Dilution Zone Agitation Critical in Blowtanks, High-Density Towers

Authors:

Thomas C. DeVries

Michael H. Doyle, P.E.

The article discusses the critical design and operation of blowtank and high-density tower agitators in pulp and paper mills, emphasizing the importance of proper dilution water control, stock consistency, and agitator sizing. It explains mechanical challenges such as fluctuating impeller forces, shock loads from high-consistency stock, and the need for reliable agitator construction to ensure consistent operation. Additionally, it covers startup procedures, troubleshooting common issues like poor consistency control or agitator failure, and best practices for maintaining efficient, stable operation.

Key Learnings

• Proper design of blowtank and high-density tower agitators is critical for consistent pulp stock processing.
• Dilution water control and correct agitator sizing are essential to maintain desired stock consistency.
• Mechanical design must account for fluctuating impeller forces, shock loads, and cavitation to prevent failures.
• Stock type, temperature, consistency, and residence time directly impact required agitator horsepower.
• Agitator and pipe locations, including recirculation and dilution lines, significantly affect flow patterns and efficiency.
• Use of a horseshoe fillet can reduce stagnant zones and optimize agitator performance when residence time allows.
• Startup procedures should gradually transition from recirculation to pumpout to prevent upsets and maintain consistency.
• Common issues include poor consistency control, upsets, and mechanical failures, often caused by improper dilution ratios or dual agitator interference.
• Operating with one properly sized agitator is generally safer than using multiple agitators in low-consistency stock.
• Regular monitoring, maintenance, and conservative horsepower selection are crucial for reliable operation.