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Carbon steel (CS) piping and equipment are used extensively in refineries and petrochemical plants, where fluid temperatures vary from moderate to high for various processes.
The American Society of Mechanical Engineers (ASME) code B31.3 limits the use of CS material in piping systems operating up to a maximum temperature of 427 [degrees]C, due to the conversion of carbides to graphite that may occur after prolonged exposure to temperatures above 427 [degrees]C. Stress analysis of such systems becomes critical because the allowable stress values are much lower at temperatures above 427 [degrees]C.
In the hydrocarbon process industries (HPI), some systems will be exposed to temperatures above 427 [degrees]C for a short duration because of various process upsets. A few examples of these systems are:
A pressure safety valve (PSV) discharge of high-high pressure (HHP) steam, design pressure = 105 kg/cm[superscript]2 at 510 [degrees]C
The acetylene converter, or the conversion of acetylene into ethylene by a cracking process, or an exothermic reaction, of a cracker plant
An ethylene oxide reactor of a monoethylene glycol (MEG) plant during run-away reaction, pre-ignition or post-ignition
A high-purity isobutylene (HPIB) unit's selective hydrogenation reactor during regeneration, approximately 100 hr/yr at 450 [degrees]C.
It is recommended to use an alloy steel (Cr-Mo) material for piping and equipment rather than for CS. Considering the long-term, creep-fatigue approach for the design of such piping systems for short-term, high-temperature applications requires the use of Cr-Mo alloy piping and equipment.
Designing application systems
A logically safe, but not an ultra-conservative, methodology in the form of case studies for the design of such piping systems and equipment should be followed. Different approaches for the design of such systems are graphitization, creep and time dependency, and the life fraction rule.
The graphitization phenomenon is dependent on material type, time and temperature. Graphitization generally results from the decomposition of pearlite (iron + iron carbide) into the equilibrium structure of iron +...