Give & take of saturation runouts.

Inside This Story

* Saturation runouts from the bottom of a furnace can be a sign that something is wrong with the refractory materials. But it can also signify a problem with the design of the entire refractory system.

* Detailed within is how saturation runouts occur and how eliminating elements of the refractory system can prevent them.

Does it seem odd that a furnace lined with a castable refractory can experience runouts even though cracks are nowhere to be found? Is it even more intriguing that this can happen when there is no damage to the refractory materials? It is possible for runouts to occur when the refractory materials remain intact. Thus, the problem is not with the refractory materials, but rather with the entire refractory system.

It happens because the castable lining is not made up solely of the refractory grain. Voids or pores that are filled with air occupy some of that lining volume. Those pores, which are interconnected from a porous network around the refractory grains, can and will saturate to form a saturation network. As the molten iron finds its way through these pores, it leaks from the furnace. This escape of molten iron is known as a saturation runout. While numerous factors are involved in how much the lining will saturate, the major contributors are the interconnected porosity of the lining material and the thermal gradient within the lining.

This article details how the saturation runout process takes place and how the dept of the molten metal saturation can be controlled to avoid it.

Porosity & Thermal Gradient

Porosity is the ratio of the pore volume to the total volume. The installed porosity for a low-moisture castable refractory is between 12 17%. Assuming that all of the pores are interconnected, the maximum depth to which molten metal can saturate the lining is the depth at which the refractory temperature is equal to the freezing point of the molten metal. In the case of cast iron, this temperature is 2,066F (1,130C). Whether or not the saturation network will reach this dept depends upon numerous factors, such as:

* whether or not all of the pores are interconnected;

* whether or not the refractory contains something that tends to plug the pores;

* the fluidity of the molten iron;

* the contact angle between the refractory and the molten iron.

Despite tills knowledge, many refractory designs still do not assume that all of the pores are interconnected and that the depth of the saturation network is controlled by the freezing of the iron.

When the lining is monolithic, a single refractory system extends from the hot face to the cold face. If it is assumed that the thermal gradient ranges from 2,700F (1,482C) at the hot face to 350F (176C) at the cold face, the saturation network will freeze well within the lining. While this would qualify as a good lining design, it would have high heat losses.

To counter the heat losses, insulating materials are placed between the hot face materials and the furnace shell. The lining then becomes known as composite lining, which typically consists of a hot face castable material, a backup brick, an insulating fire brick and insulating board or paper. In order to take advantage of an insulated lining system, a thermal gradient analysis is typically conducted to ensure the molten iron will freeze before reaching the insulating materials because they will not be able to contain the molten metal.

In the case of a 65-ton useable pressure pour furnace, the lining was considered sufficient, but it still allowed at runout. The bottom refractory was 20.1875 in. thick, consisting of 13.1875 in. of a hot face castable, 3 in. of backup brick, 3 in. of insulating fire brick and 1 in. of insulating board. The thermal gradient data for the lining is shown in Table 1.

Since the temperature between the backup brick and the insulating fire brick (1,990F/1,087C) is below the melting point of iron, this lining would have historically been considered a good design. In reality, the lining suffered a major runout, allowing molten metal to escape through the bottom of the furnace. Because the temperature of the hot face castable is higher than the melting point of iron, all of the pores in the hot face material are subject to being filled with molten iron. Since the temperature of a portion of the backup brick is higher than the melting point of iron, the pores in the backup brick and brick mortar also are subject to being filled with molten into. The porosity of the backup brick is very low, so little or no saturation of the brick takes place. But the porosity of the brick mortar is high, allowing considerable saturation of the mortar to take place.

Saturation Effects

Saturation of a refractory by molten iron alters the thermal conductivity, which in turn alters the thermal gradient. The thermal conductivity for an alumina hot face castable is about 22 Btu-in./hr/[ft.sup.2]/F. The thermal conductivity for molten iron is about 200 Btu-in./hr/[ft.sup.2]/F. Thus, the thermal conductivity for a saturated hot face lining will be higher than that for the refractory, but lower than that for the molten iron. Since the thermal conductivity of the saturated portion of the lining is not known, it can be estimated by using the concept of partial thermal conductivity. This concept equates the thermal conductivity of a saturated refractory to the sum of the partial conductivity for the refractory plus the partial conductivity for the molten iron.

The equation is written as:

T[C.sub.SR] = T[C.sub.SR] + T[C.sub.I] x pp. In the equation:

* T[C.sub.SR]--represents the thermal conductivity of the saturated refractory;

* T[C.sub.R]--represents the thermal conductivity of the refractory (22 Btu in./hr/[ft.sup.2]/F);

* T[C.sub.I]--represents the thermal conductivity of the iron (200 Bin-in./ hr/[ft.sup.2]/F);

* pp--represents percent porosity.

