thermal fatigue stress cracks start appearing in the neighborhood of these seam welds on the pipes exposed to temperature. So, adding slag retainers to a panel that has been working without them, not necessarily will increase its life, and in some cases, could reduce it.
The Roof & Shell water cooled panels were developed many years ago, with the intention to replace the refractory coverage of the EAF Roof & walls above the slag line, looking for an operational cost reduction. At the beginning, there were different designs of panels; some of them made of cast iron with an internal pipe, some others made of plate and the most common used at these days made of pipe.
Since the beginning, it was found that a good slag coverage of the pipes on the panel act as an insulation layer and help to increase its useful life. The first designs of pipe panels had a pipe-to-pipe arrangement (without gaps between them), making it very difficult to keep the slag on the naked pipes. So, the slag retainers become mandatory to help form the slag coverage required.
Some years later, it became obvious that the WC panels were taking away some heat from the furnace and it became important to look for ways to reduce the amount of heat that the panels were taking away. The spaced pipe design started to replace the original pipe-to-pipe design, because it reduced the number of pipes facing the inside of the furnace, therefore the net water-cooled area was lessened and the heat removed was reduced proportionally too.
It was found also that in some cases the cavities formed between the pipes with the spaced pipe design worked as an anchor for the slag and it was possible to have a good slag coverage without the use of slag retainers.
The main factor to obtain a good slag coverage is the viscosity of the slag; if the slag is too fluid, it is more difficult to develop the slag coverage layer. However, the spacing between the pipes is important too, if the gap is too narrow, the slag anchoring is poor and the panel performs similarly to a pipe-to-pipe design; that is why some panels with spaced pipe design are not able to develop a good slag coverage.
Sometimes the problem can be solved increasing the gap between the pipes, but in some others, it is required to add slag retainers.
Before taking a decision about adding or not slag retainers, it is convenient to verify the actual slag coverage on the panels. For example, look at the Roof when it swings out and if the pipes look naked it means that the slag is too fluid and then probably the Roof will have a better performance with slag retainers, but if there is a slag layer in a high percentage of its surface, the slag retainers are not the best solution.
There are two alternate designs of spaced pipe panels, one of them uses round bars on the back of the pipes to seal the gap, and the other uses a flat bar or a complete cover plate on the back of the pipes; this last design performs better for slag retention due to the larger cavity formed.
If you need further information or assistance on this decision, do not hesitate to call us. We are here to provide you with the best solution.