In order to safely extend the interval between outages on a digester it is necessary to address the
requirements of the insurance companies and authorized inspection agencies involved. Additionally, the
State and Local jurisdictions may have requirements that need to be identified and met.
Extending outage intervals also requires knowledge of the original design and installation practices, as well
as documentation on maintenance, repairs and alterations that have previously been made to the vessel.
The inspections required, and process for making repair decisions need to be agreed upon by all parties
involved as part of an overall risk management strategy.
While this paper is primarily intended to address issues associated with welded batch type digesters utilized
in the kraft (sulfate) pulping process, many of the principals discussed herein are universal to other
Digesters represent one of the most serious and potentially severe exposures in a pulp mill. The controlling
or minimizing of the hazards and potential losses associated with this exposure is of primary importance to
insurance industry professionals.
In order to understand the thought process that insurance industry professionals face in controlling this
exposure, it is necessary to have a basic understanding of how the insurance industry views risk.
Risk is defined in the insurance industry as the probability of failure multiplied by the consequence of that
The probability of failure is simply the likelihood that a component or system will not function as intended.
While this is easy to understand, it is not always an exact science; and in the case of digester failures is
usually subject to individual experience or obtainable historical data.
Consequence is the amount of damage or loss resulting from a failure. In the insurance industry, this is not
only the direct equipment damage and resulting cost of repair, but can also include other indirect costs such
as lost production (business interruption) time and associated revenue, personnel injury, environmental
impact, or extra production expenses.
As is shown from the chart in Figure 1, the areas of highest risk are where the probability of failure and the
consequence of failure are the highest. Lowering either of these parameters effectively lowers the overall