How to control the speed and load during the operation of pulping system of papermaking machinery?

In the operation of pulping system of papermaking machinery, the control of rotating speed and load is the core link to ensure the stable operation of equipment, reduce the loss of wearing parts and improve the pulping quality. The rotating speed and load characteristics of different equipment (such as pulper, disc refiner, pump, etc.) are quite different, so it is necessary to formulate targeted control strategies based on their working principles and technological requirements. The following are specific methods:

First, clear the correlation logic between speed and load

In the pulping system, the rotational speed is usually positively correlated with the load (when the rotational speed increases, the interaction intensity between equipment and materials increases, and the load increases accordingly), but it needs to be matched within the design threshold:

Excessive rotating speed: it may lead to excessive crushing of materials (such as short fiber cutting), intensified vibration of equipment (wearing bearings), soaring energy consumption, and even lead to overload shutdown.

Too low rotational speed: it will reduce the pulping efficiency (such as insufficient pulp crushing and insufficient pulp grinding), and at the same time, it may lead to a sudden increase in local load due to material accumulation (such as the screw conveyor is stuck).

Core principle: according to the material characteristics (concentration, hardness, impurity content) and process indicators (pulp crushing degree, fiber length, output), control the speed and load at the balance point of "efficiency-loss-quality".

Second, the equipment control strategy

1. Pulper (high consistency/low consistency)

The pulper breaks raw materials through the high-speed rotation of the rotor, and the control of the speed and load should focus on "pulp breaking uniformity" and "overload prevention":

Speed control:

Low consistency pulper (concentration 5%-10%): the rotating speed is usually designed to be 500-1500 r/min, which needs to be adjusted according to the type of raw materials (for example, the rotating speed of waste paper pulping is slightly lower, and the rotating speed of wood chip pulping needs to be higher).

High consistency pulper (concentration 15%-30%): The rotating speed is low (300-800 r/min) to avoid the load surge caused by high-speed rotation under high concentration, which can be fine-tuned in real time by frequency conversion governor.

Load control:

Core parameters: motor current (indirectly reflecting load) and slurry level.

Control logic:

When the current exceeds 80% of the rated value, automatically reduce the speed or suspend the feeding (to avoid overload);

When the liquid level is too low (for example, lower than 1/3 of the height of the rotor), reduce the rotating speed to prevent "idling" (when there is no material, the rotor rubs against the air, resulting in overheating damage of the seal);

When there are many impurities in raw materials (such as metals and stones), the emergency speed reduction or shutdown will be triggered by the sudden warning of current (instantaneous surge).

2. disc refiner (fine/coarse grinding)

Disc refiner grinds fibers through the relative rotation of the moving disk and the fixed disk. The rotational speed and load directly affect the grinding degree (knocking degree) and fiber shape, and the control needs to be more precise:

Speed control:

Coarse mill (for unbleached pulp): the rotating speed is high (1200-1800 r/min), and the long fiber is cut by strong shearing force;

Refiner (to improve fiber binding force): The rotating speed is low (800-1200 r/min), and the fiber splitting and brushing are emphasized to avoid excessive cutting.

Key: Adjust the rotating speed in real time by variable frequency motor to match the grinding needs of different pulps (for example, softwood pulp needs a lower rotating speed to avoid excessive fiber damage).

Load control:

Core parameters: disc gap, motor power, slurry concentration.

Control logic:

The clearance of the grinding disc is a direct means to adjust the load: the clearance is too small (the load suddenly increases) → it is necessary to reduce the speed or increase the clearance; Excessive clearance (insufficient load) → it is necessary to increase the speed or reduce the clearance (automatically adjusted by hydraulic/pneumatic device);

When the slurry concentration fluctuates (for example, the concentration increases), the load will increase, so it is necessary to reduce the rotating speed synchronously to maintain the power stability (to avoid excessive load setting).

3. Conveying equipment (screw conveyor, slurry pump)

The core of conveying equipment is "stable conveying" to avoid blockage or idling caused by mismatch between rotating speed and load;

Screw conveyor:

Rotational speed control: it is usually designed to be 10-50 r/min (low speed to avoid excessive extrusion and crushing of materials), and the rotational speed is fixed by the reduction motor, or fine-tuned according to the feed amount (such as the output of the front-end pulper) (slightly increase the rotational speed if there is more feed, and slow down if there is less feed).

Load control: the load of the screw shaft is monitored by the torque sensor. When the load exceeds the threshold (for example, due to material caking and blockage), it will automatically stop and reverse the blockage (to avoid motor burning).

