Screw presses used to dewater manure are notorious for "purging" or "blowing the plug". The condition is one where all the manure being pumped to the press simply flows out through the cake discharge, with no liquid being separated through the screen. Whether it is 20 gpm or 600 gpm does not matter: if it starts at midnight and no one notices until dawn, the mess is terrible.
Improving the screw press design to address this problem has been difficult because a press will operate for weeks or months without an occurrence. Gradually a number of different factors have been observed that contribute to blowing the plug.
- In the winter the cake being discharged can freeze and jam the discharge door open.
- When the screw becomes worn, manure can flow between the screw and the screen, wetting the plug and allowing it to wash away.
- When the manure pit is near empty, the flow from a centrifugal pump can go down by 75% compared to when the pit is full. The plug will tend to blow when the pit is low.
- A change in feed or bedding (like going from sawdust to manure) results in reduced dewatering and wetter cake.
The latest improvements in press design that address the problem are (a) Extending the screw shaft through the discharge so that the cake door is breaking up a donut, rather than a solid plug, of manure, and (b) Double flighting the screw over the first half of the screen where the manure is wettest. This produces drier cake, earlier in the dewatering process, which is less likely to purge.
Another useful remedy is to elevate the discharge end of the press. This has been found to be particularly effective when the pit is low and the flow of solids to the press is reduced. At this time the cake discharge becomes so slow that water wicks (soaks) into the cake on the lower side of the screw, allowing part of the plug to become soft and to blow out. Without sufficient solids being fed into the press, the plug does not re-form. A 5º incline to the press can make a world of difference, and it can cause no harm.
The photo at the top right hand corner of our brochure shows an installation with an elevated discharge. This was retrofit, during start-up, in a successful effort to eliminate blowing the plug.