A tale of two plants: Using LOPA for SIL assignment


  • This article was originally presented at the American Institute of Chemical Engineers, 2011 Spring Meeting, 7th Global Congress on Process Safety, Chicago, IL, March 13–16, 2011.


Two plants operated by the same company have “identical” ammonium nitrate (AN) solution pump installations. Yet the teams performing the layer of protection analysis (LOPA) concluded that the safety instrumented functions (SIFs) to protect those pumps needed different safety integrity levels (SILs). Despite the similarity of the installations, the teams discovered legitimate differences that warranted different conclusions about what SIL assignment should be. For those with a specific interest in AN, this article reviews the consequences of AN pump hazards, their initiating causes, and the types of independent layers of protection (IPLs) that can be brought to bear. Of more general interest will be the discussion of the impact of risk tolerance criteria, frequency modifiers, and IPLs on final SIL assignment, and why dictating a fixed SIL assignment to certain types of hazards or installations is inappropriate, especially in the development of industry standards or government regulations. © 2012 American Institute of Chemical Engineers Process Saf Prog, 2012