Features Of Steel Mill Descaling Pump Operation
Steel slab or billet is heated to 1250*C in reheating furnaces and transferred to roller tables on its onward journey to rolling mill stand where it will be rolled in multiple passes to achieve lower thickness or desired shape. Slight oxidizing atmosphere is maintained in reheating furnace. Scale formed in the reheating furnace need to be washed before sending the material to mill stands. This is done by striking the steel surface with very high velocity water jets created by passing high pressure water through descaling nozzles manufactured by Lechler and Spraying Systems among others. The descaling operation needs pumps and valves that can deliver water at 160 bar to 400 bar. Failure to remove the scale completely will result into scale getting rolled with the steel affecting surface quality and creating holes when the steel rolled for further processing in mills.
Steel slab or billet first travels on roller tables through descaling box in which descaling headers are installed at top and bottom. When the piece enters the box, descaling valve opens and allow high pressure water to pass through specially designed nozzles that create high velocity jet which strikes the surface and scale is removed. The discharge continues from 20 seconds to 60 seconds depending upon length of the piece.
After a small lapse of time the piece reaches near the mill stand. Descaling headers are installed at the entry of mill stand. Just before entry of head end into mill bite, another descaling valve opens and scale at top and bottom surfaces is washed with high pressure jets. The discharge continues until tail end passes through the roll bite. Since rolling is done in multiple passes, this sequence of opening and closing of valves is repeated 3 to 5 times till the desired thickness or shape is not achieved.
In one cycle lasting for 5 to 7 minutes, descaling valves open and close for 5 to 7 times some times at the interval of 30 seconds or less. In addition water used for descaling is of poor quality. The pumps and valves are not only designed for high pressure but are rugged enough to withstand the frequent change of pressure due to discharge going from zero to full and then again to zero at short and frequent intervals.