Sealing the Unseen: Inside the Continuous Digester in Pulp and Paper Mills

Few people see the continuous digester, which can be the most demanding part of pulp production.

Deep inside the system, white liquor breaks down lignin while black liquor carries away the spent chemicals. These are some of the harshest conditions in the entire mill. High temperatures, extreme pH levels and heavy loads of sand, wood fibres and other abrasives are standard.

If mechanical seals fail, your process grinds to a halt, maintenance costs spike, water usage increases, and safety and environmental risks escalate. Getting the proper sealing solution is about equipment performance and efficiency and protecting your team.

The Digester Environment

Every pump within the digester system plays a critical role, and each one pushes the mill's sealing technology to its limits.

Top Circulation and Chip Chute Pumps

These pumps deal with white liquor and wood chips in some of the kraft mill's most abrasive and high-pressure environments. Typical conditions could be:

• 15 bar/220 psi
• 120°C operating temps
• pH 12–14
• Heavy solids and a high chance of pressure spikes

Sealing here is no small task. Any failure risks leaks and lost production.

Make-Up Liquor Pumps

Typically operating at 10 bar and 100°C with high-speed motors (up to 3600 rpm), these pumps demand mechanical seals that can withstand high thermal stress. They also manage low NPSH (Net Positive Suction Head) to avoid cavitation damage.

Other Black Liquor Circulation Pumps

Bottom circulating, trim liquor, quench heater, wash heater, cold blow cooler and cold blow pumps are exposed to even more extreme conditions—temperatures up to 170°C and highly caustic black liquor mixed with wood fibres and sand.

John Crane's Engineered Solutions

Generic solutions can fail quickly in environments that are extreme. John Crane's mechanical sealing technologies are built to meet these demands head-on, helping pulp mills keep uptime high and maintenance low, even in the toughest parts of the process. Our solutions are designed to withstand abrasive slurry, chemicals, high pressures and high temperatures.

Type SE2 and Type JCS2P Dual Mechanical Seals

John Crane's Type SE2 and Type JCS2 mechanical seals are purpose-built for aggressive environments. Their double-balanced, OEM-engineered design offers long-term performance even in the face of high-pressure surges. Tailored for Sulzer APP and APT pumps, they deliver both reliability and safety in equal measure.

Type SB2 and Type SB2A Heavy-Duty Cartridge Seals

The John Crane Type SB2 and Type SB2A mechanical seals feature dual-cartridge, double-balanced construction with proven performance in caustic, abrasive and high-speed environments. Whether handling white liquor at high pump speed or switching between slurry duties, such as mineral processing, these seals are ideal for use with environmentally hazardous, abrasive fluids. They're also widely used with other process equipment, such as agitators.

Type 5620USP with Upstream Pumping

The Type 5620USP dual-cartridge seal with Upstream Pumping (USP), is part of the universal cartridge series. This modular mechanical seal family includes interchangeable elastomer bellows, metal bellows and elastomer O-ring pusher seal designs. With a seal face constructed of SiC and carbon, the Type 6520USP ensures non-contact sealing, reducing wear and improving reliability in liquids with suspended solids. The USP effect prevents harmful particles from reaching the seal faces by maintaining a clean barrier fluid, which enhances the mechanical seal's lifespan and reduces maintenance needs.

Beyond the Seal: Smarter Water Control

Seal water is essential, but uncontrolled use can drive up costs, create waste and damage product quality.

John Crane's Safeunit™ and John Crane's Safeunit Ultima™ are pressure and flow control devices that can do more than monitor seal water—they actively reduce usage, cut fresh and wastewater costs and help prevent internal leaks before they cause problems. They offer visual and electronic monitoring, which supports planned maintenance strategies that keep your process moving without interruption.

Real-world Impact:

One of Asia's largest paper manufacturers faced significant challenges with excessive water consumption, where over 40% of water usage stemmed from 15 kraft paper production machines and more than 1,000 pumps. Leaks from improper flow control led to high water costs, safety issues and environmental concerns. By installing John Crane's Safeunit water control and monitoring system, the mill achieved an impressive 76% reduction in water usage. John Crane's Safeunit continuously monitors and adjusts seal water flow and pressure, creating optimal operating conditions for increased process efficiency and sustainability. Read the case study for more details.

Why This Matters to Your Pulp and Paper Mill

When mechanical seals are underperforming, the effects can ripple out:

• Lost production from unplanned stops
• High seal water usage and cost
• Increased risk of environmental discharge
• Higher energy consumption
• Damaged equipment from cavitation or pressure spikes

John Crane's sealing solutions are trusted in pulp and paper mills around the world because they're designed specifically for these hurdles. If your digester or liquor circulation pumps cost you uptime, water or maintenance hours, it could be time to rethink your sealing strategy.

Explore our pulp and paper solutions and see how John Crane can help protect your process, reduce costs and boost reliability across your mill.