Applications of Magflow Meters in the Pulp and Paper

In pulp and paper production, accurate measurement of pulp flow plays a vital role. High-consistency pulp flows continuously through enclosed pipelines, which significantly increases measurement interference. As a result, the pulp and paper industry places higher demands on flow measurement instruments. Most liquids used in pulp and paper production have electrical conductivity greater than 20 µS/cm, which meets the basic measurement requirements of magflow meters.

Electromagnetic flow meters do not reduce the nominal pipe diameter and contain no components that could be clogged by fibers, a feature that is especially important for pulp flow measurement. For these reasons, the use of electromagnetic flow meters for measuring pulp flow has become increasingly widespread in recent years.

Measurement Principle of Magflow Meters

A magflow meter is designed to measure the flow of conductive liquids and slurries. It operates based on Faraday’s law of electromagnetic induction: when a conductor moves through a magnetic field and cuts magnetic flux lines, an induced electromotive force is generated. This principle applies when a conductive fluid flows through a measuring tube in a direction perpendicular to the magnetic field.

Key Features of Magflow Meters

1. The measuring tube is a smooth, straight pipe with no moving or flow-obstructing parts, resulting in minimal pressure loss. For large-diameter pipelines, this provides significant energy-saving benefits.

2. The output signal is linear with flow rate, and a wide range of pipe sizes can be measured, from DN6 to DN2200.

3. A wide measuring range is available; by selecting different linings, the meter can handle high-temperature and corrosive media.

4. Installation requirements are relatively low. Based on practical experience, a straight pipe length of 5D upstream and 3D downstream (where D is the nominal pipe diameter) is sufficient to meet accuracy requirements.

5. High measurement accuracy, typically ±0.2% or ±0.5%.

6. The measured medium must be a conductive liquid, generally with conductivity greater than 20 µS/cm. The fluid should not contain large amounts of ferromagnetic materials or excessive air bubbles. Magflow meters are not suitable for measuring gases, steam, or non-conductive media.

Process Characteristics and Challenges in the Papermaking Process

(1) Papermaking Process

Modern papermaking mainly consists of two process stages: the pulping section and the papermaking section.

Pulping is the process of separating fibers from plant-based raw materials to produce pulp. Pulping methods are generally classified into mechanical, chemical, and chemi-mechanical processes, producing mechanical pulp, chemical pulp, and chemi-mechanical pulp, respectively.

The papermaking section primarily involves proportionally blending different pulps, followed by dewatering on the forming wire, pressing and shaping, and drying to produce base paper. The base paper then undergoes downstream processes such as coating and calendaring to become finished paper.

(2) Measurement Challenges

High temperature and high pressure: After mechanical refining, pulp temperatures often exceed 80 °C. The extraction and circulation of black liquor also take place under high-temperature and high-pressure conditions.

Liner wear: High-consistency pulp, recycled paper slurry, and digester blow lines can cause severe abrasion to the liner of electromagnetic flow meters.

Slurry noise interference: When high-consistency pulp and fibers pass through an electromagnetic flow meter, they can easily rub against the electrodes, generating spike noise interference.

Magflow Meter Selection for Papermaking Applications

Considering the specific characteristics and challenges of the papermaking process, the selection of magflow meters for this industry can be analyzed as follows:

1. Liner Selection

The measured media in papermaking processes are characterized by high temperature, high pressure, and the presence of large amounts of corrosive chemicals. Therefore, electromagnetic flow meters typically use high-temperature-resistant PTFE liners and are designed in a remote (separate transmitter) configuration.

Although PTFE liners offer excellent high-temperature resistance, they are not resistant to negative pressure. In certain special applications—such as medium-consistency vertical pipe outlets—where the medium has high consistency and temperature and vacuum conditions may occur intermittently, a PFA liner with a stainless-steel support mesh is required to prevent PTFE liner bulging.

Based on many years of practical experience, PTFE, PFA, or F46 liners are commonly used for electromagnetic flow meters in the papermaking industry. In addition, installing electromagnetic flow meters with necked grounding rings can significantly improve liner wear resistance.

2. Electrode Selection

Electrode selection for electromagnetic flow meters in the papermaking industry mainly considers two factors: corrosion resistance and anti-scaling performance. Large quantities of chemicals are added during the papermaking process, such as NaOH, sodium silicate, concentrated H₂SO₄, and HCl.

Different chemicals require different electrode materials. For strongly acidic media, tantalum electrodes are recommended; for alkaline media, titanium electrodes are generally used; and for conventional water measurement, 316L stainless steel electrodes are sufficient. Improper electrode selection can lead to electrode corrosion, which may ultimately cause moisture ingress and meter failure.

Precautions for Using Magflow Meters

When installing a magflow meter, a straight pipe length of 5D upstream and 3D downstream is required (D refers to the nominal pipe diameter). The electromagnetic flow meter must be installed in a location that ensures a completely full pipe, and it should not be installed at the highest point of the pipeline.

If the electromagnetic flow meter is installed on a non-conductive pipeline, grounding rings or grounding electrodes must be used. When measuring two media with different electrical conductivities, the electromagnetic flow meter should be installed upstream of the process or at a location where the media are thoroughly mixed.

For fluids that are prone to adhesion, sedimentation, or scaling, the actual flow velocity through the electromagnetic flow meter should not be lower than 2 m/s; a velocity of 3–4 m/s or higher is recommended. For highly abrasive fluids, the typical operating velocity should be kept below 2–3 m/s.

The installation location should be kept away from variable-frequency drives (VFDs), high-power motors, large transformers, power cables, and other sources of strong electromagnetic interference.

Application Case

In the papermaking industry, the media requiring flow measurement are mainly classified into slurries (such as wood pulp and bamboo pulp) and black liquor. In 2023, a well-known paper mill in Guangxi contacted our company seeking a solution for black liquor flow measurement. Their black liquor measurement involved two process stages:

Evaporation section black liquor: medium temperature of 120 °C and concentration below 3%.

Digestion (cooking) section black liquor: medium temperature of 170 °C and concentration below 3%.

The customer required stable and accurate flow measurement, good agreement with level measurement, display fluctuation of less than 2%, and a service life of more than three years.

For the black liquor in the evaporation section, our company recommended the Aimag-C electromagnetic flow meter with a remote transmitter, PTFE liner, and 316L stainless steel electrodes. For the black liquor in the digestion section, considering the high-temperature characteristics, we customized a thickened PTFE liner and applied thermal insulation and heat dissipation treatments to the excitation coil and magnetic circuit system inside the sensor.

For the electrodes, spherical 316L stainless steel electrodes were used. To prevent poor electrode insulation caused by liner thermal expansion, a specially designed electrode locking structure was adopted. After reviewing our solution, the customer proceeded with the purchase. The meters are currently operating stably with accurate measurement results.