Feeding system having pumps in parallel for a continuous digester
The feed system is for a continuous digester where at least two pumps are arranged in parallel at the bottom of a pre-treatment vessel and a stirrer is provided in direct connection to inlets to pumps. The system makes it possible to provide a feed system with an improved accessibility and operational reliability, and to operate the main part of the pumps at optimal efficiency even if the production capacity is reduced.
1. A method for feeding wood chips to a continuous digester, comprising,
feeding wood chips into a vessel having a rotatable bottom scraper at a bottom of the vessel,
suspending the wood chips in the vessel to create a chips suspension;
arranging at least two single pumps in parallel connected directly to the bottom of the vessel, the two single pumps having no pump serially connected thereto upstream or downstream thereof,
the bottom scraper rotating to break up a column of chips descending in the vessel,
a stirrer, in operative engagement with the bottom scraper, sweeping over outlets at the bottom of the vessel to keep the chips suspension in motion and substantially evenly distributing the chips suspension between the outlets in communication with the two pumps,
each pump transferring the chips suspension in a transfer line extending from the vessel to the top of a digester,
cooking the wood chips in the digester to form a pulp, and
continuously feeding out pulp from a bottom of the digester.
2. The method for feeding wood chips according to claim 1 wherein at least three pumps transfer the chips suspension to the top of the digester.
3. The method for feeding wood chips according to claim 2 wherein at least four pumps transfer the chips suspension to the top of the digester.
4. The method for feeding wood chips according to claim 2 wherein the method further comprises connecting the pumps circumferentially, symmetrically and in a horizontal plane to the bottom of the vessel.
5. The method for feeding wood chips according to claim 2 wherein each transfer line is provided with an outlet opening that opens directly into the top of the digester so that the chips suspension fall into the digester.
6. The method for feeding wood chips according to claim 2 wherein the method further comprises providing the vessel with a bucket-shaped outlet that has an upper inlet, a cylindrical mantle surface and a bottom, the cylindrical mantle surface having two outlets defined therein, connecting pump inlets of the pumps to the outlets of the cylindrical mantle surface, connecting pump outlets of the pumps to the transfer lines.
7. The method for feeding wood chips according to claim 2 wherein each pump transfer the chips suspension in a first section of the transfer lines extending to the top of the digester, the first section of the transfer lines merging at a merging point to form a combined second section extending to the top of the digester.
8. The method for feeding wood chips according to claim 1 wherein the method further comprises adding a fluid, controlled by a level transmitter, to establish a liquid level (LIQ LEV ) in the digester.