Multivariate statistical process control (MSPC) techniques for batch processes have been extended to monitoring a semi-batch process by focussing on periodic process set point changes. This procedure can be extended to continuous processes that have repeated upsets or perturbations. The MSPC technique was demonstrated for a nuclear waste storage tank that undergoes periodic agitation from a mixing pump. The procedure described here used multi-way principal components analysis to develop a statistical model of the process based on historical data. The model can be used to determine if changes have occurred in the system. At present this procedure is used off-line for monitoring but it could be implemented on-line.