A black liquor recovery boiler (BLRB) used in the Kraft paper process experienced multiple water wall tube leaks, requiring an emergency shutdown. A metallurgical evaluation of the ruptured tubes revealed indications of long term overheating, consistent with restrictions in flow through the tubes. Deposits were found on the interior of the hot side of the ruptured tubes, along with significant quantities of deposits inside the wall tube supply headers. Issues involving the raw water treatment equipment prompted by-passing the feedwater treatment system for at least four months, preceding the tube ruptures. The untreated raw water provided the source of the deposits responsible for the tube leaks.