How to monitor and adjust the rotating speed and load during the operation of the pulping system of papermaking machinery?

During the operation of pulping system of papermaking machinery, the monitoring and adjustment of rotating speed and load is the key link to ensure the stable operation of the system and improve the quality and efficiency of pulping. The following is a detailed description from three aspects: monitoring methods, adjustment strategies and precautions:

A, speed and load monitoring method

Grasping the running state of equipment through real-time monitoring and providing data basis for adjustment mainly depends on sensors, instruments and automation systems:

1. Speed monitoring

Sensor direct monitoring:

Speed sensors (such as Hall sensor, photoelectric encoder and magnetoelectric sensor) are installed on the rotating parts of motor shaft, reducer output shaft or key equipment (such as pulper impeller and refiner grinding disc) to collect speed signals in real time.

The signal is transmitted to the instrument or PLC system of the control cabinet, and the current speed (unit: r/min) is displayed in real time through the display screen, and the upper and lower alarm thresholds of the speed are set, and an audible and visual alarm is triggered when the speed exceeds the limit.

Indirect monitoring:

For the motor driven by frequency conversion, the output speed of the motor can be directly read through the control panel of the frequency converter (the transmission ratio between the motor and the equipment needs to be calibrated and converted into the actual speed of the equipment).

For the old equipment without sensors, the rotating speed can be estimated by observing the running sound and vibration frequency of the equipment and combining with experience (the accuracy is low and it is only used as an aid).

2. Load monitoring

Motor current/power monitoring:

The load is directly reflected in the current or power of the driving motor. By installing current transformers and power transmitters, the working current and active power of the motor can be monitored in real time.

When the load increases (such as too high slurry concentration and too many impurities), the motor current/power will exceed the rated value, and the system will automatically give an alarm (such as overload protection relay action).

Torque monitoring:

Torque sensors are installed on key transmission components (such as couplings and transmission shafts) to directly measure the torque value when the equipment is running (the greater the load, the greater the torque), and the data is transmitted to the control system for analysis.

Pressure/liquid level monitoring:

For the slurry pump and screening equipment, the load change can be indirectly reflected by monitoring the pressure sensor in the pipeline (such as the outlet pressure of the slurry pump) or the liquid level sensor in the slurry pool (for example, the sudden pressure drop may lead to abnormal load due to pipeline blockage).

Second, the adjustment strategy of speed and load

According to the monitoring data, combined with the pulping process requirements (such as pulp concentration, fiber shape and output target), the rotating speed and load are adjusted by mechanical or electrical means to ensure that they match:

1. Speed adjustment method

Electric speed regulation (mainstream mode):

Frequency conversion speed regulation: by adjusting the output frequency of motor frequency converter, the motor speed is changed (such as pulper and refiner). For example, when the pulp concentration is too high and the load increases, the rotating speed can be appropriately reduced to reduce the processing capacity per unit time and avoid overload; If the output needs to be increased and the load allows, the speed can be increased (within the rated speed range of the equipment).

DC speed regulation: For the equipment driven by DC motor, the rotating speed is changed by adjusting armature voltage or excitation current (it has been gradually replaced by frequency conversion speed regulation).

Mechanical speed regulation (auxiliary mode):

For the equipment without electric speed regulation, the speed ratio of transmission parts such as pulleys and gears can be changed (for example, increasing the diameter of driving wheel and increasing the speed of driven wheel), but it needs to be stopped and adjusted, which is suitable for long-term scenes with fixed process parameters.

2. Load adjustment method

Adjustment based on slurry characteristics:

If the load is too high (for example, the motor current exceeds the standard), check the slurry concentration and impurity content first: reduce the slurry concentration by increasing the dilution water, or strengthen the screening and purification to remove impurities (such as metals and gravel), so as to reduce equipment wear and load pressure.

If the load is too low (for example, the current is too small), it may be due to insufficient slurry concentration, so that the dilution water can be reduced, the slurry inlet concentration can be increased, or the opening of the slurry inlet valve can be adjusted to increase the slurry inlet amount per unit time, so that the load can be maintained in the rated range (usually 70%-90% of the rated load, with the highest efficiency).

Based on equipment linkage adjustment:

The pulping system is a continuous process (such as pulping → screening → pulping → purification), and it is necessary to ensure that the rotating speed of the front and rear equipment matches the load. For example, when the speed of the pulper increases, it is necessary to simultaneously increase the speed and pulp feed of the subsequent screening equipment to avoid the accumulation of pulp in the intermediate link (leading to a sudden increase in load); On the other hand, if the load of a link is too high, it is necessary to reduce the speed of upstream equipment and reduce the amount of incoming materials.

Automatic closed-loop control:

Modern pulping systems mostly use PLC+HMI to form a closed-loop control system: set the target speed and load range (for example, the speed of the refiner is 800r/min, and the motor current is 30-40A). The system compares the monitored value with the target value in real time, and automatically adjusts the output frequency of the frequency converter (changing the speed) or the opening of the pulp inlet valve (changing the load) to achieve accurate control.

Third, matters needing attention

Avoid frequent start and stop: the adjustment of speed and load should be smooth transition (for example, the inverter adopts soft start/soft stop mode), so as to avoid excessive impact load of equipment caused by sudden rise and fall, which will aggravate the wear of components.

Matching process parameters: the adjustment of speed and load should be combined with the pulping process requirements (for example, the speed of refiner affects the degree of fiber cut and the load affects the refining pressure), and the pulp quality should not be sacrificed simply for the sake of load stability.

Regular calibration equipment: sensors and instruments need to be calibrated regularly (such as monthly calibration of current transformers and annual calibration of torque sensors) to ensure accurate monitoring data; The wear of transmission parts (such as gears and belts) will lead to the error of speed transmission, which needs to be checked and adjusted regularly.

Emergency treatment: when abnormal speed/load (such as overspeed and overload) is detected, stop the machine immediately for inspection, and restart it after troubleshooting (such as removing impurities and repairing transmission faults) to avoid equipment damage (such as motor burning and impeller breaking).

Through the above monitoring and adjustment means, the dynamic balance between the rotating speed and the load of the pulping system can be realized, which can not only ensure the safe operation of the equipment, but also stabilize the pulping quality and improve the production efficiency.