Continuously increasing demands are forcing mills to push their board machines to their maximum limits. This makes it hard to keep a balance between high productivity, good quality and running into breaks that cause unnecessary downtime.
A board machine is very complex and when considering investments to increase the machine capacity it is hard to determine where to put the focus to gain maximum output. Metso has developed a service product called Bottleneck Analysis, where each unit process of a board machine is thoroughly evaluated in order to define the most critical bottlenecks. The importance of each identified bottleneck is ranked and actions to improve the machine performance are evaluated. Finally, the most interesting investment options are gathered into machine scenarios with different scopes.
Tailor-made study with focus on capacity, quality and efficiency
Each analysis is tailor-made to fit the circumstances at the specific board machine. The focus of the study can either be capacity, efficiency or quality, or, as in most cases, a combination of all three. For instance, the task for a study in Korea was to evaluate what actions were needed in order to convert the board machine from WLC to SBS. The study showed that the main areas that needed to be reinforced were the stock preparation and the finishing part of the machine. Extra capacity for both softwood and hardwood bleached kraft pulp was needed in the stock preparation. SBS board puts much heavier demands on the printing surface than WLC. Therefore, the finishing area is of high importance and the results showed that modifications were needed in the calendering, coating, reeling and winding sections.
Another example is a study of an SBS machine in the USA, where the focus area was efficiency. When benchmarking the machine performance, the study showed that the momentary production was considerably above average but the annual production was still at an average level. The reason for this was the very high amount of downtime due to breaks, which has a negative impact on time efficiency. It turned out that the main issue that contributed to the high number of breaks was a poorly performing wet-stack calender. Replacing the wet-stack with a conventional hard nip calender, together with closing the open draws in the press section, was estimated to increase the efficiency corresponding to a production increase of 7,000 ton/year without increasing the machine speed.
However, most of the Bottleneck Analyses made have had a capacity focus in one way or another. A typical situation is that a mill already has predefined production targets, but needs assistance to create the best possible roadmap to realize their plans.
How to relieve bottlenecks and rank different investments
Many mills are quite aware of where their major bottlenecks are located in the machine, but the tricky part is how to relieve them in the best way. For instance, most machines are dryer-limited for heavyweight products. There are several ways to increase the drying capacity directly or indirectly. Examples of direct modifications to increase the drying capacity are to add drying cylinders, increase the fabric tension, change the felt configuration and increase steam pressure. Indirect ways to increase the drying capacity are to strengthen the dewatering capacity and modify the press concept to increase the after-press dryness. It is not hard to come up with investments that improve the capacity. The hard part is to rank how different investments affect the overall capacity and performance of the board machine. This is systematically evaluated and explained in a Bottleneck Analysis, which is one of the main reasons why nearly thirty mills all over the world have performed a Bottleneck Analysis together with Metso.