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The principle of the high-pressure water descaling system for plate mills in the roughing and finishing mill (RM/FM) can be explained as follows:
1. **Oxidation Removal**:
- The scale on the steel surface is produced due to the oxidation of the base material during the re-heating of the billet/material. To prevent this layer from fusing into the base material during the rolling process, a descaling system is used.
2. **High-Pressure Water Jet**:
- High-pressure water jets are used to strike the surface between the scale and the base material with high pressure through strategically placed nozzles at different angular positions. This causes cooling of the scale and loosens it.
3. **Mechanisms of Descaling**:
- There are several mechanisms involved in the descaling process:
- **Differential Contraction**: Different thermal contraction between the scale and the steel supports the detachment of these two layers, initiated by the cooling from the water flow of the spray nozzles.
- **Thermal Shock**: Water penetrating the scale layer through cracks is converted from liquid to gas (steam) as it hits the hot steel surface, causing the scale layer to rupture and flake off.
- **Mechanical Force**: The scale is crushed and fragmented by the mechanical force of the water jet, and the layer is sheared off. The impact force, typically measured in N/mm² or mN/mm², is the dominating parameter influencing this mechanism.
- **Flushing Away**: For the top side of the strip, it's important that the loose scale particles are finally flushed away from the steel surface to prevent them from being pressed in again by the following rolling stand, which would lead to surface and/or roll defects.
4. **System Design Features**:
- To remove mill scale from steel, large volumes of water must be sprayed at high pressure onto the slabs, ingots, or billets over a period of 60 to 120 seconds. Special attention must be paid to the design and configuration of the pumps and their structural environment at the planning stage.
5. **Benefits of Descaling System**:
- Improves the quality of rolled products by preventing the fusion of oxidized scale into the base material.
- Reduces rejection by preventing surface defects of the parent material.
- Increases the life of pressure rolls in the rolling mill and reduces downtime for repair and maintenance.
- Improves productivity and efficiency by eliminating the trimming pass for billets.
6. **Types of Descaling Systems**:
- **Direct Descaling**: Descaling is done directly from high-pressure triplex plunger pumps through valves and descaling headers, with an operating pressure range of 100 Bar to 400 Bar.
- **Indirect Descaling**: Descaling is done through an accumulator, which accumulates water at high pressure by high-pressure reciprocating pumps and uses the pressurized water as needed through emergency valves, operational valves, and descaling headers, with an operating pressure range of 100 Bar to 330 Bar.
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