Using API RP571 in PHAs

API-RP571, Degradation Mechanisms Affecting Fixed Equipment in the Refining Industry, addresses approximately 60 degradation mechanisms that can affect equipment in petroleum refineries. As the title suggests, the mechanisms are well known in refineries and a number of them can also occur in other industries. The overwhelming majority of these mechanisms are time based but there are at least 3 event based mechanisms that should be taken into consideration when conducting a PHA. Those three are chloride stress corrosion cracking (Cl-SCC), polythionic acid stress corrosion (PASCC) and low temperature brittle fracture.

With the enormous increase in hydroprocessing capacity in the industry in recent decades, there has been a corresponding increase in the quantity of piping and equipment fabricated from austenitic stainless steels which are susceptible to both Cl-SCC and PASCC.

Critical factors for Cl-SCC are chloride content, pH, temperature, stress, O2 content and alloy composition. Drains are particularly susceptible during start up and shutdown if not purged.

Critical factors for PASCC are components with surface sulfide scale which can react with air and moisture to produce polythionic acid, stress and metals which have become sensitized due to chromium carbide precipitation (high temperature operation and in particular heat affected zones of welds).

Critical factors for low temperature brittle fracture are grain size of metallic components, thickness of components (thicker components are more susceptible), temperatures below the Charpy ductile to brittle transition temperature especially in cases of auto-refrigeration in units processing light hydrocarbons such as methane, ethane/ethylene, propane or butane. Compounding this threat are variations in piping specs used by various companies. A recent survey of the piping specs used by 3 major oil companies indicates all 3 are ASME compliant, but presumably 2 of the companies have factored in their experiences and have required stricter standards for materials in potentially low temperature operation. Age of the equipment may also be a factor. Equipment manufactured to ASME B&PV Code, Section VIII, Div 1 prior to December, 1987 may be more susceptible.

To illustrate the potential dilemma, one oil company allows A106 piping to be used at minus 20 degrees F (consistent with ASME non-shock conditions), while the boiling point of propane is minus 44 degrees F and the ductile/brittle transition temperature is +50 degrees F.

Since all three of these mechanisms are likely to occur during startup, shutdown or upset conditions they need to be taken into consideration when conducting a PHA with special attention to the actual materials of construction.