Complex sentence: Once the blastfurnaces were fired up the process of making the pig-iron was continuous until such time that the brick lining had to be replaced, when it would be blown out and cooled to allow the repair work to be carried out.
Split into simpler sentences: Once the blastfurnaces were fired up the process of making the pig-iron was continuous until such time that the brick lining had to be replaced. It would then be blown out and cooled to allow the repair work to be carried out.