During the operation of a pressure screen, you may encounter a situation where the pressure before the screen is relatively stable, but the pressure after the screen fluctuates periodically.

Why Does This Phenomenon Occur

The primary function of a pressure screen is to utilize the interaction between the pressure screen basket, the pressure screen rotor, and the slurry flow field to allow slurry that meets the specifications to pass through the pressure screen basket while retaining larger impurities, fiber bundles, or other foreign matter.

During normal operation, the pre-screen pressure is typically influenced by a combination of the upstream slurry pump, pipeline pressure, control valves, and the level measurement system. Therefore, when these conditions are generally stable, the pre-screen pressure does not tend to fluctuate significantly.

However, the pressure after screening is also affected by the following factors.

• Effective flow area of the pressure screen basket’s apertures or slots
• Concentration difference between the slurry inside and outside the pressure screen basket
• Axial and radial pulsations generated by the pressure screen rotor
• Instantaneous resistance in the post-screen piping and downstream equipment
• Amount and location of dilution water added
• Flocculation, deflocculation, and reflocculation of slurry

Therefore, the pressure downstream of the screen is not determined by any single factor, but is the result of the combined effects of the pressure upstream of the screen, the flow conditions through the pressure screen basket, the operating conditions of the pressure screen rotor, and the resistance of the downstream system.

A change in any one of these steps can cause fluctuations in slurry flow and pressure. All of these are ultimately reflected on the pressure gauge downstream of the screen.

This also explains why some problems occur before or within the screen but are ultimately most evident on the pressure gauge downstream of the screen.

Causes of Pressure Fluctuation in a Pressure Screen 

Partial Blockage of the Pressure Screen Basket

A clogged pressure screen basket does not necessarily increase pre-screen pressure.

If blockage occurs uniformly across the entire surface of the pressure screen basket, the basket’s overall flow capacity will gradually decrease, the resistance to slurry flow through the screen basket will increase, and the pre-screen pressure will typically rise gradually as well.

However, if the blockage occurs only in a localized area of the pressure screen basket, the pressure behavior may differ: it may not rise continuously but instead fluctuate periodically up and down.

This is because a localized blockage does not cause the flow capacity of the entire pressure screen basket to decrease simultaneously.

Typical examples include

• Long fibers, plastic strands, or adhesive substances are lodged in the screen openings of the pressure screen basket
• Small, sticky particles form a film at the entrance to the pressure screen basket slot openings
• A ring-shaped layer of slurry residue forms on the screen basket in certain areas
• Under the action of the pressure screen rotor, the fiber flocs repeatedly approach and move away from the pressure screen basket surface

All of these issues can reduce flow capacity in a specific section of the screen drum, while other areas remain unobstructed.

When the pressure screen rotor reaches a certain position, localized shear and pressure changes acting on the slurry cause the deposits to detach or reattach temporarily. 

At this point, the effective flow area of the pressure screen basket continuously changes.

Pressure Screen Rotor Wear

Pressure screen rotor wear is typically understood as a “decrease in screening efficiency” or a “change in energy consumption.” 

However, during actual operation, pressure screen rotor wear can also alter the flow field inside the screen, causing slight fluctuations in the pressure downstream of the screen.

The pressure screen rotor propels the slurry forward, and also creates a stable slurry sweeping and shearing action on the surface of the pressure screen basket.

The following pressure screen rotor blade issues can alter the flow field inside the pressure screen. 

• Rotor blade edge wear
• The gap between the pressure screen rotor blades increases
• Rotor blade defects
• Uneven gap between rotor and pressure screen basket

After the pressure screen rotor wears down, the pulsating pressure applied to different parts of the pressure screen basket is no longer uniform.

Particular attention should be paid to one situation in which

A slight change in the pressure screen rotor current does not necessarily indicate that the rotor is operating normally.

If the pressure screen rotor wear occurs gradually, the motor’s average current may still remain within the normal range, but the instantaneous current values will exhibit slight periodic fluctuations. 

In this case, it is more useful to observe the trend than to examine individual data points.