Slurry pump (centrifugal pump, screw pump):

Rotational speed control: the rotational speed of centrifugal pump is usually 1450-2900 r/min (according to the lift demand), and the rotational speed of screw pump is low (300-900 r/min to avoid shearing overheating of high-viscosity slurry), so it can adapt to different conveying capacity through frequency conversion control.

Load control:

Monitor the outlet pressure (if the pressure is too high → the pipeline is blocked or the rotating speed is too high, it is necessary to slow down or stop the machine to dredge);

Avoid "idling" (the load drops suddenly when there is no slurry, and the mechanical seal is damaged due to dry friction). Through the linkage control of the liquid level sensor, the pump will be automatically stopped when it is lower than the minimum liquid level.

4. Screening equipment (pressure screen, vibrating screen)

The rotating speed of screening equipment affects the screening efficiency and impurity separation effect;

Pressure screen: The rotating speed of the rotor is usually 300-600 r/min. If the rotating speed is too high, the abrasion of the screen plate will be aggravated (friction between the rotor and the screen surface), and if the rotating speed is too low, the fiber on the screen surface cannot be effectively removed (leading to blockage). Fine-tune the rotating speed by monitoring the cleanliness of the screened slurry (such as tailings ratio): if the tailings ratio is high, increase the rotating speed appropriately (enhance the sweeping effect).

Load control: Take the pressure difference between the inlet and outlet as the core index (excessive pressure difference → blocked screen surface and increased load). At this time, it is necessary to reduce the feed rate or increase the rotating speed (cleaning screen surface) to avoid deformation of screen frame due to excessive load.

Third, the application of automation control technology

Modern pulping system realizes the precise regulation of speed and load through the linkage of PLC control system and sensor, and reduces the error of manual intervention;

Sensor real-time monitoring:

Install current sensor (monitoring motor load), speed sensor (real-time feedback equipment speed), pressure/liquid level/concentration sensor (related material characteristics), and transmit data to the control system in real time.

Closed loop control logic:

Set the target parameters (such as rated power of disc refiner and outlet pressure of slurry pump), and the control system will automatically adjust the motor speed (through frequency converter) according to the real-time data;

When the load (current) exceeds 10% of the rated value, the speed will be automatically reduced by 10%-20%;

When the concentration increases (for example, the concentration of pulper exceeds 12%), the rotating speed is reduced synchronously to maintain the load stability;

When the rotational speed deviates from the set value (for example, due to belt slipping), an early warning is given and the motor output is adjusted.

Linkage control:

Match the rotating speed of front and rear equipment with the load (for example, the rotating speed of the pulper is increased → the rotating speed of the delivery pump is increased synchronously to avoid slurry accumulation);

Emergency response: If the load of a certain equipment suddenly increases (such as entering a hard object), the control system will trigger a chain shutdown (such as pulper → delivery pump → disc refiner shutdown in turn) to reduce equipment damage.

Four, manual operation and maintenance

Start-up and shutdown specifications:

Start-up: follow the process of "low speed start → gradually increase the speed → stabilize the load" (for example, disc refiner needs to introduce slurry first, then start and slowly increase the speed to the working value to avoid dry grinding);

Shutdown: first reduce the speed to idle speed → stop feeding → empty materials → shut down (to avoid excessive starting load caused by material residue).

Regular calibration and maintenance:

Check the transmission system (belt, gearbox): Loose belt will lead to false high speed (actual speed is lower than the set value, and the load is underestimated), so it needs to be tensioned regularly; Insufficient lubrication of gearbox will increase mechanical loss and lead to abnormal load increase.

Calibrate the sensor: ensure the accuracy of current, speed, pressure and other data (such as sensor drift will lead to the failure of control logic).

Optimization of process parameters:

Reduce the speed of key equipment (such as pulper) in advance according to the change of raw materials (such as the increase of impurities in waste paper) to avoid sudden load impact;

Trial before mass production: Test the pulping quality (such as fiber length and beating degree) at different speeds to determine the optimal speed-load combination (for example, when a softwood pulp is ground, the fiber bonding force is the best at 1500 r/min and 80% rated load).

summary

The control of speed and load of pulping system should follow the principle of "equipment characteristics as the foundation, process requirements as the key link, automatic regulation as the core and manual specification as the guarantee". By accurately matching the speed and load, it can not only avoid excessive equipment loss (such as bearing wear and component fracture), but also ensure the stability of pulping efficiency and quality, and finally achieve cost reduction and efficiency increase. The equipment configuration of different pulping lines (such as chemical pulping and waste paper pulping) is quite different, so it is necessary to formulate personalized control schemes in combination with specific production lines.