You don’t know what you don’t know – a case study for a deeper understanding of varnish formation at a refinery CONTEXT & OBJECTIVES Operator: A large European refinery operating a turbine and two critical process gas compressors.Issue: ISO code drift from 17/16/15 to 20/19/18, initially misinterpreted as particulate contamination. Standard diagnosis: MPC value considered acceptable; no abnormal wear detected; full system