Occasionally reports are received describing purging of a screw press during operation with low consistency feed. The reason this happens is a combination of blinded screens with head pressure on the inlet to the screw press. Most commonly the reports come from paper mills, where low consistency feed tends to occur when the percentage of fiber in the solids has been reduced, leaving proportionally more ash (clay).

The screens of a press can become blinded when the feed to the press is too thin, for instance less than 1% consistency. The low consistency causes the problem because the press relies on fiber being swept across the screen by the screw in order to keep the screen clear for liquid to pass.

Low consistency streams typically contain fine particles that remain in suspension. It is customarily a cloudy liquid, like dishwater. The solids do not accumulate sufficiently so that the flights can begin to convey them. Instead, the flights simply stir the fine particles in suspension. The solids that do make it to the screen cover it like a blanket. Head pressure serves to keep the blanket plastered in place.

With the screen in the blinded condition and head pressure on the inlet, the compression section of the press becomes a "cylinder of liquid". The plug at the discharge is the only thing holding back the liquid.

If the head pressure overcomes the ability of the cone to maintain the plug around the entire annulus, then liquid purges through the press. With sufficient head pressure, the surge of liquid past the cone can be unexpected and have quite high velocity.

Solutions to the problem involve assuring a thicker feed to the press. This can be done by prethickening with a sidehill screen, rotary drum screen, or gravity table. Allowing better settling in a clarifier (decanter) tank will also do the job.