Measurement of Pulp Stock to the Blow Tank

The construction of the ADMAG AXG magnetic flowmeter makes it an ideal meter for use in the high temperatures and abrasive conditions seen in a digester blow line.
 

Introduction

The pulp and paper industry involves harsh conditions for magnetic flowmeters (magmeters) due to corrosive chemicals, high temperatures, high pressures, and abrasive slurries. One especially challenging application is measuring pulp stock flow from the digester (where wood chips are chemically broken down) to the blow tank (where pulp is collected after digestion). This process involves thick, fibrous, and abrasive pulp stock, often at elevated temperatures and pressures, which can degrade or interfere with standard magmeters. 

Fig. 1 – Pulp & Paper Stock Processing

Yokogawa offers a highly reliable and accurate magmeter designed to withstand these extreme conditions, ensuring consistent performance and precise flow measurement in this demanding application.

Challenges

In the papermaking process, pulp production begins in a digester, where wood chips are cooked at high temperatures and pressures in a caustic solution known as white liquor. This solution dissolves lignin, the binding component in wood, separating the cellulose fibers. To ensure the digester remains fully filled, spent cooking solution (black liquor) may be added to the white liquor. Steam heats the mixture to initiate the cooking process, reducing the wood chips to a mush-like pulp. The cooked pulp is then washed, diluted, and cooled with a weak liquor before being transferred to the blow tank.

The transfer of pulp stock from the digester to the blow tank is regulated by a blow valve at the digester's bottom, controlled by a magnetic flowmeter. Accurate measurement and control of pulp stock flow provide several benefits:

1. Higher yields: Precise flow control optimizes pulp output.
2. Reduced chemical usage: Accurate measurement minimizes overuse of caustic solutions.
3. Increased throughput: Efficient flow control speeds up production.
4. More uniform product: Consistent flow ensures stable pulp quality.

Fig. 2 – Conventional Magmeter vs ADMAG AXG

Conventional magmeters struggle in this application due to:

1. Exposed electrode seals: Corrosive white and black liquors can degrade seals, leading to leaks or failures.
2. Liner abrasion: Abrasive pulp stock wears down the meter's liner, shortening its lifespan.
3. Slurry noise: The fibrous, non-homogeneous pulp stock causes signal instability, reducing measurement accuracy and control reliability.

Conventional style magnetic flow meters have difficulties measuring pulp stock due to problems related to exposed electrode seals and liner abrasion, resulting in a short life expectancy. Additionally, slurry noise can produce an unstable output, resulting in a loss of accuracy and control.

Solution

The Yokogawa ADMAG magnetic flowmeter is specifically designed to excel in the harsh conditions of the digester blow line in pulp and paper production, where high temperatures, high pressures, and abrasive pulp stock challenge conventional magmeters. Here’s a clarified explanation of its advantages over traditional designs:

Flush Electrode Design:

• The ADMAG eliminates exposed electrode seals by using a flush electrode design, where the electrode surface is level with the meter’s inner lining. This prevents abrasive pulp stock from eroding the sealing area, enhancing reliability and preventing leaks.
• The flush design also reduces slurry noise by minimizing collisions between wood/pulp particles and the electrode, as there’s no protruding surface for particles to strike (See Fig. 3).

Fig. 3 – Metal hat earth rings designed to minimize lining abrasion

Dual Frequency Excitation:
The ADMAG employs a dual-frequency excitation system, combining the benefits of low-frequency (better zero stability) and high-frequency (noise reduction) meters.
This results in:

• Fast response time: Allowing real-time flow control.
• High accuracy: 0.5% of reading, far superior to the 1% of span in typical AC meters.
• Zero stability: Ensures consistent measurements even in fluctuating conditions.
• Immunity to slurry noise: The dual-frequency approach mitigates noise from pulp stock, maintaining stable and accurate output.

Fig. 4 – ADMAG lasted more than 5 years in a digester blow line where others lasted 6 months.

Injection-Molded PFA Liner: 
Unlike the thinner, extruded PTFE liners in conventional meters, the ADMAG uses a thicker, injection-molded PFA liner supported by a retaining grid.
This rugged construction: 

• Prevents the abrasive pulp stock from eroding the liner or penetrating to the internal stainless steel tube.
• Enhances durability, making the flowtube capable of withstanding the severe conditions of the digester blow line.

Fig. 5 – Enlarged view of meter illustrating worn liner mechanically retained by bridge-plate. 

Key Advantages

The Yokogawa ADMAG AXG magmeter is tailored for the digester blow line’s extreme conditions, offering:

• A flush electrode design that eliminates seal failures and reduces slurry noise.
• Dual-frequency excitation for high accuracy (0.5% of reading), fast response, zero stability, and immunity to slurry noise.
• A robust, injection-molded PFA liner with a retaining grid for superior abrasion resistance, ensuring long-term reliability.

These features enable precise flow control, resulting in higher yields, reduced chemical usage, increased throughput, and a more uniform pulp product, thereby addressing the limitations of conventional magmeters in this challenging application.