If there were 15% porosity, the equation would be written as: T[C.sub.SR] = 22+200 x 15% = 52 Btu-in./hr/[ft.sup.2]/F.

In this example, 15% iron saturation more than doubles the thermal conductivity of that portion of the lining. Table 2 compares the thermal gradient of the unsaturated bottom lining from Table 1 to that lining saturated with 15% molten iron.

As the hot face castable portion of the lining saturates with molten iron, the temperature between the hot face castable and the backup brick climbs from 2,209F (1,209C) to 2,227F (1,219C). Consequently, the freezing point of the saturation network (2,066F/ 1,130C) moves from within the backup brick to within the insulating fire brick. Since the insulating fire brick cannot freeze off and contain the molten iron. a runout results.

This is when metlacasters have to determine if the refractory system has too much insulation. A refractory lining system is considered to be over insulated when the freezing point of the saturation network falls within the fire brick. In order to solidify the iron in the backup brick, the thermal gradient must be changed by either decreasing or removing backup insulation. Table 3 compares the thermal gradient of the same 20.1875-in. thick saturated bottom lining without the 1-in. thick insulating board.

By eliminating the insulating board, the interface temperature between the backup brick and the insulating fire brick is reduced from 2,227F (1,219C) to 1,980F (1,082C), which is well below the freezing point of iron. Herein, the leading edge of the saturation network will freeze within the mortar of the backup brick and not reach the insulating fire brick. Removing the insulating board will have two effects. The furnace will not fail due to a saturation runout, but the heat loss from that portion of the furnace will increase from 937 to 1,339 Btu/hr/sq. ft.

Protecting Against Runouts

Many saturation runouts through the bottom of the furnace occur because of over-insulated lining designs. In order to combat runouts, metalcasters need to ask a question--which is more costly, runouts or heat loss?

To stop runouts, over-insulted bottom linings that are designed for low thermal losses need to be traded in favor of less-insulated lining systems with more thermal losses. Likewise, no backup brick or insulating board should be allowed on the furnace bottom within 12 in. of the throat opening.

To ensure that saturation runouts do not occur through the furnace bottom, the thermal gradient study must be made using the thermal conductivity for a saturated refractory in that portion of the lining that is at and above the melting point of iron. And finally, the saturation network must never be allowed to reach the backup insulating materials.

Table 1. Thermal Gradient Data

Location                                             Unsaturated

Molten Iron Temperature                                2,700F
Hot Face Castable/Backup Brick Temperature             2,909F
Backup Brick/Insulating Fire Brick Temperature         1,990F
Insulating Fire Brick/Insulating Board Temperature     1,138F
Shell Temperature                                       327F
Heat Loss (Btu/sq-ft/hr)                                 810

Table 2. Thermal Gradient Data With Unsaturated and 15% Saturated
Lining

Location                                              Unsaturated
Molten Iron Temperature                                 2,700F
Hot Face Castable/Backup Brick Temperature              2,209F
Backup Brick/Insulating Fire Brick Temperature          1,990F
Insulating Fire Brick/Insulating Board Temperature      1,138F
Shell Temperature                                        327F
Heat Loss (Btu/sq-ft/hr)                                  810

Location                                             15% Saturated
Molten Iron Temperature                                 2,700F
Hot Face Castable/Backup Brick Temperature              2,465F
Backup Brick/Insulating Fire Brick Temperature          2,227F
Insulating Fire Brick/Insulating Board Temperature      2,289F
Shell Temperature                                        351F
Heat Loss (Btu/sq-ft/hr)                                  937

Table 3. Thermal Gradient Data With and Without Insulating Board

Location                                              With Board
Molten Iron Temperature                                 2,700F
Hot Face Castable/Backup Brick Temperature              2,465F
Backup Brick/Insulating Fire Brick Temperature          2,227F
Insulating Fire Brick/Insulating Board Temperature      2,289F
Shell Temperature                                        351F
Heat Loss (Btu/sq-ft/hr)                                  937

Location                                             Without Board
Molten Iron Temperature                                 2,700F
Hot Face Castable/Backup Brick Temperature              2,335F
Backup Brick/Insulating Fire Brick Temperature          1,980F
Insulating Fire Brick/Insulating Board Temperature        --
Shell Temperature                                        423F
Heat Loss (Btu/sq-ft/hr)                                 1,339

For More Information

"Improved Method for Lining Channel Induction Furnaces," E.R. Webster Jr., AFS Transactions (02-101), 2002.

William J. Duca is the president of Duca Manufacturing, Boardman, Ohio.