It takes a lot of water to make paper – by some accounts, 14,000 gallons to produce a ton – and that means a lot of valves. In the papermaking industry, valves are regulating flow, maintaining quality and enabling wastewater treatment. Gate and ball valves help control costs, manage processes, and ensure environmental compliance.
As with other manufacturing industries, valves also are used outside the core processes. Sanitation and fire suppression are must-have functions, but for this article, we’ll focus on the industry-specific applications.
The papermaking process
Paper comes mostly from lumber, although a lot of recycled content gets mixed in. That lumber is chipped before being passed into a digester. Here, a liquid chemical the industry calls “white liquor” breaks the bonds between wood fibers, after which the pulp is blown against a wall to complete the separation.
The used liquor then goes through a series of treatment steps so it doesn't have to be disposed of. In parallel, the wood pulp is washed and bleached. After this, it's prepared as paper stock, which involves blending in various additives to give it the properties needed, and then it goes to the paper machine.
Paper machines are talked about in terms of the wet end, where the stock goes in, and the dry end, where paper comes out. In between, there's a particularly important valve: the basis weight valve. Often a highly specialized ball valve, this regulates the rate at which paper stock is deposited onto a moving belt. That's critical because it determines the thickness and thus basis weight of the finished paper.
Cost and environmental issues
Cost pressures are intense in papermaking, and manufacturers look for every opportunity to save money. Three areas stand out:
- Recovery and reuse of materials used during the process, particularly water
- More precise process control to improve product consistency and reduce waste
- Increased equipment reliability, which prevents downtime and cuts maintenance costs
Valves play a part in achieving each of these.
Digesters have a capping valve to let chips in. This is typically a ball valve, although gate valves also are used.
Liquor recovery is especially hard on valves, due to the corrosive and abrasive nature of the liquid. Scaling is a particular problem, and ball valves are often a good choice, as rotation scrapes them clean. Metal seats are almost always used in other rotary valves to provide durability.
Bleaching is another process that challenges valve materials. Valve bodies are typically 316 grade stainless, although some use corrosion-resistant liners.
In stock preparation, you'll most often see knife gate valves. These handle abrasive slurries such as kaolin clay, so wear can be a problem.
Modern paper mills invest a lot in water treatment. This is driven partly by environmental compliance, but it also saves money by cutting use through recycling. This part of the papermaking process uses a lot of control and check valves for flow regulation and process control.
Another fluid used extensively in papermaking is steam. Many mills generate their own process steam from wood bark. Steam is also a byproduct of the drying stage, where it's captured and condensed into water.
Valve selection challenges
Papermakers consider three main aspects before choosing a valve: pressure, temperature, and chemical resistance. Only by fully considering these can they arrive at the best combination of valve design and material for each application. When they get it right, the payoff includes higher reliability, lower costs, and more consistent paper, but getting to that point isn't easy.