Risk-based maintenance and inspection for kraft recovery boilers : on the development of an RBMI method and its applications

Abstract

A critical bottleneck-asset in any Kraft paper mill concerning risk for severe production loss in case of unavailability is the recovery boiler which is a thermochemical reactor producing steam and reduced molten salt smelt from black liquor, a biofuel similarly challenging as waste wood or municipal waste. This is especially problematic as the probability to get a water leak due to thinning or cracking in such boiler is quite challenging to predict as operations parameters are severe and the consequence in case of such leak may with a probability of around 2% be not only loss of one to five production days but a fatal smelt-water-reaction explosion potentially destroying the asset and its availability. Fortunately though, the probability for a leak and associated risk being the product of probability with statistical consequence may be not only evaluated and analysed but also significantly reduced by application of risk based maintenance and inspection methods enhanced by structural reliability and probabilistic decision theory. Thus an industrial approach to reliability centred maintenance based on typical degradation mechanisms and regimes due to the challenging boiler tube environment on hand was developed and applied to a large industrial Kraft recovery boiler to show and evaluate its potential to innovate and optimise both reliability and availability for this key-asset, thereby minimising production losses and overall cost.