Tissue machines represent one of the most demanding application points when it comes to press roll covers. They are usually faster than machines producing printing paper or board. Their press rolls usually have a rather small diameter, leading to an extremely high loading frequency of the nips. The operating environment of the cover is challenging due to water, steam and the heat generated by the Yankee cylinder.
Furthermore, a great number of operational requirements are placed on the roll covers. For example, the suction roll nip acting as the 1st press must raise the dry content of the web to 40–45%. When it comes to runnability and quality requirements for tissue paper, the covers must be evenly hard and significantly softer than the covers used in machines producing printing paper or board. The pressure level in a nip formed with a soft cover is low, which is beneficial for paper thickness.
The aforementioned factors often lead to short grinding intervals due to wearing or changes in cover hardness, an insufficient dewatering capacity and cooling problems related to the roll in the 2nd nip. In addition, other problems have been caused by sudden debonding of roll covers - especially in the press roll in the 2nd nip.
Polyurethane took over the press roll surface positions in printing paper machines decades ago. However, poly covers suitable for tissue machines have been less common. In recent years, Valmet has actively developed new polyurethane materials suitable for tissue applications, and now these materials offer indisputable benefits when compared to older products on the market.
The best possible wear resistance and even hardness are the most important qualities in the process conditions to which the suction roll and press roll positions in tissue machines are subjected. The wear resistance of Valmet’s modern polyurethane materials is unbeatable. Their grinding intervals can even be twice as long as with traditional covers.
The new products do not harden in use like most traditional rubber covers; instead, the hardness remains even despite the demanding operating conditions. When it comes to hardness, most polyurethane coatings are applied using a different P&J value compared to traditional rubber coatings. This does not mean that they act differently in a pressed nip, but the material’s response to the P&J hardness measurement is different.
In tissue applications, traditionally only holes have been used on suction and press roll covers in order to reach a sufficient water handling capacity. The strength properties and softness of the covers have set limits for grooving, and cracking in the neck areas between the grooves as well as groove closures have been noted earlier on grooved covers. With Valmet’s new polyurethane materials, a high strength level and special groove geometry enable using grooves in soft covers.
The water handling capacity reached by using grooves is important especially in suction roll positions where the roll cover handles most of the water. As dewatering in the roll nip is intensified, the need to dry the paper with the heat energy of the Yankee cylinder decreases, resulting in lower production costs.
Traditionally press rolls equipped with internal water cooling have posed the biggest challenge for the technical features of the covers. Cooling has been essential in order to prevent overheating of the cover due to the viscous nature of the cover materials. One common problem resulted from using a water cooling system on the roll body; water manages to enter the cover and make its way between the connecting layers. This has often led to sudden debonding of the cover.
Valmet’s new polyurethane cover designed especially for press rolls generates only a fraction of the heat usually generated in the press nip when using traditional materials. The internal water cooling of the roll is thus not needed even at high speeds, and coating problems related to cooling are thus eliminated. Water cooling becoming unnecessary leads to considerable savings in the form of reduced maintenance and pumping costs.
Furthermore, the rolling resistance of the cover in the nip is lower, meaning that the energy need of the drive motors running the rolls decreases and thereby reduces energy costs.
For more information on how Valmet Press Roll Cover PH or PF-V can improve your tissue production, call your Valmet representative.