Crown Seal Issues

Silica is the most common material of choice for melter crowns and has been for the better part of the last century. When oxy-fuel combustion became more common in the 1990’s, new problems with this “ideal” crown material became apparent. The absence of nitrogen in the fuel oxidant resulted in an internal furnace atmosphere that had a 5x concentration of corrosive materials: sodium sulfate in soda lime glasses and hydrogen chloride in borosilicates. The first oxy furnaces had crown issues and no one likes that.

One plan of attacked was hot sealing of the crown with a pumpable fused silica refractory. This would form a monolith over the standard crown and then the insulation would be applied over it. The idea is that even a small rathole could allow vapors to penetrate the brick, dissolving it to form big ratholes. By sealing the crown with liquid, all the joints between bricks and would be sealed and thus there would not be ratholing. Often the castable was laid end wall to endwall with no expansion joints as a complete monolith.

Looked good at the time!

It would look nice and smooth but there was an issue. Several layers of insulation would be applied over this or else the heat loss would be extreme.

An unsealed melter crown lights up the factory.

The mistake is assuming that this seal layer doesn’t expand when it heats up. It does and without expansion joints the seal layer will rise above the crown and leave a gap between the crown and the seal. It would all be hidden under the layers of insulation and crown seal. Also somewhere under the seal the nasty vapors from the melter atmosphere would cool enough to condense and begin the dissolve the crown from within. At first it would be a game of whack-a-mole with little holes popping up through the insulation at times some distance from where the started in the crown.

There’s a sizable gap between the insulation and the crown.

What can happen is that a large enough area under the seal dissolves and the arch of the crown is lost. When this happens a large section or even the entire crown can catastrophically fail. This can all be avoided by just leaving expansion joints in the seal coat and filling them in after the insulation and laid on top. A simple solution to a big problem.