A 1990s uncoated freesheet machine in the United States wanted to decrease their press moisture and contacted Valmet for assistance. Valmet met with mill personnel and recommended installation of a Valmet Belt Roll Doctor for the shoe press belt.
The doctor was installed on the machine's Valmet Sym Nip Roll in 2005. The installation included a new platform and stairs, as well as a steam heated crosswalk under the doctor and also a new doctor on the 1st press Valmet SymZL Shoe Press Roll. According to the machine manager the installation went fine.
The Valmet Belt Roll Doctor is adequately reducing press moisture and expectations have been met by the installation, says the machine manager. The doctor is used once the press felt has broken in and they are then nip dewatering.
The Valmet Belt Roll Doctor included standard bearings and journals with a Sta-Clean SP DST type holder with two pneumatic hoses, one to load and the other to unload the blade against the belt. Hydraulic cylinders on the tend and drive side pivot the doctor beam toward and away from the roll. A lubricating shower above the doctor enhances roll cleaning.
The Valmet Doctor Blade Poly blade was 5mm thick, and operates at a blade angle of 10° at a pressure of 50-100 N/m (0.282-0.571 pli). It is an economical plastic doctor blade used in the forming and press sections of paper, board and tissue machines. Its novel material composition (ultra-high-density polyethylene) offers efficient dewatering over an extended lifetime. Using the same blade for more than two months is not recommended because the blade may start aquaplaning.
There is also a Valmet Doctor Holder Fit doctor on the first press Valmet SymZL Shoe Press Roll (grooved) with an air blade. It operates at a blade angle of 23° and pressure of ~90 N/m (0.51 pli). Valmet Doctor Holder Fit blade holders are designed for paper, board and tissue machines' cleaning positions that do not require the load adjustment precision of a hose-loaded blade holder. They are particularly well-suited for challenging applications that require regular cleaning of the holder.
Valmet Doctor Holder Fit blade holders are very lightweight at 20% of the weight of traditional metal holders, making them easy and safe to handle. The holder is highly impact resistant and quick to recover from even the strongest physical shocks. The holder is composed of two parts: a composite holder profile and a removable support plate. The construction of the holder provides good vibration dampening properties and exceptional corrosion resistance.
The single-blade Valmet Belt Roll Doctor consists of a beam, blade holder, blade and pneumatic hoses to load and unload the blade. Hydraulic cylinders pivot the doctor beam between an operating and maintenance position.
The primary purpose of the doctor is to keep the roll surface clean. In some doctor locations, it also releases the web from the roll surface and guides it to the pulper.
To retract the doctor: the roll is stopped, the doctor is retracted and the lubricating shower is turned off. The blade must not be kept in contact with the roll surface when the roll is not rotating.
Blade angle is critical for a doctor, and is obtained by using a measuring gauge. Set the angle at an accuracy of 1°. The angles on the front and back sides must match at an accuracy of 0.5°.
If you are interested in finding out more about the Valmet Belt Roll Doctor and Valmet's doctoring alternatives, contact Farhad Fereydoonzad, Product Manager – Pressing, or your Valmet representative.