Roll covers in the press section have a strong impact on production line efficiency, capacity and product quality. Along with press section clothing, they are key components for dewatering through the wet pressing process. Especially in center roll-based presses with an open draw, roll covers also directly affect machine runnability. The most important rolls in your machine deserve superior covers.
Valmet press roll covers range from economical rubber covers for less demanding applications, to the toughest roll covers providing the most eﬃcient dewatering technology. Covers can be designed for the unique needs of any application. Cover topography can be tailored for maximal dewatering, and non-felted rolls can have surfaces with release characteristics adjusted for the specific grade being produced.
Valmet offers soft press roll polyurethane covers for demanding process conditions such as LNP rolls and suction press rolls. They can operate under the highest loads and at the highest speeds – and still save energy and need less maintenance.
A key to higher energy efficiency, and often a significant enabler of a capacity increase, is optimal wet pressing performance. Increased nip dewatering improves drying capacity and increases sheet dryness, with a remarkable effect on energy costs. For example, increasing post-press dryness by 1.5 percent unit means savings of approximately €1.3 million in drying energy costs (case linerboard 400,000t/a).
The latest PU press roll cover products like the Valmet Press Roll Cover PP and PK have a lower friction coefficient, thanks to intensive R&D work that has resulted in lower rotation resistance under high nip loads and thus a reduction in the press drive’s power consumption. Allowing for a 15% reduction of drive energy compared to typical rubber, CO2 reduction -0.2 kg/t per roll.
The redesign of the suction press roll cover has not only solved a persistent problem of shadow marking at Siam Kraft board mill. It has also helped the mill meet their goals for sustainability, cost-effectiveness and the circular economy by reducing fiber consumption.
The Indian board-maker Vijay Anand has overcome the challenge of traditional rubber covers by upgrading with a Valmet roll cover. Press Roll Cover PP premium polyurethane covers have provided remarkable benefits with an outstanding payback from the investment.
Water removal at the forming or press section is significantly cheaper than at the dryer section. Since removing one liter of water from the sheet at the dryer section costs five times more than at the press section, it pays to optimize dewatering at an early stage.
Paper, board and tissue makers all have their own roll surface structures that they are used to having on their press section rolls. In many cases, those structures have been there from day one and may no longer be the most effective ones, taking into account advances in materials, felts and knowledge.