Continuous digester screen DP rise after chip-furnish change
A two-vessel continuous digester produces 1,050 adt/d at Kappa 32. Over nine days the upper cooking-zone screen differential pressure rose from 62 to 148 kPa while lower extraction flow fell 15% at constant white liquor flow and steady blow consistency. Three weeks earlier the mill increased sawdust and fines to 35% of the chip furnish to use up inventory. The digester has eight hours of spare blow-tank capacity and no planned outage for 12 days. Operations wants to cut production 10% and raise co-current liquor flow to keep the screens open; the area manager wants a controlled shutdown now. A brief increase in extraction flow did not reduce the DP. Top separator pressure is normal, but chip-meter amps have risen 6%. The mill has no spare screen set and a 14-hour minimum cool-down and wash before entry.
1. Which measurements would you take first to separate chip-column compaction from screen plugging, and what result would point to each?
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Take a stepwise test during steady operation: (1) record DP from top separator to upper screen and from upper screen to lower extraction separately; (2) log extraction flow against valve position; (3) measure extraction filtrate suspended solids; (4) reduce production 10% for 30 minutes and see whether DP falls proportionally. Screen plugging gives a high screen-bank DP at low extraction flow, little change when flow is increased, and a drop after backflush; the column DP stays near normal. Column compaction gives rising chip-meter torque, high top-separator level or pressure, and DP that tracks production and chip furnish; a flow increase may help briefly, then DP returns. The 6% amp rise points to compaction or a hang-up, but the failure of extraction flow to respond suggests screens are partly plugged. Because both can coexist, do not rely on a single indicator.
2. What operating decision do you recommend for the next 12 days, and what risk are you accepting?
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Recommend a controlled shutdown within 48 hours, after building blow-tank inventory and arranging about 6 hours of purchased pulp or a reduced downstream rate. The 148 kPa DP, 15% loss of extraction, and failed flow response show a mechanical restriction that production cuts will not clear; continuing for 12 days risks channeling, uneven Kappa, screen collapse, and an uncontrolled blow-down. A 10% cut may buy hours, not 12 days. Accept the roughly 6-hour production loss and the 14-hour cool-down. If management insists on running, set hard trip limits: DP above 170 kPa, extraction below 70% of baseline, or chip-meter amps up 12%; any one triggers shutdown. Do not raise co-current flow blindly, because it can increase channeling and carry chips into the extraction header.
3. On shutdown, what inspection and acceptance criteria would you set for the screens and shell?
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After cool-down, wash, and gas test, inspect screen slots for bent bars, peened metal, and plugged slots; check backing plates and support rings for cracks; check the extraction header for chip carryover. PT/MT all screen-support welds and the shell band behind the screens; UT the shell for erosion at screen edges; check the top separator and chip column for hang-up. Acceptance: no cracks in supports or shell, screen slot width within OEM tolerance, open area above 90%, no through-wall pitting, and extraction header clear. Replace damaged screen sections rather than straightening bent bars. Record a new baseline DP after restart.