Kare Väisänen

Global Product Manager

Metso Paper Inc.

Finland

Doctoring is a necessary sub-process in paper and board making. Doctors were introduced to paper machines more than a hundred years ago to improve water removal and remove fibres from roll surfaces. This was done to speed up machines and the papermaking process. The basic function of doctors still remains the same today, but doctoring components and machine design have naturally developed considerably. Paper and board making are also facing new issues. The increasing use of recycled fibre brings sticky problems, while increased machine speeds present a variety of challenges for reliable doctoring. However, lowering overall paper and board making costs, including doctoring costs, seems to be the primary goal of the day.

Doctoring-Related Costs

If you ask a papermaker how much money he is spending on doctoring, the answer will usually be limited to the money spent on doctor blades. Understandably so, since this is a very easy figure to calculate or get directly from an ERP system. However, blade costs are only the tip of the iceberg. Many other cost factors related to doctoring are more or less hidden, or costs that are assumed to be unaffected by changes in doctoring. These costs, nevertheless, contribute to the manufacturing cost of each tonne of paper or board produced.

Let’s start with the biggest single cost factor related to doctoring, namely energy. Approximately one-third of the energy consumed by the dryer section’s sectional drives is needed to overcome the additional friction caused by doctoring. In this area alone, we are easily talking about more money than the cost of blades for the whole machine line. If water removal is aided with compressed air operated air blades, the amount of money spent on doctoring energy at the press and forming sections will also be substantial. Energy is very expensive today all over the world, which is why energy consumption should receive the most attention when looking to reduce total doctoring costs.

The whole paper or board making line needs maintenance. For doctors, this means continuous follow-up, monitoring, and maintenance. Maintenance also means spare parts, which cost money. Bearings, seals, oscillators, loading tubes, and blade holders need to be replaced every now and then to keep them working properly. Improperly working doctors may cause production losses or roll cover damage, and considerable costs. Maintenance costs are unavoidable, but can they be lowered? 

Figure 1. Typical construction of a paper or board machine doctor.

Click Image to Enlarge

Figure 2. A case example from a board machine. Savings exceed cost of blades for the whole dryer section for two years.

Figure 3. Extractable composite holder with individually replaceable components.

Costs always go up when items are ordered and put into stock. For wearing parts, like doctor blades, constant order handling is surprisingly expensive for both the buyer and seller. For spare parts stocked at the mill site, costs rise along with increasing inventory values. Today, more and more attention is paid to reducing the value of paper and board mill inventories, so this cost factor is worth noting as well.

Effective Doctoring with Lower Energy Costs

Dryer section energy usage is the easiest place to start making savings. All doctors should be in good condition and blade loads should be optimized, which means that they should be just high enough to keep the cylinders clean without excess friction and energy usage. Once these parameters have been addressed, the optimal blade materials should be selected. Traditional dryer section blade materials, like bronze and fibreglass, are cheap and relatively good at keeping the cylinders clean, but any savings on blade materials are easily lost through higher energy usage. Adding carbon fibre and/or select additives to the blade material lowers friction significantly. Higher carbon content also extends blade lifetimes and improves cleanliness, thereby helping heat transfer from cylinder to paper and reducing the need for steam, i.e. energy.

It’s not only carbon fibre that provides energy savings through lower friction (a typical dryer section blade contains only 15 to 30 percent carbon fibre). An even more important factor is the resin system that keeps the fibre layers together. The composition of this resin system is a key issue for low friction. That is why this matrix should be carefully selected. The best way to find blades with the lowest energy consumption is to challenge blade suppliers to trials and then compare unit costs, lifetimes, and energy costs. The dryer section may, and usually does, hold hidden potential for savings totalling tens or even hundreds of thousands of Euros, depending on the number of doctors, machine speed, and current design solutions employed. Figure 2 shows one actual example of this type of analysis for a board machine. In this example, switching the blade material from the cheapest unit price to a more expensive one brought enough annual savings to pay for a 2 ½-year supply of the new blades.

The potential for doctoring-related energy savings is not as obvious at the wet end as it is at the dry end. However, opportunities also exist in this area. Greater attention should be paid to the doctoring of suction rolls, grooved rolls and belts, and to their water removal potential. Blade materials have improved a lot, and there are now new ways to keep rolls working as planned. With these novel solutions available, existing air blades should be replaced due to their huge compressed air needs. At the forming section, blade materials should be changed to ones that require less lubrication water.

Doctor Maintenance

Maintenance costs can obviously be managed by planning maintenance actions carefully and concentrating on critical positions. However, spare parts’ needs are highly dependent on the construction of doctors. The most economical solution is to select components where you can replace only the damaged part instead of the whole component. One example of this is a composite blade holder (see Figure 3). They are usually very simple and can be extracted from the machine. This significantly reduces maintenance and cleaning times. Regular maintenance, like hose changes, is also very fast and easy. These holders allow you to change just the top plate or blade carrier part. Additional benefits of composite holders include their imperviousness to corrosion and light handling.

The necessity of doctor oscillation tends to be carefully evaluated these days. Is it really needed or just an old habit? In any case, oscillating doctors gobble up energy and bring incremental maintenance costs. In fact, many mills have been apt to run their oscillators to failure and then permanently shut them down to save money.

Other Costs Related to Doctoring

Most mills in the Western world are these days looking to release net working capital by reducing the amount of money tied up in inventory. This is done, in part, by transferring the responsibility for stocking parts and supplies to suppliers, which is a good trend that brings tangible savings.

However, this model can be expanded even further. Suppliers are willing, and able, to extend their responsibility to all spares required for doctoring, for example. This offers mills lower inventory values and ensures the availability of spares in stock. With only one supplier for all of the components needed for doctoring, the amount of paperwork is reduced and very expensive rush orders can be avoided.

Why would a supplier be ready to do this? Being the sole supplier for all doctoring components will bring the supplier enough volume to warrant committing its capital to the required inventory. This is a situation where everybody wins.

Consider Doctoring as a Process to Be Optimized

In conclusion, start by considering the biggest cost factors first when selecting paper or board machine doctoring components. Rather than the price of blades per unit or meter, the relevant question is how to avoid production losses and roll cover damage while minimizing energy costs. You may end up spending more on blades, but you will get your investment back many times over through energy savings and increased production output.