Thick Stock Screening, 1999 Papermakers Conference Proceedings

Lassi Halonen

Paper machine approach systems have many different types of debris causing quality and runnability problems. A slotted thick stock screening system can reduce these problems significantly. Proper design of a thick stock screening system takes into account the type of debris to be removed. The slot size can be chosen according to the debris size distribution. A consideration has to be given to various aspects in each individual papermachine, the furnish and grades of paper made on that machine. A special consideration needs to be given to the stability of the system in order to prevent variations in the consistency and freeness. The flowsheet and controls have to be geared towards maximum stability.

Mill experiences prove, that a thick stock screening system can significantly reduce dirt particles which are bigger than 0.05…0.4 mm² in the sheet depending on the type of debris and slot size used in the screens. Printing press breaks caused by shive type material can be reduced significantly or virtually eliminated. Paper machine runnability will increase as a result of a thick stock screening system. In some cases the paper breaks caused by stock related reasons could be reduced by 90%.