ValDual: Low-cost doctoring in challenging soft cover positions
Plastic has traditionally been considered the only blade material suitable for the doctoring of soft roll covers as it will not damage them. However, plastic blades fail to provide optimal doctoring results due to insufficient doctoring loads, or then bend and let most of the water pass by under heavier loading.
Another alternative is to use an air blade, which will yield good results until it inevitably gets clogged. Furthermore, the use of compressed air to aid water removal dramatically increases operating costs and carbon fiber air blades can damage the roll surface, which will then require regrinding. It is thus no wonder that many mills prefer to run their soft-covered suction rolls without doctoring. This practice keeps the roll covers safe, but it also results in poor
A new material applicable to all challenging covers
Metso’s new ValDual blade solves roll doctoring problems in difficult positions. It is made of an UHMW-HDPE (ultra-high-molecular-weight polyethylene - high density polyethylene) compound applicable to all challenging covers, including polyurethane, transfer belts, belt rolls as well as hard covers, G-bands and the steel shells of suction and grooved rolls. With ValDual, there is no risk of cover damage to any type of surface.
Valdual has two patented functions and it can serve as a doctor blade or a foil blade.
Foil blade or doctor blade
ValDual’s patented construction and protected design make it ideal for the doctoring of grooved and suction rolls. It can be used as a traditional doctor blade or a foil blade.
When used as a doctor blade, its stiffness enables higher doctoring loads and thus provides better water removal than traditional plastic blades while still treating the roll cover just as gently. Due to its geometry, ValDual will not let water through like a traditional plastic blade.
When ValDual is used as a foil blade, its stiffness in the machine direction and flexibility in the cross direction enable it to follow the roll surface and reach a more optimal doctoring result than traditional foil blades.
|ValDual blades are lightweight and easy to handle. |
Operating costs lowered to one tenth of earlier costs
Since ValDual does not require any additional blows, its operating costs are significantly lower than those of an air blade. Paper mills have reported very high
operating costs with air blades, up to EUR 50,000 per position annually. ValDual can reduce these annual operating costs to about one tenth while still maintaining the same doctoring result.
Doctoring costs fell from EUR 60,000 to EUR 4,800
A paper mill used an air blade to doctor its soft-covered press suction roll. The mill’s reported annual doctoring costs amounted to EUR 60,000 (EUR 40,000 for compressed air and EUR 20,000 for blades). The suction roll had also twice required regrinding after the carbon fiber air blade had touched the soft roll surface. After a ValDual blade replaced the air blade, total annual operating costs fell dramatically and now amount to EUR 4,800. And there is no further risk of roll cover damage.
“Water removal and sheet dry content after the press section have been at least at the same level as with an air blade,” says the operations manager for the mill in question.