Analysis of Refinery Equipment to Improve Unit Start-Ups and Shut Downs

Analysis of Refinery Equipment to Improve Unit Start-ups and Shut Downs

Gerald W. Wilks, Senior Engineer

Lemont Refinery, CITGO Petroleum Corporation

Abstract

In the Oil Industry much of the equipment operates at high enough temperatures to avoid the risk of brittle fracture during typical operations. However, there are start-up and shut‑down periods that subject pressure vessels and piping to combinations of stress and low temperature that could lead to brittle fracture.  In the past, the risk of equipment failure led to the development of start-up and shut-down procedures that sometimes require long delays. To minimize the time required for unit start-up and shut‑down, brittle fracture analysis of refinery equipment is being conducted today to determine the acceptable operating temperatures over the range of pressures.  The guidelines in API 579 are usually used for these analyses. The results of these analyses are the generation of graphs called “safe pressurization curves”.  Operations that keep the operating conditions above these curves can be used to shorten start-up and shut-down time periods thereby saving refineries money.  There is also a risk of brittle fracture during auto-refrigeration events associated with pressure changes in light hydrocarbon systems, and brittle fracture analyses can also deal with auto‑refrigeration risks. This paper provides background information on brittle fracture analysis along with several examples of brittle fracture analysis of refinery equipment.  Examples that show the effects of past process conditions on the results of brittle fracture analysis are also reviewed.   

Keywords:  Fracture, brittle, pressure vessel, piping, refinery, hydrogen, hydrotesting, auto-refrigeration, carbon steel, low alloy steel.