The 3-stage Clean-In-Place (CIP) model of Xin (2003. A Study of the Mechanisms of Chemical Cleaning of Milk Protein Fouling Deposits Using a Model Material (Whey Protein Concentrate Gel). Ph.D. Thesis. The University of Auckland. Chapter 5, pp 99-137) and Xin et al. (2004. Am Inst. Chem. Eng. J. 50 (8), 1961-1973) is used for the first time to illustrate a Friday 13th risk assessment. The aim was to reveal the risk of failure with auto set CIP cleaning times (t(T)) to remove whey protein deposits on metal surfaces during the alkali cleaning step due to stochastic, within system changes. The approach is to define an underlying unit operations model together with a computationally convenient risk factor (p) so that for all failures to remove the deposit in a set time with practical tolerance, p > 0. CIP is simulated using Monte Carlo simulation with Latin Hypercube sampling of the temperature of the alkali cleaning fluid (0.5-wt% NaOH) together with a tolerance 2% on t(T), Results illustrate that with a widely used cleaning fluid temperature of 75 degrees C some 2% of all alkali CIP operations will fail over the long term. These failures cannot be attributed to human error or faulty fillings. This new risk assessment illustrates that apparent continuous CIP operation is actually a mix of successful and unsuccessful operations. The application of this novel Friday 13 risk methodology to quantitatively reduce vulnerability to these failures and to assess control and other process intervention strategies to improve reliability and safety is discussed. Crown Copyright (C) 2014 Published by Elsevier Lid. All rights reserved.